Transcriber's Notes:

1. Subscripts have been marked with an underscore character in front with text surrounded in curly braces, for example: H_{2}O (formula of water). 

2. Inconsistent hyphenation of words preserved. 

3. Several misprints fixed. A full list of corrections can be found at the end of the text. 

[Illustration: LIGHT AND LIBERTY]

 The Century Books of Useful Science

 ARTIFICIAL LIGHT

 ITS INFLUENCE UPON CIVILIZATION

 BY M. LUCKIESH

 DIRECTOR OF APPLIED SCIENCE. NELA RESEARCH LABORATORY, NATIONAL LAMP WORKS OF GENERAL ELECTRIC COMPANY

 Author of "Color and Its Applications, " "Light and Shade and Their Applications, " "The Lighting Art, " "The Language of Color, " etc. 

 _ILLUSTRATED WITH PHOTOGRAPHS_

 NEW YORK THE CENTURY CO. 1920

 Copyright, 1920, by THE CENTURY CO. 

 DEDICATED

 TO THOSE WHO HAVE ENCOURAGED ORGANIZED SCIENTIFIC RESEARCH FOR THE ADVANCEMENT OF CIVILIZATION

PREFACE

In the following pages I have endeavored to discuss artificial light forthe general reader, in a manner as devoid as possible of intricatedetails. The early chapters deal particularly with primitive artificiallight and their contents are generally historical. The science oflight-production may be considered to have been born in the latter partof the eighteenth century and beginning with that period a few chapterstreat of the development of artificial light up to the present time. Until the middle of the nineteenth century _mere_ light was available, but as the century progressed, the light-sources through the applicationof science became more powerful and efficient. Gradually _mere_ lightgrew to _more_ light and in the dawn of the twentieth century _adequate_light became available. In a single century, after the development ofartificial light began in earnest, the efficiency of light-productionincreased fifty-fold and the cost diminished correspondingly. The nextgroup of chapters deals with various economic influences of artificiallight and with some of the byways in which artificial light is servingmankind. On passing through the spectacular aspects of lighting wefinally emerge into the esthetics of light and lighting. 

The aim has been to show that artificial light has become intricatelyinterwoven with human activities and that it has been a powerfulinfluence upon the progress of civilization. The subject is tooextensive to be treated in detail in a single volume, but an effort hasbeen made to present a discussion fairly complete in scope. It is hopedthat the reader will gain a greater appreciation of artificial light asan economic factor, as an artistic medium, and as a mighty influenceupon the safety, efficiency, health, happiness, and general progress ofmankind. 

 M. LUCKIESH. 

ACKNOWLEDGMENTS

It is a pleasant duty to acknowledge the coöperation of variouscompanies in obtaining the photographs which illustrate this book. Withthe exception of Plates 2 and 7, which are reproduced from the excellentworks of Benesch and Allegemane respectively, the illustrations of earlylighting devices are taken from an historical collection in thepossession of the National Lamp Works of the General Electric Co. Tothis company the author is indebted for Plates 1, 3, 4, 5, 6, 9, 11, 15, 18b, 20, 21, 29; to Dr. McFarlan Moore for Plate 10; to Macbeth EvansGlass Co. For Plate 12; to the Corps of Engineers, U. S. Army, for Plate13; to Lynn Works of G. E. Co. For Plates 14, 16; to Edison Lamp Works ofG. E. Co. For Plates 17, 24; to Cooper Hewitt Co. For Plate 18a; toR. U. V. Co. For Plate 19; to New York Edison Co. For Plates 22, 26, 30;to W. D'A. Ryan and the Schenectady Works of G. E. Co. For Plates 23, 25, 31; to National X-Ray Reflector Co. For Plate 28. Besides the companiesand the individuals particularly involved in the foregoing, the authoris glad to acknowledge his appreciation of the assistance of othersduring the preparation of this volume. 

CONTENTS

CHAPTER PAGE

 I LIGHT AND PROGRESS 3

 II THE ART OF MAKING FIRE 15

 III PRIMITIVE LIGHT-SOURCES 24

 IV THE CEREMONIAL USE OF LIGHT 38

 V OIL-LAMPS OF THE NINETEENTH CENTURY 51

 VI EARLY GAS-LIGHTING 63

 VII THE SCIENCE OF LIGHT-PRODUCTION 80

 VIII MODERN GAS-LIGHTING 97

 IX THE ELECTRIC ARCS 111

 X THE ELECTRIC INCANDESCENT FILAMENT LAMPS 127

 XI THE LIGHT OF THE FUTURE 143

 XII LIGHTING THE STREETS 152

 XIII LIGHTHOUSES 163

 XIV ARTIFICIAL LIGHT IN WARFARE 178

 XV SIGNALING 194

 XVI THE COST OF LIGHT 208

 XVII LIGHT AND SAFETY 225

XVIII THE COST OF LIVING 238

 XIX ARTIFICIAL LIGHT AND CHEMISTRY 256

 XX LIGHT AND HEALTH 269

 XXI MODIFYING ARTIFICIAL LIGHT 284

XXII SPECTACULAR LIGHTING 298

XXIII THE EXPRESSIVENESS OF LIGHT 310

 XXIV LIGHTING THE HOME 325

 XXV LIGHTING--A FINE ART? 341

 READING REFERENCES 357

 INDEX 359

LIST OF ILLUSTRATIONS

Light and Liberty _Frontispiece_ FACING PAGEPrimitive fire-baskets 16

Crude splinter-holders 16

Early open-flame oil and grease lamps 17

A typical metal multiple-wick open-flame oil-lamp 32

A group of oil-lamps of two centuries ago 33

Lamps of a century or two ago 56

Elaborate fixtures of the age of candles 57

Flame arc 128

Direct current arc 128

On the testing-racks of the manufacturer ofincandescent filament lamps 129

Carbon-dioxide tube for accurate color-matching 160

The Moore nitrogen tube 160

Modern street lighting 161

A completed lighthouse lens 176

Torro Point Lighthouse, Panama Canal 176

American search-light position on Western Front in1919 177

American standard field search-light and power unit 177

Signal-light for airplane 232

Trench light-signaling outfit 232

Aviation field light-signal projector 232

Signal search-light for airplane 232

Unsafe, unproductive lighting worthy of the dark ages 233

The same factory made safe, cheerful, and moreproductive by modern lighting 233

Locomotive electric headlight 240

Search-light on a fire-boat 240

Building ships under artificial light at Hog IslandShipyard 241

Artificial light in photography 256

Sterilizing water with radiant energy from quartzmercury-arcs 257

Judging color under artificial daylight 272

Artificial daylight 273

Fireworks and illuminated battle-fleet atHudson-Fulton Celebration 288

Fireworks exhibition on May Day at Panama-PacificExposition 289

The new flood lighting contrasted with the oldoutline lighting 304

Niagara Falls flooded with light 305

Artificial light honoring those who fell and thosewho returned 320

The expressiveness of light in churches 321

Obtaining two different moods in a room by a portablelamp which supplies direct and indirect componentsof light 336

The lights of New York City 337

Artificial light in community affairs 352

Panama-Pacific Exposition 353

ARTIFICIAL LIGHT

I

LIGHT AND PROGRESS

The human race was born in slavery, totally subservient to nature. Theearliest primitive beings feasted or starved according to nature'sbounty and sweltered or shivered according to the weather. When nightfell they sought shelter with animal instinct, for not only wereactivities almost completely curtailed by darkness but beyond its screenlurked many dangers. It is interesting to philosophize upon adistinction between a human being and the animal just below him in thescale, but it may serve the present purpose to distinguish the humanbeing as that animal in whom there is an unquenchable and insatiabledesire for independence. The effort to escape from the bondage of natureis not solely a human instinct; animals burrow or build retreats throughthe instinct of self-preservation. But this instinct in animals is soonsatisfied, whereas in human beings it has been leading ever onwardtoward complete emancipation. 

The progress of civilization is a long chain of countless achievementseach one of which has increased man's independence. Early man perhapsdid not conceive the idea of fire and then set out to produce it. Hisinfant mind did not operate in this manner. But when he accidentallystruck a spark, produced fire by friction, or discovered it in someother manner, he saw its possibility. It is thrilling to pictureprimitive man at his first bonfire, enjoying the warmth, or at leastinterested in it. But how wonderful it must have become as twilight'scurtain was drawn across the heavens! This controllable fire emitted_light_. It is easy to imagine primitive man pondering over thisphenomenon with his sluggish mind. Doubtless he cautiously picked up aflaming stick and timidly explored the crowding darkness. Perhaps hecarried it into his cave and behold! night had retreated from his abode!No longer was it necessary for him to retire to his bed of leaves whendaylight failed. The fire not only banished the chill of night but was apower over darkness. Viewed from the standpoint of civilization, itsdiscovery was one of the greatest strides along the highway of humanprogress. The activities of man were no longer bounded by sunrise andsunset. The march of civilization had begun. 

In the present age of abundant artificial light, with its manifoldlight-sources and accessories which have made possible countlessapplications of light, mankind does not realize the importance of thiscomfort. Its wonderful convenience and omnipresence have resulted inindifference toward it by mankind in general, notwithstanding the factthat it is essential to man's most important and educative sense. Byextinguishing the light and pondering upon his helplessness in theresulting darkness, man may gain an idea of its overwhelming importance. Those unfortunate persons who suffer the terrible calamity of blindnessafter years of dependence upon sight will testify in heartrending termsto the importance of light. Milton, whose eyesight had failed, laments, O first created beam and thou great Word "Let there be light, " and light was over all, Why am I thus bereaved thy prime decree?

Perhaps only through a similar loss would one fully appreciate thetremendous importance of light to him, but imagination should be capableof convincing him that it is one of the most essential andpleasure-giving phenomena known to mankind. 

A retrospective view down the vista of centuries reveals by contrast thecomplexity with which artificial light is woven into human activities ofthe present time. Written history fails long before the primitive racesare reached, but it is safe to trust the imagination to penetrate thefog of unwritten history and find early man huddled in his cave asdaylight wanes. Impelled by the restless spirit of progress, thisprimitive being grasped the opportunity which fire afforded to extendhis activities beyond the boundaries of daylight. The crude art upon thewalls of his cave was executed by the flame of a smoking fagot. The fireon the ledge at the entrance to his abode became a symbol of home, asthe fire on the hearth has symbolized home and hospitality throughoutsucceeding ages. The accompanying light and the protection from coldcombined to establish the home circle. The ties of mated animalsexpanded through these influences to the bonds of family. Thus light waswoven early into family life and has been throughout the ages amoralizing and civilizing influence. To-day the residence functions as ahome mainly under artificial light, for owing to the conditions ofliving and working, the family group gathers chiefly after daylight hasfailed. 

From the pine knot of primitive man to the wonderfully convenientlight-sources of to-day there is a great interval, consisting, asappears retrospectively, of small and simple steps long periods apart. Measured by present standards and achievements, development was slow atfirst and modern man may be inclined to impatience as he views thehistory of light and human progress. But the achievements of earlycenturies, which appear so simple at the present time, were really greataccomplishments when considered in the light of the knowledge of thoseremote periods. Science as it exists to-day is founded upon provedfacts. The scientist, equipped with a knowledge of physical and chemicallaws, is led by his imagination into the darkness of the unexploredunknown. This knowledge illuminates the pathway so that hypotheses areintelligently formed. These evolve into theories which are graduallyaltered to fit the accumulating facts, for along the battle area ofprogress there are innumerable scouting-parties gaining secrets fromnature. These are supported by individuals and by groups, who verify, amplify, and organize the facts, and they in turn are followed byinventors who apply them. Liaison is maintained at all points, but theattack varies from time to time. It may be intense at certain places andother sectors may be quiet for a time. There are occasional reverses, but the whole line in general progresses. Each year witnesses theacquirement of new territory. It is seen that through the centuriesthere is an ever-growing momentum as knowledge, efficiency, andorganization increase the strength of this invading army of scientistsand inventors. 

The burning fagot rescued mankind from the shackles of darkness, and thegrease-lamp and tallow-candle have done their part. Progress was slow inthose early centuries because the great minds of those agesphilosophized without a basis of established facts: scientific progressresulted more from an accumulation of accidental discoveries than by adirected attack of philosophy supported by the facts established byexperiment. It was not until comparatively recent times, at most threecenturies ago, that the great intellects turned to systematicallyorganized scientific research. Such men as Newton laid the foundationfor the tremendous strides of to-day. The store of facts increased andas the attitude changed from philosophizing to investigating, theorganized knowledge grew apace. All of this paved the way for themomentous successes of the present time. 

The end is not in sight and perhaps never will be. The unexplored regionextends to infinity and, judged by the past, the momentum of discoverywill continue to increase for ages to come, unless the human race decaysthrough the comfort and ease gained from utilizing the magic secretswhich are constantly being wrested from nature. Among the achievementsof science and invention, the production and application of artificiallight ranks high. As an influence upon civilization, no singleachievement surpasses it. 

Without artificial light, mankind would be comparatively inactive aboutone half its lifetime. To-day it has been fairly well established thatthe human organism can flourish on eight hours' sleep in a period oftwenty-four hours. Another eight hours spent in work should settle man'sobligation to the world. The remaining hours should be his own. Artificial light has made such a distribution of time possible. Theworking-periods in many cases may be arranged in the interests ofeconomy, which often means continuous operations. The sun need not beconsidered when these operations are confined to interiors or localizedoutdoors. 

Thus, artificial light has been an important factor in the greatindustrial development of the present time. Man now burrows into theearth, navigates under water, travels upon the surface of land and sea, and soars among the clouds piloted by light of his own making. Progressdoes not halt at sunset but continues twenty-four hours each day. Building, printing, manufacturing, commerce, and other activities areprosecuted continuously, the working-shifts changing at certain periodsregardless of the rising or setting sun. Adequate artificial lightingdecreases spoilage, increases production, and is a powerful factor inthe prevention of industrial accidents. 

It has ever been true since the advent of artificial light that theintellect has been largely nourished after the completion of the day'swork. The highly developed artificial lighting of the present time mayaccount for much of the vast industry of publication. Books, magazines, and newspapers owe much to convenient and inexpensive artificial light, for without it fewer hours would be available for recreation andadvancement through reading. Schools, libraries, and art museums may beattended at night for the betterment of the human race. The immortalLincoln, it is said, gained his early education largely by the light ofthe fireplace. But all were not endowed with the persistence of Lincoln, so that illiteracy was more common in his day than in the present age ofadequate illumination. 

The theatrical stage not only depends for its effectiveness uponartificial light but owes its existence and development largely to thisagency. In the moving-picture theater, pictures are projected upon thescreen by means of it and even the production of the pictures isindependent of daylight. These and a vast number of recreationalactivities owe much, and in some cases their existence, to artificiallight. 

Not many centuries ago the streets at night were overrun by thieves andto venture outdoors after dark was to court robbery and even bodilyharm. In these days of comparative safety it is difficult to realize theinfluence that abundant illumination has had in increasing the safety oflife and property. Maeterlinck in his poetical drama, "The Bluebird, "appropriately has made _Light_ the faithful companion of mankind. ThePalace of Night, into which _Light_ is not permitted to enter, is theabode of many evils. Thus the poet has played upon the primitiveinstincts of the impressiveness of light and darkness. 

By combining the symbolism of light, color, and darkness with theinstincts which have been inherited by mankind from its superstitiousancestry of the age of mythology, another field of application ofartificial light is opened. Light has gradually assumed such attributesas truth, knowledge, progress, enlightenment. Throughout the early ageslight was more or less worshiped and thus artificial lights became wovenin many religious ceremonies. Some of these have persisted to thepresent time. The great pageants of peace celebrations and world'sexpositions appropriately feature artificial light. In drawing upon thepotentiality of the expressiveness and impressiveness of light andcolor, artificial light is playing a major part. Doubtless the futuregenerations will be entertained by gorgeous symphonies of light. Experiments are performed in this direction now and then, and it isreasonable to expect that after many centuries of cultivation of theappreciation of light-symphonies, these will take a place among thearts. The elaborate and complicated music of the present time isappreciated by civilized nations only after many centuries of slowcultivation of taste and understanding. 

Light-therapy is to-day a distinct science and art. The germicidalaction of light-rays and of some of the invisible rays which ordinarilyaccompany the luminous rays is well proved. Wounds are treatedeffectively and water is sterilized by the ultraviolet radiant energy inmodern artificial illuminants. 

Thousands of lighthouses, light-ships, and light-buoys are scatteredalong sea-coasts, rivers, and channels. They guide the wheelman and warnthe lookout of shoals and reefs. Some of these send forth flashes oflight whose intensities are measured in millions of candle-power. Manyare unattended for days and even months. These powerful lights dominatedby automatic mechanisms have replaced the wood-fires which weremaintained a few centuries ago upon certain prominent points. 

Signal-lights now guide the railroad train through the night. A burningflare dropped from the rear of a train keeps the following train at asafe distance. Huge search-lights penetrate the night air for manymiles. When these are equipped with shutters, a code may be flashed fromone ship to another or between the vessel and land. A code from apowerful search-light has been read a hundred miles away because theflashes were projected upon a layer of high clouds and were thus visiblefar beyond the horizon. 

Artificial light played its part in the recent war. Huge search-lightequipments were devised for portability. This mobile apparatus wasutilized against enemy aircraft and in various other ways. Smallhand-lamps are used to send out a pencil of light as directed by a pairof sights and the code is flashed by means of a trigger. Raiding-partiesare no longer concealed by the curtain of darkness, for rockets andstar-shells are used to illuminate large areas. Flares sent upward todrift slowly downward supported by parachutes saved and cost many livesduring the recent war. Rockets are used by ships in distress and also bybeleaguered troops. 

Experiments are being prosecuted to ascertain the possibilities ofartificial light in the forcing of plant-growth, and even chickens aremade to work longer hours by its use. 

Artificial light is now modified in color or spectral character to meetmany requirements. Daylight has been reproduced in spectral quality sothat certain processes requiring accurate discrimination of color arenow prosecuted twenty-four hours a day under artificial daylight. Colored light is made of the correct quality which does not affectphotographic plates of various sensibilities. Monochromatic light isutilized in photo-micrography for the best rendition of detail. Light-waves have been utilized as standards of length because they areinvariable and fundamental. Numerous other interesting adaptations ofartificial light are in daily use. 

This is in reality the age of artificial light, for mankind has not onlybecome independent of daylight in certain respects, but has improvedupon natural light. The controllability of artificial light makes itsuperior to natural light in many ways. In fact, uses have been made ofartificial light which are impossible with natural light. Light-sourcesmay be made of a vast variety of shapes, and those may be transportedwherever desired. They may be equipped with reflectors and other opticaldevices to direct or to diffuse the light as required. 

Thus, artificial light to-day has numerous advantages over light whichhas been furnished by the Creator. It is sometimes stated that it cannever compete with daylight in cheapness, inasmuch as the latter costsnothing. But this is not true. Even in the residence, daylight costssomething, because windows are more expensive than plain walls. Theexpense of washing windows is an appreciable percentage of the cost ofgas or electricity. And there is window-breakage to be considered. 

In the more elaborate buildings of the congested portions of cities, daylight is satisfactory a lesser number of hours than in the outlyingdistricts. In some stores, offices, and factories artificial light isused throughout the day. Still, the daylighting-equipment is installedand maintained. Furthermore, when it is considered that much expensivearea is given to light-courts and much valuable wall space to windows, it is seen that the cost of daylight in congested cities is in realityconsiderable. Of course, the daylighting-equipment has value inventilating, but ventilation may be taken care of in a very satisfactorymanner as a separate problem. 

The cost of skylights in museums and other large buildings is fargreater than that of ordinary ceilings and walls, and the extraallowance for heating is appreciable. The expense of maintenance of someskylights is considerable. Thus it is seen that the cost and maintenanceof daylighting-equipment, the loss of valuable rental space and of wallarea, and the increased expense of heating are factors which challengethe statement that daylight costs nothing. In fact, it is not surprisingto find that occasionally the elimination of daylighting--the relianceupon artificial light alone--has been seriously contemplated. When thepossibilities of the latter are considered, it is reasonable to expectthat it will make greater and greater inroads and that many buildings ofthe future will be equipped solely with artificial-lighting systems. 

Naturally, with the tremendous development of artificial light duringthe present age, a new profession has arisen. The lighting expert isevolving to fill the needs. He is studying the problems of producing andutilizing artificial illumination. He deals with the physics oflight-production. His studies of utilization carry him into the vastfields of physiology and psychology. His is a profession whicheventually will lead into numerous highways and byways of enterprise, because the possibilities of lighting extend into all those activitieswhich make their appeal to consciousness through the doorway of vision. These possibilities are limited only by the boundaries of human endeavorand in the broadest sense extend even beyond them, for light is one ofthe most prominent agencies in the scheme of creation. It contributeslargely to the safety, the efficiency, and the happiness of civilizedbeings and beyond all it is a powerful civilizing agency. 

II

THE ART OF MAKING FIRE

Scattered over the earth at the present time various stages ofcivilization are to be found, from the primitive savages to the mosthighly cultivated peoples. Although it is possible that there are tribesof lowly beings on earth to-day unfamiliar with fire or ignorant of itsuses, savages are generally able to make fire. Thus the use of fire mayserve the purpose of distinguishing human beings from the lower animals. Surely the savage of to-day who is unable to kindle fire or whopossesses a mind as yet insufficiently developed to realize itspossibilities, is quite at the mercy of nature's whims. He lives merelyby animal prowess and differs little in deeds and needs from the beastsof the jungle. In this imaginary journey to the remote regions beyondthe outskirts of civilization it soon becomes evident that thedevelopment of artificial light may be a fair measure of civilization. 

In viewing the development of artificial light it is seen that precedingthe modern electrical age, man depended universally upon burningmaterial. Obviously, the course of civilization has been highly complexand cannot be symbolized adequately by the branching tree. From itsobscure beginning far in the impenetrable fog of prehistoric times, ithas branched here and there. These various branches have been subjectedto many different influences, with the result that some flourished andendured, some retrogressed, some died, some went to seed and fell totake root and to begin again the upward climb. The ultimate result isthe varied civilization of the present time, a study of which aids inpenetrating the veil that obscures the ages of unrecorded writing. Likewise, material relics of bygone ages supply some threads of thestory of human progress and mythology aids in spanning the misty gapbetween the earliest ages of man and the period when historic writingswere begun. Throughout these various stages it becomes manifest that thedevelopment of artificial light is associated with the progress ofmankind. 

According to a certain myth, Prometheus stole fire from heaven andbrought this blessing to earth. Throughout the mythologies of variousraces, fire and, as a consequence, light have been associated withdivinity. They have been subjects of worship perhaps more generally thananything else, and these early impressions have survived in theceremonial uses of light and fire even to the present time. The originof fire as represented in any of the myths of the superstitious beingsof early ages is as suitable as any other, inasmuch as definiteknowledge is unavailable. Active volcanoes, spontaneous combustion, friction, accidental focusing of the sun's image, and other means mayhave introduced primitive beings to fire. A study of savage tribes ofthe present age combined with a survey of past history of mythology, ofmaterial relics, and of the absence of lamps or other lighting utensilsleads to the conclusion that the earliest source of light was the woodfire. 

[Illustration: PRIMITIVE FIRE-BASKETS]

[Illustration: CRUDE SPLINTER-HOLDERS]

[Illustration: EARLY OPEN-FLAME OIL AND GREASE LAMPS]

Even to-day the savages of remote lands have not advanced further thanthe wood-fire stage, and they may be found kneeling upon the groundenergetically but skilfully rubbing sticks together until the frictionkindles a fire. In using these fire-sticks they convert mechanicalenergy into heat energy. This is a fundamental principle of physics, employed by them as necessity demands, but they are totally ignorant ofit as a scientific law. The things which these savages learn are theresult of accidental discovery. Until man pondered over such simplefacts and coördinated them so that he could extend his knowledge bygeneral reasoning, his progress could not be rapid. But the sluggishmind of primitive man is capable of devising improvements, howeverslowly, and the art of making fire by means of rubbing fire-sticksgradually became more refined. Mechanical improvements resulted fromexperience, with the consequence that finally one stick was rubbed toand fro in a groove, or was rapidly twirled between the palms of thehands while one end was pressed firmly into a hole in a piece of wood. In the course of a few seconds or a minute, depending upon skill andother conditions, a fire was obtained. It is interesting to note howcivilized man is often compelled by necessity to adopt the methods ofprimitive beings. The rubbing of sticks is an emergency measure of themaster of woodcraft at the present time, and the production of fire inthis manner is the proud accomplishment or ambition of every Boy Scout. 

Where only such crude means of kindling fire were available it becamethe custom in some cases to maintain a fire burning continuously in apublic place. Around this pyrtaneum the various civil, political, andreligious affairs were carried on by the light and warmth of the publicfire. Many quaint customs evolved, apparently, from this ancientprocedure. 

The tinder-box of modern centuries doubtless originated in very earlytimes, for it is inconceivable that the earliest beings did not becomeaware of the production of sparks when certain stones were strucktogether. In the stone age, when human beings spent much of their timechiseling implements and utensils from stone by means of tools of thesame substance, it appears certain that this means of producing fire wasever apparent. Many of their sharp implements, such as knives andarrow-heads, were made of quartz and similar material and it is likelythat the use of two pieces of quartz for producing a spark originated inthose remote periods. Alaskan and Aleutian tribes are known to haveemployed two pieces of quartz covered with native sulphur. When thesewere struck together with skill, excellent sparks were obtained. 

Later, when iron and steel became available, the more modern tinder-boxwas developed. An early application of the flint-and-steel principle wasmade by certain Esquimo tribes who obtained fire by striking a piece ofquartz against a piece of iron pyrites. The latter is a yellow sulphideof iron, of crystalline form, best known as "fool's gold. " Doubtless, the more primitive beings used dried grass, leaves, and moss asinflammable material upon which the sparks were showered. In latercenturies the tinder-box was filled with charred grass, linen, andpaper. There was a long interval between the development of fire-sticksand that of the tinder-box as measured by the progress of civilization. During recent centuries ordinary brown paper soaked in saltpeter anddried was utilized satisfactorily as an inflammable material. Suchdevices have been employed in past ages in widely separated regions ofthe earth. Elaborate specimens of tinder-boxes from Jamaica, Japan, China, Europe, and various other countries are now reposing in thecollections in the possession of museums and of individuals. 

If the radiant energy from the sun is sufficiently concentrated uponinflammable material, the latter will ignite. Such concentration may beachieved by means of a convex lens or a concave mirror. This method ofproducing fire does not antedate the more primitive methods such asstriking quartz or rubbing wooden sticks, because the materials requiredare not readily found or prepared, but it is of very remote origin. Aristophanes in his comedy "The Clouds, " which is a satire aimed at thescience and philosophy of his period (488-385 B. C. ), mentionsthe "burning lens. " Nearly every one is familiar with an achievementattributed to Archimedes in which he destroyed the ships at Syracuse byfocusing the image of the sun upon them by means of a concave mirror. The ancient Egyptians were proficient in the art of glass-making, so itis likely that the "burning-glass" was employed by them. Even a crudelens of glass will focus an image of the sun sufficiently well to causeinflammable material to ignite. 

The energy in sunlight varies enormously, even on clear days, becausethe water-vapor in the atmosphere absorbs some of the radiant energyemitted by the sun. This absorbed radiation is chiefly known asinfra-red energy, which does not arouse the sensation of light. When thewater-vapor content of the atmosphere is high, the sun, though it mayappear as bright to the eye, in reality is not as hot as it would be ifthe water-vapor were not present. However, a fire may be kindled byconcentrating only the visible rays in sunlight because of the enormousintensity of sunlight. A convex lens fashioned from ice by means of asharp-edged stone and finally shaped by melting the surfaces as they arerubbed in the palms of the hands, will kindle a fire in highlyinflammable material if the sun is high and the atmosphere is fairlyclear. Burning-glasses are used to a considerable extent at the presenttime in certain countries and it is reported that British soldiers weresupplied with them during the Boer War. Indicative of the predominantuse to which the glass lens was applied in the past is the employment ofthe term "burning-glass" instead of lens in the scientific writings aslate as a century or two ago. 

As civilization advanced, leading intellects began to inquire into themysteries of nature and the periods of pure philosophy gave way to anera of methodical research. Alchemy and superstition began to retirebefore the attacks of those pioneers who had the temerity to believethat the scheme of creation involved a vast network of invariable laws. In this manner the powerful sciences of physics and chemistry were borna few centuries ago. Among other things the production of fire and lightreceived attention and the "dark ages" were doomed to end. The crude, uncertain, and inconvenient methods of making fire were replaced bysteadily improving scientific devices. 

Matches were at first cumbersome, dangerous, and expensive, but thesegradually evolved into the safety matches of the present time. Althoughthey were primarily intended for lighting fires and various kinds oflamps, billions of them are now used yearly as convenient light-sources. Smoldering hemp or other material treated with niter and othersubstances was an early form of match used especially for dischargingfirearms. The modern wax-taper is an evolutionary form of this type oflight-source. 

Phosphorus has long played a dominant rôle in the preparation ofmatches. The first attempt at making them in their modern form appearsto have occurred about 1680. Small pieces of phosphorus were used inconnection with small splints of wood dipped in sulphur. This type ofmatch did not come into general use until after the beginning of thenineteenth century, owing to its danger and expense. White or yellowphosphorus is a deadly poison; therefore the progress of the phosphorusmatch was inhibited until the discovery of the relatively harmless formknown as red phosphorus. The first commercial application of this formwas made in about 1850. 

An early ingenious device consisted of a piece of phosphorus containedin a tube. A piston fitted snugly into the tube, by means of which theair could be compressed and the phosphorus ignited. Sulphur matches wereignited from the burning tinder, the latter being fired by flint andsteel. In 1828 another form of match consisted of a glass tubecontaining sulphuric acid and surrounded by a mixture of chlorate ofpotash and sugar. A pair of nippers was supplied with each box of these"matches, " by means of which the tip of the glass tube could be brokenoff. This liberated the acid, which upon mixing with the otheringredients set fire to them. To this contrivance a roll of paper wasattached which was ignited by the burning chemicals. 

The lucifer or friction matches appeared in about 1827, but successfulphosphorus matches were first made in about 1833. The so-called safetymatch of the present time was invented in the year 1855. To-day, thetotal daily output of matches reaches millions and perhaps billions. Automatic machinery is employed in preparing the splints of wood and indipping them into molten paraffin wax and finally into the ignitingcomposition. 

During recent years the principle of the tinder-box has been revived ina device in which sparks are produced by rubbing the mineral cerite (ahydrous silicate of cerium and allied metals) against steel. Thesesparks ignite a gas-jet or a wick soaked in a highly inflammable liquidsuch as gasolene or alcohol. This device is a tinder-box of the modernscientific age. 

Naturally with the advent of electricity, electrical sparks came intouse for lighting gas-jets and mantles and in isolated instances theyhave served as light-sources. Doubtless, every one is familiar with theparlor stunt of igniting a gas-jet from the discharge from thefinger-tips of static electricity accumulated by shuffling the feetacross the floor-rug. 

Although many of these methods and devices have been used primarily formaking fire, they have served as emergency or momentary light-sources. In the outskirts of civilization some of them are employed at thepresent time and various modern light-sources require a method ofignition. 

III

PRIMITIVE LIGHT-SOURCES

Many are familiar with the light of the firefly or of its larvæ, theglow-worm, but few persons realize that a vast number of insects andlower organisms are endowed with the superhuman ability of producinglight by physiological processes. Apparently the chief function of theselighting-plants within the living bodies is not to provide light in thesense that the human being uses it predominantly. That is, thesewonderful light-sources seem to be utilized more for signaling, forluring prey, and for protection than for strictly illuminating-purposes. Much study has been given to the production of light by animals, becausethe secrets will be extremely valuable to mankind. As one floats overtide-water on a balmy evening after dark and watches the pulsating spotsof phosphorescent light emitted by the lowly jellyfishes, hisimaginative mood formulates the question, "Why are these lowly organismsendowed with such a wonderful ability?"

Despite his highly developed mind and body and his boasted superiority, man must go forth and learn the secrets of light-production before hemay emancipate himself from darkness. If man could emit light inrelative proportion to his size as compared with the firefly, he wouldneed no other torch in the coal-mine. How independent he would be inextreme darkness where his adapted eyes need only a feeblelight-source! Primitive man, desiring a light-source and having no meansof making fire, imprisoned the glowing insects in a perforated gourd orreceptacle of clay, and thus invented the first lantern perhaps beforehe knew how to make fire. The fireflies of the West Indies emit acontinuous glow of considerable luminous intensity and the natives haveused these imprisoned insects as light-sources. Thus mankind hasexhibited his superiority by adapting the facilities at hand to thegrowing requirements which his independent nature continuouslynourished. His insistent demand for independence in turn has nourishedhis desire to learn nature's secrets and this desire has increased inintensity throughout the ages. 

The act of imprisoning a glowing insect was in itself no greater stridealong the highway of progress than the act of picking a tasty fruit fromits tree. However, the crude lantern perhaps directed his primitive mindto the possibilities of artificial light. The flaming fagot from thefire was the ancestor of the oil-lamp, the candle, the lantern, and theelectric flash-light. It is a matter of conjecture how much time elapsedbefore his feeble intellect became aware that resinous wood afforded abetter light-source than woods which were less inflammable. Nevertheless, pine knots and similar resinous pieces of wood eventuallywere favored as torches and their use has persisted until the presenttime. In some instances in ancient times resin was extracted from woodand burned in vessels. This was the forerunner of the grease-and theoil-lamp. In the woods to-day the craftsman of the wilds keeps on thelookout for live trees saturated with highly inflammable ingredients. 

Viewed from the present age, these smoking, flickering light-sourcesappear very crude; nevertheless they represent a wide gulf between theirusers and those primitive beings who were unacquainted with the art ofmaking fire. Although the wood fire prevailed as a light-sourcethroughout uncounted centuries, it was subjected to more or lessimprovement as civilization advanced. When the wood fire was broughtindoors the day was extended and early man began to develop his crudearts. He thought and planned in the comfort and security of his cave orhut. By the firelight he devised implements and even decorated his stonesurroundings with pictures which to-day reveal something of the thoughtsand activities of mankind during a civilization which existed manythousand years ago. 

When it was too warm to have a roaring fire upon the hearth, man devisedother means for obtaining light without undue warmth. He placed glowingembers upon ledges in the walls, upon stone slabs, or even uponsuspended devices of non-inflammable material. Later he split longsplinters of wood from pieces selected for their straightness of grain. These burning splinters emitting a smoking, feeble light were crude butthey were refinements of considerable merit. A testimonial of theirsatisfactoriness is their use throughout many centuries. Until veryrecent times the burning splinter has been in use in Scotland and inother countries, and it is probable that at present in remote districtsof highly civilized countries this crude device serves the meager needsof those whose requirements have been undisturbed by the progress ofcivilization. Scott, in "The Legend of Montrose, " describes a tablescene during a feast. Behind each seat a giant Highlander stood, holdinga blazing torch of bog-pine. This was also the method of lighting in theHomeric age. 

Crude clay relics representing a human head, from the mouth of which thewood-splinters projected, appear to corroborate the report that theflaming splinter was sometimes held in the mouth in order that bothhands of a workman would be free. Splinter-holders of many types havesurvived, but most of them are of the form of a crude pedestal with anotch or spring clip at its upper end. The splinter was held in thisclip and burned for a time depending upon its length and the characterof the wood. It was the business of certain individuals to preparebundles of splinters, which in the later stages of civilization weresold at the market-place or from house to house. Those who have observedthe frontiersman even among civilized races will be quite certain thatthe wood for splinters was selected and split with skill, and that thesplinters were burned under conditions which would yield the mostsatisfactory light. It is a characteristic of those who live close tonature, and are thus limited in facilities, to acquire a surprisingefficiency in their primitive activities. 

An obvious step in the use of burning wood as a light-source was toplace such a fire on a shelf or in a cavity in the wall. Later whenmetal was available, gratings or baskets were suspended from the ceilingor from brackets and glowing embers or flaming chips were placed uponthem. Some of these were equipped with crude chimneys to carry away thesmoke, and perhaps to increase the draft. In more recent centuries thefirst attempt at lighting outdoor public places was by means of metalbaskets in which flaming wood emitted light. It was the duty of thewatchman to keep these baskets supplied with pine knots. In earlycenturies street-lighting was not attempted, and no serious effortsworthy of consideration as adequate lighting were made earlier thanabout a century ago. As a consequence the "link-boy" came intoexistence. With flaming torch he would escort pedestrians to their homeson dark nights. This practice was in vogue so recently that the"link-boy" is remembered by persons still living. In England theprofession appears to have existed until about 1840. 

Somewhat akin to the wood-splinter, and a forerunner of the candle, wasthe rushlight. In burning wood man noticed that a resinous or fattymaterial increased the inflammability and added greatly to the amount oflight emitted. It was a logical step to try to reproduce this conditionby artificial means. As a consequence rushes were cut and soaked inwater. They were then peeled, leaving lengths of pith partiallysupported by threads of the skin which were not stripped off. Thesesticks of pith were placed in the sun to bleach and to dry, and afterthey were thoroughly dry they were dipped in scalding grease, which wassaved from cooking operations or was otherwise acquired for the purpose. A reed two or three feet long held in the splinter-holder would burn forabout an hour. Thus it is seen that man was beginning to progress in thedevelopment of artificial light. In developing the rushlight he waslaying the foundation for the invention of the candle. Pliny hasmentioned the burning of reeds soaked in oil as a feature of funeralrites. Many crude forerunners of the candle were developed in variousparts of the world by different races. For example, the Malays made atorch by wrapping resinous gum in palm leaves, thus devising a crudecandle with the wick on the outside. 

Many primitive uses of vegetable and animal fats were forerunners of theoil-lamp. In the East Indies the candleberry, which contains oily seeds, has been burned for light by the natives. In many cases burning fish andbirds have served as lamps. In the Orkney Islands the carcass of astormy petrel with a wick in its mouth has been utilized as alight-source, and in Alaska a fish in a split stick has provided a crudetorch for the natives. These primitive methods of obtaining artificiallight have been employed for centuries and many are in use at thepresent time among uncivilized tribes and even by civilized beings inthe remote outskirts of civilization. Surely progress is limited where aburning fish serves as a torch, or where, at best, the light-sources arefeeble, smoking, flickering, and ill-smelling!

Progress insisted upon a light-source which was free from the defects ofthe crude devices already described and the next developments wereimprovements to the extent at least that combustion was more thorough. The early oil-lamps and candles did not emit much smoke, but they werestill feeble light-sources and not always without noticeable odors. Nevertheless, they marked a tremendous advance in the production ofartificial light. Although they were not scientific developments in themodern sense, the early oil-lamp and the candle represented the greatpossibilities of utilizing knowledge rather than depending upon the rawproducts of nature in unmodified forms. The advent of these twolight-sources in reality marked the beginning of the civilization whichwas destined to progress and survive. 

Although such primitive light-sources as the flaming splinter and theglowing ember have survived until the present age, lamps consisting of awick dipped into a receptacle containing animal and vegetable oils havebeen in use among the more advanced peoples since prehistoric times. Oil-lamps are to be seen in the earliest Roman illustrations. During theheight of ancient civilization along the eastern shores of theMediterranean Sea, elaborate lighting was effected by means of theshallow grease-or oil-lamp. It is difficult to estimate the age in whichthis form of light-source originated, but some lamps in existence incollections at the present time appear to have been made as early asfour or five thousand years before the Christian era. It is noteworthythat such lamps did not differ materially in essential details fromthose in use as late as a few centuries ago. 

At first the grease used was the crude fat from animals. Vegetable oilsalso were burned in the early lamps. The Japanese, for example, extracted oil from nuts. When the demands of civilization increased, extensive efforts were made to obtain the required fats and oils. Amphibious animals of the North and the huge mammals of the sea wereslaughtered for their fat, and vegetable sources were cultivated. Later, sperm and colza were the most common oils used by the advancedraces. The former is an animal oil obtained from the head cavities ofthe sperm-whale; the latter is a vegetable oil obtained from rape-seed. Mineral oil was introduced as an illuminant in 1853, and the modern lampcame into use. 

The grease-and oil-lamps in general were of such a form that they couldbe carried with ease and they had flat bottoms so that they would restsecurely. The simplest forms had a single wick, but in others many wicksdipped into the same receptacle. The early ones were of stone, butlater, lamps were modeled from clay or terra cotta and finally frommetals. They were usually covered and the wick projected through a holein the top near the edge. Large stone vases filled with a hundred poundsof liquid fat are known to have been used in early times. As a part ofthe setting in the celebration of festivals the ancient nations of Asiaand Africa placed along the streets bronze vases filled with liquid fat. The Esquimaux to-day use this form of lamp, in which whale-oil and sealblubber is the fuel. Incidentally, these lamps also supply the onlyartificial heat for their huts and igloos. The heat from these feeblelight-sources and from their bodies keeps these natives of the arcticswarm within the icy walls of their abodes. 

Very beautiful oil-lamps of brass, bronze, and pewter evolved in suchcountries as Egypt. Many of these were designed for and used inreligious ceremonies. The oil-lamps of China, Scotland, and othercountries in later centuries were improved by the addition of a panbeneath the oil-receptacle, to catch drippings from the wick or oilwhich might run over during the filling. The Chinese lamps weresometimes made of bamboo, but the Scottish lamps were made of metal. Aflat metal lamp, called a crusie, was one of the chief products ofblacksmiths and was common in Scotland until the middle of thenineteenth century. This type of lamp was used by many nations and hasbeen found in the catacombs of Rome. The crusie was usually suspended byan iron hook and the flow of oil to the wick could be regulated bytilting. The wick in the Scottish lamps consisted of the pith of rushes, cloth, or twisted threads. These early oil-lamps were almost alwaysshallow vessels into which a short wick was dipped, and it was not untilthe latter part of the eighteenth century that other forms came intogeneral use. The change in form was due chiefly to the introduction ofscientific knowledge when mineral oil was introduced. As early as 1781the burning of naptha obtained by distilling coal at low temperatureswas first discussed, but no general applications were made until a laterperiod. This was the beginning of many marked improvements in oil-lamps, and was in reality the birth of the modern science of light-production. 

[Illustration: A TYPICAL METAL MULTIPLE-WICK OPEN-FLAME OIL-LAMP]

[Illustration: A GROUP OF OIL-LAMPS OF TWO CENTURIES AGO]

As the activities of man became more complex he met from his growingstore of knowledge the increasing requirements of lighting. Inconsequence, many ingenious devices for lighting were evolved. Forexample, in England in the seventeenth century man was already burrowinginto the earth for coal and of course encountered coal-gases. Theseinflammable gases were first known for the direful effects which they sooften produced rather than for their useful qualities. Although theywere known to miners long before they received scientific attention, theearliest account of them in the Transactions of the Royal Society waspresented in the year 1667. A description of early gas-lighting has beenreserved for a later chapter, but the foregoing is noted at this pointto introduce a novel early method of lighting in coal-mines whereinflammable gases were encountered. In discussing this coal-gas anotherearly writer stated that "it will not take fire except by flame" andthat "sparks do not affect it. " One of the early solutions of theproblem of artificial lighting under such conditions is summarized asfollows:

 Before the invention of Sir Humphrey Davy's Safety Lamp, this property of the gas gave rise to a variety of contrivances for affording the miners sufficient light to pursue their operations; and one of the most useful of these inventions was a mill for producing light by sparks elicited by the collision of flint and steel. 

Such a stream of sparks may appear a very crude and unsatisfactorysolution as judged by present standards, but it was at least aningenious application of the facilities available at that time. Variousother devices were resorted to in the coal-mines before the introductionof a safety lamp. 

In discussing the candle it is necessary again to go back to an earlyperiod, for it slowly evolved in the course of many centuries. It is thenatural descendant of the rushlight, the grease-lamp, and variousprimitive devices. Until the advent of the more scientific age ofartificial lighting, the candle stood preëminent among earlylight-sources. It did not emit appreciable smoke or odor and it wasconveniently portable and less fragile than the oil-lamp. Candles havebeen used throughout the Christian era and some authorities are inclinedto attribute their origin to the Phoenicians. It is known that theRomans used them, especially the wax-candles, in religious ceremonies. The Phoenicians introduced them into Byzantium, but they disappearedunder the Turkish rule and did not come into use again until the twelfthcentury. 

The wax-candle was very much more expensive than the tallow-candle untilthe fifteenth century, when its relative cost was somewhat reduced, bringing it within the means of a greater proportion of the people. Nevertheless it has long been used, chiefly by the wealthy; thedeparting guest of the early Victorian inn would be likely to find anitem on his bill such as this: "For a gentleman who called himself agentleman, wax-lights, 5/. " Poor men used tallow dips or went to bed inthe dark. It is interesting to note the importance of the candle in thehousehold budget of early times in various sayings. For example, "Thegame is not worth the candle, " implies that the cost of candle-light wasnot ignored. In these days little attention is given to the cost ofartificial light under similar conditions. If a person "burns a candleat both ends" he is wasteful and oblivious to the consequences ofextravagance whether in material goods or in human energy. 

With the rise of the Christian church, candles came to be used inreligious ceremonies and many of the symbolisms, meanings, and customssurvive to the present time. Some of the finest art of past centuries isfound in the old candlesticks. Many of these antiques, which ofttimeswere gifts to the church, have been preserved to posterity by thechurch. The influence of these lighting accessories is often noted inmodern lighting-fixtures, but unfortunately early art often suffers fromadaptation to the requirements of modern light-sources, or the eyesightsuffers from a senseless devotion to art which results in the use ofmodern light-sources, unshaded and glaring, in places where it wasunnecessary to shade the feeble candle. 

The oldest materials employed for making candles are beeswax and tallow. The beeswax was bleached before use. The tallow was melted and strainedand then cotton or flax fibers were dipped into it repeatedly, until thedesired thickness was obtained. In early centuries the pith of rusheswas used for wicks. Tallow is now used only as a source of stearine. Spermaceti, a fatty substance obtained from the sperm-whale, wasintroduced into candle-making in about 1750 and great numbers of mensearched the sea to fill the growing demands. Paraffin wax, a mixture ofsolid hydrocarbons obtained from petroleum, came into use in 1854 andstearine is now used with it. The latter increases the rigidity anddecreases the brittleness of the candle. Some of the modern candles aremade of a mixture of stearine and the hard fat extracted fromcocoanut-oil. Modern candles vary in composition, but all are theproduct of much experience and of the application of scientificknowledge. The wicks are now made chiefly of cotton yarn, braided orplaited by machinery and chemically treated to aid in completecombustion when the candle is burned. Their structure is the result oflong experience and they are now made so that they bend and dip into themolten fuel and are wholly consumed. This eliminates the necessity oftrimming. 

Candles have been made in various ways, including dipping, pouring, drawing, and molding. Wax-candles are made by pouring, because waxcannot be molded satisfactorily. Drawing is somewhat similar to dipping, except that the process is more or less continuous and is carried out bymachinery. Molding, as the term implies, involves the use of molds, ofthe size and shape desired. 

The candlestick evolved from the most primitive wooden objects toelaborately designed and decorated works of art. The primitivecandlestick was crude and was no more than a holder of some kind forkeeping the candle upright. Later a form of cup was attached to the stemof the holder, to catch the dripping wax or fat. The latter improvementhas persisted throughout the centuries. The modern candle is by no meansan unsatisfactory light-source. Those who have had experience with it inthe outskirts of civilization will testify that it possesses severaldesirable characteristics. Supplies of candles are transported withoutdifficulty; the lighted candle is easily carried about; and the light ina quiescent atmosphere is quite satisfactory, if common sense is used inshading and placing the candle. Although in a sense a primitivelight-source, it is a blessing in many cases and, incidentally, it isextensively used to-day in industries, in religious ceremonies, as adecorative element at banquets, and in the outposts of civilization. 

This account of the evolution of light-sources has crossed the thresholdof what may be termed modern scientific light-production in the case ofthe candle and the oil-lamp. There is a period of a century or moreduring which scientific progress was slow, but those years paved the wayfor the extraordinary developments of the last few decades. 

IV

THE CEREMONIAL USE OF LIGHT

Inasmuch as the symbolisms and ceremonial uses of light originated inthe childhood of the human race and were nourished throughout the age ofmythology, the early light-sources are associated more with this phaseof artificial light than modern ones. For this reason it appearsappropriate to present this discussion before entering into the laterstages of the development and utilization of artificial light. Furthermore, many of the traditions of lighting at the present time aresurvivors of the early ages. Lighting-fixtures show the influence ofthis byway of lighting, and in those cases where the ceremonial use oflight has survived to the present time, modern light-sources cannot beemployed wisely in replacing more primitive ones without considerationof the origin and existence of the customs. In fact, candles are likelyto be used for hundreds of years to come, owing to the sentimentconnected with them and to the established customs founded uponcenturies of traditional use. 

Doubtless, the sun as a source of heat and light and of the blessingswhich these bring to earth, is responsible largely for the divinesignificance bestowed upon light. Darkness very deservingly acquiredmany uncomplimentary attributes, for danger lurked behind its veil andit was the suitable abode of evil spirits. It harbored all that was theantithesis of goodness, happiness, and security. Light naturally becamesacred, life-giving, and symbolic of divine presence. Fire was toprimitive beings the most impressive phenomenon over which they had anycontrol, and it was sufficiently mysterious in its operation to warranta connection with the supernatural. Thus it was very natural that theseearlier beings worshiped it as representing divine presence. The sun, asRa, was one of the chief gods of the ancient Egyptians; and theAssyrians, the Babylonians, the ancient Greeks, and many other earlypeoples gave a high place to this deity. Among simpler races the sun wasoften the sole object of worship, and those peoples who worship Light asthe god of all, in a sense are not far afield. Fire-worshipers generallyconsidered fire as the purest representation of heavenly fire, theorigin of everything that lives. 

Light was considered such a blessing that lamps were buried with thedead in order that spirits should be able to have it in the next world. This custom has prevailed widely but the fact that the lamps wereunlighted indicates that only the material aspect was considered. It isinteresting to note that the lamps and other light-sources in pagantemples and religious processions were not symbolical but were offeringsto the gods. In later centuries a deeper symbolical meaning becameattached to light and burning lamps were placed upon the tombs ofimportant personages. The burying of lamps with the dead appears to haveoriginated in Asia. The Phoenicians and Romans apparently continuedthe custom, but no traces of it have been found in Greece and Egypt. 

Fire and light have been closely associated in various religious creedsand their ceremonies. The Hindu festival in honor of the goddess ofprosperity is attended by the burning of many lamps in the temples andhomes. The Jewish synagogues have their eternal lamps and in theirrituals fire and light have played prominent rôles. The devout Brahmanmaintains a fire on the hearth and worships it as omniscient and divine. He expects a brand from this to be used to light his funeral pyre, whosefire and light will make his spirit fit to enter his heavenly abode. Hekeeps a fire burning on the altar, worships Agni, the god of fire, andmakes fire sacrifices on various occasions such as betrothals andmarriages. To the Mohammedans lighted lamps symbolize holy places, andthe Kaaba at Mecca, which contains a black stone supposed to have beenbrought from heaven, is illuminated by thousands of lamps. Many of theuses to which light was put in ancient times indicate its rarity andsacred nature. Doubtless, the increasing use of artificial light atfestivals and celebrations of the present time is partly the result oflingering customs of bygone centuries and partly due to a recognition ofan innate appeal or attribute of light. Certainly nothing is moregenerally appropriate in representing joy and prosperity. 

Throughout all countries ancient races had woven natural light and fireinto their rites and customs, so it became a natural step to utilizeartificial light and fire in the same manner. It would be tedious andmonotonous to survey the vast field of ancient worship of light, for theunderlying ideas are generally similar. The mythology of the Greeks isillustrative of the importance attached to fire and light by thecultivated peoples of ancient times. The myth of Prometheus emphasizesthe fact that in those remote periods fire and light were regarded as ofprime importance. According to this myth, fire and light were containedin heaven and great cunning and daring were necessary in order to obtainit. Prometheus stole this heavenly fire, for which act he was chained tothe mountain and made to suffer. The Greeks mark this event as thebeginning of human civilization. All arts are traced to Prometheus, andall earthly woe likewise. As past history is surveyed it appears naturalto think of scientific men who have become martyrs to the quest ofhidden secrets. They have made great sacrifices for the future benefitof civilization and not a few of them have endured persecution even inrecent times. The Greeks recognized that a new era began with theacquisition of artificial light. Its divine nature was recognized and itbecame a phenomenon for worship and a means for representing divinepresence. The origin of fire and light made them holy. The fire on thealtar took its place in religious rites and there evolved manyceremonial uses of lamps, candles, and fire. 

The Greeks and Romans burned sacred lamps in the temples and utilizedlight and fire in many ceremonies. The torch-race, in which young menran with lighted torches, the winner being the one who reached the goalfirst with his torch still alight, originated in a Grecian ceremony oflighting the sacred fire. There are many references in ancient Roman andGrecian literature to sacred lamps burning day and night in sanctuariesand before statues of gods and heroes. On birthdays and festivals thehouses of the Romans were specially ornamented with burning lamps. TheVestal Virgins in Rome maintained the sacred fire which had been broughtby fugitives from Troy. In ancient Rome when the fire in the Temple ofVesta became extinguished, it was rekindled by the rubbing of a piece ofwood upon another until fire was obtained. This was carried into thetemple by the Vestal Virgin and the sacred fire was rekindled. The fireproduced in this manner, for some reason, was considered holy. 

The early peoples displayed many lamps on feast-days and an example ofextravagance in this respect is an occasion when King Constantinecommanded that the entire city of Constantinople be illuminated bywax-candles on Christmas Eve. Candelabra, of the form of the branchingtree, were commonly in use in the Roman temples. 

The ceremonial use of light in the Christian church evolved both fromadaptations of pagan customs and of the natural symbolisms of fire andlight. However, these acquired a deeper meaning in Christianity than inearly times because they were primarily visible representations ormanifestations of the divine presence. The Bible contains manyreferences to the importance and symbolisms of light and fire. Accordingto the First Book of Moses, the achievement of the Creator immediatelyfollowing the creation of "the heavens and the earth" was the creationof light. The word "light" is the forty-sixth word in Genesis. Christ is"the true light" and Christians are "children of light" in war againstthe evil "powers of darkness. " When St. Paul was converted "thereshined about him a great light from heaven. " The impressiveness andsymbolism of fire and light are testified to in many biblicalexpressions. Christ stands "in the midst of seven candle-sticks" with"his eyes as a flame of fire. " When the Holy Ghost appeared before theapostles "there appeared unto them cloven tongues of fire. " When St. Paul was preaching the gospel of Christ at Alexandria "there were manylights" suggesting a festive illumination. 

According to the Bible, the perpetual fire which came originally fromheaven was to be kept burning on the altar. It was holy and those whoseduty it was to keep it burning were guilty of a grave offense if theyallowed it to be extinguished. If human hands were permitted to kindleit, punishment was meted out. The two sons of Aaron who "offered strangefire before the Lord" were devoured by "fire from the Lord. " Theseven-branched candlestick was lighted eternally and these burninglight-sources were necessary accompaniments of worship. 

The countless ceremonial uses of fire and light which had evolved in thepast centuries were bound to influence the rites and customs of theChristian church. The festive illumination of pagan temples in honor ofgods was carried over into the Christian era. The Christmas tree ofto-day is incomplete without its many lights. Its illumination is ahomage of light to the source of light. The celebration of Easter in theChurch of the Holy Sepulchre in Jerusalem is a typical example offire-worship retained from ancient times. At the climax of the servicescomes the descent of the Holy Fire. The central candelabra suddenlybecomes ablaze and the worshipers, each of whom carries a wax taper, light their candles therefrom and rush through the streets. The fire isconsidered to be of divine origin and is a symbol of resurrection. Thecustom is similar in meaning to the light which in older times wasmaintained before gods. 

During the first two or three centuries of the Christian era theceremonial use of light does not appear to have been very extensive. Writings of the period contain statements which appear to ridicule thisuse to some extent. For example, one writer of the second century statesthat "On days of rejoicing . .. We do not encroach upon daylight withlamps. " Another, in the fourth century, refers with sarcasm to the"heathen practice" in this manner: "They kindle lights as though to onewho is in darkness. Can he be thought sane who offers the light of lampsand candles to the Author and Giver of all light?"

That candles were lighted in cemeteries is evidenced by an edict whichforbade their use during the day. Lamps of the early centuries of theChristian era have been found in the catacombs of Rome which are thoughtto have been ceremonial lamps, for they were not buried with the dead. They were found only in niches in the walls. During these same centurieselaborate candelabra containing hundreds of candles were kept burningbefore the tombs of saints. Notwithstanding the doubt that exists as tothe extent of ceremonial lighting in the early centuries of theChristian era, it is certain that by the beginning of the fifth centurythe ceremonial use of light in the Christian church had become veryextensive and firmly established. That this is true and that there werestill some objections is indicated by many controversies. Some thoughtthat lamps before tombs were ensigns of idolatry and others felt that noharm was done if religious people thus tried to honor martyrs andsaints. Some early writings convey the idea that the ritualistic use oflights in the church arose from the retention of lights necessary atnocturnal services after the hours of worship had been changed todaytime. 

Passing beyond the early controversial period, the ceremonial use oflight is everywhere in evidence at ordinary church services. On specialoccasions such as funerals, baptisms, and marriages, elaboratealtar-lighting was customary. The gorgeous candelabra and the eternallamp are noted in many writings. Early in the fifth century the popeordered that candles be blessed and provided rituals for this ceremony. Shortly after this the Feast of Purification of the Virgin wasinaugurated and it became known as Candlemas because on this day thecandles for the entire year were blessed. However, it appears that theblessing of candles was not carried out in all churches. Altar lightswere not generally used until the thirteenth century. They wereoriginally the seven candles carried by church officials and placed nearthe altar. 

The custom of placing lighted lamps before the tombs of martyrs wasgradually extended to the placing of such lamps before various objectsof a sacred or divine relation. Finally certain light-sources themselvesbecame objects of worship and were surrounded by other lamps, and thesymbolisms of light grew apace. A bishop in the sixth century heraldedthe triple offering to God represented by the burning wax-candle. Hepointed out that the rush-wick developed from pure water; that the waxwas the product of virgin bees; and that the flame was sent from heaven. Each of these, he was certain, was an offering acceptable to God. Wax-candles became associated chiefly with religious ceremonies. The waxlater became symbolic of the Blessed Virgin and of the body of Christ. The wick was symbolical of Christ's soul, the flame represented hisdivine character, and the burning candle thus became symbolical of hisdeath. The lamp, lantern, and taper are frequently symbols of piety, heavenly wisdom, or spiritual light. Fire and flames are emblems of zealand fervor or of the sufferings of martyrdom and the flaming heartsymbolizes fervent piety and spiritual or divine love. 

By the time the Middle Ages were reached the ceremonial uses of lightbecame very complex, but for the Roman Catholic Church they may bedivided into three general groups: (1) They were symbolical of God'spresence or of the effect of his presence; of Christ or of "the childrenof light"; or of joy and content at festivals. (2) They may be offeredin fulfillment of a religious vow; that is, as an act of worship. (3)They may possess certain divine power because of their being blessed bythe church, and therefore may be helpful to soul and body. The threeconceptions are indicated in the prayers offered at the blessing of thecandles on Candlemas as follows: (1) "O holy Lord . .. Who . .. By thycommand didst cause this liquid to come by the labor of bees to theperfection of wax, . .. We beseech thee . .. To bless and sanctify thesecandles for the use of men, and the health of bodies and souls. .. . " (2)". .. These candles, which we thy servants desire to carry lighted tomagnify thy name; that by offering them to thee, being worthily inflamedwith the holy fire of thy most sweet charity, we may deserve. .. . " (3) "OLord Jesus Christ, the true light, . .. Mercifully grant, that as theselights enkindled with visible fire dispel nocturnal darkness, so ourhearts illuminated by visible fire, " etc. 

In general, the ceremonial uses of lights in this church were originatedas a forceful representation of Christ and of salvation. On the eve ofEaster a new fire, emblematic of the arisen Christ, is kindled, and allcandles throughout the year are lighted from this. During the service ofHoly Week thirteen lighted candles are placed before the altar and asthe penitential songs are sung they are extinguished one by one. Whenbut one remains burning it is carried behind the altar, thus symbolizingthe last days of Christ on earth. It is said that this ceremony has beentraced to the eighth century. On Easter Eve, after the new fire islighted and blessed, certain ceremonies of light symbolize theresurrection of Christ. From this new fire three candles are lighted andfrom these the Paschal Candle. The origin of the latter is uncertain, but it symbolizes a victorious Christ. From it all the ceremonial lightsof the church are lighted and they thereby are emblematic of thepresence of the light of Christ. 

Many interesting ceremonial uses may be traced out, but space permits aglimpse of only a few. At baptismal services the paschal candle isdipped into the water so that the latter will be effective as aregenerative element. The baptized child is reborn as a child of light. Lighted candles are placed in the hands of the baptized persons or oftheir god-parents. Those about to take vows carry lights before thechurch official and the same idea is attached to the custom of carryingor of holding lights on other occasions such as weddings and firstcommunion. Lights are placed around the bodies of the dead and arecarried at the funeral. They not only protect the dead from the powersof darkness but they symbolize the dead as still living in the light ofChrist. The use of lighted candles around bodies of the dead stillsurvives to some extent among Protestants, but their significance hasbeen lost sight of. Even in the eighteenth century funerals in Englandwere accompanied by lighted tapers, but the carrying of lights in otherprocessions appears to have ceased with the Reformation. In some partsof Scotland it is still the custom to place two lighted candles on atable beside a corpse on the day of the funeral. 

With the importance of light in the ritual of the church it is notsurprising that the extinction of lights is a part of the ceremony ofexcommunication. Such a ceremony is described in an early writing thus:"Twelve priests should stand about the bishop, holding in their handslighted torches, which at the conclusion of the anathema orexcommunication they should cast down and trample under foot. " When theexcommunicant is reinstated, a lighted candle is placed in his hands asa symbol of reconciliation. These and many other ceremonial uses oflight have been and are practised, but they are not always mandatory. Furthermore, the customs have varied from time to time, but the fewwhich have been touched upon illustrate the impressive part that lighthas played in religious services. 

During the Reformation the ceremonial use of lights was greatly alteredand was abolished in the Protestant churches as a relic of superstitionand papal authority. In the Lutheran churches ceremonial lights werelargely retained, in the Church of England they have been subjected tomany changes largely through the edicts of the rulers. In the latterchurch many controversies were waged over ceremonial lights and theiruse has been among the indictments of a number of officials of thechurch in impeachment cases before the House of Commons. Many uses oflight in religious ceremonies were revived in cathedrals after theRestoration and they became wide-spread in England in the nineteenthcentury. As late as 1889 the Archbishop of Canterbury ruled that certainceremonial candles were lawful according to the Prayer-Book of EdwardVI, but the whole question was left open and unsettled. 

These byways of artificial light are complex and fascinating becausetheir study leads into many channels and far into the obscurity of thechildhood of the human race. A glimpse of them is important in a surveyof the influence of artificial light upon the progress of civilizationbecause in these usages the innate and acquired impressiveness of lightis encountered. Although many ceremonial uses of light remain, it isdoubtful if their significance and especially their origin areappreciated by most persons. Nevertheless, no more interesting phase ofartificial light is encountered than this, which reaches to thefoundation of civilization. 

V

OIL-LAMPS OF THE NINETEENTH CENTURY

It will be noted that the light-sources throughout the early ages wereflames, the result of burning material. This principle oflight-production has persisted until the present time, but in the latterpart of the nineteenth century certain departures revolutionizedartificial lighting. However, it is not the intention to enter themodern period in this chapter except in following the progress of theoil-lamp through its period of scientific development. The oil-lamp andthe candle were the mainstays of artificial lighting throughout manycenturies. The fats and waxes which these light-sources burned were manybut in the later centuries they were chiefly tallow, sperm-oil, spermaceti, lard-oil, olive-oil, colza-oil, bees-wax and vegetablewaxes. Those fuels which are not liquid are melted to liquid form by theheat of the flame before they are actually consumed. The candle is ofthe latter type and despite its present lowly place and its primitivecharacter, it is really an ingenious device. Its fuel remainsconveniently solid so that it is readily shipped and stored; there isnothing to spill or to break beyond easy repair; but when it is lightedthe heat of its flame melts the solid fuel and thus it becomes an"oil-lamp. " Animal and vegetable oils were mainly used until the middleof the nineteenth century, when petroleum was produced in sufficientquantities to introduce mineral oils. This marked the beginning of anera of developments in oil-lamps, but these were generally the naturaloffspring of early developments by Ami Argand. 

Before man discovered that nature had stored a tremendous supply ofmineral oil in the earth he was obliged to hunt broadcast for fats andwaxes to supply him with artificial light. He also was obliged to endureunpleasant odors from the crude fuels and in early experiments with fatsand waxes the odor was carefully noted as an important factor. Tallowwas a by-product of the kitchen or of the butcher. Stearine, aconstituent of tallow, is a compound of glyceryl and stearic acid. It isobtained by breaking up chemically the glycerides of animal fats andseparating the fatty acids from glycerin. Fats are glycerides; that is, combinations of oleic, palmetic, and stearic acids. Inasmuch as theformer is liquid at ordinary temperatures and the others are solid, itfollows that the consistency or solidity of fats depend upon therelative proportions of the three constituents. The sperm-whale, whichlives in the warmer parts of all the oceans, has been huntedrelentlessly for fuels for artificial lighting. In its head cavitiessperm-oil in liquid form is found with the white waxy substance known asspermaceti. Colza-oil is yielded by rape-seed and olive-oil is extractedfrom ripe olives. The waxes are combinations of allied acids with basessomewhat related to glycerin but of complex composition. Fats and waxesare more or less related, but to distinguish them carefully would leadfar afield into the complexities of organic chemistry. All these animaland vegetable products which were used as fuels for light-sources arerich in carbon, which accounts for the light-value of their flames. Thebrightness of such a flame is due to incandescent carbon particles, butthis phase of light-production is discussed in another chapter. Theseoils, fats, and waxes are composed by weight of about 75 to 80 per cent. Carbon; 10 to 15 per cent. Hydrogen; and 5 to 10 per cent. Oxygen. 

Until the middle of the eighteenth century the oil-lamps were shallowvessels filled with animal or vegetable oil and from these reservoirsshort wicks projected. The flame was feeble and smoky and the odors weresometimes very repugnant. Viewing such light-sources from the presentage in which light is plentiful, convenient, and free from the greatdisadvantages of these early oil-lamps, it is difficult to imagine thepossibility of the present civilization emerging from that periodwithout being accompanied by progress in light-production. Theimprovements made in the eighteenth century paved the way for greaterprogress in the following century. This is the case throughout the ages, but there are special reasons for the tremendous impetus whichlight-production has experienced in the past half-century. These are theacquirement of scientific knowledge from systematic research and theapplication of this knowledge by organized development. 

The first and most notable improvement in the oil-lamp was made byArgand in 1784. Our nation was just organizing after its successfulstruggle for independence at the time when the production of light as ascience was born. Argand produced the tubular wick and contributed thegreatest improvement by being the first to perform the apparently simpleact of placing a glass chimney upon the lamp. His burner consisted oftwo concentric metal tubes between which the wick was located. The innertube was open, so that air could reach the inner surface of the wick aswell as the outer surface. The lamp chimney not only protected the flamefrom drafts but also improved combustion by increasing the supply ofair. It rested upon a perforated flange below the burner. If the glasschimney of a modern kerosene lamp be lifted, it will be noted that theflame flickers and smokes and that it becomes steady and smokeless whenthe chimney is replaced. The advantages of such a chimney are obviousnow, but Argand for his achievements is entitled to a place among thegreat men who have borne the torch of civilization. He took the firststep toward adequate artificial light and opened a new era in lighting. 

The various improvements of the oil-lamp achieved by Argand combined toeffect complete combustion, with the result that a steady, smokelesslamp of considerable luminous intensity was for the first timeavailable. Many developments followed, among which was a combination ofreservoir and gravity feed which maintained the oil at a constant level. In later lamps, upon the adoption of mineral oil, this was foundunnecessary, perhaps owing to the construction of the wick and to thephysical characteristics of the oil which favored capillary action inthe wick. However, the height of the oil in the reservoir of modernoil-lamps makes some difference in the amount of light emitted. 

The Carcel lamp, which appeared in 1800, consisted of a double pistonoperated by clockwork. This forced the oil through a tube to the burner. Franchot invented the moderator lamp in 1836, which, because of itssimplicity and efficiency soon superseded many other lamps designed forburning animal and vegetable oils. The chief feature of the moderatorlamp is a spiral spring which forces the oil upward through a verticaltube to the burner. These are still used to some extent in France, butowing to the fact that "mechanical" lamps eventually were very generallyreplaced by more simple ones, it does not appear necessary to describethese complex mechanisms in detail. 

When coal is distilled at moderate temperatures, volatile liquids areobtained. These hydrocarbons, being inflammable, naturally attractedattention when first known, and in 1781 their use as fuel for lamps wassuggested. However, it was not until 1820 that the light oils obtainedby distilling coal-tar, a by-product of the coal-gas industry which wasthen in its early stage of development, were burned to some extent inthe Holliday lamp. In this lamp the oil is contained in a reservoir fromthe bottom of which a fine metal tube carries the oil down to arose-burner. The oil is heated by the flame and the vaporized mineraloil which escapes through small orifices is burned. This type of lamphas undergone many physical changes, but its principle survives to thepresent time in the gasolene and kerosene burners hanging on a pole bythe side of the street-peddler's stand. 

Although petroleum products were not used to any appreciable extent forilluminating-purposes until after the middle of the nineteenth century, mineral oil is mentioned by Herodotus and other early writers. In 1847petroleum was discovered in a coal-mine in England, but the supplyfailed in a short time. However, the discoverer, James Young, had foundthat this oil was valuable as a lubricant and upon the failure of thissource he began experiments in distilling oil from shale found in coaldeposits. These were destined to form the corner-stone of the oilindustry in Scotland. In 1850 he began producing petroleum in thismanner, but it was not seriously considered for illuminating-purposes. However, in Germany about this time lamps were developed for burning thelighter distillates and these were introduced into several countries. But the price of these lighter oils was so great that little progresswas made until, in 1859, Col. E. L. Drake discovered oil in Pennsylvania. By studying the geological formations and concluding that oil should beobtained by boring, Drake gave to the world a means of obtainingpetroleum, and in quantities which were destined to reduce the price ofmineral oil to a level undreamed of theretofore. To his imagination, which saw vast reservoirs of oil in the depths of the earth, the worldowes a great debt. Lamps were imported from Germany to all parts of thecivilized world and the kerosene lamp became the prevailinglight-source. Hundreds of American patents were allowed for oil-lampsand their improvements in the next decade. 

[Illustration: LAMPS OF A CENTURY OR TWO AGO]

[Illustration: ELABORATE FIXTURES OF THE AGE OF CANDLES]

The crude petroleum, of course, is not fit for illuminating purposes, but it contains components which are satisfactory. The variouscomponents are sorted out by fractional distillation and the oil forburning in lamps is selected according to its volatility, viscosity, stability, etc. It must not be so volatile as to have a dangerouslylow flashing-point, nor so stable as to hinder its burning well. In thisfractional distillation a vast variety of products are now obtained. Gasolene is among the lighter products, with a density of about 0. 65;kerosene has a density of about 0. 80; the lubricating-oils from 0. 85 to0. 95; and there are many solids such as vaseline and paraffin which arewidely used for many purposes. This process of refining oils is now thesource of paraffin for making candles, in which it is usually mixed withsubstances like stearin in order to raise its melting-point. 

Crude petroleum possesses a very repugnant odor; it varies in color fromyellow to black; and its specific gravity ranges from about 0. 80 to1. 00, but commonly is between 0. 80 and 0. 90. Its chemical constitutionis chiefly of carbon and hydrogen, in the approximate ratio of about sixto one respectively. It is a mixture of paraffin hydrocarbons having thegeneral formula of C_{n}H_{2n+2} and the individual members of thisseries vary from CH_{4} (methane) to C_{15}H_{32} (pentadecane), although the solid hydrocarbons are still more complex. Petroleum isfound in many countries and the United States is particularly blessedwith great stores of it. 

The ordinary lamp consisting of a wick which draws up the mineral oiland feeds it to a flame is efficient and fairly free from danger. Itrequires care and may cause disaster if it is upset, but it has beenblamed unjustly in many accidents. A disadvantage of the kerosene lampover electric lighting, for example, is the relatively greaterpossibility of accidents through the carelessness of the user. Thispoint is brought out in statistics of fire-insurance companies, whichshow that the fires caused by kerosene lamps are much more numerous thanthose from other methods of lighting. If in a modern lamp of properconstruction a close-fitting wick is used and the lamp is extinguishedby turning down and blowing across the chimney, there is little dangerin its use excepting accidental breakage or overturning. 

In oil-lamps at the present time mineral oils are used which possessflashing-points above 75°F. The highly volatile components of petroleumare dangerous because they form very explosive mixtures with air atordinary temperatures. A mineral oil like kerosene, to be used withsafety in lamps, should not be too volatile. It is preferable that aninflammable vapor should not be given off at temperatures under 120°F. The oil must be of such physical characteristics as to be drawn up tothe burner by capillarity from the reservoir which is situated below. Itis volatilized by the heat of the flame into a mixture of hydrogen andhydrocarbon gases and these are consumed under the heat of the processof consumption by the oxygen in the air. The resulting products of thiscombustion, if it is complete, are carbon dioxide and water-vapor. Foreach candle-power of light per hour about 0. 24 cubic foot of carbondioxide and 0. 18 cubic foot of water-vapor are formed by a modernoil-lamp. That an open flame devours something from the air is easilydemonstrated by enclosing it in an air-tight space. The flame graduallybecomes feeble and smoky and finally goes out. It will be noted that aburning lamp will vitiate the atmosphere of a closed room by consumingthe oxygen and returning in its place carbon dioxide. This is similar tothe vitiation of the atmosphere by breathing persons and tests indicatethat for each two candle-power emitted by a kerosene flame the vitiationis equal to that produced by one adult person. Inasmuch as oil-lamps areordinarily of 10 to 20 candle-power, it is seen that one lamp willconsume as much oxygen as several persons. 

In order that oil-lamps may produce a brilliant light free from smoke, combustion must be complete. The correct quantity of oil must be fed tothe burner and it must be properly vaporized by heat. If insufficientoil is fed, the intensity of the light is diminished and if too much isavailable at the burner, smoke and other products of incompletecombustion will be emitted. The wick is an important factor, for, through capillarity, it feeds oil forcefully to the burner against theaction of gravity. This action of a wick is commonly looked upon withindifference but in reality it is caused by an interesting and reallywonderful phenomenon. Wicks are usually made of high-grade cotton fiberloosely spun into coarse threads and these are woven into a loose plait. The wick must be dry before being inserted into the burner; and it isdesirable that it be considerably longer than is necessary merely toreach the bottom of the reservoir. A flame burning in the open willsmoke because insufficient oxygen is brought in contact with it. Theinjurious products of this incomplete combustion are carbon monoxide andoil vapors, which are a menace to health. 

To supply the necessary amount of oxygen (air) to the flame, a forceddraft is produced. Chimneys are simple means of accomplishing this, andthis is their function whether on oil-lamps or factories. Other means offorced draft have been used, such as small fans or compressed air. Inthe railway locomotive the short smoke-stack is insufficient forsupplying large quantities of air to the fire-box so the exhausted steamis allowed to escape into the stack. With each noisy puff of smoke aquantity of air is forcibly drawn into the fire-box through the burningfuel. In the modern oil-lamp the rush of air due to the "pull" of thechimney is broken and the air is diffused by the wire gauze or holes atthe base of the burner. These metal parts, being hot, also serve to warmthe oil before it reaches the burning end of the wick, thus serving toaid vaporization and combustion. 

The consumption of oil per candle-power per hour varies considerablywith the kind of lamp and with the character of the oil. The averageconsumption of oil-lamps burning a mineral oil of about 0. 80 specificgravity and a rather high flashing-point is about 50 to 60 grams of oilper candle-power per hour for well-designed flame-lamps. Kerosene weighsabout 6. 6 pounds per gallon; therefore, about 800 candle-power hours pergallon are obtained from modern lamps employing wicks. Kerosene lampsare usually of 10 to 20 candle-power, although they are made up to 100candle-power. These luminous intensities refer to the maximum horizontalcandle-power. The best practice now deals with the total light output, which is expressed in lumens, and on this basis a consumption of onegallon of kerosene per hour would yield about 8000 lumens. 

Oil-lamps have been devised in which the oil is burned as a sprayejected by air-pressure. These burn with a large flame; however, aserious feature is the escape of considerable oil which is not burned. These lamps are used in industrial lighting, especially outdoors, andpossess the advantage of consuming low-grade oils. They produce about700 to 800 candle-power hours per gallon of oil. Lamps of this type ofthe larger sizes burn with vertical flames two or three feet high. Theoil is heated as it approaches the nozzle and is fairly well vaporizedon emerging into the air. The names of Lucigen, Wells, Doty, and othersare associated with this type of lamp or torch, which is a step in thedirection of air-gas lighting. 

During the latter part of the nineteenth century numerous developmentswere made which paralleled the progress in gas-lighting. Experimentswere conducted which bordered closely upon the next epochal event inlight-production--the appearance of the gas mantle. One of these was theuse of platinum gauze by Kitson. He produced an apparatus similar to theoil-spray lamp, on a small and more delicate scale. The hot blue flamewas not very luminous and he attempted to obtain light by heating amantle of fine platinum gauze. Although these mantles emitted abrilliant light for a few hours, their light-emissivity was destroyed bycarbonization. After the appearance of the Welsbach mantle, Kitson'slamp and others met with success by utilizing it. From this point, attention was centered upon the new wonder, which is discussed in alater chapter after certain scientific principles in light-productionhave been discussed. 

The kerosene or mineral-oil lamp was a boon to lighting in thenineteenth century and even to-day it is a blessing in many homes, especially in villages, in the country, and in the remote districts ofcivilization. Its extensive use at the present time is shown by the factthat about eight million lamp-chimneys are now being manufactured yearlyin this country. It is convenient and safe when carelessness is avoided, and is fairly free from odor. Its vitiation of the atmosphere may becounteracted by proper ventilation and there remains only thedisadvantage of keeping it in order and of accidental breakage andoverturning. The kerosene lantern is widely used to-day, but the dangerdue to accident is ever-present. The consequences of such accidents areoften serious and are exemplified in the terrible conflagration inChicago in 1871, when Mrs. O'Leary's cow kicked over a lantern andstarted a fire which burned the city. Modern developments in lightingare gradually encroaching upon the territory in which the oil-lamp hasreigned supreme for many years. Acetylene plants were introduced to aconsiderable extent some time ago and to-day the self-containedhome-lighting electric plant is being installed in large numbers in thecountry homes of the land. 

VI

EARLY GAS-LIGHTING

Owing to the fact that the smoky, flickering oil-lamp persistedthroughout the centuries and until the magic touch of Argand in thelatter part of the eighteenth century transformed it into a commendablelight-source, the reader is prepared to suppose that gas-lighting is ofrecent origin. Apparently William Murdock in England was the first toinstall pipes for the conveyance of gas for lighting purposes. In anarticle in the "Philosophical Transactions of the Royal Society ofLondon" dated February 25, 1808, in which he gives an account of thefirst industrial gas-lighting, he states:

 It is now nearly sixteen years, since, in a course of experiments I was making at Redruth in Cornwall, upon the quantities and qualities of the gases produced by distillation from different mineral and vegetable substances, I was induced by some observation I had previously made upon the burning of coal, to try the combustible property of the gases produced from it. .. . 

Inasmuch as he is credited with having lighted his home by means ofpiped gas, this experimental installation may be considered to have beenmade in 1792. In his first trial he burned the gas at the open ends ofthe pipes; but finding this wasteful, he closed the ends and in eachbored three small holes from which the gas-flames diverged. It is saidthat he once used his wife's thimble in an emergency to close the end ofthe pipe; and, the thimble being much worn and consequently containing anumber of small holes, tiny gas-jets emerged from the holes. Thisincident is said to have led to the use of small holes in his burners. He also lighted a street lamp and had bladders filled with gas "to carryat night, with which, and his little steam carriage running on the road, he used to astonish the people. " Apparently unknown to Murdock, previousobservations had been made as to the inflammability of gas from coal. Long before this Dr. Clayton described some observations on coal-gas, which he called "the spirit of coals. " He filled bladders with this gasand kept them for some time. Upon his pricking one of them with a pinand applying a candle, the gas burned at the hole. Thus Clayton had aportable gas-light. He was led to experiment with distillation of coalfrom some experiences with gas from a natural coal bed, and he thusdescribes his initial laboratory experiment:

 I got some coal, and distilled it in a retort in an open fire. At first there came over only phlegm, afterwards a black _oil_, and then likewise, a _spirit_ arose which I could no ways condense; but it forced my lute and broke my glasses. Once when it had forced my lute, coming close thereto, in order to try to repair it, I observed that the spirit which issued out _caught fire_ at the _flame_ of the _candle_, and continued burning with violence as it _issued out_ in a _stream_, which I blew out, and lighted again alternately several times. 

He then turned his attention to saving some of the gas and hit upon theuse of bladders. He was surprised at the amount of gas which wasobtained from a small amount of coal; for, as he stated, "the spiritcontinued to rise for several hours, and filled the bladders almost asfast as a man could have blown them with his mouth; and yet the quantityof coals distilled was inconsiderable. "

Although this account appeared in the Transactions of the Royal Societyin 1739, there is strong evidence that Dr. Clayton had written it manyyears before, at least prior to 1691. 

But before entering further into the early history of gas-lighting, itis interesting to inquire into the knowledge possessed in theseventeenth century pertaining to natural and artificial gas. Doubtlessthere are isolated instances throughout history of encounters withnatural gas. Surely observant persons of bygone ages have noted a smallflame emanating from the end of a stick whose other end was burning in abonfire or in the fireplace. This is a gas-plant on a small scale; forthe gas is formed at the burning end of the wooden stick and isconducted through its hollow center to the cold end, where it will burnif lighted. If a piece of paper be rolled into the form of a tube andinclined somewhat from a horizontal position, inflammable gas willemanate from the upper end if the lower end is burning. By applying amatch near the upper end, we can ignite this jet of gas. However, it iscertain that little was known of gas for illuminating purposes beforethe eighteenth century. 

The literature of an ancient nation is often referred to as revealingthe civilization of the period. Surely the scientific literature whichdeals with concrete facts is an exact indicator of the technicalknowledge of a period! That little was known of natural gas anddoubtless of artificial gas in the seventeenth century is shown by abrief report entitled "A Well and Earth in Lancashire taking Fire at aCandle, " by Tho. Shirley in the Transactions of the Royal Society in1667. Much of the quaint charm of the original is lost by inability topresent the text in its original form, but it is reproduced as closelyas practicable. The report was as follows:

 About the latter End of _Feb. _ 1659, returning from a Journey to my House in Wigan, I was entertained with the Relation of an odd Spring situated in one Mr. _Hawkley's_ Ground (if I mistake not) about a Mile from the Town, in that Road which leads to _Warrington_ and _Chester_: The People of this Town did confidently affirm, That the Water of this Spring did burn like Oil. 

 When we came to the said Spring (being 5 or 6 in Company together) and applied a lighted Candle to the Surface of the Water; there was 'tis true, a large Flame suddenly produced, which burnt the Foot of a Tree, growing on the Top of a neighbouring Bank, the Water of which Spring filled a Ditch that was there, and covered the Burning-place; I applied the lighted Candle to divers Parts of the Water contained in the said Ditch, and found, as I expected, that upon the Touch of the Candle and the Water the Flame was extinct. 

 Again, having taken up a Dish full of water at the flaming Place, and held the lighted Candle to it, it went out. Yet I observed that the Water, at the Burning-place, did boil, and heave, like Water in a Pot upon the Fire, tho' by putting my Hand into it, I could not perceive it so much as warm. 

 This Boiling I conceived to proceed from the Eruption of some bituminous or sulphureous Fumes; considering this Place was not above 30 or 40 Yards distant from the Mouth of a Coal-Pit there: And indeed _Wigan_, _Ashton_, and the whole Country, for many Miles compass, is underlaid with Coal. Then, applying my Hand to the Surface of the Burning-place of the Water, I found a strong Breath, as it were a Wind, to bear against my Hand. 

 When the Water was drained away, I applied the Candle to the Surface of the dry Earth, at the same Point where the Water burned before; the Fumes took fire, and burned very bright and vigorous. The Cone of the Flame ascended a Foot and a half from the Superficies of the Earth; and the Basis of it was of the Compass of a Man's Hat about the Brims. I then caused a Bucket full of Water to be pour'd on the Fire, by which it was presently quenched. I did not perceive the Flame to be discoloured like that of sulphurous Bodies, nor to have any manifest Scent with it. The Fumes, when they broke out of the Earth, and press'd against my Hand, were not, to my best Remembrance, at all hot. 

Turning again to Dr. Clayton's experiments, we see that he pointed outstriking and valuable properties of coal-gas but apparently gave noattention to its useful purposes. Furthermore, his account appears tohave attracted no particular notice at the time of its publication in1739. Dr. Richard Watson in 1767 described the results of experimentswhich he had been making with the products arising from the distillationof coal. In his process he permitted the gas to ascend through curvedtubes, and he particularly noted "its great inflammability as well aselasticity. " He also observed that "it retained the former propertyafter it had passed through a great quantity of water. " His publishedaccount dealt with a variety of facts and computations pertaining to thequantities of coke, tar, etc. , produced from different kinds of coal andwas a scientific work of value, but apparently the usefulness of theproperty of inflammability of coal-gas did not occur to him. 

It is usually the habit of the scientific explorer of nature to returnfrom excursions into her unfrequented recesses with new knowledge, toplace it upon exhibition, and to return for more. The inventor passes byand sees applications for some of these scientific trophies which areproductive of momentous consequences to mankind. Sir Humphrey Davydescribed his primitive arc-lamp three quarters of a century beforeBrush developed an arc-lamp for practical purposes. Maxwell and Hertzrespectively predicted and produced electromagnetic waves long beforeMarconi applied this knowledge and developed "wireless" telegraphy. In asimilar manner scientific accounts of the production and properties ofcoal-gas antedated by many years the initial applications made byMurdock to illuminating purposes. 

Up to the beginning of the nineteenth century the civilized world hadonly a faint glimpse of the illuminating property of gas, butpracticable gas-lighting was destined soon to be an epochal event in theprogress of lighting. The dawn of modern science was coincident with thedawn of a luminous era. 

At Soho foundry in 1798 Murdock constructed an apparatus which enabledhim to exhibit his lighting-plan on a larger scale and to experiment onpurifying and burning the gas so as to eliminate odor and smoke. Sohowas an unique institution described as a place

 to which men of genius were invited and resorted from every civilized country, to exercise and to display their talents. The perfection of the manufacturing arts was the great and constant aim of its liberal and enlightened proprietors, Messrs. Boulton and Watt; and whoever resided there was surrounded by a circle of scientific, ingenious, and skilful men, at all times ready to carry into effect the inventions of each other. 

The Treaty of Amiens, which gave to England the peace she was sorely inneed of, afforded Murdock an opportunity in 1802 favorable for making apublic display of gas-lighting. The illumination of the Soho works onthis occasion is described as "one of extraordinary splendour. " Thefronts of the extensive range of buildings were ornamented with a largenumber of devices which displayed the variety of forms of gas-lights. Atthat time this was a luminous spectacle of great novelty and thepopulace came from far and wide "to gaze at, and to admire, thiswonderful display of the combined effects of science and art. "

Naturally, Murdock had many difficulties to overcome in these earlydays, but he possessed skill and perseverance. His first retorts fordistilling coal were similar to the common glass retort of the chemist. Next he tried cast-iron cylinders placed perpendicularly in a commonfurnace, and in each were put about fifteen pounds of coal. In 1804 heconstructed them with doors at each end, for feeding coal andextracting coke respectively, but these were found inconvenient. In hisfirst lighting installation in the factory of Phillips and Lee in 1805he used a large retort of the form of a bucket with a cover on it. Inside he installed a loose cage of grating to hold the coal. Whencarbonization was complete the coke could be removed as a whole byextracting this cage. This retort had a capacity of fifteen hundredpounds of coal. He labored with mechanical details, varied the size andshape of the retorts, and experimented with different temperatures, withthe result that he laid a solid foundation for coal-gas lighting. Forhis achievements he is entitled to an honorable place among thetorch-bearers of civilization. 

The epochal feature of the development of gas-lighting is that here wasa possibility for the first time of providing lighting as a publicutility. In the early years of the nineteenth century the foundation waslaid for the great public-utility organizations of the present time. Furthermore, gas-lighting was an improvement over candles and oil-lampsfrom the standpoints of convenience, safety, and cost. The latter pointsare emphasized by Murdock in his paper presented before the RoyalSociety in 1808, in which he describes the first industrial installationof gas-lighting. He used two types of burners, the Argand and thecockspur. The former resembled the Argand lamp in some respects and thelatter was a three-flame burner suggesting a fleur-de-lis. In thisinstallation there were 271 Argand burners and 636 cockspurs. Each ofthe former "gave a light equal to that of four candles; and each of thelatter, a light equal to two and a quarter of the same candles; makingtherefore the total of the gas light a little more than 2500 candles. "The candle to which he refers was a mold candle "of six in the pound"and its light was considered a standard of luminous intensity when itwas consuming tallow at the rate of 0. 4 oz. (175 grains) per hour. Thusthe candle became very early a standard light-source and has persistedas such (with certain variations in the specifications) until thepresent time. However, during recent years other standard light-sourceshave been devised. 

According to Murdock, the yearly cost of gas-lighting in this initialcase was 600 pounds sterling after allowing generously for interest oncapital invested and depreciation of the apparatus. The cost offurnishing the same amount of light by means of candles he computed tobe 2000 pounds sterling. This comparison was on the basis of an averageof two hours of artificial lighting per day. On the basis of three hoursof artificial lighting per day, the relative cost of gas-andcandle-lighting was about one to five. Murdock was characteristicallymodest in discussing his achievements and his following statement shouldbe read with the conditions of the year 1808 in mind:

 The peculiar softness and clearness of this light with its almost unvarying intensity, have brought it into great favour with the work people. And its being free from the inconvenience and danger, resulting from sparks and frequent snuffing of candles, is a circumstance of material importance, as tending to diminish the hazard of fire, to which cotton mills are known to be exposed. 

Although this installation in the mill of Phillips and Lee is the firstone described by Murdock, in reality it is not the first industrialgas-lighting installation. During the development of gas apparatus atthe Soho works and after his luminous display in 1802, he graduallyextended gas-lighting to all the principal shops. However, this in asense was experimental work. Others were applying their knowledge andingenuity to the problem of making gas-lighting practicable, but Murdockhas been aptly termed "the father of gas-lighting. " Among the pioneerswas Le Bon in France, Becher in Munich, and Winzler or Winsor, a Germanwho was attracted to the possibilities of gas-lighting by an exhibitionwhich Le Bon gave in Paris in 1802. Winsor learned that Le Bon had beengranted a patent in Paris in 1799 for making an illuminating gas fromwood and tried to obtain the rights for Germany. Being unsuccessful inthis, he set about to learn the secrets of Le Bon's process, which hedid, perhaps largely owing to an accumulation of information directlyfrom the inventor during the negotiations. Winsor then turned to Englandas a fertile field for the exploitation of gas-lighting and afterconducting experiments in London for some time he made plans to organizethe National Heat and Light Co. 

Winsor was primarily a promoter, with little or no technical knowledge;for in his claims and advertisements he disregarded facts with afacility possessed only by the ignorant. He boasted of his inventionsand discoveries in the most hyperbolical language, which was bound toprovoke a controversy. Nevertheless, he was clever and in 1803 hepublicly exhibited his plan of lighting by means of coal-gas at theLyceum Theatre in London. He gave lectures accompanied by interestingand instructive experiments and in this manner attracted the public tohis exhibition. All this time he was promoting his company, but hispromoting instinct caused his representations to be extravagant anddeceptive, which exposed him to the ridicule and suspicion of learnedmen. His attempt to obtain certain exclusive rights by Act of Parliamentfailed because of opposition of scientific men toward his claims and ofthe stand which Murdock justly made in self-protection. These years ofcontroversy yield entertaining literature for those who choose to readit, but unfortunately space does not permit dwelling upon it. Theinvestigations by committees of Parliament also afford amusingside-lights. Throughout all this Murdock appeared modest andconservative and had the support of reputable scientific men, but Winsormaintained extravagant claims. 

During one of these investigations Sir Humphrey Davy was examined by acommittee from the House of Commons in 1809. He refuted Winsor's claimsfor a superior coke as a by-product and stated that the production ofgas by the distillation of coal had been well known for thirty or fortyyears and the production of tar as long. He stated that it was theopinion of the Council of the Royal Society that Murdock was the firstperson to apply coal-gas to lighting in actual practice. As secretary ofthe Society, Sir Humphrey Davy stated that at the last session it hadbestowed the Count Rumford medal upon Murdock for "his economicalapplication of the gas light. "

Winsor proceeded to float his company without awaiting the Act ofParliament and in 1807 lighted a street in Pall Mall. Through theopposition which he aroused, and owing to the just claims of priority onthe part of Murdock, the bill to incorporate the National Heat and LightCo. With a capital of 200, 000 pounds sterling was thrown out. However, he succeeded in 1812 in receiving a charter very much modified in form, for the Chartered Gas Light and Coke Co. Which was the forerunner of thepresent London Gas Light and Coke Co. 

The conditions imposed upon this company as presented in an earlytreatise on gas-lighting (by Accum in 1818) were as follows:

 The power and authorities granted to this corporate body are very restricted and moderate. The individuals composing it have no exclusive privilege; their charter does not prevent other persons from entering into competition with them. Their operations are confined to the metropolis, where they are bound to furnish not only a stronger and better light to such streets and parishes as chuse to be lighted with gas, but also at a cheaper price than shall be paid for lighting the said streets with oil in the usual manner. The corporation is not permitted to traffic in machinery for manufacturing or conveying the gas into private houses, their capital or joint stock is limited to £200, 000, and his Majesty has the power of declaring the gas-light charter void if the company fail to fulfil the terms of it. 

The progress of this early company was slow at first, but with theappointment of Samuel Clegg as engineer in 1813 an era of technicaldevelopments began. New stations were built and many improvements wereintroduced. By improving the methods of purifying the gas a greatadvance was made. The utility of gas-lighting grew apace as theprejudices disappeared, but for a long time the stock of the companysold at a price far below par. About this time the first gas explosiontook place and the members of the Royal Society set a precedent whichhas lived and thrived: they appointed a committee to make an inquiry. But apparently the inquiry was of some value, for it led "to some usefulalterations and new modifications in its apparatus and machinery. "

Many improvements were being introduced during these years and one ofthem in 1816 increased the gaseous product from coal by distilling thetar which was obtained during the first distillation. In 1816 Cleggobtained a patent for a horizontal rotating retort; for an apparatus forpurifying coal-gas with "cream of lime"; and for a rotative gas-meter. 

Before progressing too far, we must mention the early work of WilliamHenry. In 1804 he described publicly a method of producing coal-gas. Besides making experiments on production and utilization of coal-gas forlighting, he devoted his knowledge of chemistry to the analysis of thegas. He also made analytical studies of the relative value of wood, peat, oil, wax, and different kinds of coal for the distillation of gas. His chemical analyses showed to a considerable extent the properties ofcarbureted hydrogen upon which illuminating value depended. The resultsof his work were published in various English journals between 1805 and1825 and they contributed much to the advancement of gas-lighting. 

Although Clegg's original gas-meter was complicated and cumbersome, itproved to be a useful device. In fact, it appears to have been the mostoriginal and beneficial invention occasioned by early gas-lighting. Later Samuel Crosley greatly improved it, with the result that it wasintroduced to a considerable extent; but by no means was it universallyadopted. Another improvement made by Clegg at this time was a devicewhich maintained the pressure of gas approximately constant regardlessof the pressure in the gasometer or tank. Clegg retired from the serviceof the gas company in 1817 after a record of accomplishments whichglorifies his name in the annals of gas-lighting. Murdock is undoubtedlyentitled to the distinction of having been the first person who appliedgas-lighting to large private establishments, but Clegg overcame manydifficulties and was the first to illuminate a whole town by this means. 

In London in 1817 over 300, 000 cubic feet of coal-gas was beingmanufactured daily, an amount sufficient to operate 76, 500 Argandburners yielding 6 candle-power each. Gas-lighting was now excitinggreat interest and was firmly established. Westminster Bridge waslighted by gas in 1813, and the streets of Westminster during thefollowing year. Gas-lighting became popular in London by 1816 and in thecourse of the next few years it was adopted by the chief cities andtowns in the United Kingdom and on the Continent. It found its way intothe houses rather slowly at first, owing to apprehension of theattendant dangers, to the lack of purification of the gas, and to theindifferent service. It was not until the latter half of the nineteenthcentury that it was generally used in residences. 

The gas-burner first employed by Murdock received the name "cockspur"from the shape of the flame. This had an illuminating value equivalentto about one candle for each cubic foot of gas burned per hour. The nextstep was to flatten the welded end of the gas-pipe and to bore a seriesof holes in a line. From the shape of the flames this form of burnerreceived the name "cockscomb. " It was somewhat more efficient than thecockspur burner. The next obvious step was to slit the end of the pipeby means of a fine saw. From this slit the gas was burned as a sheet offlame called the "bats-wing. " In 1820 Nielson made a burner whichallowed two small jets to collide and thus form a flat flame. Theefficiency of this "fish-tail" burner was somewhat higher than that ofthe earlier ones. Its flame was steadier because it was less influencedby drafts of air. In 1853 Frankland showed an Argand burner consistingof a metal ring containing a series of holes from which jets of gasissued. The glass chimney surrounded these, another chimney, extendingsomewhat lower, surrounded the whole, and a glass plate closed thebottom. The air to be fed to the gas-jets came downward between the twochimneys and was heated before it reached the burner. This increased theefficiency by reducing the amount of cooling at the burner by the airrequired for combustion. This improvement was in reality the forerunnerof the regenerative lamps which were developed later. 

In 1854 Bowditch brought out a regenerative lamp and, owing to theexcessive publicity which this lamp obtained, he is generally creditedwith the inception of the regenerative burner. This principle wasadopted in several lamps which came into use later. They were all basedupon the principle of heating both the gas and the air required forcombustion prior to their reaching the burner. The burner is somethinglike an inverted Argand arranged to produce a circular flame projectingdownward with a central cusp. The air- and gas-passages are directlyabove the flame and are heated by it. In 1879 Friedrich Siemens broughtout a lamp of this type which was adapted from a device originallydesigned for heating purposes, owing to the superior light which wasproduced. This was the best gas-lamp up to that time. Later, Wenham, Cromartie, and others patented lamps operating on this same principle. 

Murdock early modified the Argand burner to meet the requirements ofburning gas and by using the chimney obtained better combustion and asteadier flame than from the open burners. He and others recognized thatthe temperature of the flame had a considerable effect upon the amountof light emitted and non-conducting material such as steatite wassubstituted for the metal, which cooled the flame by conducting heatfrom it. These were the early steps which led finally to theregenerative burner. 

The increasing efficiency of the various gas-burners is indicated by thefollowing, which are approximately the candle-power based upon equalrates of consumption, namely, one cubic foot of gas per hour:

 Candle-power per cubic foot of gas per hour

 Fish-tail flames, depending upon size 0. 6 to 2. 5 Argand, depending upon improvements 2. 9 to 3. 5 Regenerative 7 to 10

It is seen that the possibilities of gas lighting were recognized inseveral countries, all of which contributed to its development. Some ofthe earlier accounts have been drawn chiefly from England, but these areintended merely to serve as examples of the difficulties encountered. Doubtless, similar controversies arose in other countries in whichpioneers were also nursing gas-lighting to maturity. However, it iscertain that much of the early progress of lighting of this characterwas fathered in England. Gas-lighting was destined to become a thrivingindustry, and is of such importance in lighting that another chapter isgiven its modern developments. 

VII

THE SCIENCE OF LIGHT-PRODUCTION

In previous chapters much of the historical development of artificiallighting has been presented and several subjects have been traced to themodern period which marks the beginning of an intensive attack byscientists upon the problems pertaining to the production of efficientand adequate light-sources. Many historical events remain to be touchedupon in later chapters, but it is necessary at this point for the readerto become acquainted with certain general physical principles in orderthat he may read with greater interest some of the chapters whichfollow. It is seen that from a standpoint of artificial lighting, the"dark age" extended well into the nineteenth century. Oil-lamps andgas-lighting began to be seriously developed at the beginning of thelast century, but the pioneers gave attention chiefly to mechanicaldetails and somewhat to the chemistry of the fuels. It was not until thescience of physics was applied to light-sources that rapid progress wasmade. 

All the light-sources used throughout the ages, and nearly all modernones, radiate light by virtue of the incandescence of solids or of solidparticles and it is an interesting fact that carbon is generally thesolid which emits light. This is due to various physical characteristicsof carbon, the chief one being its extremely high melting-point. However, most practicable light-sources of the past and present may bedivided into two general classes: (1) Those in which solids or solidparticles are heated by their own combustion, and (2) those in which thesolids are heated by some other means. Some light-sources include bothprinciples and some perhaps cannot be included under either principlewithout qualification. The luminous flames of burning material such asthose of wood-splinters, candles, oil-lamps, and gas-jets, and theglowing embers of burning material appear in the first class; andincandescent gas-mantles, electric filaments, and arc-lamps to someextent are representative of the second class. Certain "flaming" arcsinvolve both principles, but the light of the firefly, phosphorescence, and incandescent gas in "vacuum" tubes cannot be included in thissimplified classification. The status of these will become clear later. 

It has been seen that flames have been prominent sources of artificiallight; and although of low luminous efficiency, they still have much tocommend them from the standpoints of portability, convenience, andsubdivision. The materials which have been burned for light, whethersolid or liquid, are rich in carbon, and the solid particles of carbonby virtue of their incandescence are responsible for the brightness of aflame. A jet of pure hydrogen gas will burn very hot but with so low abrightness as to be barely visible. If solid particles are injected intothe flame, much more light usually will be emitted. A gas-burner of theBunsen type, in which complete combustion is obtained by mixing air inproper proportions with the gas, gives a hot flame which is of a paleblue color. Upon the closing of the orifice through which air isadmitted, the flame becomes bright and smoky. The flame is now less hot, as indicated by the presence of smoke or carbon particles, andcombustion is not complete. However, it is brighter because the solidparticles of carbon in passing upward through the flame become heated totemperatures at which they glow and each becomes a miniature source oflight. 

A close observer will notice that the flame from a match, a candle, or agas-jet, is not uniformly bright. The reader may verify this by lightinga match and observing the flame. There is always a bluish or darkerportion near the bottom. In this less luminous part the air is combiningwith the hydrogen of the hydrocarbon which is being vaporized anddisintegrated. Even the flame of a candle or of a burning splinter is aminiature gas-plant, for the solid or liquid hydrocarbons are vaporizedbefore being burned. Owing to the incoming colder air at this point, theflame is not hot enough for complete combustion. The unburned carbonparticles rise in its draft and become heated to incandescence, thusaccounting for the brighter portion. In cases of complete combustionthey are eventually oxidized into carbon dioxide before they are able toescape. If a piece of metal be held in the flame, it immediately becomescovered with soot or carbon, because it has reduced the temperaturebelow the point at which the chemical reaction--the uniting of carbonwith oxygen--will continue. An ordinary flat gas-flame of the"bats-wing" type may vary in temperature in its central portion from300°F. At the bottom to about 3000°F. At the top. The central portionlies between two hotter layers in which the vertical variation is not sogreat. The brightness of the upper portion is due to incandescent carbonformed in the lower part. 

When scientists learned by exploring flames that brightness was due tothe radiation of light by incandescent solid matter, the way was openfor many experiments. In the early days of gas-lighting investigationswere made to determine the relation of illuminating value to thechemical constitution of the gas. The results combined with a knowledgeof the necessity for solid carbon in the flame led to improvements inthe gas for lighting purposes. Gas rich in hydrocarbons which in turnare rich in carbon is high in illuminating value. Heating-effect dependsupon heat-units, so the rating of gas in calories or other heat-unitsper cubic foot is wholly satisfactory only for gas used for heating. Thechemical constitution is a better indicator of illuminating value. 

As scientific knowledge increased, efforts were made to get solid matterinto the flames of light-sources. Instead of confining efforts to thecarbon content of the gas, solid materials were actually placed in theflame, and in this manner various incandescent burners were developed. Apiece of lime placed in a hydrogen flame or that of a Bunsen burner isseen to become hot and to glow brilliantly. By producing a hotter flameby means of the blowpipe, in which hydrogen and oxygen are consumed, thepiece of lime was raised to a higher temperature and a more intenselight was obtained. In Paris there was a serious attempt atstreet-lighting by the use of buttons of zirconia heated in anoxygen-coal-gas flame, but it proved unsuccessful owing to the rapiddeterioration of the buttons. This was the line of experimentation whichled to the development of the lime-light. The incandescent burner waswidely employed, and until the use of electricity became common thelime-light was the mainstay for the stage and for the projection oflantern slides. It is in use even to-day for some purposes. The originof the phrase "in the lime-light" is obvious. The luminous intensity ofthe oxyhydrogen lime-light as used in practice was generally from 200 to400 candle-power. The light decreases rapidly as the burner is used, ifa new surface of lime is not presented to the flame from time to time. At the high temperatures the lime is somewhat volatile and the surfaceseems to change in radiating power. Zirconium oxide has been found toserve better than lime. 

Improvements were made in gas-burners in order to obtain hotter flamesinto which solid matter could be introduced to obtain bright light. Manymaterials were used, but obviously they were limited to those of afairly high melting-point. Lime, magnesia, zirconia, and similar oxideswere used successfully. If the reader would care to try an experiment inverification of this simple principle, let him take a piece of magnesiumribbon such as is used in lighting for photography and ignite it in aBunsen flame. If it is held carefully while burning, a ribbon of ash(magnesia) will be obtained intact. Placing this in the faintly luminousflame, he will be surprised at the brilliance of its incandescence whenit has become heated. The simple experiment indicates the possibilitiesof light-production in this direction. Naturally, metals of highmelting-point such as platinum were tried and a network of platinumwire, in reality a platinum mantle, came into practical use in about1880. The town of Nantes was lighted by gas-burners using theseplatinum-gauze mantles, but the mantles were unsuccessful owing to theirrapid deterioration. This line of experimentation finally bore fruit ofimmense value for from it the gas-mantle evolved. 

A group of so-called "rare-earths, " among which are zirconia, thoria, ceria, erbia, and yttria (these are oxides of zirconium, etc. ) possess anumber of interesting chemical properties some of which have beenutilized to advantage in the development of modern artificial light. They are white or yellowish-white oxides of a highly refractorycharacter found in certain rare minerals. Most of them are verybrilliant when heated to a high temperature. This latter feature iseasily explained if the nature of light and the radiating properties ofsubstances are considered. Suppose pieces of different substances, forexample, glass and lime, are heated in a Bunsen flame to the sametemperature which is sufficiently great to cause both of them to glow. Notwithstanding the identical conditions of heating, the glass will beonly faintly luminous, while the piece of lime will glow brilliantly. The former is a poor radiator; furthermore, the lime radiates arelatively greater percentage of its total energy in the form ofluminous energy. 

The latter point will become clearer if the reader will refresh hismemory regarding the nature of light. Any luminous source such as thesun, a candle flame, or an incandescent lamp is sending forthelectromagnetic waves not unlike those used in wireless telegraphyexcepting that they are of much shorter wave-length. The eye is capableof recording some of these waves as light just as a receiving station istuned to record a range of wave-lengths of electromagnetic energy. Theelectromagnetic waves sent forth by a light-source like the sun are notall visible, that is, all of them do not arouse a sensation of light. Those that do comprise the visible spectrum and the differentwave-lengths of visible radiant energy manifest themselves by arousingthe sensations of the various spectral colors. The radiant energy ofshortest wave-length perceptible by the visual apparatus excites thesensation of violet and the longest ones the sensation of deep red. Between these two extremes of the visible spectrum, the chief spectralcolors are blue, green, yellow, orange, and red in the order ofincreasing wave-lengths. Electromagnetic energy radiated by alight-source in waves of too great wave-length to be perceived by theeye as light is termed as a class "infra-red radiant energy. " Those tooshort to be perceived as light are termed as a class "ultravioletradiant energy. " A solid body at a high temperature emitselectro-magnetic energy of all wave-lengths, from the shortestultra-violet to the longest infra-red. 

Another complication arises in the variation in visibility or luminosityof energy of wave-lengths within the range of the visible spectrum. Obviously, no amount of energy incapable of exciting the sensation oflight will be visible. The energy of those wave-lengths near the endsof the visible spectrum will be inefficient in producing light. Thatenergy which excites the sensation of yellow-green produces the greatestluminosity per unit of energy and is the most efficient light. Thevisibility or luminous efficiency of radiant energy may be rangedapproximately in this manner according to the colors aroused:yellow-green, yellow, green, orange, blue-green, red, blue, deep red, violet. 

Newton, an English scientist, first described the discovery of thevisible spectrum and this is of such fundamental importance in thescience of light that the first paragraph of his original paper in the"Transactions of the Royal Society of London" is quoted as follows:

 In the Year 1666 (at which time I applied my self to the Grinding of Optick Glasses of other Figures than Spherical) I procured me a Triangular Glass-Prism, to try therewith the celebrated Phaenomena of Colours. And in order thereto, having darkened my Chamber, and made a small Hole in my Window-Shuts, to let in a convenient Quantity of the Sun's Light, I placed my Prism at its Entrance, that it might be thereby refracted to the opposite Wall. It was at first a very pleasing Divertisement, to view the vivid and intense Colours produced thereby; but after a while applying my self to consider them more circumspectly, I became surprised to see them in an oblong Form; which, according to the receiv'd Law of Refractions, I expected should have been circular. They were terminated at the Sides with streight Lines, but at the Ends the Decay of Light was so gradual, that it was difficult to determine justly what was the Figure, yet they seemed Semicircular. 

Even Newton could not have had the faintest idea of the greatdevelopments which were to be based upon the spectrum. 

Now to return to the peculiar property of rare-earth oxides--namely, their unusual brilliance when heated in a flame--it is easy tounderstand the reason for this. For example, when a number of substancesare heated to the same temperature they may radiate the same amount ofenergy and still differ considerably in brightness. Many substances are"selective" in their absorbing and radiating properties. One may radiatemore luminous energy and less infra-red energy, and for another thereverse may be true. The former would appear brighter than the latter. The scientific worker in light-production has been searching for such"selective" radiators whose other properties are satisfactory. Therare-earths possess the property of selectivity and are fortunatelyhighly refractory. Welsbach used these in his mantle, whose efficiencyis due partly to this selective property. Recent work indicates thatmuch higher efficiencies of light-production are still attainable by theprinciples involved in the gas-mantle. 

Turning again to flames, another interesting physical phenomenon is seenon placing solutions of different chemical salts in the flame. Forexample, if a piece of asbestos is soaked in sodium chloride (commonsalt) and is placed in a Bunsen flame, the pale-blue flame suddenlybecomes luminous and of a yellow color. If this is repeated with othersalts, a characteristic color will be noted in each case. The yellowflame is characteristic of sodium and if it is examined by means of aspectroscope, a brilliant yellow line (in fact, a double line) will beseen. This forms the basis of spectrum analysis as applied in chemistry. 

Every element has its characteristic spectrum consisting usually oflines, but the complexity varies with the elements. The spectra ofelements also exhibit lines in the ultra-violet region which may bestudied with a photographic plate, with a photo-electric cell, and byother means. Their spectral lines or bands also extend into theinfra-red region and here they are studied by means of the bolometer orother apparatus for detecting radiant energy by the heat which itproduces upon being absorbed. Spectrum analysis is far more sensitivethan the finest weighing balance, for if a grain of salt be dissolved ina barrel of water and an asbestos strip be soaked in the water and heldin a Bunsen flame, the yellow color characteristic of sodium will bedetectable. A wonderful example of the possibilities of this method isthe discovery of helium in the sun before it was found on earth! Itsspectral lines were detected in the sun's spectrum and could not beaccounted for by any known element. However, it should be stated thatthe spectrum of an element differs generally with the manner obtained. The electric spark, the arc, the electric discharge in a vacuum tube, and the flame are the means usually employed. 

The spectrum has been dwelt upon at some length because it is of greatimportance in light-production and probably will figure strongly infuture developments. Although in lighting little use has been made ofthe injection of chemical salts into ordinary flames, it appears certainthat such developments would have risen if electric illuminants had notentered the field. However, the principle has been applied with greatsuccess in arc-lamps. In the first arc-lamps plain carbon electrodeswere used, but in some of the latest carbon-arcs, electrodes of carbonimpregnated with various salts are employed. For example, calciumfluoride gives a brilliant yellow light when used in the carbons of the"flame" arc. These are described in detail later. 

Following this principle of light-production the vacuum tubes weredeveloped. Crookes studied the light from various gases by enclosingthem in a tube which was pumped out until a low vacuum was produced. Onconnecting a high voltage to electrodes in each end, an electricaldischarge passed through the residual gas making it luminous. Thedifferent gases show their characteristic spectra and their desirabilityas light-producers is at once evident. 

However, the most general principle of light-production at the presenttime is the radiation of bodies by virtue of their temperature. If apiece of wire be heated by electricity, it will become very hot beforeit becomes luminous. At this temperature it is emitting only invisibleinfra-red energy and has an efficiency of zero as a producer of light. As it becomes hotter it begins to appear red, but as its temperature israised it appears orange, until if it could be heated to the temperatureof the sun, about 10, 000°F. , it would appear white. All this time itsluminous efficiency is increasing, because it is radiating not only anincreasing percentage of visible radiant energy but an increasing amountof the most effective luminous energy. But even when it appears white, alarge amount of the energy which it radiates is invisible infra-red andultra-violet, which are ineffective in producing light, so at best thesubstance at this high temperature is inefficient as a light-producer. 

In this branch of the science of light-production substances are soughtnot only for their high melting-point, but for their ability to radiateselectively as much visible energy as possible and of the most luminouscharacter. However, at best the present method of utilizing thetemperature radiation of hot bodies has limitations. 

The luminous efficiencies of light-sources to-day are still very low, but great advances have been made in the past half-century. There mustbe some radical departures if the efficiency of light-production is toreach a much higher figure. A good deal has been said of the firefly andof phosphorescence. These light-sources appear to emit only visibleenergy and, therefore, are efficient as radiators of luminous radiantenergy. But much remains to be unearthed concerning them before theywill be generally applicable to lighting. If ultra-violet radiation isallowed to impinge upon a phosphorescent material, it will glow with aconsiderable brightness but will be cool to the touch. A substance ofthe same brightness by virtue of its temperature would be hot; hencephosphorescence is said to be "cold" light. 

An acquaintance with certain terms is necessary if the reader is tounderstand certain parts of the text. The early candle gradually becamea standard, and uniform candles are still satisfactory standards wherehigh accuracy is not required. Their luminous intensity andilluminating value became units just as the foot was arbitrarily adoptedas a unit of length. The intensity of other light-sources wasrepresented in terms of the number of candles or fraction of a candlewhich gave the same amount of light. But the luminous intensity of thecandle was taken only in the horizontal direction. In the same mannerthe luminous intensities of light-sources until a short time ago wereexpressed in candles as measured in a certain direction. Incandescentlamps were rated in terms of mean horizontal candles, which would besatisfactory if the luminous intensity were the same in all directions, but it is not. Therefore, the candle-power in one direction does notgive a measure of the total light-output. 

If a source of light has a luminous intensity of one candle in alldirections, the illumination at a distance of one foot in any directionis said to be a foot-candle. This is the unit of illumination intensity. A lumen is the quantity of light which falls on one square foot if theintensity of illumination is one foot-candle. It is seen that the areaof a sphere with a radius of one foot is 4 pi or 12. 57 square feet;therefore, a light-source having a luminous intensity of one candle inall directions emits 12. 57 lumens. This is the satisfactory unit, for itmeasures total quantity of light, and luminous efficiencies may beexpressed in terms of lumens per watt, lumens per cubic foot of gas perhour, etc. 

Of course, the efficiencies of light-sources are usually of interest tothe consumer if they are expressed in terms of cost. But from apractical point of view there are many elements which combine to makeanother important factor, namely, satisfactoriness. Therefore, theefficiency of artificial lighting from the standpoint of the consumershould be the ratio of satisfactoriness to cost. However, the scientistis interested chiefly in the efficiency of the light-source which may beexpressed in lumens per watt, or the amount of light obtained from agiven rate of consumption or of emission of energy. This method ofrating light-sources penalizes those radiating considerable energy whichdoes not produce the sensation of light or which at best is ofwave-lengths that are inefficient in this respect. That radiant energywhich is wholly of a wave-length of maximum visibility, or, in otherwords, excites the sensation of yellow-green, is the most efficient inproducing luminous sensation. Of course, no illuminants are availablewhich approach this theoretical ideal and it is not likely that thiswould be a practical ideal. Under monochromatic yellow-green light themagical drapery of color would disappear and the surroundings would be amonochrome of shades of this hue. Having no colors with which tocontrast this color, the world would be colorless. This should beobvious when it is considered that an object which is red under anilluminant containing all colors such as sunlight would be black or darkgray under monochromatic yellow-green light. The red under presentconditions is kept alive by contrast with other colors, because thelatter live by virtue of the fact that most of our present illuminantscontain their hues. It is assumed that the reader knows that a redobject, for example, appears red because it reflects (or transmits) redrays and absorbs the other rays in the illuminant. In other words, coloris due to selective absorption reflection, or transmission. 

Perhaps the ideal illuminant, which is most generally satisfactory forgeneral activities, is a white light corresponding to noon sunlight. Ifthis is chosen as the scientific ideal, the illuminants of the presenttime are much more "efficient" than if the most efficient light is theideal. 

The luminous efficiency of the radiant energy most efficient inproducing the sensation of light (yellow-green) is about 625 lumens perwatt. That is, if energy of this wave-length alone were radiated by ahypothetical light-source, each watt would produce 625 lumens. Theluminous efficiency of the most efficient white light is about 265lumens per watt; in other words, if a hypothetical light-source radiatedenergy of only the visible wave-lengths and in proportions to producethe sensation of white, each watt would produce 265 lumens. If such awhite light were obtained by pure temperature radiation--that is, by anormal radiator at a temperature of 10, 000°F. , which is impracticable atpresent--the luminous efficiency would be about 100 lumens per watt. Thenormal radiator which emits energy by virtue of its temperature withoutselectively radiating more or less energy in any part of the spectrumthan indicated by the theoretical radiation laws is called a"black-body" or normal radiator. Modern illuminants have luminousefficiencies ranging from 5 to 30 lumens per watt, so it is seen thatmuch is to be done before the limiting efficiencies are reached. 

The amount of light obtained from various gas-burners for each cubicfoot of gas consumed per hour varies for open gas-flames from 5 to 30lumens; for Argand burners from 35 to 40 lumens; for regenerative lampsfrom 50 to 75 lumens; and for gas-mantles from 200 to 250 lumens. 

In the development of light-sources, of course, any harmful effects ofgases formed by burning or chemical action must be avoided. Some of thefumes from arcs are harmful, but no commercial arc appears to bedangerous when used as it is intended to be used. Gas-burners rob theatmosphere of oxygen and vitiate it with gases, which, however, areharmless if combustion is complete. That adequate ventilation isnecessary where oxygen is being consumed is evident from the datapresented by authorities on hygiene. A standard candle when burningvitiates the air in a room almost as much as an adult person. Anordinary kerosene lamp vitiates the atmosphere as much as a half-dozenpersons. An ordinary single mantle burner causes as much vitiation astwo or three persons. 

In order to obtain a bird's-eye view of progress in light-production, the following table of relative luminous efficiencies of severallight-sources is given in round numbers. These efficiencies are in termsof the most efficient (yellow-green) light. 

 Efficiency in per cent. Sperm-candle 0. 02 Open gas-flame . 04 Incandescent gas-mantle . 19 Carbon filament lamp . 05 Vacuum Mazda lamp 1. 3 Gas-filled Mazda lamp 2 to 3 Arc-lamps 2 to 7 White light radiated by "black-body" 16 Most efficient white light 40 Firefly 95 Most efficient light (yellow-green) 100

The luminous efficiency of a light-source is distinguished from that ofa lamp. The former is the ratio of the light produced to the amount ofenergy radiated by the light-source. The latter is the ratio of thelight produced to the total amount of energy consumed by the device. Inother words, the luminous efficiency of a lamp is less than that of thelight-source because the consumption of energy in other parts of thelamp besides the light-source are taken into account. These additionallosses are appreciable in the mechanisms of arc-lamps but are almostnegligible in vacuum incandescent filament lamps. They are unknown forthe firefly, so that its luminous efficiency only as a light-source canbe determined. Its efficiency as a lighting-plant may be and perhaps israther low. 

VIII

MODERN GAS-LIGHTING

As has been seen, the lighting industry, as a public service, was bornin London about a century ago and companies to serve the public wereorganized on the Continent shortly after. From this early beginninggas-light remained for a long time the only illuminant supplied by apublic-service company. It has been seen that throughout the ages littleadvance was made in lighting until oil-lamps were improved by Argand inthe eighteenth century. Candles and open-flame oil-lamps were in usewhen the Pyramids were built and these were common until the approach ofthe nineteenth century. In fact, several decades passed after the firstgas-lighting was installed before this form of lighting began todisplace the improved oil-lamps and candles. It was not until about 1850that it began to invade the homes of the middle and poorer classes. During the first half of the nineteenth century the total light in anaverage home was less than is now obtained from a single light-sourceused in residences; still, the total cost of lighting a residence hasdecreased considerably. If the social and industrial activities ofmankind are visualized for these various periods in parallel with thedevelopment of artificial lighting, a close relation is evident. Didartificial light advance merely hand in hand with science, invention, commerce, and industry, or did it illuminate the pathway?

Although gas-lighting was born in England it soon began to receiveattention elsewhere. In 1815 the first attempt to provide a gas-works inAmerica was made in Philadelphia; but progress was slow, with the resultthat Baltimore and New York led in the erection of gas-works. There areon record many protests against proposals which meant progress inlighting. These are amusing now, but they indicate the inertia of thepeople in such matters. When Bollman was projecting a plan for lightingPhiladelphia by means of piped gas, a group of prominent citizenssubmitted a protest in 1833 which aimed to show that the consequences ofthe use of gas were appalling. But this protest failed and in 1835 agas-plant was founded in Philadelphia. Thus gas-lighting, which to SirWalter Scott was a "pestilential innovation" projected by a madman, weathered its early difficulties and grew to be a mighty industry. Continued improvements and increasing output not only altered the courseof civilization by increased and adequate lighting but they reduced thecost of lighting over the span of the nineteenth century to a smallfraction of its initial cost. 

Think of the city of Philadelphia in 1800, with a population of aboutfifty thousand, dependent for its lighting wholly upon candles andoil-lamps! Washington's birthday anniversary was celebrated in 1817 witha grand ball attended by five hundred of the élite. An old report of theoccasion states that the room was lighted by two thousand wax-candles. The cost of this lighting was a hundred times the cost of as much lightfor a similar occasion at the present time. Can one imagine the presentcomplex activities of a city like Philadelphia with nearly two millioninhabitants to exist under the lighting conditions of a century ago?To-day there are more than fifty thousand street lamps in the city--onefor each inhabitant of a century ago. Of these street lamps abouttwenty-five thousand burn gas. This single instance is representative ofgas-lighting which initiated the "light age" and nursed it through thevicissitudes of youth. The consumption of gas has grown in the UnitedStates during this time to three billion cubic feet per day. Forstrictly illuminating purposes in 1910 nearly one hundred billion cubicfeet were used. This country has been blessed with large supplies ofnatural gas; but as this fails new oil-fields are constantly beingdiscovered, so that as far as raw materials are concerned the future ofgas-lighting is assured for a long time to come. 

The advent of the gas-mantle is responsible for the survival ofgas-lighting, because when it appeared electric lamps had already beeninvented. These were destined to become the formidable light-sources ofthe approaching century and without the gas-mantle gas-lighting wouldnot have prospered. Auer von Welsbach was conducting a spectroscopicstudy of the rare-earths when he was confronted with the problem ofheating these substances. He immersed cotton in solutions of these saltsas a variation of the regular means for studying elements by injectingthem into flames. After burning the cotton he found that he had areplica of the original fabric composed of the oxide of the metal, andthis glowed brilliantly when left in the flame. 

This gave him the idea of producing a mantle for illuminating purposesand in 1885 he placed such a mantle in commercial use. His first mantleswere unsatisfactory, but they were improved in 1886 by the use ofthoria, an oxide of thorium, in conjunction with other rare-earthoxides. His mantle was now not only stronger but it gave more light. Later he greatly improved the mantles by purifying the oxides andfinally achieved his great triumph by adding a slight amount of ceria, an oxide of cerium. Welsbach is deserving of a great deal of credit forhis extensive work, which overcame many difficulties and finally gave tothe world a durable mantle that greatly increased the amount of lightpreviously obtainable from gas. 

The physical characteristics of a mantle depend upon the fabric and uponthe rare-earths used. It must not shrink unduly when burned, and the ashshould remain porous. It has been found that a mantle in which thoria isused alone is a poor light-source, but that when a small amount of ceriais added the mantle glows brilliantly. By experiment it was determinedthat the best proportions for the rare-earth content are one part ofceria and ninety-nine parts of thoria. Greater or less proportions ofceria decreased the light-output. The actual percentage of these oxidesin the ash of the mantle is about 10 per cent. , making the content ofceria about one part in one thousand. 

Mantles are made by knitting cylinders of cotton or of other fiber andsoaking these in a solution of the nitrates of cerium and thorium. Oneend of the cylinder is then sewed together with asbestos thread, whichalso provides the loop for supporting the mantle over the burner. Afterthe mantle has dried in proper form, it is burned; the organic matterdisappears and the nitrates are converted into oxides. After this"burning off" has been accomplished and any residual blackening isremoved, the mantle is dipped into collodion, which strengthens it forshipping and handling. The collodion is a solution of gun-cotton inalcohol and ether to which an oil such as castor-oil has been added toprevent excessive shrinkage on drying. 

The materials and structure of the fabric of mantles have been subjectedto much study. Cotton was first used; then ramie fibers were introduced. The ramie mantle was found to possess a greater life than the cottonmantle. Later the mantles were mercerized by immersion in ammonia-waterand this process yielded a stronger material. The latest development isthe use of an artificial silk as the base fabric, which results in amantle superior to previous mantles in strength, flexibility, permanenceof form, and permanence of luminous property. This artificial silkmantle will permit of handling even after it has been in use for severalhundred hours. This great advance appears to be due to the fact thatafter the artificial-silk fibers have been burned off, the fibers aresolid and continuous instead of porous as in previous mantles. 

The color-value of the light from mantles may be varied considerably byaltering the proportions of the rare-earths. The yellowness of the lighthas been traced to ceria, so by varying the proportions of ceria, thecolor of the light may be influenced. 

The inverted mantle introduced greater possibilities into gas-lighting. The light could be directed downward with ease and many units such asinverted bowls were developed. In fact, the lighting-fixtures and thelighting-effects obtainable kept pace with those of electric lighting, notwithstanding the greater difficulties encountered by the designer ofgas-lighting fixtures. Many problems were encountered in designing aninverted burner operating on the Bunsen principle, but they were finallysatisfactorily solved. In recent years a great deal of study has beengiven to the efficiency of gas-burners, with the result that a highlevel of development has been reached. 

Several methods of electrical ignition have been evolved which ingeneral employ the electric spark. Electrical ignition and developmentsof remote control have added great improvements especially tostreet-lighting by means of gas. Gas-valves for remote control areactuated by gas pressure and by electromagnets. In general, thegas-lighting engineers have kept pace marvelously with electriclighting, when their handicaps are considered. 

Various types of burners have appeared which aimed to burn more gas in agiven time under a mantle and thereby to increase the output of light. These led to the development of the pressure system in which thepressure of gas was at first several times greater than usual. The gasis fed into the mixing tube under this higher pressure in a manner whichalso draws in an adequate amount of air. In this way the combustion atthe burner is forced beyond the point reached with the usual pressure. Ordinary gas pressure is equal to that of a few inches of water, buthigh-pressure systems employ pressures as great as sixty inches ofwater. Under this high-pressure system, mantle-burners yield as high as500 lumens per cubic foot of gas per hour. 

The fuels for gas-lighting are natural gas, carbureted water-gas, andcoal-gas obtained by distilling coal, but there are different methods ofproducing the artificial gases. Coal-gas is produced analytically bydistilling certain kinds of coal, but water-gas and producer-gas aremade synthetically by the action of several constituents upon oneanother. Carbureted water-gas is made from fixed carbon, steam, and oiland also from steam and oil. Producer-gas is made by the action of steamor air or both upon fixed carbon. Water-gas made from steam and oil isusually limited to those places where the raw materials are readilyavailable. The composition of a gas determines its heating andilluminating values, and constituents favorable to one are notnecessarily favorable to the other. Coal-gas usually is of lowerilluminating value than carbureted water-gas. It contains more hydrogen, for example, than water-gas and it is well known that hydrogen giveslittle light on burning. 

It has been seen in a previous chapter that the distillation of gas fromcoal for illuminating purposes began in the latter part of theeighteenth century. From this beginning the manufacture of coal-gas hasbeen developed to a great and complex industry. The method isessentially destructive distillation. The coal is placed in a retort andwhen it reaches a temperature of about 700°F. Through heating by anoutside fire, the coal begins to fuse and hydrocarbon vapors begin toemanate. These are generally paraffins and olefins. As the temperatureincreases, these hydrocarbons begin to be affected. The chemicalcombinations which have long existed are broken up and there arerearrangements of the atoms of carbon and hydrogen. The actual chemicalreactions become very complex and are somewhat shrouded in uncertainty. In this last stage the illuminating and heating values of the gas aredetermined. Usually about four hours are allowed for the completedistillation of the gaseous and liquid products from a charge of coal. Many interesting chemical problems arise in this process and theinfluences of temperature and time cannot be discussed within the scopeof this book. Besides the coal-gas, various by-products are obtaineddepending upon the raw materials, upon the procedure, and upon themarket. 

After the coal-gas is produced it must be purified and the sulphuretedhydrogen at least must be removed. One method of accomplishing this isby washing the gas with water and ammonia, which also removes some ofthe carbon dioxide and hydrocyanic acid. Various other undesirableconstituents are removed by chemical means, depending upon theconditions. The purified gas is now delivered to the gas-holder; but, ofcourse, all this time the pressure is governed, in order that thepressure in the mains will be maintained constant. 

Much attention has been given to the enrichment of gas for illuminatingpurposes; that is, to produce a gas of high illuminating value fromcheap fuel or by inexpensive processes. This has been done bydecomposing the tar obtained during the distillation of coal and addingthese gases to the coal-gas; by mixing carbureted water-gas withcoal-gas; by carbureting inferior coal-gases; and by mixing oil-gas withinferior coal-gas. 

Water-gas is of low illuminating value, but after it is carbureted itburns with a brilliant flame. The water-gas is made by raising thetemperature of the fuel bed of hard coal or coke by forced air, which isthen cut off, while steam is passed through the incandescent fuel. Thisyields hydrogen and carbon monoxide. To make carbureted water-gas, oil-gas is mixed with it, the latter being made by heating oil inretorts. 

A great many kinds of gas are made which are determined by therequirements and the raw materials available. The amount of illuminatinggas yielded by a ton of fuel, of course, varies with the method ofmanufacture, with the raw material, and with the use to which the fuelis to be put. The production of coal-gas per ton of coal is of the orderof magnitude of 10, 000 cubic feet. A typical yield by weight of acoal-gas retort is, 10, 000 cubic feet of gas 17 per cent. Coke 70 " " tar 5 " " ammoniacal liquid 8 " "

The coke is not pure carbon but contains the non-volatile minerals whichwill remain as ash when the coke is burned, just as if the original coalhad been burned. On the crown of the retort used in coal-gas production, pure carbon is deposited. This is used for electric-arc carbons and forother purposes. From the tar many products are derived such as anilinedyes, benzene, carbolic acid, picric acid, napthalene, pitch, anthracene, and saccharin. 

A typical analysis of the gas distilled from coal is very approximatelyas follows, Hydrocarbons 40 per cent. Hydrogen 50 " " Carbon monoxide 4 " " Nitrogen 4 " " Carbon dioxide 1 " " Various other gases 1 " "

It is seen that illuminating gas is not a definite compound but amixture of a number of gases. The proportion of these is controlled inso far as possible in order to obtain illuminating value and some ofthem are reduced to very small percentages because they are valueless asilluminants or even harmful. The constituents are seen to consist oflight-giving hydrocarbons, of gases which yield chiefly heat, and ofimpurities. The chief hydrocarbons found in illuminating gas are, ethylene C_{2}H_{4} crotonylene C_{4}H_{6} propylene C_{3}H_{6} benzene C_{6}H_{6} butylene C_{4}H_{8} toluene C_{7}H_{8} amylene C_{5}H_{10} xylene C_{8}H_{10} acetylene C_{2}H_{2} methane C H_{4} allylene C_{3}H_{4} ethane C_{2}H_{6}

A gas which has played a prominent part in lighting is acetylene, produced by the interaction of water and calcium carbide. No other gaseasily produced upon a commercial scale yields as much light, volume forvolume, as acetylene. It has the great advantage of being easilyprepared from raw material whose yield of gas is considerably greaterfor a given amount than the raw materials which are used in making otherilluminating gases. The simplicity of the manufacture of acetylene fromcalcium carbide and water gives to this gas a great advantage in somecases. It has served for individual lighting in houses and in otherplaces where gas or electric service was unavailable. Where space islimited it also had an advantage and was adopted to some extent onautomobiles, motor-boats, ships, lighthouses, and railway cars beforeelectric lighting was developed for these purposes. 

The color of the acetylene flame is satisfactory and it is extremelybrilliant compared with most flames. An interesting experiment is foundin placing a spark-gap in the flame and sending a series of sparksacross it. If the conditions are proper the flame will became very muchbrighter. When the gas issues from a proper jet under sufficientpressure, the flame is quite steady. Its luminous efficiency gives it anadvantage over other open gas-flames in lighting rooms, because for thesame amount of light it vitiates the air and exhausts the oxygen to aless degree than the others. Of course, in these respects the gas-mantleis superior. 

The reaction which takes place when water and calcium carbide arebrought together is a double decomposition and is represented by, CaC_{2} + H_{2}O = C_{2}H_{2} + CaO

It will be seen that the products are acetylene gas and calcium oxide orlime. The lime, being hydroscopic and being in the presence of water orwater-vapor in the acetylene generator, really becomes calcium hydroxideCa(OH)_{2}, commonly called slaked lime. If there are impurities in thecalcium carbide, it is sometimes necessary to purify the gas before itmay be safely used for interior lighting. 

The burners and mantles used in acetylene lighting are essentially thesame as those for other gas-lighting, excepting, of course, that theyare especially adapted for it in minor details. 

The chief source of calcium carbide in this country is the electricfurnace. Cheap electrical energy from hydro-electric developments, suchas the Niagara plants, have done much to make the earth yield itselements. Aluminum is very prevalent in the soil of the earth's surface, because its oxide, alumina, is a chief constituent of ordinary clay. Butthe elements, aluminum and oxygen, cling tenaciously to each other andonly the electric furnace with its excessively high temperatures hasbeen able to separate them on a large commercial scale. Similarly, calcium is found in various compounds over the earth's surface. Limestone abounds widely, hence the oxide and carbonate of lime arewide-spread. But calcium clings tightly to the other elements of itscompounds and it has taken the electric furnace to bring it tosubmission. The cheapness of calcium carbide is due to the developmentof cheap electric power. It is said that calcium carbide was discoveredas a by-product of the electric furnace by accidentally throwing waterupon the waste materials of a furnace process. The discovery of acommercial scale of manufacture of calcium carbide has been a boon toisolated lighting. Electric lighting has usurped its place on theautomobile and is making inroads in country-home lighting. Doubtless, acetylene will continue to serve for many years, but its future does notappear as bright as it did many years ago. 

The Pintsch gas, used to some extent in railroad passenger-cars in thiscountry, is an oil-gas produced by the destructive distillation ofpetroleum or other mineral oil in retorts heated externally. The productconsists chiefly of methane and heavy hydrocarbons with a small amountof hydrogen. In the early days of railways, some trains were not runafter dark and those which were operated were not always lighted. Atfirst attempts were made at lighting railway cars with compressedcoal-gas, but the disadvantage of this was the large tank required. Obviously, a gas of higher illuminating-value per volume was desiredwhere limited storage space was available, and Pintsch turned hisattention to oil-gas. Gas suffers in illuminating-value upon beingcompressed, but oil-gas suffers only about half the loss that coal-gasdoes. In about 1880 Pintsch developed a method of welding cylinders andbuoys which satisfied lighthouse authorities and he was enabled tofurnish these filled with compressed gas. Thus the buoy was its owngas-tank. He devised lanterns which would remain lighted regardless ofwind and waves and thus gained a start with his compressed-gas systems. He compressed the gas to a pressure of about one hundred and fiftypounds per square inch and was obliged to devise a reducer which woulddeliver the gas to the burner at about one pound per square inch. Thisregulator served well throughout many years of exacting service. Thesystem began to be adopted on ships and railroads in 1880 and for manyyears it has served well. 

Although gas-lighting has affected the activities of mankindconsiderably by intensifying commerce and industry and by advancingsocial progress, the illuminants which eventually took the lead haveextended the possibilities and influences of artificial light. In thebrief span of a century civilized man is almost totally independent ofnatural light in those fields over which he has control. What anothercentury will bring can be predicted only from the accomplishments of thepast. These indicate possibilities beyond the powers of imagination. 

IX

THE ELECTRIC ARCS

Early in 1800 Volta wrote a letter to the President of the Royal Societyof London announcing the epochal discovery of a device now known as thevoltaic pile. This letter was published in the Transactions and itcreated great excitement among scientific men, who immediately beganactive investigations of certain electrical phenomena. Volta showed thatall metals could be arranged in a series so that each one would indicatea positive electric potential when in contact with any metal followingit in the series. He constructed a pile of metal disks consisting ofzinc and copper alternated and separated by wet cloths. At first hebelieved that mere contact was sufficient, but when, later, it was shownthat chemical action took place, rapid progress was made in theconstruction of voltaic cells. The next step after his pile wasconstructed was to place pairs of strips of copper and zinc in cupscontaining water or dilute acid. Volta received many honors for hisdiscovery, which contributed so much to the development of electricalscience and art--among them a call to Paris by Bonaparte to exhibit hiselectrical experiments, and to receive a medal struck in his honor. 

While Volta was being showered with honors, various scientific men withgreat enthusiasm were entering new fields of research, among which wasthe heating value of electric current and particularly of electricsparks made by breaking a circuit. Late in 1800 Sir Humphrey Davy wasthe first to use charcoal for the sparking points. In a lecture beforethe Royal Society in the following year he described and demonstratedthat the "spark" passing between two pieces of charcoal was larger andmore brilliant than between brass spheres. Apparently, he was producinga feeble arc, rather than a pure spark. In the years which immediatelyfollowed many scientific men in England, France, and Germany werepublishing the results of their studies of electrical phenomenabordering upon the arc. 

By subscription among the members of the Royal Society, a voltaicbattery of two thousand cells was obtained and in 1808 Davy exhibitedthe electric arc on a large scale. It is difficult to judge from thereports of these early investigations who was the first to recognize thedifference between the spark and the arc. Certainly the descriptionsindicate that the simple spark was not being experimented with, but thesource of electric current available at that time was of such highresistance that only feeble arcs could have been produced. In 1809 Davydemonstrated publicly an arc obtained by a current from a Volta pile ofone thousand plates. This he described as "a most brilliant flame, offrom half an inch to one and a quarter inches in length. "

In the library of the Royal Society, Davy's notes made during the yearsof 1805 and 1812 are available in two large volumes. These were arrangedand paged by Faraday, who was destined to contribute greatly to thefuture development of the science and art of electricity. In one ofthese volumes is found an account of a lecture-experiment by Davy whichcertainly is a description of the electric arc. An extract of thisaccount is as follows:

 The spark [presumably the arc], the light of which was so intense as to resemble that of the sun, . .. Produced a discharge through heated air nearly three inches in length, and of a dazzling splendor. Several bodies which had not been fused before were fused by this flame. .. . Charcoal was made to evaporate, and plumbago appeared to fuse in vacuo. Charcoal was ignited to intense whiteness by it in oxymuriatic acid, and volatilized by it, but without being decomposed. 

From a consideration of his source of electricity, a voltaic pile of twothousand plates, it is certain that this could not have been an electricspark. Later in his notes Davy continued:

 . .. The charcoal became ignited to whitness, and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space at least equal to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle. 

This is surely a description of the electric arc. Apparently theelectrodes were in a horizontal position and the arc therefore washorizontal. Owing to the rise of the heated air, the arc tended to risein the form of an arch. From this appearance the term "arc" evolved andDavy himself in 1820 definitely named the electric flame, the "arc. "This name was continued in use even after the two carbons were arrangedin a vertical co-axial position and the arc no more "arched. " Aninteresting scientific event of 1820 was the discovery by Arago and byDavy independently that the arc could be deflected by a magnet and thatit was similar to a wire carrying current in that there was a magneticfield around it. This has been taken advantage of in certain modernarc-lamps in which inclined carbons are used. In these arcs a magnetkeeps the arc in place, for without the magnet the arc would tend toclimb up the carbons and go out. 

In 1838 Gassiot made the discovery that the temperature of the positiveelectrode of an electric arc is much greater than that of the negativeelectrode. This is explained in electronic theory by the bombardment ofthe positive electrode by negative electrons or corpuscles ofelectricity. This temperature-difference was later taken into account indesigning direct-current arc-lamps, for inasmuch as most of the lightfrom an ordinary arc is emitted by the end of the positive electrode, this was placed above the negative electrode. In this manner most of thelight from the arc is directed downward where desired. In the fewinstances in modern times where the ordinary direct-current arc has beenused for indirect lighting, in which case the arc is above an invertedshade, the positive carbon is placed below the negative one. Gassiotfirst proved that the positive electrode is hotter than the negative oneby striking an arc between the ends of two horizontal wires of the samesubstance and diameter. After the arc operated for some time, thepositive wire was melted for such a distance that it bent downward, butthe negative remained quite straight. 

Charcoal was used for the electrodes in all the early experiments, butowing to the intense heat of the arc, it burned away rapidly. Aprogressive step was made in 1843 when electrodes were first made byFoucault from the carbon deposited in retorts in which coal wasdistilled in the production of coal-gas. However, charcoal, owing to itssoft porous character, gives a longer arc and a larger flame. In 1877the "cored" carbons were introduced. These consist of hard molded carbonrods in which there is a core of soft carbon. In these are combined theadvantages of charcoal and hard carbon and the core in burning away morerapidly has a tendency to hold the arc in the center. Modern carbons forordinary arc-lamps are generally made of a mixture of retort-carbon, soot, and coal-tar. This paste is forced through dies and the carbonsare baked at a fairly high temperature. A variation in the hardness ofthe carbons may be obtained as the requirements demand by varying theproportions of soot and retort-carbon. Cored carbons are made byinserting a small rod in the center of the die and the carbons areformed with a hollow core. This may be filled with a softer carbon. 

If two carbons connected to a source of electric current are broughttogether, the circuit is completed and a current flows. If the twocarbons are now slightly separated, an arc will be formed. As the arcburns the carbons waste away and in the case of direct current, thepositive decreases in length more rapidly than the negative one. This isdue largely to the extremely high temperature of the positive tip, where the carbon fairly boils. A crater is formed at the positive tipand this is always characteristic of the positive carbon of the ordinaryarc, although it becomes more shallow as the arc-length is increased. The negative tip has a bright spot to which one end of the arc isattached. By wasting away, the length of the arc increases and likewiseits resistance, until finally insufficient current will pass to maintainthe arc. It then goes out and to start it the carbons must be broughttogether and separated. The mechanisms of modern arc-lamps perform thesefunctions automatically by the ingenious use of electromagnets. 

The interior of the arc is of a violet color and the exterior is agreenish yellow. The white-hot spot on the negative tip is generallysurrounded by a fringe of agitated globules which consist of tar andother ingredients of carbons. Often material is deposited from thepositive crater upon the negative tip and these accretions may build upa rounded tip. This deposit sometimes interferes with the properformation of the arc and also with the light from the arc. It is oftenresponsible for the hissing noise, although this hissing occurs with anylength of arc when the current is sufficiently increased. The hissingseems to be due to the crater enlarging under excessive current until itpasses the confines of the cross-section of the carbon. It thus tends torun up the side, where it comes in contact with oxygen of the air. Inthis manner the carbon is directly burned instead of being vaporized, asit is when the hot crater is small and is protected from the air by thearc itself. The temperature of the positive crater is in theneighborhood of 6000° to 7000°F. The brightness of the arc underpressure is the greatest produced by artificial means and is veryintense. By putting the arc under high pressure, the brightness of thesun may be attained. The temperature of the hottest spot on the negativetip is about a thousand degrees below that of the positive. 

No great demand arose for arc-lamps until the development of the Grammedynamo in 1870, which provided a practicable source of electric current. In 1876 Jablochkov invented his famous "electric candle" consisting oftwo rods of carbon placed side by side but separated by insulatingmaterial. In this country Brush was the pioneer in the development ofopen arc-lamps. In 1877 he invented an arc-lamp and an efficient form ofdynamo to supply the electrical energy. The first arc-lamps wereordinary direct-current open arcs and the carbons were made fromhigh-grade coke, lampblack, and syrup. The upper positive carbon inthese lamps is consumed at a rate of one to two inches per hour. Inasmuch as about 85 per cent. Of the total light is emitted by theupper (positive) carbon and most of this from the crater, the lowercarbon is made as small as possible in order not to obstruct any morelight than necessary. The positive carbon of the open arc is often coredand the negative is a smaller one of solid carbon. This combinationoperates quite satisfactorily, but sometimes solid carbons are usedoutdoors. The voltage across the arc is about 50 volts. 

In 1846 Staite discovered that the carbons of an arc enclosed in a glassvessel into which the air was not freely admitted were consumed lessrapidly than when the arc operated in the open air. After theappearance of the dynamo, when increased attention was given to thedevelopment of arc-lamps, this principle of enclosing the arcs was againconsidered. The early attempts in about 1880 were unsuccessful becauselow voltages were used and it was not until the discovery was made thatthe negative tip builds up considerably for voltages under 65 volts, that higher voltages were employed. In 1893 marked improvements wereconsummated and Jandus brought out a successful enclosed arc operatingat 80 volts. Marks contributed largely to the success of the enclosedarc by showing that a small current and a high voltage of 80 to 85 voltswere the requisites for a satisfactory enclosed arc. 

The principle of the enclosed arc is simple. A closely fitting glassglobe surrounds the arc, the fit being as close as the feeding of thecarbons will permit. When the arc is struck the oxygen is rapidlyconsumed and the heated gases and the enclosure check the supply offresh air. The result is that the carbons are consumed about one tenthas rapidly as in the open arc. There is no crater formed on the positivetip and the arc wanders considerably. The efficiency of the enclosed arcas a light-producer is lower than that of the open arc, but it foundfavor because of its slow rate of consumption of the carbons andconsequent decreased attention necessary. This arc operates a hundredhours or more without trimming, and will therefore operate a week ormore in street-lighting without attention. When it is considered thatopen arcs for all-night burning were supplied with two pairs ofcarbons, the second set going into use automatically when the first wereconsumed, the value of the enclosed arc is apparent. However, the openarc has served well and has given way to greater improvements. It israpidly disappearing from use. 

The alternating-current arc-lamp was developed after the appearance ofthe direct-current open-arc and has been widely used. It has no positiveor negative carbons, for the alternating current is reversing indirection usually at the rate of 120 times per second; that is, itpasses through 60 complete cycles during each second. No marked cratersform on the tips and the two carbons are consumed at about the samerate. The average temperature of the carbon tips is lower than that ofthe positive tip of a direct-current arc, with the result that theluminous efficiency is lower. These arcs have been made of both the openand enclosed type. They are characterized by a humming noise due to theeffect of alternating current upon the mechanism and also upon the airnear the arc. This humming sound is quite different from the occasionalhissing of a direct-current arc. When soft carbons are used, the arc islarger and apparently this mass of vapor reduces the hummingconsiderably. The humming is not very apparent for the enclosedalternating-current arc. The alternating arc can easily be detected byclosely observing moving objects. If a pencil or coin be moved rapidly, a number of images appear which are due to the pulsating character ofthe light. At each reversal of the current, the current reaches zerovalue and the arc is virtually extinguished. Therefore, there is amaximum brightness midway between the reversals. 

Various types of all these arcs have been developed to meet thedifferent requirements of ordinary lighting and to adapt this method oflight-production to the needs of projection, stage-equipment, lighthouses, search-lights, and other applications. 

Up to this point the ordinary carbon arc has been considered and it hasbeen seen that most of the light is emitted by the glowing end of thepositive carbon. In fact, the light from the arc itself is negligible. Alogical step in the development of the arc-lamp was to introduce saltsin order to obtain a luminous flame. This possibility as applied toordinary gas-flames had been known for years and it is surprising thatit had not been early applied to carbons. Apparently Bremer in 1898 wasthe first to introduce fluorides of calcium, barium, and strontium. Thesalts deflagrate and a luminous flame envelops the ordinary feeblearc-flame. From these arcs most of the light is emitted by the arcitself, hence the name "flame-arcs. "

By the introduction of metallic salts into the carbons the possibilitiesof the arc-lamp were greatly extended. The luminous output of such lampsis much greater than that of an ordinary carbon arc using the sameamount of electrical energy. Furthermore, the color or spectralcharacter of the light may be varied through a wide range by the use ofvarious salts. For example, if carbons are impregnated with calciumfluoride, the arc-flame when examined by means of a spectroscope will beseen to contain the characteristic spectrum of calcium, namely, somegreen, orange, and red rays. These combine to give to this arc a veryyellow color. As explained in a previous chapter, the salts for thispurpose may be wisely chosen from a knowledge of their fundamental orcharacteristic flame-spectra. 

These lamps have been developed to meet a variety of needs and theirluminous efficiencies range from 20 to 40 lumens per watt, being severaltimes that of the ordinary carbon open-arc. The red flame-arc owes itscolor chiefly to strontium, whose characteristic visible spectrumconsists chiefly of red and yellow rays. Barium gives to the arc afairly white color. The yellow and so-called white flame-arcs have beenmost commonly used. Flame-arcs have been produced which are close todaylight in color, and powerful blue-white flame-arcs have satisfied theneeds of various chemical industries and photographic processes. Thesearcs are generally operated in a space where the air-supply isrestricted similar to the enclosed-arc principle. Inasmuch as poisonousfumes are emitted in large quantities from some flame-arcs, they are notused indoors without rather generous ventilation. In fact, theflame-arcs are such powerful light-sources that they are almost entirelyused outdoors or in very large interiors especially of the type of openfactory buildings. They are made for both direct and alternating currentand the mechanisms have been of several types. The electrodes areconsumed rather rapidly so they are made as long as possible. In onetype of arc, the carbons are both fed downward, their lower ends forminga narrow V with the arc-flame between their tips. Under theseconditions the arc tends to travel vertically and finally to "stretch"itself to extinction. However, the arc is kept in place by means of amagnet above it which repels the arc and holds it at the ends of thecarbons. 

The chief objection to the early flame-arcs was the necessity forfrequent renewal of the carbons. This was overcome to a large extent inthe Jandus regenerative lamp in which the arc operates in a glassenclosure surrounded by an opal globe. However, in addition to the innerglass enclosure, two cooling chambers of metal are attached to it. Airenters at the bottom and the fumes from the arc pass upward and into thecooling chambers, where the solid products are deposited. The air onreturning to the bottom is thus relieved of these solids and the innerglass enclosure remains fairly clean. The lower carbon is impregnatedwith salts for producing the luminous flame and the upper carbon iscored. The life of the electrodes is about seventy-five hours. 

The next step was the introduction of the so-called "luminous-arc" whichis a "flame-arc" with entirely different electrodes. The lower(negative) electrode consists of an iron tube packed chiefly withmagnetite (an iron oxide) and titanium oxide in the approximateproportions of three to one respectively. The magnetite is a conductorof electricity which is easily vaporized. The arc-flame is large and thetitanium gives it a high brilliancy. The positive electrode, usually theupper one, is a short, thick, solid cylinder of copper, which isconsumed very slowly. This lamp, known as the magnetite-arc, has aluminous efficiency of about 20 lumens per watt with a clear glassglobe. 

The mechanisms which strike the arc and feed the carbons are ingeniousdevices of many designs depending upon the kind of arc and upon thecharacter of the electric circuit to which it is connected. Latedevelopments in electric incandescent filament lamps have usurped someof the fields in which the arc-lamp reigned supreme for years and itsfuture does not appear as bright now as it did ten years ago. High-intensity arcs have been devised with small carbons for specialpurposes and considered as a whole a great amount of ingenuity has beenexpended in the development of arc-lamps. There will be a continueddemand for arc-lamps, for scientific developments are opening new fieldsfor them. Their value in photo-engraving, in the moving-pictureproduction studios, in moving-picture projection, and in certain aspectsof stage-lighting is firmly established, and it appears that they willfind application in certain chemical industries because the arc is apowerful source of radiant energy which is very active in its effectsupon chemical reactions. 

The luminous efficiencies of arc-lamps depend upon so many conditionsthat it is difficult to present a concise comparison; however, thefollowing may suffice to show the ranges of luminous output per wattunder actual conditions of usage. These efficiencies, of course, areless than the efficiencies of the arc alone, because the losses in themechanism, globes, etc. , are included. 

 Lumens per watt Open carbon arc 4 to 8 Enclosed carbon arc 3 to 7 Enclosed flame-arc (yellow or white) 15 to 25 Luminous arc 10 to 25

Another lamp differing widely in appearance from the preceding arcs maybe described here because it is known as the mercury-arc. In this lampmercury is confined in a transparent tube and an arc is started bymaking and breaking a mercury connection between the two electrodes. Thearc may be maintained of a length of several feet. Perhaps the firstmercury-arc was produced in 1860 by Way, who permitted a fine jet ofmercury to fall from a reservoir into a vessel, the reservoir andreceiver being connected to the poles of a battery. The electric currentscattered the jet and between the drops arcs were formed. He exhibitedthis novel light-source on the mast of a yacht and it received greatattention. Later, various investigators experimented on the productionof a mercury-arc and the first successful ones were made in the form ofan inverted U-tube with the ends filled with mercury and the remainderof the tube exhausted. 

Cooper Hewitt was a successful pioneer in the production of practicablemercury-arcs. He made them chiefly in the form of straight tubes ofglass up to several feet in length, with enlarged ends to facilitatecooling. The tubes are inclined so that the mercury vapor whichcondenses will run back into the enlarged end, where a pool of mercuryforms the negative electrode. The arc may be started by tilting the tubeso that a mercury thread runs down the side and connects with thepositive electrode of iron. The heat of the arc volatilizes the mercuryso that an arc of considerable length is maintained. The tilting is doneby electromagnets. Starting has also been accomplished by means of aheating coil and also by an electric spark. The lamps are stabilized byresistance and inductance coils. 

One of the defects of the light emitted by the incandescent vapor ofmercury is its paucity of spectral colors. Its visible spectrum consistschiefly of violet, blue, green, and yellow rays. It emits virtually nored rays, and, therefore, red objects appear devoid of red. The humanface appears ghastly under this light and it distorts colors in general. However, it possesses the advantages of high efficiency, of reasonablylow brightness, of high actinic value, and of revealing detail clearly. Various attempts have been made to improve the color of the light byadding red rays. Reflectors of a fluorescent red dye have been used withsome success, but such a method reduces the luminous efficiency of thelamp considerably. The dye fluoresces red under the illumination ofultra-violet, violet, and blue rays; that is, it has the property ofconverting radiation of these wave-lengths into radiant energy of longerwave-lengths. By the use of electric incandescent filament lamps inconjunction with mercury-arcs, a fairly satisfactory light is obtained. Many experiments have been made by adding other substances to themercury, such as zinc, with the hope that the spectrum of the othersubstance would compensate the defects in the mercury spectrum. Howeverno success has been reached in this direction. 

By the use of a quartz tube which can withstand a much highertemperature than glass, the current density can be greatly increased. Thus a small quartz tube of incandescent mercury vapor will emit as muchlight as a long glass tube. The quartz mercury-arc produces a lightwhich is almost white, but the actual spectrum is very different fromthat of white sunlight. Although some red rays are emitted by the quartzarc, its spectrum is essentially the same as that of the glass-tube arc. Quartz transmits ultra-violet radiation, which is harmful to the eyes, and inasmuch as the mercury vapor emits such rays, a glass globe shouldbe used to enclose the quartz tube when the lamp is used for ordinarylighting purposes. 

It is fortunate that such radically different kinds of light-sources areavailable, for in the complex activities of the present time all are indemand. The quartz mercury-arc finds many isolated uses, owing to itswealth of ultra-violet radiation. It is valuable as a source ofultra-violet for exciting phosphorescence, for examining thetransmission of glasses for this radiation, for sterilizing water, formedical purposes, and for photography. 

X

THE ELECTRIC INCANDESCENT FILAMENT LAMPS

Prior to 1800 electricity was chiefly a plaything for men of scientifictendencies and it was not until Volta invented the electric pile orbattery that certain scientific men gave their entire attention to thestudy of electricity. Volta was not merely an inventor, for he was oneof the greatest scientists of his period, endowed with an imaginationwhich marked him as a genius in creative work. By contributing theelectric battery, he added the greatest impetus to research inelectrical science that it has ever received. As has already been shown, there began a period of enthusiastic research in the general field ofheating effects of electric current. The electric arc was born in thecradle of this enthusiasm, and in the heating of metals by electricitythe future incandescent lamp had its beginning. 

Between the years 1841 and 1848 several inventors attempted to makelight-sources by heating metals. These crude lamps were operated bymeans of Grove and Bunsen electric cells, but no practicableincandescent filament lamps were brought out until the development ofthe electric dynamo supplied an adequate source of electric current. Aselectrical science progressed through the continued efforts ofscientific men, it finally became evident that an adequate supply ofelectric current could be obtained by mechanical means; that is, byrotating conductors in such a manner that current would be generatedwithin them as they cut through a magnetic field. Even the pioneerinventors of electric lamps made great contributions to electricalpractice by developing the dynamo. Brush developed a satisfactory dynamocoincidental with his invention of the arc-lamp, and in a similarmanner, Edison made a great contribution to electrical practice indevising means of generating and distributing electricity for thepurpose of serving his filament lamp. 

[Illustration: DIRECT CURRENT ARC

Most of the light being emitted by the positive (upper) electrode]

[Illustration: FLAME ARC

Most of the light being emitted by the flame]

[Illustration: ON THE TESTING-RACKS OF THE MANUFACTURER OF INCANDESCENTFILAMENT LAMPS

Thousands of lamps are burned out for the sake of making improvements. Theelectrical energy used is equivalent to that consumed by a city of 30, 000inhabitants]

Edison in 1878 attacked the problem of producing light from a wire orfilament heated electrically. He used platinum wire in his firstexperiments, but its volatility and low melting-point (3200°F. ) limitedthe success of the lamps. Carbon with its extremely high melting-pointhad long attracted attention and in 1879 Edison produced a carbonfilament by carbonizing a strip of paper. He sealed this in a vessel ofglass from which the air was exhausted and the electric current was ledto the filament through platinum wires sealed in the glass. Platinum wasused because its expansion and contraction is about the same as glass. Incidentally, many improvements were made in incandescent lamps andthirty years passed before a material was found to replace the platinumleading-in wires. The cost of platinum steadily increased and finally inthe present century a substitute was made by the use of two metals whosecombined expansion was the same as that of platinum or glass. In 1879and 1880 Edison had succeeded in overcoming the many difficultiessufficiently to give to the world a practicable incandescent filamentlamp. About this time Swan and Stearn in England had also produced asuccessful lamp. 

In Edison's early experiments with filaments he used platinum wirecoated with carbon but without much success. He also made thin rods of amixture of finely divided metals such as platinum and iridium mixed withsuch oxides as magnesia, zirconia, and lime. He even coiled platinumwire around a piece of one of these oxides, with the aim of obtaininglight from the wire and from the heated oxide. However, theseexperiments served little purpose besides indicating that the filamentwas best if it consisted solely of carbon and that it should becontained in an evacuated vessel. 

One of the chief difficulties was to make the carbon filaments. Some ofthe pioneers, such as Sawyer and Mann, attempted to cut these from apiece of carbon. However, Edison and also Swan turned their attention toforming them by carbonizing a fiber of organic matter. Filaments cutfrom paper and threads of cotton and silk were carbonized for thispurpose. Edison scoured the earth for better materials. He tried afibrous grass from South America and various kinds of bamboo from otherparts of the world. Thin filaments of split bamboo eventually proved thebest material up to that time. He made many lamps containing filamentsof this material, and even until 1910 bamboo was used to some extent incertain lamps. 

Of these early days, Edison said:

 It occurred to me that perhaps a filament of carbon could be made to stand in sealed glass vessels, or bulbs, which we were using, exhausted to a high vacuum. Separate lamps were made in this way independent of the air-pump, and, in October, 1879, we made lamps of paper carbon, and with carbons of common sewing thread, placed in a receiver or bulb made entirely of glass, with the leading-in wires sealed in by fusion. The whole thing was exhausted by the Sprengel pump to nearly one-millionth of an atmosphere. The filaments of carbon, although naturally quite fragile owing to their length and small mass, had a smaller radiating surface and higher resistance than we had dared hope. We had virtually reached the position and condition where the carbons were stable. In other words, the incandescent lamp as we still know it to-day [1904], in essentially all its particulars unchanged, had been born. 

After Edison's later success with bamboo, Swan invented a process ofsquirting filaments of nitrocellulose into a coagulating liquid, afterwhich they are carbonized. Very fine uniform filaments can be made bythis process and although improvements have been made from time to time, this method has been employed ever since its invention. In these lateryears cotton is dissolved in a suitable solvent such as a solution ofzinc chloride and this material is forced through a small diamond die. This thread when hardened appears similar to cat-gut. It is cut intoproper lengths and bent upon a form. It is then immersed in plumbago andheated to a high temperature in order to destroy the organic matter. Acarbon filament is the result. From this point to the finished lamp manyoperations are performed, but a discussion of these would lead farafield. The production of a high vacuum is one of the most importantprocesses and manufacturers of incandescent lamps have mastered the artperhaps more thoroughly than any other manufacturers. At least, theirexperience in this field made it possible for them to produce quicklyand on a large scale such devices as X-ray tubes during the recent war. 

During the early years of incandescent lamps, improvements were madefrom time to time which increased the life and the luminous efficiencyof the carbon filaments, but it was not until 1906 that any radicalimprovement was achieved. In that year in this country a process wasdevised whereby the carbon filament was made more compact. In fact, fromits appearance it received the name "metallized filament. " These carbonfilaments are prepared in the same manner as the earlier ones but arefinally "treated" by heating in an atmosphere of hydrocarbons such ascoal-gas. The filament is heated by electric current and the heat breaksdown the hydrocarbons, with the result that carbon is deposited upon thefilament. This "treated" filament has a coating of hard carbon and itselectrical resistance is greater than that of the untreated filament. 

The luminous efficiency of a carbon filament is a function of itstemperature and it increases very rapidly with increasing temperature. For this reason it is a constant aim to reach high filamenttemperatures. Of all the materials used in filaments up to the presenttime, carbon possesses the highest melting-point (perhaps as high as7000°F. ), but the carbon filament as operated in practice has a lowerefficiency than any other filament. This is because the highesttemperature at which it can be operated and still have a reasonable lifeis much lower than that of metallic filaments. The incandescent carbonin the evacuated bulb sublimes or volatilizes and deposits upon thebulb. This decreases the size of the filament eventually to thebreaking-point and the blackening of the bulb decreases the output oflight. The treated filament was found to be a harder form of carbon thatdid not volatilize as rapidly as the untreated filament. It immediatelybecame possible to operate it at a higher temperature with a resultingincrease of luminous efficiency. This "graphitized" carbon filament lampbecame known as the gem lamp in this country and many persons havewondered over the word "gem. " The first two letters stand for "GeneralElectric" and the last for "metallized. " This lamp was welcomed withenthusiasm in its day, but the day for carbon filaments has passed. Theadvent of incandescent lamps of higher efficiency has made ituneconomical to use carbon lamps for general lighting purposes. Althoughthe treated carbon filament was a great improvement, its reign was cutshort by the appearance of metal filaments. 

In 1803 a new element was discovered and named tantalum. It is a dark, lustrous, hard metal. Pure tantalum is harder than steel; it may bedrawn into fine wire; and its melting-point is very high (about5100°F. ). It is seen to possess properties desirable for filaments, butfor some reason it did not attract attention for a long time. A centuryelapsed after its discovery before von Bolton produced the firsttantalum filament lamp. Owing to the low electrical resistance oftantalum, a filament in order to operate satisfactorily on a standardvoltage must be long and thin. This necessitates storing away aconsiderable length of wire in the bulb without permitting the loops tocome into contact with each other. After the filaments have been inoperation for a few hundred hours they become brittle and faultsdevelop. When examined under a microscope, parts of the filamentoperated on alternating current appear to be offset. The explanation ofthis defect goes deeply into crystalline structure. The tantalumfilament was quickly followed by osmium and by tungsten in this country. 

The osmium filament appeared in 1905 and its invention is due toWelsbach, who had produced the marvelous gas-mantle. Owing to itsextreme brittleness, osmium was finely divided and made into a paste oforganic material. The filaments were squirted through dies and, afterbeing formed and dried, they were heated to a high temperature. Theorganic matter disappeared and the fine metallic particles weresintered. This made a very brittle lamp, but its high efficiency servedto introduce it. 

In 1870 when Scheele discovered a new element, known in this country astungsten, no one realized that it was to revolutionize artificiallighting and to alter the course of some of the byways of civilization. This metal--which is known as "wolfram" in Germany, and to some extentin English-speaking countries--is one of the heaviest of elements, having a specific gravity of 19. 1. It is 50 per cent. Heavier thanmercury and nearly twice as heavy as lead. It was early used in Germansilver to the extent of 1 or 2 per cent. To make platinoid, an alloypossessing a high resistance which varies only slightly as thetemperature changes. This made an excellent material for electricalresistors. The melting-point of tungsten is about 5350°F. , which makesit desirable for filaments, but it was very brittle as prepared in theearly experiments. It unites very readily with oxygen and with carbon athigh temperatures. 

The first tungsten lamps appeared on the market in 1906, but thesecontained fragile filaments made by the squirting process. When thesquirted filament of tungsten powder and organic matter was heated in anatmosphere of steam and hydrogen to remove the binding material, abrittle filament of tungsten was obtained. The first lamps were costlyand fragile. After years of organized research tungsten is now drawninto the finest wires, possessing a tensile strength perhaps greaterthan any other material. Filaments are now made into many shapes and thegreatest strides in artificial lighting have been due to scientificresearch on a huge scale. 

The achievements which combined to perfect the tungsten lamp to thepoint where it has become the mainstay of electric lighting are notattached to names in the Hall of Fame. Organization of scientificresearch in the industrial laboratories is such that often many personscontribute to the development of an improvement. Furthermore, time isusually required for a full perspective of applications of scientificknowledge. In the early days organized research was not practised andthe great developments of those days were the works of individuals. To-day, even in pure science, some of the greatest contributions aremade by industrial laboratories; but sometimes these do not become knownto the public for many years. The whole scheme of scientific developmenthas changed materially. For example, the story of the development ofductile tungsten, which has revolutionized lighting, is complex and moreor less shrouded in secrecy at the present time. Many men havecontributed toward this accomplishment and the public at the presenttime knows little more than the fact that tungsten filaments, which werebrittle yesterday, are now made of ductile tungsten wire drawn into thefinest filaments. 

The earlier tungsten filaments were made by three rival processes. Bythe first, a deposit of tungsten was "flashed" on a fine carbonfilament, the latter being eliminated finally by heating in anatmosphere of hydrogen and water-vapor. By the second, colloidaltungsten was produced by operating an arc between tungsten electrodesunder water. The finely divided tungsten was gathered, partially dried, and squirted through dies to form filaments. These were then sintered. The third was the "paste" process already described. These methodsproduced fragile filaments, but their luminous efficiency was higherthan that of previous ones. However, in this country ductile tungstenwas soon on its way. An ingot of tungsten is subjected to vigorousswaging until it takes the form of a rod. This is finally drawn intowire. 

Much of this development work was done by the laboratories of theGeneral Electric Company and they were destined to contribute anothergreat improvement. The blackening of the lamp bulbs was due to theevaporation of tungsten from the filament. All filaments up to this timehad been confined in evacuated bulbs and the low pressure facilitatesevaporation, as is well known. It had long been known that an inert gasin the bulb would reduce the evaporation and remedy other defects;however, under these conditions, there would be a considerable loss ofenergy through conduction of heat by the gases. In the vacuum lampnearly all the electrical energy is converted into radiant energy, whichis emitted by the filament and any dissipation of heat is an energyloss. A high vacuum was one of the chief aims up to this time, but aradical departure was pending. 

If an ordinary tungsten-lamp bulb be filled with an inert gas such asnitrogen, the filament may be operated at a very much higher temperaturewithout any more deterioration than takes place in a vacuum at a lowertemperature. This gives a more efficient _light_ but a less efficient_lamp_. The greater output of light is compensated by losses byconduction of heat through the gas. In other words, a great deal moreenergy is required by the filament in order to remain at a giventemperature in a gas than in a vacuum. However, elaborate studies of thedependence of heat-losses upon the size and shape of the filament and ofthe physics of conduction from a solid to a gas, established thefoundation for the gas-filled tungsten lamp. The knowledge gained inthese investigations indicated that a thicker filament lost a relativelyless percentage of energy by conduction than a thin one for equalamounts of emitted light. However, a practical filament must havesufficient resistance to be used safely on lighting circuits alreadyestablished and, therefore, the large diameter and high resistance wereobtained by making a helical coil of a fine wire. In fact, thegas-filled tungsten lamp may be thought of as an ordinary lamp with itslong filament made into a short helical coil and the bulb filled withnitrogen or argon gas. 

This development was not accidental and from a scientific point of viewit is not spectacular. It did not mark a new discovery in the same senseas the discovery of X-rays. However, it is an excellent example of thegreat rewards which come to systematic, thorough study of rathercommonplace physical laws in respect to a given condition. Suchachievements are being duplicated in various lines in the laboratoriesof the industries. Scientific research is no longer monopolized byeducational institutions. The most elaborate and best-equippedlaboratories are to be found in the industries sometimes surrounded bythe smoke and noise and vigorous activity which indicate thatachievements of the laboratory are on their way to mankind. Thesmoke-laden industrial district, pulsating with life, is the proudexhibit of the present civilization. It is the creation of those whodiscover, organize, and apply scientific facts. But how many appreciatethe debt that mankind owes not only to the individual who dedicates hislife to science but to the far-sighted manufacturer who risks his moneyin organized quest of new benefits for mankind? A glimpse into a vastorganization of research, which, for example, has been mainlyresponsible for the progress of the incandescent lamp would alter theattitude of many persons toward science and toward the large industrialcompanies. 

The progress in the development of electric incandescent lamps is shownin the following table, where the dates and values are more or lessapproximate. It should be understood that from 1880 to the present timethere has been a steady progress, which occasionally has been greatlyaugmented by sudden steps. 

APPROXIMATE VALUES

 Lumens per Date Filament Temperature watt 1880 Carbon 3300°F. 3. 0 1906 Carbon (graphitized) 3400 4. 5 1905 Tantalum 3550 6. 5 1905 Osmium 3600 7. 5 1906 Tungsten (vacuum) 3700 8. 0 1914 Tungsten (gas-filled) up to 5300°F. 10 to 25

Throughout the development of incandescent filament lamps many ingeniousexperiments were made which resulted usually in light-sources ofscientific interest but not of practical value. One of the latest is thetungsten arc in an inert gas. By means of a heating coil, a small arc isstarted between two electrodes consisting of tungsten, but this as yethas not been shown to be practicable. 

Another type of filament lamp was developed by Nernst in 1897. It was aningenious application of the peculiar properties of rare-earth oxides. His first lamp consisted essentially of a slender rod of magnesia. Thissubstance does not conduct electricity at ordinary temperatures, butwhen heated to incandescence it becomes conducting. Upon sufficientheating of this filament by external means while a proper voltage isimpressed upon it, the electric current passes through it and thereafterthis current will maintain its temperature. Thus such a filament becomesa conductor and will continue to glow brilliantly by virtue of theelectrical energy which it converts into heat. Later lamps consisted of"glowers" about one inch long made from a mixture of zirconia andyttria, and finally a mixture of ceria, thoria, and zirconia was used. The glower is heated initially by a coil of platinum wire located nearit but not in contact with it. Owing to the fact that this glowerdecreases rapidly in resistance as its temperature is increased, it isnecessary to place in series with it a substance which increases inresistance with increasing current. This is called a "ballastingresistance" and is usually an iron wire in a glass bulb containinghydrogen. The heater is cut out by an electromagnet when the glower goesinto operation. This lamp is a marvel of ingenuity and when at itszenith it was installed to a considerable extent. Its light isconsiderably whiter than that of the carbon filament lamps. However, itsdoom was sounded when metallic filament lamps appeared. 

An interesting filament was developed by Parker and Clark by using as acore a small filament of carbon. This flashed in an atmospherecontaining a vapor of a compound of silicon, became coated with silicon. This filament was of high specific resistance and appeared to havepromise. It has not been introduced commercially and doubtless it cannotcompete with the latest tungsten lamps. 

Electric incandescent lamps are the present mainstay of electricillumination and, it might be stated, of progress in lighting. Wonderfulachievements have been accomplished in other modes of lighting and theforegoing statement is not meant to depreciate those achievements. However, the incandescent filament lamp has many inherent advantages. The light-source is enclosed in an air-tight bulb which makes for asafe, convenient lamp. The filament is capable of subdivision, with theresult that such lamps vary from the minutest spark of the smallestminiature lamp to the enormous output of the largest gas-filled tungstenlamp. The outputs of these are respectively a fraction of a lumen andtwenty-five thousand lumens; that is, the luminous intensity varies froman equivalent of a small fraction of a standard candle to a singlelight-source emitting light equivalent to two thousand standard candles. 

Statistics are cold facts and are usually uninteresting in a volume ofthis character, but they tell a story in a concise manner. Thedevelopment of the modern incandescent lamp has increased the intensityof light available with a great decrease in cost, and this progressivedevelopment is shown easily by tables. For example, since the advent ofthe tungsten lamp the average candle-power and luminous efficiency ofall the lamps sold in this country has steadily increased, while theaverage wattages of the lamps have remained virtually stationary. 

AVERAGE CANDLE-POWER, WATTS, AND EFFICIENCY OF ALL THE LAMPS SOLD INTHIS COUNTRY

 Lumens Year Candle-power Watts per watt 1907 18. 0 53 3. 33 1908 19. 0 53 3. 52 1909 21. 0 52 3. 96 1910 23. 0 51 4. 42 1911 25. 0 51 4. 82 1912 26. 0 49 5. 20 1913 29. 4 47 6. 13 1914 38. 2 48 7. 80 1915 42. 2 47 8. 74 1916 45. 8 49 9. 60 1917 48. 7 51 10. 56

It will be noted that the luminous intensity of incandescent filamentlamps has steadily increased since the carbon lamp was superseded, andthat in a period of ten years of organized research behind the tungstenlamp the luminous efficiency (lumens per watt) has trebled. In otherwords, everything else remaining unchanged, the cost of light in tenyears was reduced to one third. But the reduction in cost has been morethan this, as will be shown later. During the same span of years thepercentage of carbon filament lamps of the total filament lamps solddecreased from 100 per cent. In 1907 to 13 per cent. In 1917. At thesame time the percentage of tungsten (Mazda) lamps increased fromvirtually zero in 1907 to about 87 per cent. In 1917. The tantalum lamphad no opportunity to become established, because the tungsten lampfollowed its appearance very closely. In 1910 the sales of the formerreached their highest mark, which was only 3. 5 per cent. Of all thelamps sold in the United States. From a lowly beginning the number ofincandescent filament lamps sold for use in this country has grownrapidly, reaching nearly two hundred million in 1919. 

XI

THE LIGHT OF THE FUTURE

In viewing the development of artificial light and its manifold effectsupon the activities of mankind, it is natural to look into the future. Jules Verne possessed the advantage of being able to write into fictionwhat his riotous imagination dictated, and so much of what he picturedhas come true that his success tempts one to do likewise in prophesyingthe future of lighting. Surely a forecast based alone upon the pastachievements and the present indications will fall short of the actualrealizations of the future! If the imagination is permitted to view thefuture without restrictions, many apparently far-fetched schemes may bedevised. It may be possible to turn to nature's supply of daylight andto place some of it in storage for night use. One millionth part ofdaylight released as desired at night would illuminate sufficiently allof man's nocturnal activities. The fictionist need not heed thescientist's inquiry as to how this daylight would be bottled. Instead ofgiving time to such inquiries he would pass on to another scheme, whereby earth would be belted with optical devices so that day couldnever leave. When the sun was shining in China its light would begathered on a large scale and sent eastward and westward in these greatoptical "pipe-lines" to the regions of darkness, thus banishing nightforever. The writer of fiction need not bother with a consideration ofthe economic situation which would demand such efforts. This line ofconjecture is interesting, for it may suggest possibilities toward whichthe present trend of artificial lighting does not point; however, theauthor is constrained to treat the future of light-production on asomewhat more conservative basis. 

At the present time the light-source of chief interest in electriclighting is the incandescent filament lamp; but its luminous efficiencyis limited, as has been shown in a previous chapter. When light isemitted by virtue of its temperature much invisible radiant energyaccompanies the visible energy. The highest luminous efficiencyattainable by pure temperature radiation will be reached when thetemperature of a normal radiator reaches the vicinity of 10, 000°F. To11, 000°F. The melting-points of metals are much lower than this. Thetungsten filament in the most efficient lamps employing it is operatingnear its melting-point at the present time. Carbon is a most attractiveelement in respect to melting-point, for it melts at a temperaturebetween 6000°F. And 7000°F. Even this is far below the most efficienttemperature for the production of light by means of pure temperatureradiation. There are possibilities of higher efficiency being obtainedby operating arcs or filaments under pressure; however, it appears thathighly efficient light of the future will result from a radicaldeparture. 

Scientists are becoming more and more intimate with the structure ofmatter. They are learning secrets every year which apparently areleading to a fundamental knowledge of the subject. When these mysteriesare solved, who can say that man will not be able to create elements tosuit his needs, or at least to alter the properties of the elementsalready available? If he could so alter the mechanism of radiation thata hot metal would radiate no invisible energy, he would have made atremendous stride even in the production of light by virtue of hightemperature. This property of selective radiation is possessed by someelements to a slight degree, but if treatment could enhance thisproperty, luminous efficiency would be greatly increased. Certainly theprinciple of selectivity is a byway of possibilities. 

A careful study of commonplace factors may result in a great step inlight-production without the creation of new elements or compounds, justas such a procedure doubled the luminous efficiency of the tungstenfilament when the gas-filled lamp appeared. There are a few elementsstill missing, according to the Periodic Law which has been so valuablein chemistry. When these turn up, they may be found to possess valuableproperties for light-production; but this is a discouraging byway. 

It is natural to inquire whether or not any mode of light-production nowin use has a limiting luminous efficiency approaching the ultimate limitwhich is imposed by the visibility of radiation. The eye is able toconvert radiant energy of different wave-lengths into certain fixedproportions of light. For example, radiant energy of such a wave-lengthas to excite the sensation of yellow-green is the most efficient and onewatt of this energy is capable of being converted by the visualapparatus into about 625 lumens of light. Is this efficiency ofconversion of the visual apparatus everlastingly fixed? For the answerit is necessary to turn to the physiologist, and doubtless he wouldsuggest the curbing of the imagination. But is it unthinkable that thevisual processes will always be beyond the control of man? However, toturn again to the physics of light-production, there are still severalprocesses of producing light which are appealing. 

Many years ago Geissler, Crookes, and other scientists studied thespectra of gases excited to incandescence by the electric discharge inso-called vacuum tubes. The gases are placed in transparent glass orquartz tubes at rather low pressures and a high voltage is impressedupon the ends of these tubes. When the pressure is sufficiently low, thegases will glow and emit light. Twenty-eight years ago, D. McFarlanMoore developed the nitrogen tube, which was actually installed invarious places. But there is such a loss of energy near the cathode thatshort "vacuum" tubes of this character are very inefficient producers oflight. Efficiency is greatly increased by lengthening the tubes, soMoore used tubes of great length and a rather high voltage. As a tube ofthis sort is used, the gas gradually disappears and it must bereplenished. In order to replenish the gas, Moore devised a valve forfeeding gas automatically. An advantage of this mode of light-productionis that the color or quality of the light may be varied by varying thegas used. Nitrogen yields a pinkish light; neon an orange light; andcarbon dioxide a white light. Moore's carbon-dioxide tube is anexcellent substitute for daylight and has been used for thediscrimination of colors where this is an important factor. However, forthis purpose devices utilizing color-screens which alter the light fromthe tungsten lamp to a daylight quality are being used extensively. 

The vacuum-tube method of producing light has an advantage in theselection of a gas among a large number of possibilities, and some ofthe color effects of the future may be obtained by means of it. Claudehas lately worked on light-production by vacuum tubes and has combinedthe neon tube with the mercury-vapor tube. The spectrum of neon to alarge extent compensates for the absence of red light in the mercuryspectrum, with a result that the mixture produces a more satisfactorylight than that of either tube. However, this mode of light-productionhas not proved practicable in its present state of development. Fundamentally the limitations are those of the inherent spectralcharacteristics of gases. Doubtless the possibilities of the mechanismsof the tubes and of combining various gases have not been exhausted. Furthermore, if man ever becomes capable of controlling to some extentthe structure of elements and of compounds, this method oflight-production is perhaps more promising than others of the presentday. 

There is another attractive method of producing light and it has notescaped the writer of fiction. H. G. Wells, with his rare skill and withthe license so often envied by the writer of facts, has drawn upon thecharacteristics of fluorescence and phosphorescence. In his story "TheFirst Men in the Moon, " the inhabitants of the moon illuminate theircaverns by utilizing this phenomenon. A fluorescent liquid was preparedin large quantities. It emitted a brilliant phosphorescent glow and whenit splashed on the feet of the earth-men it felt cold, but it glowed fora long time. This is a possibility of the future and many have hadvisions of such lighting. If the ceiling of a coal-mine was lined withglowing fireflies or with phosphorescent material excited in somemanner, it would be possible to see fairly well with the dark-adaptedeyes. 

This leads to the class of phenomena included under the general term"luminescence. " The definition of this term is not thoroughly agreedupon, but light produced in this manner does not depend upon temperaturein the sense that a glowing tungsten filament emits light because it issufficiently hot. A phosphorus match rubbed in the moist palm of thehand is seen to glow, although it is at an ordinary temperature. Thismay be termed "chemi-luminescence. " Sidot blende, Balmain's paint, andmany other compounds, when illuminated with ordinary light, andespecially with ultra-violet and violet rays, will continue to glow fora long time. Despite their brightness they will be cold to the touch. This phenomenon would be termed "photo-luminescence, " although it isbetter known as "phosphorescence. " It should be noted that the latterterm was carelessly originated, for phosphorus has nothing to do withit. The glow of the Geissler tube or electrically excited gas at lowpressure would be an example of "electro-luminescence. " The luminosityof various salts in the Bunsen-flame is due to so-called luminescenceand there are many other examples of light-production which are includedin the same general class. Inasmuch as light is emitted fromcomparatively cold bodies in these cases, it is popularly known as"cold" light. 

There are many instances of light being emitted without beingaccompanied by appreciable amounts of invisible radiant energy and it isnatural to hope for practical possibilities in this direction. As yetlittle is known regarding the efficiency of light-production byphosphorescence. The luminous efficiency of the radiant energy emittedby phosphorescent substances has been studied, but it seems strange thatamong the vast works on phosphorescent phenomena, scarcely any mentionis made of the efficiency of producing light in this manner. Forexample, assume that phosphorescent zinc sulphide is excited by thelight from a mercury-arc. All the energy falling upon it is not capableof exciting phosphorescence, as may be readily shown. Assuming that aknown amount of radiant energy of a certain wave-length has beenpermitted to fall upon the phosphorescent material, then in the dark thelatter may be seen to glow for a long time. An interesting point toinvestigate is the relation of the output to input; that is, the ratioof the total emitted light to the total exciting energy. This is aneglected aspect in the study of light-production by this means. 

The firefly has been praised far and wide as the ideal light-source. Itis an efficient radiator of light, for its light is "cold"; that is, itdoes not appear to be accompanied by invisible radiant energy. Butlittle is said about its efficiency as a light-producer. Who knows howmuch fuel its lighting-plant consumes? The chemistry of light-productionby living organisms is being unraveled and this part of the phenomenonwill likely be laid bare before long. For an equal amount of energyradiated, the firefly emits a great many times more light than the mostefficient lamp in use at the present time, but before the firefly ispronounced ideal, the efficiency of its light-producing process must beknown. 

There are many ways of exciting phosphorescence and fluorescence, thelatter being merely an unenduring phosphorescence, which ceases when theexciting energy is cut off. Ultra-violet, violet, and blue rays aregenerally the most effective radiant energy for excitation purposes. X-rays and the high-frequency discharge are also powerful excitants. Asalready stated, virtually nothing is known of the efficiency of thismode of light-production or of the mechanism within the substance, buton the whole it is a remarkable phenomenon. 

Radium is also a possibility in light-production and in fact has beenpractically employed for this purpose for several years. It or one ofits compounds is mixed with a phosphorescent substance such as zincsulphide and the latter glows continuously. Inasmuch as the life of someof the radium products is very long, such a method of illuminatingwatch-dials, scales of instruments, etc. , is very practicable where theyare to be read by eyes adapted to darkness and consequently highlysensitive to light. Whether or not radium will be manufactured by theton in the future can only conjectured. 

Owing to the limitations imposed by physical laws of radiation and bythe physiological processes of vision the highest luminous efficiencyobtainable by heating solid materials is only about 15 per cent. Of theluminous efficiency of the most luminous radiant energy. At presentthere are no materials available which may be operated at thetemperature necessary to reach even this efficiency. Great progress inthe future of light-production as indicated by present knowledge appearsto lie in the production of light which is unaccompanied by invisibleradiant energy. At present such phenomena as fluorescence, phosphorescence, the light of the firefly, chemi-luminescence, etc. , areexamples of this kind of light-production. Of course, if science everobtains control over the constitution of matter, many difficulties willdisappear; for then man will not be dependent upon the elements andcompounds now available but will be able to modify them to suit hisneeds. 

XII

LIGHTING THE STREETS

In this age of brilliantly lighted boulevards and "great white ways"flooded with light from shop-windows, electric signs, and street-lamps, it is difficult to visualize the gloom which shrouded the streets acentury ago. As the belated pedestrian walks along the suburban highwaysin comparative safety under adequate artificial lighting, he willrealize the great influence of artificial light upon civilization if herecalls that not more than two centuries ago in London

 it was a common practice . .. That a hundred or more in a company, young and old, would make nightly invasions upon houses of the wealthy to the intent to rob them and that when night was come no man durst adventure to walk in the streets. 

Inhabitants of the cities of the present time are inclined to think thatcrime is common on the streets at night, but what would it be withoutadequate artificial light? Two centuries ago in a city like London asmoking grease-lamp, a candle, or a basket of pine knots here and thereafforded the only street-lighting, and these were extinguished by eleveno'clock. Lawlessness was hatched and hidden by darkness, and even thelantern or torch served more to mark the victim than to protect him. Ithas been said in describing the conditions of the age of dark streetsthat everybody signed his will and was prepared for death before he lefthis home. By comparison with the present, one is again encouraged tobelieve that the world grows better. Doubtless, artificial lightprojected into the crannies has had something to do with this change. 

Adequate street-lighting is really a product of the twentieth century, but throughout the nineteenth century progress was steadily made fromthe beginning of gas-lighting in 1807. In preceding centuries crudelighting was employed here and there but not generally by the publicauthorities. In the earliest centuries of written history little is saidof street-lighting. In those days man was not so much inclined toimprove upon nature, beyond protecting himself from the elements, and helighted the streets more as a festive outburst than as an economicproposition. Nevertheless, in the early writings occasionally there areindications that in the centers of advanced civilization some effortswere made to light the streets. 

The old Syrian city of Antioch, which in the fourth century of theChristian era contained about four hundred thousand inhabitants, appearsto have had lighted streets. Libanius, who lived in the early years ofthat century, wrote:

 The light of the sun is succeeded by other lights, which are far superior to the lamps lighted by Egyptians on the festival of Minerva of Sais. The night with us differs from the day only in the appearance of the light; with regard to labor and employment, everything goes on well. 

Although apparently labor was not on a strike, the soldiers causeddisturbances, for in another passage he tells of riotous soldiers who

 cut with their swords the ropes from which were suspended the lamps that afforded light in the night-time, to show that the ornaments of the city ought to give way to them. 

Another writer in describing a dispute between two religious adherentsof opposed creeds stated that they quarreled "till the streets werelighted" and the crowd of onlookers broke up, but not until they "spatin each other's face and retired. " Thus it is seen that artificial lightand civilization may advance, even though some human traits remainfundamentally unchanged. 

Throughout the next thousand years there was little attempt to light thestreets. Iron baskets of burning wood, primitive oil-lamps, and candleswere used to some extent, but during all these centuries there was noattempt on the part of the government or of individuals to light thestreets in an organized manner. In 1417 the Mayor of London ordained"lanthorns with lights to bee hanged out on the winter evenings betwixtHallowtide and Candlemasse. " This was during the festive season, soperhaps street-lighting was not the sole aim. Early in the sixteenthcentury, the streets of Paris being infested with robbers, theinhabitants were ordered to keep lights burning in the windows of allhouses that fronted on the streets. 

For about three centuries the citizens of London, and doubtless of Parisand of other cities, were reminded from time to time in officialmandates "on pains and penalties to hang out their lanthorns at theappointed time. " The watchman in long coat with halberd and lantern inhand supplemented these mandates as he made his rounds by, A light here, maids, hang out your lights, And see your horns be clear and bright, That so your candle clear may shine, Continuing from six till nine; That honest men that walk along May see to pass safe without wrong. 

In 1668, when some regulations were made for improving the streets ofLondon, the inhabitants were ordered "for the safety and peace of thecity to hang out candles duly to the accustomed hour. " Apparently thismethod of obtaining lighting for the streets was not met by theenthusiastic support of the people, for during the next few decades theLord Mayor was busy issuing threats and commands. In 1679 he proclaimedthe "neglect of the inhabitants of this city in hanging and keeping outtheir lights at the accustomed hours, according to the good and ancientusage of this City and Acts of the Common Council on that behalf. " Theresult of this neglect was "when nights darkened the streets thenwandered forth the sons of Belial, flown with insolence and wine. "

In 1694 Hemig patented a reflector which partially surrounded the openflame of a whale-oil lamp and possessed a hole in the top which aidedventilation. He obtained the exclusive rights of lighting London for aperiod of years and undertook to place a light before every tenth door, between the hours of six and twelve o'clock, from Michaelmas to LadyDay. His effort was a worthy one, but he was opposed by a certainfaction, which was successful in obtaining a withdrawal of his licensein 1716. Again the burden of lighting the streets was thrust upon theresidents and fines were imposed for negligence in this respect. Butthis procedure after a few more years of desultory lighting was againfound to be unsatisfactory. 

In 1729 certain individuals contracted to light the streets of London bytaxing the residents and paid the city for this monopoly. Householderswere permitted to hang out a lantern or a candle or to pay the companyfor doing so. But robberies increased so rapidly that in 1736 the LordMayor and Common Council petitioned Parliament to erect lamps forlighting the city. An act was passed accordingly, giving them theprivilege to erect lamps where they saw fit and to burn them from sunsetto sunrise. A charge was made to the residents, on a sliding scaledepending upon the rate of rental of the houses. As a consequence fivethousand lamps were soon installed. In 1738 there were fifteen thousandstreet lamps in London and they were burned an average of five thousandhours annually. 

In the annals of these early times street-lighting is almost invariablythe result of an attempt to reduce the number of robberies and othercrimes. In appealing for more street-lamps in 1744 the Lord Mayor andaldermen of London in a petition to the king, stated

 that divers confederacies of great numbers of evil-disposed persons, armed with bludgeons, pistols, cutlasses, and other dangerous weapons, infest not only the private lanes and passages, but likewise the public streets and places of public concourse, and commit most daring outrages upon the persons of your Majesty's good subjects, whose affairs oblige them to pass through the streets, by terrifying, robbing and wounding them; and these facts are frequently perpetrated at such times as were heretofore deemed hours of security. 

It has already been seen that gas-lighting was introduced in the streetsof London for the first time in 1807. This marks the real beginning ofpublic-service lighting companies. In the next decade interest instreet-lighting by means of gas was awakened on the Continent, and itwas not long before this new phase of civilization was well under way. Although this first gas-lighting was done by the use of open flames, itwas a great improvement over all the preceding efforts. Lawlessness didnot disappear entirely, of course, and perhaps it never will, but itskulked in the back streets. A controlling influence had now appeared. 

But early innovations in lighting did not escape criticism andopposition. In fact, innovations to-day are not always received byunanimous consent. There were many in those early days who felt thatwhat was good for them should be good enough for the younger generation. The descendants of these opponents are present to-day but fortunately indiminishing numbers. It has been shown that in Philadelphia in 1833 aproposal to install a gas-plant was met with a protest signed by manyprominent citizens. A few paragraphs of an article entitled "Argumentsagainst Light" which appeared in the Cologne _Zeitung_ in 1816 indicatethe character of the objections raised against street-lighting. 

1 From the theological standpoint: Artificial illumination is an attempt to interfere with the divine plan of the world, which has preordained darkness during the night-time. 

2 From the judicial standpoint: Those people who do not want light ought not to be compelled to pay for its use. 

3 From the medical standpoint: The emanations of illuminating gas are injurious. Moreover, illuminated streets would induce people to remain later out of doors, leading to an increase in ailments caused by colds. 

4 From the moral standpoint: The fear of darkness will vanish and drunkenness and depravity increase. 

5 From the viewpoint of the police: The horses will get frightened and the thieves emboldened. 

6 From the point of view of national economy: Great sums of money will be exported to foreign countries. 

7 From the point of view of the common people: The constant illumination of streets by night will rob festive illuminations of their charm. 

The foregoing objections require no comment, for they speak volumespertaining to the thoughts and activities of men a century ago. It isdifficult to believe that civilization has traveled so far in a singlecentury, but from this early beginning of street-lighting socialprogress received a great impetus. Artificial light-sources were feebleat that time, but they made the streets safer and by means of themsocial intercourse was extended. The people increased their hours ofactivity and commerce, industry, and knowledge grew apace. 

The open gas-jet and kerosene-flame lamps held forth on the streetsuntil within the memory of middle-aged persons of to-day. Thelamplighter with his ladder is still fresh in memory. Many of the townsand villages have never been lighted by gas, for they stepped from theoil-lamp to the electric lamp. The gas-mantle has made it possible forgas-lighting to continue as a competitor of electric-lighting for thestreets. 

In 1877 Mr. Brush illuminated the Public Square of Cleveland with anumber of arc-lamps, and these met with such success that within a shorttime two hundred and fifty thousand open-arc lamps were installed inthis country, involving an investment of millions of dollars. Adding tothis investment a much greater one in central-station equipment, a verylarge investment is seen to have resulted from this single developmentin lighting. 

This open-arc lamp was the first powerful light-source available and, appearing several years before the gas-mantle, it threatened tomonopolize street-lighting. It consumed about 500 watts and had amaximum luminous intensity of about 1200 candles at an angle of about 45degrees. Its chief disadvantage was its distribution of light, mainly atthis angle of 45 degrees, which resulted in a spot of light near thelamp and little light at a distance. A satisfactory street-lighting unitmust emit its light chiefly just below the horizontal in those caseswhere the lamps must be spaced far apart for economical reasons. Onreferring to the chapter on the electric arc it will be seen that theupper (positive) carbon of the open-arc emits most of the light. Thusmost of the light tends to be sent downward, but the lower carbonobstructs some of this with a resulting dark spot beneath the lamp. 

The gas-mantle followed closely after the arrival of the carbon arc andis responsible for the existence of gas-lighting on the streets at thepresent time. It is a large source of light and therefore its lightcannot be controlled by modern accessories as well as the light fromsmaller sources, such as the arc or concentrated-filament lamp. As aconsequence, there is marked unevenness of illumination along thestreets unless the gas-mantle units are spaced rather closely. Even withthe open-arc, without special light-controlling equipment there is abouta thousand times the intensity near the lamps when placed on the cornersof the block as there is midway between them. 

In 1879 the incandescent filament lamp was introduced and it began toappear on the streets in a short time. It was a feeble, inefficientlight-source, compared with the arc-lamp, but it had the advantage ofbeing installed on a small bracket. As a consequence of simplicity ofoperation, the incandescent lamp was installed to a considerable extent, especially in the suburban districts. 

[Illustration: THE MOORE NITROGEN TUBE

In lobby of Madison Square Garden]

[Illustration: CARBON-DIOXIDE TUBE FOR ACCURATE COLOR-MATCHING]

[Illustration: MODERN STREET LIGHTING

Tunnels of light boring through the darkness provide safe channels formodern traffic]

The open-arc lamp possessed the disadvantage of emitting a very unsteadylight and of consuming the carbons so rapidly that daily trimming wasoften necessary. In 1893 the enclosed arc appeared and although itconsumed as much electrical energy as the open-arc and emittedconsiderably less light, it possessed the great advantage of operating aweek without requiring a renewal of carbons. By surrounding the arcby means of a glass globe, little oxygen could come in contact with thecarbons and they were not consumed very rapidly. The light was fairlysteady and these arcs operated satisfactorily on alternating current. The latter feature simplified the generating and distributing equipmentof the central station. 

The magnetite or luminous arc-lamp next appeared and met withconsiderable success. It was more efficient than the preceding lamps butwas handicapped by being solely a direct-current device. Those familiarwith the generation and distribution of electricity will realize thisdisadvantage. However, its luminous intensity just below the horizontalwas about 700 candles and its general distribution of light was fairlysatisfactory. Later the flame-arcs began to appear and they wereinstalled to some extent. The arc-lamp has served well instreet-lighting from the year 1877, when the open-arc was introduced, until the present time, when the luminous-arc is the chief survivor ofall the arc-lamps. 

The carbon incandescent filament lamp was used extensively until 1909, when the tungsten filament lamp began to replace it very rapidly. However, it was not until 1914, when the gas-filled tungsten lampappeared, that this type of light-source could compete with arc-lamps onthe basis of efficiency. The helical construction of the filament madeit possible to confine the filament of a high-intensity tungsten lamp ina small space and for the first time a high degree of control of thelight of street lamps was possible. Prismatic "refractors" weredesigned, somewhat on the principle of the lighthouse refractor, sothat the light would be emitted largely just below the horizontal. Thistype of distribution builds up the illumination at distant pointsbetween successive street lamps, which is very desirable instreet-lighting. The incandescent filament lamp possesses manyadvantages over other systems. It is efficient; capable of subdivision;operates on direct and alternating current; requires little attention;and is capable of most successful use with light-controlling apparatus. 

According to the reports of the Department of Commerce the number ofelectric arc-lamps for street-lighting supplied by public electric-lightplants decreased from 348, 643 in 1912 to 256, 838 in 1917, while thenumber of electric incandescent filament lamps increased from 681, 957 in1912 to 1, 389, 382 in 1917. 

Street-lighting is not only a reinforcement for the police but itdecreases accidents and has come to be looked upon as an advertisingmedium. In the downtown districts the high-intensity "white-way"lighting is festive. The ornamental street lamps have possibilities inmaking the streets attractive and in illuminating the buildings. However, it is to be hoped that in the present age the streets of citiesand towns will be cleared of the ragged equipment of the telephone andlighting companies. These may be placed in the alleys or underground, leaving the streets beautiful by day and glorified at night by thetorches of advanced civilization. 

XIII

LIGHTHOUSES

At the present time thousands of lighthouses, light-ships, andlight-buoys guide the navigator along the waterways and into harbors andwarn him of dangerous shoals. Many wonderful feats of engineering areinvolved in their construction and in no field of artificial lightinghas more ingenuity been displayed in devising powerful beams of light. Many of these beacons of safety are automatic in operation and requirelittle attention. It has been said that nothing indicates theliberality, prosperity, or intelligence of a nation more clearly thanthe facilities which it affords for the safe approach of the mariner toits shores. Surely these marine lights are important factors in modernnavigation. 

The first "lighthouses" were beacon-fires of burning wood maintained bypriests for the benefit of the early commerce in the eastern part of theMediterranean Sea. As early as the seventh century before Christ thesebeacon-fires were mentioned in writings. In the third century before theChristian era a tower said to be of a great height was built on a smallisland near Alexandria during the reign of Ptolemy II. The tower wasnamed Pharos, which is the origin of the term "pharology" applied to thescience of lighthouse construction. Cæsar, who visited Alexandria twocenturies later, described the Pharos as a "tower of great height, ofwonderful construction. " Fire was kept burning in it night and day andPliny said of it, "During the night it appears as bright as a star, andduring the day it is distinguished by the smoke. " Apparently this towerserved as a lighthouse for more than a thousand years. It was found inruins in 1349. Throughout succeeding centuries many towers were built, but little attention was given to the development of light-sources andoptical apparatus. 

The first lighthouse in the United States and perhaps on the Westerncontinents was the Boston Light, which was completed in 1716. A few daysafter it was put into operation a news item in a Boston paper heraldedthe noteworthy event as follows:

 By virtue of an Act of Assembly made in the First Year of His Majesty's Reign, For Building and Maintaining a Light House upon the Great Brewster (called Beacon-Island) at the Entrance of the Harbour of Boston, in order to prevent the loss of the Lives and Estates of His Majesty's Subjects; the said Light House has been built; and on Fryday last the 14th Currant the Light was kindled, which will be very useful for all Vessels going out and coming in to the Harbour of Boston, or any other Harbours in the Massachusetts Bay, for which all Masters shall pay to the Receiver of Impost, one Penny per Ton Inwards, and another Penny Outwards, except Coasters, who are to pay Two Shillings each, at their clearance Out, And all Fishing Vessels, Wood Sloops, etc. Five Shillings each by the Year. 

This was the practical result of a petition of Boston merchants madethree years before. The tower was built of stone, at a cost of aboutten thousand dollars. Two years later the keeper and his family weredrowned and the catastrophe so affected Benjamin Franklin, a boy ofthirteen, that he wrote a poem concerning it. The lighthouse was badlydamaged during the Revolution, by raiding-parties, and in 1776, when theBritish fleet left the harbor, a squad of sailors blew it up. It wasrebuilt in 1783 and has since been increased in height. 

Apparently oil-lamps were used in it from the beginning, notwithstandingthe fact that candles and coal fires served for years in manylighthouses of Europe. In 1789 sixteen lamps were used and in 1811Argand lamps and reflectors were installed, with a revolving mechanism. It now ceased to be a fixed light and the day of flashing lights hadarrived. At the present time the Boston Light emits a beam of 100, 000candle-power directed by modern lenses. 

When the United States Government was organized in 1789 there were tenlighthouses owned by the Colonies, but the Boston Light was in operationthirty years before the others. Sandy Hook Light, New York Harbor, wasestablished in 1764 and its original masonry tower is still standing andin use. It is the oldest surviving lighthouse in this country. It wasbuilt with funds raised by means of two lotteries authorized by the NewYork Assembly. A few days after it was lighted for the first time thefollowing news item appeared in a New York paper:

 On Monday evening last the New York Light-house erected at Sandy Hook was lighted for the first time. The House is of an Octagon Figure, having eight equal Sides; the Diameter at the Base 29 Feet; and at the top of the Wall, 15 Feet. The Lanthorn is 7 feet high; the Circumference 33 feet. The whole Construction of the Lanthorn is Iron; the Top covered with Copper. There are 48 Oil Blazes. The Building from the Surface is Nine Stories; the whole from Bottom to Top is 103 Feet. 

From these early years the number of lighthouses has steadily grown, until now the United States maintains lights along 50, 000 miles ofcoast-line and river channels, a distance equal to twice thecircumference of the earth. It maintains at the present time about15, 000 aids to navigation at an annual cost of about $5, 000, 000. In 1916this country was operating 1706 major lights, 53 light-ships, and 512light-buoys--a total of 5323. 

The earliest lighthouses were equipped with braziers or grates in whichcoal or wood was burned. These crude light-sources were used until afterthe advent of the nineteenth century and in one case until 1846. In thefamous Eddystone tower off Plymouth, England, candles were used for thefirst time. The first Eddystone tower was completed in 1698, but it wasswept away in 1703. Another was built and destroyed by fire in 1755. Smeaton then built another in 1759. Inasmuch as Smeaton is credited withhaving introduced the use of candles, this must have occurred in theeighteenth century; still it appears that, as we have said, the BostonLight, built in 1716, used oil-lamps from its beginning. However, Smeaton installed twenty-four candles of rather large size each creditedwith an intensity of 2. 8 candles. The total luminous intensity of thelight-source in this tower was about 67 candles. Inasmuch as this wasbefore the use of efficient reflectors and lenses, it is obvious thatthe early lighthouses were rather feeble beacons. 

According to British records, oil-lamps with flat wicks were first usedin the Liverpool lighthouses in 1763. The Argand lamp, introduced inabout 1784, became widely used. The better combustion obtained with thislamp having a cylindrical wick and a glass chimney greatly increased theluminous intensity and general satisfactoriness of the oil-lamp. LaterLange added an improvement by providing a contraction toward the upperpart of the chimney. Rumford and also Fresnel devised multiple-wickburners, thus increasing the luminous intensity. In these early lampssperm-oil and colza-oil were burned and they continued to be until themiddle of the nineteenth century. Cocoanut-oil, lard-oil, and olive-oilhave also been used in lighthouses. 

Naturally, mineral oil was introduced as soon as it was available, owingto its lower cost; but it was not until nearly 1870 that a satisfactorymineral-oil lamp was in operation in lighthouses. Doty is credited withthe invention of the first successful multiple-wick lighthouse lampusing mineral oil, and his lamp and modifications of it were verygenerally used until the latter part of the nineteenth century. Theselamps are of two types--one in which oil is supplied to the burner underpressure and the other in which oil is maintained at a constant level. In some of the smallest lamps the ordinary capillarity of the wick isdepended on to supply oil to the flame. 

Coal-gas was introduced into lighthouses in about the middle of thenineteenth century. Inasmuch as the gas-mantle had not yet appeared, thegas was burned in jets. Various arrangements of the jets, such asconcentric rings forming a stepped cone, were devised. The gas-mantlewas a great boon to the mariner as well as to civilized beings ingeneral. It greatly increases the intensity of light obtainable from agiven amount of fuel and it is a fairly compact bright source whichmakes it possible to direct the light to some degree by means of opticalsystems. Owing to the elaborate apparatus necessary for making coal-gas, several other gases have been more desirable fuels for lighthouse lamps. Various simple gas-generators have been devised. Some of the high-flashmineral-oils are vaporized and burned under a mantle. Acetylene, whichis so simply made by means of calcium carbide and water, has been agreat factor in lighting for navigation. By the latter part of thenineteenth century lighthouses employing incandescent gas-burners wereemitting beams of light having luminous intensities as great as severalhundred thousand candles. These special gas-mantle light-sources havebrightness as high as several hundred candles per square inch. 

Electric arc-lamps were first introduced into lighthouse service inabout 1860, but these lamps cannot be considered to have been reallypracticable until about 1875. In 1883 the British lighthouse authoritiescarried out an extensive investigation of arc-lamps. It was found thatthe whiter light from these lamps suffered a greater absorption by theatmosphere than the yellower light from oils, but the much greaterluminous intensity of the arc-lamp more than compensated for thisdisadvantage. The final result of the investigation was the conclusionthat for ordinary lighthouse purposes the oil-and gas-lamps were moresuitable and economical than arc-lamps; but where great range wasdesired, the latter were much more advantageous, owing to their greatluminous intensity. Electric incandescent filament lamps have been usedfor the less important lights, and recently there has been someapplication of the modern high-efficiency filament lamps. 

Besides the high towers there are many minor beacons, light-ships, andlight-buoys in use. Many of these are untended and therefore mustoperate automatically. The light-ship is used where it is impracticableor too expensive to build a lighthouse. Inasmuch as it is anchored infairly deep water, it is safe in foggy weather to steer almost directlytoward its position as indicated by the fog-signal. Light-ships are moreexpensive to maintain than lighthouses, but they have the advantages ofsmaller cost and of mobility; for sometimes it may be desired to movethem. The first light-ship was established in 1732 near the mouth of theThames, and the first in this country was anchored in Chesapeake Baynear Norfolk in 1820. The early ships had no mode of self-propulsion, but the modern ones are being provided with their own power. Oil and gashave been used as fuel for the light-sources and in 1892 the U. S. Lighthouse Board constructed a light-ship with a powerful electriclight. Since that time several have been equipped with electric lightssupplied by electric generators and batteries. 

Untended lights were not developed until about 1880, when Pintschintroduced his welded buoys filled with compressed gas and therebyprovided a complete lighting-plant. With improvements in lamps andcontrols the untended light-buoys became a success. The lights burn forseveral months, and even for a year continuously; and the oil-gas usedappears to be very satisfactory. Recently some experiments have beenmade with devices which would be actuated by sunlight in such a mannerthat the light would be extinguished during the day excepting a smallpilot-flame. By this means a longer period of burning without attentionmay be obtained. Electric filament lamps supplied by batteries or bycables from the shore have been used, but the oil-gas buoy still remainsin favor. Acetylene has been employed as a fuel for light-buoys. Automatic generators have been devised, but the high-pressure system ismore simple. In the latter case purified acetylene is held in solutionunder high pressure in a reservoir containing an asbestos compositionsaturated with acetone. 

The light-sources of beacons have had the same history as those of othernavigation lights. Many of these are automatic in operation, sometimesbeing controlled by clockwork. During the last twenty years thegas-mantle has been very generally applied to beacon-lights. In thelatter part of the nineteenth century a mineral-oil lamp was devisedwith a permanent wick made by forming upon a thick wick a coating ofcarbon. The operation is such that this is not consumed and it preventsfurther burning of the wick. 

The optical apparatus of navigation lights has undergone manyimprovements in the past century. The early lights were not equippedwith either reflecting or refracting apparatus. In 1824 Drummond deviseda scheme for reflecting light in order that a distant observer mightmake a reading upon the point where the apparatus was being operated byanother person. He was led by his experiments to suggest the applicationof mirrors to lighthouses. His device was essentially a parabolic mirrorsimilar to the reflectors now widely used in automobile head-lamps, search-lights, etc. He employed the lime-light as a source of light andwas enthusiastic over the results obtained. His discussion published in1826 indicates that little practical work had been done up to that timetoward obtaining beams or belts of light by means of optical apparatus. However, lighthouse records show that as early as 1763 small silveredplane glasses were set in plaster of Paris in such a manner as to form apartially enveloping reflector. Spherical reflectors were introduced inabout 1780 and parabolic reflectors about ten years later. 

All the earlier lights were "fixed, " but as it is desirable that themariner be able to distinguish one light from another, the revolvingmechanism evolved. By its agency characteristic flashes are obtained andfrom the time interval the light is recognized. The first revolvingmechanism was installed in 1783. The early flashing lights were obtainedby means of revolving reflectors which gathered the light and directedit in the form of a beam or pencil. The type of parabolic reflector nowin use does not differ essentially from that of an automobile head-lamp, excepting that it is larger. 

Lenses appear to have been introduced in the latter part of thenineteenth century. They were at first ground from a solid piece ofglass, in concentric zones, in order to reduce the thickness. They weresimilar in principle to some of the tail-light lenses used at present onautomobiles. Later the lenses were built up by means of separate annularrings. The name of Fresnel is permanently associated with lighthouselenses because in 1822 he developed an elaborate built-up lens ofannular rings. The centers of curvature of the different rings recededfrom the axis as their distance from the center increased, in such amanner as to overcome a serious optical defect known as sphericalaberration. Fresnel devised many improvements in which he usedrefracting and reflecting prisms for the outer elements. 

The optical apparatus of lighthouses usually aims (1) to concentrate therays of light into a pencil of light, (2) to concentrate them into abelt of light, or (3) to concentrate the rays over a limited azimuth. Inthe first case a single lens or a parabolic reflector suffices, but inthe second case a cylindrical lens which condenses the light verticallyinto a horizontal sheet of light is essential. The third case is acombination of the first two. The modern lighthouse lenses are veryelaborate in construction, being built up by means of many elements intoseveral sections. For example, the central section may consist of aspherical lens ground with annular rings. In the next section refractingprisms may be used and in the outer section reflecting glass prisms areemployed. The various elements are carefully designed according to thelaws of geometrical optics. 

The flashing light has such advantages over the fixed that it isgenerally used for important beacons. A variety of methods of obtainingintermittent light have been employed, but they are not of particularinterest. Sometimes the lens or reflector is revolved and in other typesan opaque screen containing slits is revolved. In the larger lighthousesthe optical apparatus and its structure sometimes weigh several tons. When it is necessary to revolve apparatus of this weight, the wholemechanism is floated upon mercury contained in a cast-iron vessel ofsuitable size, and by an ingenious arrangement only a small portion ofmercury is required. 

The characteristics of navigation lights are varied considerably inorder to enable the mariner to distinguish them and thereby to learnexactly where he is. The fixed light is liable to be confused withothers, so it has now become a minor light. Flashes of short durationfollowed by longer periods of darkness are extensively used. The marinerby timing the intervals is able to recognize the light. This method isextended to groups of short flashes followed by longer intervals ofdarkness. In fact, short flashes have been employed to indicate acertain number so that a mariner could recognize the light by a numberrather than by means of his watch. However, a time element is generallyused. A combination of fixed light upon which is superposed a flash or agroup of flashes of white or of colored light has been used, but it isin disrepute as being unreliable. A type known as "occulating lights"consists of a fixed light which is momentarily eclipsed, but theduration of the eclipse is usually less than that of the light. Obviously, groups of eclipses may be used. Sometimes lights of differentcolors are alternated without any dark intervals. The colored ones usedare generally red and green, but these are short-range lights at best. Colored sectors are sometimes used over portions of the field, in orderto indicate dangers, and white light shows in the fairway. These areusually fixed lights for marking the channel. 

The distance at which a light may be seen at sea depends upon itsluminous intensity, upon its color or spectral composition, upon itsheight and that of the observer's eyes above the sea-level, and upon theatmospheric conditions. Assuming a perfectly clear atmosphere, thevisibility of a light-source apparently depends directly upon itscandle-power. The atmosphere ordinarily absorbs the red, orange, andyellow rays less than the green, blue, and violet rays. This isdemonstrated by the setting sun, which as it approaches closer to thehorizon changes from yellow to orange and finally to red as the amountof atmosphere between it and the eye increases. For this reason a redlight would have a greater range than a blue light of the same luminousintensity. 

Under ordinary atmospheric conditions the range of the more powerfullight-sources used in lighthouses is greater than the range as limitedby the curvature of the earth. For the uncolored illuminants the rangein nautical miles appears to be at least equal to the square root of thecandle-power. A real practical limitation which still exists is thecurvature of the earth, and the distance an object may be seen by theeye at sea-level depends upon the height of the object. The relation isapproximately expressed thus, --

Range in nautical miles = 8/7 square root of Height of object infeet. For example, the top of a tower 100 feet high is visible to an eyeat sea-level a distance of 8/7 square root of 100 = 80/7 = 11. 43miles. Now if the eye is 49 feet above sea-level, a similar computationwill show how far away it may be seen by the original eye at sea-level. This is 8/7 square root of 49 = 8 miles. Hence an eye 49 feet abovesea-level will be able to see the top of the 100-foot tower at adistance of 11. 43 + 8 or 19. 43 nautical miles. Under these conditions animaginary line drawn from the top of the tower to the eye will be justtangent to the spherical surface of the sea at a distance of 8 milesfrom the eye and 11. 43 miles from the tower. 

The luminous intensity of a light-source or of the beam of light isdirectly responsible for the range. The luminous intensity of the earlybeacon-fires and oil-lamps was equivalent to a few candles. Theimprovements in light-sources and also in reflecting and refractingoptical systems have steadily increased the candle-power of the beams, until to-day the beams from gas-lamps have intensities as high asseveral hundred thousand candle-power. The beams sent forth by modernlighthouses equipped with electric lamps and enormous light-gatheringdevices are rated in millions of candle-power. In fact, Navesink Lightat the entrance of New York Bay is rated as high as 60, 000, 000candle-power. 

Of course, light-production has increased enormously in efficiency inthe past century, but without optical devices for gathering the light, the enormous beam intensity would not be obtained. For example, considera small source of light possessing a luminous intensity of one candle inall directions. If all this light which is emitted in all directions isgathered and sent forth in a beam of small angle, say one thousandth ofthe total angle surrounding a point, the intensity of this beam would be1000 candles. It is in this manner that the enormous beam intensitiesare built up. 

There is an interesting point pertaining to short flashes of light. Tothe dark-adapted eye a brief flash is registered as of considerablyhigher intensity than if the light remained constant. In other words, the lookout on a vessel is adapted to darkness and a flash from a beamof light is much brighter than if the same beam were shining steadily. This is a physiological phenomenon which operates in favor of theflashing light. 

[Illustration: A. A COMPLETED LIGHTHOUSE LENS]

[Illustration: B. TORRO POINT LIGHTHOUSE, PANAMA CANAL]

[Illustration: AMERICAN SEARCH-LIGHT POSITION ON WESTERN FRONT IN 1919]

[Illustration: AMERICAN STANDARD FIELD SEARCH-LIGHT AND POWER UNIT]

Doubtless, the reader has noted that reliability, simplicity, and lowcost of operation are the primary considerations for light-sources usedas aids to navigation. This accounts for the continued use of oil andgas. From an optical standpoint the electric arc-lamps andconcentrated-filament lamps are usually superior to the earlier sourcesof light, but the complexity of a plant for generating electricity isusually a disadvantage in isolated places. The larger light-ships arenow using electricity generated by apparatus installed in the vessels. There seems to be a tendency toward the use of more buoys and fewerlighthouses, but the beam-intensities of the latter are increasing. 

In the hundred years since the Boston Light was built the same greatchanges wrought by the development of artificial light in otheractivities of civilization have appeared in the beacons of the mariner. The development of these aids to navigation has been wonderful, but itmust go on and on. The surface of the earth comprises 51, 886, 000 squarestatute miles of land and 145, 054, 000 square miles of water. Threefourths of the earth's surface is water and the oceans will always behighways of world commerce. All the dangers cannot be overcome, buthuman ingenuity is capable of great achievements. Wreckage will appearalong the shore-lines despite the lights, but the harvest of the shoalshas been much reduced since the time described by Robert LouisStevenson, when the coast people in the Orkneys looked upon wrecks as asource of gain. He states:

 It had become proverbial with some of the inhabitants to observe that "if wrecks were to happen, they might as well be sent to the poor island of Sanday as anywhere else. " On this and the neighboring island, the inhabitants have certainly had their share of wrecked goods. On complaining to one of the pilots of the badness of his boat's sails, he replied with some degree of pleasantry, "Had it been His [God's] will that you come na here wi these lights, we might a' had better sails to our boats and more o' other things. "

 In the leasing of farms, a location with a greater probability of shipwreck on the shore brought a much higher rent. 

XIV

ARTIFICIAL LIGHT IN WARFARE

When the recent war broke out science responded to the call and underthe stress of feverish necessity compressed the normal development of ahalf-century into a few years. The airplane, in 1914 a doubtfulplaything of daredevils, emerged from the war a perfected thing of theair. Lighting did not have the glamor of flying or the novelty ofchemical warfare, but it progressed greatly in certain directions andserved well. While artificial lighting conducted its unheraldedoffensive by increasing production in the supporting industries andhelped to maintain liaison with the front-line trenches by lending eyesto transportation, it was also doing its part at the battle front. Hugesearch-lights revealed the submarine and the aërial bomber; flaresexposed the manoeuvers of the enemy; rockets brought aid tobeleaguered vessels and troops; pistol lights fired by the aërialobserver directed artillery fire; and many other devices of artificiallight were in the fray. Many improvements were made in search-lights andin signaling devices and the elements of the festive fireworks of pastages were improved and developed for the needs of modern warfare. 

Night after night along the battle front flares were sent up to revealpatrols and any other enemy activity. On the slightest suspicion greatswarms of these brilliant lights would burst forth as though flocks ofhuge fireflies had been disturbed. They were even used as lightbarrages, for movements could be executed in comparative safety when alarge number of these lights lay before the enemy's trenches sputteringtheir brilliant light. The airman dropped flares to illuminate histarget or his landing field. The torches of past parades aided thesoldier in his night operations and rockets sent skyward radiated theirmessages to headquarters in the rear. The star-shell had the samemissions as other flares, but it was projected by a charge of powderfrom a gun. These and many modifications represent the usefulapplications of what formerly were mere "fireworks. " Those which areprimarily signaling devices are discussed in another chapter, but theothers will be described sufficiently to indicate the place whichartificial light played in certain phases of warfare. 

The illuminating compounds used in these devices are not particularlynew, consisting essentially of a combustible powder and chemical saltswhich make the flame luminous and give it color when desired. Among theingredients are barium nitrate, potassium perchlorate, powderedaluminum, powdered magnesium, potassium nitrate, and sulphur. One of thesimplest mixtures used by the English is, Barium nitrate 37 per cent. Powdered magnesium 34 per cent. Potassium nitrate 29 per cent. 

The magnesium is coated with hot wax or paraffin, which not only acts asa binder for the mixture when it is pressed into its container but alsoserves to prevent oxidation of the magnesium when the shells are stored. The barium and potassium nitrates supply the oxygen to the magnesium, which burns with a brilliant white flame. The potassium nitrate takesfire more readily than the barium nitrate, but it is more expensive thanthe latter. 

Owing to the cost of magnesium, powdered aluminum has been used to someextent as a substitute. Aluminum does not have the illuminating value ofmagnesium and it is more difficult to ignite, but it is a goodsubstitute in case of necessity. An English mixture containing theseelements is, Barium nitrate 58 per cent. Magnesium 29 per cent. Aluminum 13 per cent. 

Mixtures which are slow to ignite must be supplemented by a primarymixture which is readily ignited. For obtaining colored lights it isonly necessary to add chemicals which will give the desired color. Themixtures can be proportioned by means of purely theoreticalconsiderations; that is, just enough oxygen can be present to burn thefuel completely. However, usually more oxygen is supplied than calledfor by theory. 

The illuminating shell is perhaps the most useful of these devices tothe soldier. It has been constructed with and without parachutes, theformer providing an intense light for a brief period because it fallsrapidly. These shells of the larger calibers are equipped withtime-fuses and are generally rather elaborate in construction. The shellis of steel, and has a time-fuse at the tip. This fuse ignites acharge of black powder in the nose of the shell and this explosionejects the star-shell out of the rear of the steel casing. At the sametime the black powder ignites the priming mixture next to it, which inturn ignites the slow-burning illuminating compound. The star-shell hasa large parachute of strong material folded in the rear of the casingand the cardboard tube containing the illuminating mixture is attachedto it. The time of burning varies, but is ordinarily less than a minute. Certain structural details must be such as to endure the stresses of ahigh muzzle velocity. Furthermore, a velocity of perhaps 1000 feet persecond still obtains when the star-shell with its parachute is ejectedat the desired point in the air. 

The non-parachute illuminating shell is designed to give an intenselight for a brief interval and is especially applicable to defenseagainst air raids. Such a light aims to reveal the aircraft in orderthat the gunners may fire at it effectively. These shells are fittedwith time-fuses which fire the charge of black powder at the desiredinterval after the discharge of the shell from the gun. The contents ofthe shell are thereby ejected and ignited. The container for theilluminating material is so designed that there is rapid combustion andconsequently a brilliant light for about ten seconds. The enemy airmanin this short time is unable to obtain any valuable knowledge pertainingto the earth below and furthermore he is likely to be temporarilyblinded by the brilliant light if it is near him. 

The rifle-light which resembles an ordinary rocket, is fired from arifle and is designed for short-range use. It consists of a steelcylindrical shell a few inches long fastened to a steel rod. A parachuteis attached to the cardboard container in which the illuminating mixtureis packed and the whole is stowed away in the steel shell. Shoredelay-fuses are used for starting the usual cycle of events after therifle-light has been fired from the gun. The steel rod is injected intothe barrel of a rifle and a blank cartridge is used for ejecting thisrocket-like apparatus. Owing to inertia the firing-pin in the shelloperates and the short delay-fuse is thus fired automatically an instantafter the trigger of the rifle is pulled. 

Illuminating "bombs" of the same general principles are used by airmenin search of a landing for himself or for a destructive bomb; insignaling to a gunner, and in many other ways. They are simple inconstruction because they need not withstand the stresses of being firedfrom a gun; they are merely dropped from the aircraft. The mechanism ofignition and the cycle of events which follow are similar to those ofother illuminating shells. 

The value of such artificial-lighting devices depends both upon luminousintensity and time of burning. Although long-burning is not generallyrequired in warfare, it is obvious that more than a momentary light isusually needed. In general, high candle-power and long-burning areopposed to each other, so that the most intense lights of this characterusually are of short duration. Typical performances of two flares of thesame composition are as follows:

 Flare No. 1 Flare No. 2 Average candle-power 270, 000 95, 000 Seconds of burning 10 35 Candle-seconds 2, 700, 000 3, 325, 000 Cubic inches of compound 6 7 Candle-seconds per cubic inch 450, 000 475, 000 Candle-hours per cubic inch 125 132

The illuminating compound was the same in these two flares, whichdiffered only in the time allowed for burning. Of course, themeasurements of the luminous intensity of such flares is difficultbecause of the fluctuations, but within the errors of the measurementsit is seen that the illuminating power of the compound is about the sameregardless of the time of burning. The light-source in the case ofburning powders is really a flame, and inasmuch as the burning end hangsdownward, more light is emitted in the lower hemisphere than in theupper. The candle-power of the largest flares equals the combinedluminous intensities of 200 street arc-lamps or of 10, 000 ordinary40-watt tungsten lamps such as are used in residence lighting. 

It is interesting to note the candle-hours obtained per cubic inch ofcompound and to find that the cost of this light is less than that ofcandles at the present time and only five or ten times greater than thatof modern electric lighting. 

Illuminating shells in use during the recent war were designed formuzzle velocities as high as 2700 feet per second and were gaged toignite at any distance from a quarter of a mile to several miles. Themaximum range of illuminating shells fired from rifles was about 200yards; for trench mortars about one mile; and from field and naval gunsabout four miles. 

The search-light has long been a valuable aid in warfare and during therecent conflict considerable attention was given to its development andapplication. It is used chiefly for detecting and illuminating distanttargets, but this covers a wide range of conditions and requirements. Inorder that a search-light may be effective at a great distance, as muchas possible of the light emitted by a source is directed into a beam oflight of as nearly parallel rays as can be obtained. Reflectors areusually employed in military search-lights, and in order that the beammay be as nearly parallel (minimum divergence) as possible, the lightmust be emitted by the smallest source compatible with high intensity. This source is placed at the proper point in respect to a largeparabolic reflecter which renders the rays parallel or nearly so. 

Ever since its advent the electric arc has been employed in largesearch-lights, with which the army and the navy were supplied; however, the greatest improvements have been made under the stress of war. Thescience of aëronautics advanced so rapidly during the recent war thatthe necessity for powerful search-lights was greatly augmented and asthe conflict progressed the enemy airmen came to look upon the newlydeveloped ones with considerable concern. The rapidly moving aircraftand its high altitude brought new factors into the design of theselights. It now became necessary to have the most intense beam and to beable to sweep the heavens with it by means of delicate controllingapparatus, for the targets were sometimes minute specks moving at highspeed at altitudes as high as five miles. Furthermore, owing to theshifting battle areas, mobile apparatus was necessary. 

The control of light by means of reflectors has been studied forcenturies, but until the advent of the electric arc the light-sourceswere of such large areas that effective control was impossible. Opticaldevices generally are considered in connection with "point sources, " butinasmuch as no light can be obtained from a point, a source of smalldimensions and of high brightness is the most effective compromise. Parabolic mirrors were in use in the eighteenth century and theirproperties were known long before the first search-light worthy of thename was made in 1825 by Drummond, who used as a source of light a pieceof lime heated to incandescence in a blast flame. He finally developedthe "lime-light" by directing an oxyhydrogen flame upon a piece of limeand this device was adapted to search-lights and to indoor projection. It is said that the first search-light to be used in warfare was aDrummond lime-light which played a part in the attack on Fort Wagner atCharleston in 1863. 

In 1848 the first electric arc lamp used for general lighting wasinstalled in Paris. It was supplied with current by a large voltaiccell, but the success of the electric arc was obliged to await thedevelopment of a more satisfactory source of electricity. A score ofyears was destined to elapse, after the public was amazed by the firstdemonstration, before a suitable electric dynamo was invented. With theadvent of the dynamo, the electric arc was rapidly developed and thusthere became available a concentrated light-source of high intensityand great brilliancy. Gradually the size was increased, until at thepresent time mirrors as large as seven feet in diameter and electriccurrents as great as several hundred amperes are employed. The beamintensities of the most powerful search-lights are now as great asseveral hundred million candles. 

The most notable advance in the design of arc search-lights was achievedin recent years by Beck, who developed an intensive flame carbon-arc. His chief object was to send a much greater current through the arc thanhad been done previously without increasing the size of the carbons andthe unsteadiness of the arc. In the ordinary arc excessive currentcauses the carbons to disintegrate rapidly unless they are of largediameter. Beck directed a stream of alcohol vapor at the arc and theywere kept from oxidizing. He thus achieved a high current-density andmuch greater beam intensities. He also used cored carbons containingcertain metallic salts which added to the luminous intensity, and byrotation of the positive carbon so that the crater was kept in aconstant position, greater steadiness and uniformity were obtained. Tests show that, in addition to its higher luminous efficiency, an arcof this character directs a greater percentage of the light into theeffective angle of the mirror. The small source results in a beam ofsmall divergence; in other words, the beam differs from a cylinder byonly one or two degrees. If the beam consisted entirely of parallel raysand if there were no loss of light in the atmosphere by scattering or byabsorption, the beam intensity would be the same throughout its entirelength. However, both divergence and atmospheric losses tend to reducethe intensity of the beam as the distance from the search-lightincreases. 

Inasmuch as the intensity of the beam depends upon the actual brightnessof the light-source, the brightness of a few modern light-sources are ofinterest. These are expressed in candles per square inch of projectedarea; that is, if a small hole in a sheet of metal is placed next to thelight-source and the intensity of the light passing through this hole ismeasured, the brightness of the hole is easily determined in candles persquare inch. 

BRIGHTNESS OF LIGHT-SOURCES IN CANDLES PER SQUARE INCH

 Kerosene flame 5 to 10 Acetylene 30 to 60 Gas-mantle 30 to 500 Tungsten filament (vacuum) lamp 750 to 1, 200 Tungsten filament (gas-filled) lamp 3, 500 to 18, 000 Magnetite arc 4, 000 to 6, 000 Carbon arc for search-lights 80, 000 to 90, 000 Flame arc for search-lights 250, 000 to 350, 000 Sun (computed mean) about 1, 000, 000

As the reflector of a search-light is an exceedingly important factor inobtaining high beam-intensities, considerable attention has been givento it since the practicable electric arc appeared. The parabolic mirrorhas the property of rendering parallel, or nearly so, the rays from alight-source placed at its focus. If the mirror subtends a large angleat the light-source, a greater amount of light is intercepted andrendered parallel than in the case of smaller subtended angles; hence, mirrors are large and of as short focus as practicable. Search-lightprojectors direct from 30 to 60 per cent. Of the available light intothe beam, but with lens systems the effective angle is so small that amuch smaller percentage is delivered in the beam. Mangin in 1874 made areflector of glass in which both outer and inner surfaces were sphericalbut of different radii of curvature, so that the reflector was thickerin the middle. This device was "silvered" on the outside and therefraction in the glass, as the light passed through it to the mirrorand back again, corrected the spherical aberration of the mirroredsurface. These have been extensively used. Many combinations of curvedsurfaces have been developed for special projection purposes, but theparabolic mirror is still in favor for powerful search-lights. The tipof the positive carbon is placed at its focus and the effective angle inwhich light is intercepted by the mirror is generally about 125 degrees. Within this angle is included a large portion of the light emitted bythe light-source in the case of direct-current arcs. If this angle isincreased for a mirror of a given diameter by decreasing its focallength, the divergence of the beam is increased and the beam-intensityis diminished. This is due to the fact that the light-source now becomesapparently larger; that is, being of a given size it now subtends alarger angle at the reflector and departs more from the theoreticalpoint. 

When the recent war began the search-lights available were intendedgenerally for fixed installations. These were "barrel" lights withreflectors several feet in diameter, the whole output sometimes weighingas much as several tons. Shortly after the entrance of this countryinto the war, a mobile "barrel" search-light five feet in diameter wasproduced, which, complete with carriage, weighed only 1800 pounds. Laterthere were further improvements. An example of the impetus which thestress of war gives to technical accomplishments is found in thedevelopment of a particular mobile searchlight. Two months after the WarDepartment submitted the problems of design to certain large industrialestablishments a new 60-inch search-light was placed in production. Itweighed one fifth as much as the previous standard; it had one twentieththe bulk; it was much simpler; it could be built in one fourth the time;and it cost half as much. Remote control of the apparatus has beenhighly developed in order that the operator may be at a distance fromthe scattered light near the unit. If he is near the search-light, thisveil of diffused light very seriously interferes with his vision. 

Mobile power-units were necessary and the types developed used theautomobile engine as the prime mover. In one the generator is located infront of the engine and supported beyond the automobile chassis. Inanother type the generator is located between the automobiletransmission and the differential. A standard clutch and gear-shiftlever is employed to connect the engine either with the generator orwith the propeller shaft of the truck. The first type included a115-volt, 15-kilowatt generator, a 36-inch wheel barrel search-light, and 500 feet of wire cable. The second type included a 105-volt, 20-kilowatt generator, a 60-inch open searchlight, and 600 feet ofcable. This type has been extended in magnitude to include a 50-kilowattgenerator. When these units are moved, the search-light and itscarriage are loaded upon the rear of the mobile generating equipment. Anidea of the intensities obtainable with the largest apparatus is gainedfrom illumination produced at a given distance. For example, the15-kilowatt search-light with highly concentrated beam, produced anillumination at 930 feet of 280 foot-candles. At this point this is theequivalent of the illumination produced by a source having a luminousintensity of nearly 250, 000, 000 candles. 

Of course, the range at which search-lights are effective is the factorof most importance, but this depends upon a number of conditions such asthe illumination produced by the beam at various distances, theatmospheric conditions, the position of the observer, the size, pattern, color, and reflection-factor of the object, and the color, pattern, andreflection-factor of the background. These are too involved to bediscussed here, but it may be stated that under ordinary conditionsthese powerful lights are effective at distances of several miles. According to recent work, it appears that the range of a search-light inrevealing a given object under fixed conditions varies about as thefourth root of its intensity. 

Although the metallic parabolic reflector is used in the most powerfulsearch-lights, there have been many other developments adapted towarfare. Fresnel lenses have been used above the arc for search-lightswhose beams are directed upward in search of aircraft, thus replacingthe mirror below the arc, which, owing to its position, is always indanger of deterioration by the hot carbon particles dropping upon it. For short ranges incandescent filament lamps have been used withsuccess. Oxyacetylene equipment has found application, owing to itsportability. The oxyacetylene flame is concentrated upon a small pelletof ceria, which provides a brilliant source of small dimensions. A tankcontaining about 1000 liters of dissolved acetylene and anothercontaining about 1100 liters of oxygen supply the fuel. A beam having anintensity of about 1, 500, 000 candles is obtained with a consumption of40 liters of each of the gases per hour. At this rate the search-lightmay be operated twenty hours without replenishing. 

Although the beacon-light for nocturnal airmen is a development whichwill assume much importance in peaceful activities, it was developedchiefly to meet the requirements of warfare. These do not differmaterially from those which guide the mariner, except that the travelerin the aërial ocean is far above the plane on which the beacon rests. For this reason the lenses are designed to send light generally upward. In foreign countries several types of beacons for aërial navigation havebeen in use. In one the light from the source is freely emitted in allupward directions, but the light normally emitted into the lowerhemisphere is turned upward by means of prisms. In a more elaboratetype, belts of lenses are arranged so as to send light in all directionsabove the horizontal plane. A flashing apparatus is used to designatethe locality by the number or character of the flashes. Electricfilaments and acetylene flames have been used as the light-sources forthis purpose. In another type the light is concentrated in one azimuthand the whole beacon is revolved. Portable beacons employing gas wereused during the war on some of the flying-fields near the battle front. 

All kinds of lighting and lighting-devices were used depending upon theneeds and material available. Even self-luminous paint was used forvarious purposes at the front, as well as for illuminating watch-dialsand the scales of instruments. Wooden buttons two or three inches indiameter covered with self-luminous paint could be fixed whereverdesired and thus serve as landmarks. They are visible only at shortdistances and the feebleness of their light made them particularlyvaluable for various purposes at the battle front. They could be used inthe hand for giving optical signals at a short distance where silencewas essential. Self-luminous arrows and signs directed troops and trucksat night and even stretcher-bearers have borne self-luminous marks ontheir backs in order to identify them to their friends. 

Somewhat analogous to this application of luminous paint is the use ofblue light at night on battle-ships and other vessels in action or nearthe enemy. Several years ago a Brazilian battle-ship built in thiscountry was equipped with a dual lighting-system. The extra one useddeep-blue light, which is very effective for eyes adapted to darkness orto very low intensities of illumination and is a short-range light. Owing to the low luminous intensity of the blue lights they do not carryfar; and furthermore, it is well established that blue light does notpenetrate as far through ordinary atmosphere as lights of other colorsof the same intensity. 

The war has been responsible for great strides in certain directions inthe development and use of artificial light and the era of peace willinherit these developments and will adapt them to more constructivepurposes. 

XV

SIGNALING

From earliest times the beacon-fire has sent forth messages fromhilltops or across inaccessible places. In this country, when the Indianwas monarch of the vast areas of forest and prairie, he spread newsbroadcast to roving tribesmen by means of the signal-fire, and heflashed his code by covering and uncovering it. Castaways, whether infiction or in reality, instinctively turn to the beacon-fire as a modeof attracting a passing ship. On every hand throughout the ages thissimple means of communication has been employed; therefore, it is notsurprising that mankind has applied his ingenuity to the perfection ofsignaling by means of light, which has its own peculiar fields andadvantages. Of course, wireless telephony and telegraphy will replacelight-signaling to some extent, but there are many fields in which thelast-named is still supreme. In fact, during the recent war much use wasmade of light in this manner and devices were developed despite the manyother available means of signaling. One of the chief advantages of lightas a signal is that it is so easily controlled and directed in astraight line. Wireless waves, for example, are radiated broadcast to beintercepted by the enemy. 

The beginning of light-signaling is hidden in the obscurity of the past. Of course, the most primitive light-signals were wood fires, but it islikely that man early utilized the mirror to reflect the sun's image andthus laid the foundation of the modern heliograph. The Book of Job, which is probably one of the oldest writings available, mentions moltenmirrors. The Egyptians in the time of Moses used mirrors of polishedbrass. Euclid in the third century before the Christian era is said tohave written a treatise in which he discussed the reflection of light byconcave mirrors. John Peckham, Archbishop of Canterbury in thethirteenth century, described mirrors of polished steel and of glassbacked with lead. Mirrors of glass coated with an alloy of tin andmercury were made by the Venetians in the sixteenth century. Huygens inthe seventeenth century studied the laws of refraction and reflectionand devised optical apparatus for various purposes. However, it was notuntil the eighteenth century that any noteworthy attempts were made tocontrol artificial light for practical purposes. Dollond in 1757 was thefirst to make achromatic lenses by using combinations of differentglasses. Lavoisier in 1774 made a lens about four feet in diameter byconstructing a cell of two concave glasses and filling it with water andother liquids. It is said that he ignited wood and melted metals byconcentrating the sun's image upon them by means of this lens. Aboutthat time Buffon made a built-up parabolic mirror by means of severalhundred small plane mirrors set at the proper angles. With this he setfire to wood at a distance of more than two hundred feet byconcentrating the sun's rays. He is said also to have made a lens from asolid piece of glass by grinding it in concentric steps similar to thedesigns worked out by Fresnel seventy years later. These are examples ofthe early work which laid the foundation for the highly perfectedcontrol of light of the present time. 

While engaged in the survey of Ireland, Thomas Drummond in 1826 devisedapparatus for signaling many miles, thus facilitating triangulation. Distances as great as eighty miles were encountered and it appeareddesirable to have some method for seeing a point at these greatdistances. Gauss in 1822 used the reflection of the sun's image from aplane mirror and Drummond also tried this means. The latter wassuccessful in signaling 45 miles to a station which because of hazecould not be seen, or even the hill upon which it rested. Havingdemonstrated the feasibility of the plan, he set about making a devicewhich would include a powerful artificial light in order to beindependent of the sun. In earlier geodetic surveys Argand lamps hadbeen employed with parabolic reflectors and with convex lenses, butapparently these did not have a sufficient range. Fresnel and Aragoconstructed a lens consisting of a series of concentric rings which werecemented together, and on placing this before an Argand lamp possessingfour concentric wicks, they obtained a light which was observed atforty-eight miles. 

Despite these successes, Drummond believed the parabolic mirror and amore powerful light-source afforded the best combination for asignal-light. In searching for a brilliant light-source he experimentedwith phosphorus burning in oxygen and with various brilliantpyrotechnical preparations. However, flames were unsteady and generallyunsuitable. He then turned in the direction which led to his developmentof the lime-light. In his first apparatus he used a small sphere of limein an alcohol flame and directed a jet of oxygen through the flame uponthe lime. He thereby obtained, according to his own description in 1826, a light so intense that when placed in the focus of a reflector the eye could with difficulty support its splendor, even at a distance of forty feet, the contour being lost in the brilliancy of the radiation. 

He then continued to experiment with various oxides, including zirconia, magnesia, and lime from chalk and marble. This was the advent of thelime-light, which should bear Drummond's name because it was one of thegreatest steps in the evolution of artificial light. 

By means of this apparatus in the survey, signals were rendered visibleat distances as great as one hundred miles. Drummond proposed the use ofthis light-source in the important lighthouses at that time and foresawmany other applications. The lime-light eventually was extensively usedas a light-signaling device. The heliograph, which utilizes the sun as alight-source, has been widely used as a light-signaling apparatus andDrummond perhaps was the first to utilize artificial light with it. Thedisadvantage of the heliograph is the undependability of the sun. Withthe adoption of artificial light, various optical devices have come intouse. 

Philip Colomb perhaps is deserving of the credit of initiating modernsignaling by flashing a code. He began work on such a system in 1858 andas an officer in the British Navy worked hard to introduce it. Finally, in 1867, the British Navy adopted the flashing-system, in which alight-source is exposed and eclipsed in such a manner as to representdots and dashes analogous to the Morse code. At first the rate oftransmission of words was from seven to ten per minute. Recently muchmore sensitive apparatus is available, and with such devices the rate islimited only by the sluggishness of the visual process. This initialsystem was very successful in the British Navy and it was soon foundthat a fleet could be handled with ease and safety in darkness or infog. Inasmuch as the "dot-and-dash" system requires only two elements, it may be transmitted by various means. A lantern may be swung in shortand long arcs or dipped accordingly. 

The blinker or pulsating light-signal consists of a single light-sourcemechanically occulted. It is controlled by means of a telegraph-key andthe code may be rapidly transmitted. The search-light affords a meansfor signaling great distances, even in the daytime. The light is usuallymechanically occulted by a quick-acting shutter, but recently anothersystem has been devised. In the latter the light itself is controlled bymeans of an electrical shunt across the arc. In this manner the light isdimmed by shunting most of the current, thereby producing the sameeffect as actually eclipsing the light with a mechanical shutter. Bymeans of the search-light signals are usually visible as far as thelimitations of the earth's curvature will permit. By directing the beamagainst a cloud, signals have been observed at a distance of one hundredmiles from the search-light despite intervening elevated land or thecurvature of the ocean's surface. By means of small search-lights it iseasy to send signals ten miles. 

This kind of apparatus has the advantage of being selective; that is, the signals are not visible to persons a few degrees from the directionof the beam. One of the most recent developments has been a specialtungsten filament in a gas-filled bulb placed at the focus of a smallparabolic mirror. The beam is directed by means of sights and theflashes are obtained by interrupting the current by means of atrigger-switch. The filament is so sensitive that signals may be sentfaster than the physiological process of vision will record. With theadvent of wireless telegraphy light-signaling for long distances wastemporarily eclipsed, but during the recent war it was revived and muchdevelopment work was prosecuted. 

The Ardois system consists of four lamps mounted in a vertical line ashigh as possible. Each lamp is double, containing a red and a whitelight, and these lights are controlled from a keyboard. A red lightindicates a dot in the Morse code and a white light indicates a dash. The keys are numbered and lettered, so that the system may be operatedby any one. Various other systems employing colored lights have beenused, but they are necessarily short-range signals. Another example isthe semaphore. When used at night, tungsten lamps in reflectors indicatethe positions of the arms. The advantage of these signals over theflashing-system is that each signal is complete and easy to follow. Theflashing-system is progressive and must be carefully followed in orderto obtain the meaning of the dots and dashes. 

Smaller signal-lamps using acetylene have been employed in the forestryservice and in other activities where a portable device is necessary. Inone type, a mixture-tank containing calcium carbide and water is ofsufficient capacity for three hours of signaling. A small pilot-light ispermitted to burn constantly and the flashes are obtained by operating akey which increases the gas-pressure. The light flares as long as thekey is depressed. The range of this apparatus is from ten to twentymiles. An electric lamp supplied from a storage battery has beendesigned for geodetic operations in mountainous districts where it isdesired to send signals as far as one hundred miles. Tests show thatthis device is a hundred and fifty times more powerful than the ordinaryacetylene signal-lamp, and it is thought that with this new electriclamp haze and smoke will seldom prevent observations. 

Certain fixed lights are required by law on a vessel at night. When itis under way there must be a white light at the masthead, a starboardgreen light, a port red light, a white range-light, and a white light atthe stern. The masthead light is designed to emit light through ahorizontal arc of twenty points of the compass, ten on each side of deadahead. This light must be visible at a distance of five miles. The portand starboard lights operate through a horizontal arc of twenty pointsof the compass, the middle of which is dead ahead. They are screened soas not to be visible across the bow and they must be intense enough tobe visible two miles ahead. The masthead light is carried on theforemast and the range-light on the mainmast, at an elevation fifteenfeet higher than the former. The range-light emits light toward allpoints of the compass and must be intense enough to be seen at adistance of three miles. The stern light is similar to the masthead, butits light must not be visible forward of the beam. When a vessel istowing another it must display two or three lights in a vertical linewith the masthead light and similar to it. The lights are spaced aboutsix feet apart, and two extra ones indicate a short tow and three a longone. A vessel over a hundred and fifty feet long when at anchor isrequired to display a white light forward and aft, each visible aroundthe entire horizon. These and many other specifications indicate howartificial light informs the mariner and makes for order in shipping. Without artificial light the waterways would be trackless and chaoswould reign. 

The distress signals of a vessel are rockets, but any burning flame alsoserves if rockets are unavailable. Fireworks were known many centuriesago and doubtless the possibilities of signaling by means of rocketshave long been recognized. An early instance of scientific interest inrockets and their usefulness is that of Benjamin Robins in 1749. Whilehe was witnessing a display of fireworks in London it occurred to himthat it would be of interest to measure the height to which the rocketsascended and to determine the ranges at which they were visible. Hismeasurements indicated that the rockets ascended usually to a height of440 yards, but some of them attained altitudes as high as 615 yards. Hethen had some special ones made and despatched letters to friends inthree different localities, at distances as great as 50 miles, askingthem to observe at a certain time, when the rockets were to be sent upin the outskirts of London. Some of these rockets rose to altitudes asgreat as 600 yards and were distinctly seen by observers 38 miles away. Later he made rockets which ascended as high as 1200 yards and concludedthat this was a practical means of signaling. Since that time andespecially during the recent war, rockets have served well in signalingmessages. 

The self-propelled rockets have not been altered in essential featuressince the remote centuries when the Chinese first used them incelebrations. A cylindrical shell is mounted on a wooden stick and whenthe powder in the shell burns the hot gases are ejected so violentlydownward that the reaction drives the shell upward. At a certain pointin the air, various signals burst forth, which vary in character andcolor. One of the advantages of the rocket is that it contains withinitself the force of propulsion; that is, no gun is necessary to projectit. The illuminating compounds and various details are similar to thoseof the illuminating shells described in another chapter. 

At present the rocket is not scientifically designed to obtain thegreatest efficiency of propulsion, but its simplicity in this respect isone of its chief advantages. If the self-propelled rocket becomes theprojectile of the future, as some have ventured to predict, muchconsideration must be given to the design of the orifice through whichthe gases violently escape in order that the best efficiency ofpropulsion may be attained. There are other details in whichimprovements may be made. The combustion products of the black powderwhich are not gaseous equal about one third the weight of the powder. This represents inefficient propulsion. Furthermore, during recent yearsmuch information has been gained pertaining to the air-resistance whichcan be applied to advantage in designing the form of rockets. 

Besides the various rockets, signal-lights have been constructed to befired from guns and pistols. During the recent war the airman in thedark heights used the pistol signal-light effectively for communication. These devices emitted stars either singly or in succession, and thecolor of these stars as well as their number and sequence gavesignificance to the signal. Some of these light-signals were providedwith parachutes and were long-burning; that is, light was emitted for aminute or two. There are many variations possible and a great manydifferent kinds of light-signals of this character were used. In thefront-line trenches and in advances they were used when telephoneservice was unavailable. The airman directed artillery fire by means ofhis pistol-light. Rockets brought aid to the foundered ship or to thelife-boats. The signal-tube which burned red, green, or white was heldin the hand or laid on the ground and it often told its story. For manyyears such a device dropped from the rear of the railroad train has keptthe following train at a safe distance. A device was tried out in thetrenches, during the war, which emitted a flame. This could be varied incolor to serve as a signal and the apparatus had sufficient capacity forthirty hours' burning. This could also be used as a weapon, or whenreduced in intensity it served as a flash-light. 

For many years experiments have been made upon the use of the invisiblerays which accompany visible rays. The practicability of signaling withinvisible rays depends upon producing them efficiently in sufficientquantity and upon separating them from the visible rays which accompanythem. Some successful results were obtained with a 6-volt electric lamppossessing a coiled filament at the focus of a lens three inches indiameter and twelve inches in focal length. This gave a very narrow beamvisible only in the neighborhood of the observation post to which thesignals were directed. The beam was directed by telescopic sights. During the day a deep red filter was placed over the lamp and the lightwas invisible to an observer unless he was equipped with a similar redscreen to eliminate the daylight. It is said that signals weredistinguished at a distance of six miles. By night a screen was usedwhich transmitted only the ultraviolet rays, and the observer'stelescope was provided with a fluorescent screen in its focal plane. Theultraviolet rays falling upon this screen were transformed into visiblerays by the phenomenon of fluorescence. The range of this device wasabout six miles. For naval convoys lamps are required to radiate towardall points of the compass. For this purpose a quartz mercury-arc whichis rich in ultraviolet rays was surrounded with a chimney whichtransmitted the ultraviolet rays efficiently and absorbed all visiblerays excepting violet light. The lamp appeared a deep violet color atclose range, but the faintly visible light which it transmitted was notseen at a distance. A distant observer picks up the invisibleultraviolet "light" by means of a special optical device having afluorescent screen of barium-platino-cyanide. This device had a range ofabout four miles. 

Light-signals are essential for the operation of railways at night andthey have been in use for many years. In this field the significance oflight-signals is based almost universally on color. The setting of aswitch is indicated by the color of the light that it shows. With theintroduction of the semaphore system, in which during the day theposition of the arm is significant, colored glasses were placed on theopposite end of the arm in such a manner that a certain colored glasswould appear before the light-source for a certain position of the arm. A kerosene flame behind a glass lens was the lamp used, and, forexample, red meant "Stop, " green counseled "Caution, " and clear or whiteindicated "All clear. " For many years the kerosene lamp has been used, but recently the electric filament lamp is being installed to someextent for this purpose. In fact, on one railroad at least, tungstenlamps are used for light-signals by day as well as by night. Threesignals--red, green, and white--are placed in a vertical line and behindeach lens are two lamps, one operating at high efficiency and one at lowefficiency to insure against the failure of the signal. The normaldaylight range is about three thousand feet and under the worstconditions when opposed to direct sunlight, the range is not less thantwo thousand feet. It is said that these lights are seen more easilythan semaphore arms under all circumstances and that they show two orthree times as far as the latter during a snow-storm. 

The standard colors for light-signals as adopted by the Railway SignalAssociation are red, yellow, green, blue, purple, and lunar white. Theseare specified as to the amount of the various spectral colors which theytransmit when the light-source is the kerosene flame. Obviously, thecolors generally appear different when another illuminant is used. Theblue and purple are short-range signals, but the effective range of thebest railway signal employing a kerosene flame is only about four miles. 

It has been shown that the visibility of point sources of white light inclear atmosphere, for distances up to a mile at least, is proportionalto their candle-power and inversely proportional to the square of thedistance. Apparently the luminous intensities of signal-lamps requiredin clear weather in order that they may be visible must be 0. 43 candlesfor one nautical mile, 1. 75 candles for two nautical miles, and 11candles for five nautical miles. From the data available it appears thata red or a white signal-light will be easily visible at a distance innautical miles equal to the square root of its candle-power in thatdirection. The range in nautical miles of a green light apparently isproportional to the cube root of the candle-power. Whether or not theserelations between the range in miles and the luminous intensity incandles hold for greater distances than those ordinarily encountered hasnot been determined, but it is interesting to note that the square rootof the luminous intensity of the Navesink Light at the entrance to NewYork Harbor is about 7000. Could this light be seen at a distance ofseven thousand miles through ordinary atmosphere?

The most distinctive colored lights are red, yellow, green, and blue. Tothese white (clear) and purple have been added for signaling-purposes. Yellow is intense, but it may be confused with "white" or clear. Blueand purple as obtained from the present practicable light-sources are oflow intensity. This leaves red, green, and clear as the most generallysatisfactory signal-lights. 

There are numerous other applications, especially indoors. Some of thesehave been devised for special needs, but there are many others which aregeneral, such as for elevators, telephones, various call systems, andtraffic signals. Light has the advantages of being silent andcontrollable as to position and direction, and of being a visible signalat night. Thus, in another field artificial light has responded to thedemands of civilization. 

XVI

THE COST OF LIGHT

Artificial light is so superior to natural light in many respects thatmankind has acquired the habit of retiring many hours after darkness hasfallen, a result of which has brought forth the issue known as "daylightsaving. " Doubtless, daylight should be used whenever possible, but thereare two sides to the question. In the first place, it costs something tobring daylight indoors. The architectural construction of windows andskylights increases the cost of daylight. Light-courts, by sacrificingvaluable floor-area, add to the expense. The maintenance of windows andsky lights is an appreciable item. Considering these and other factors, it can be seen that daylight indoors is expensive; and as it is alsoundependable, a supplementary system of artificial lighting is generallynecessary. In fact, it is easy to show in some cases that artificiallighting is cheaper than natural lighting. 

The average middle-class home is now lighted artificially for about$15. 00 to $25. 00 per year, with convenient light-sources which areavailable at all times. There is no item in the household budget whichreturns as much satisfaction, comfort, and happiness in proportion toits cost as artificial light. It is an artistic medium of greatpotentiality, and light in a narrow utilitarian sense is always aby-product of artistic lighting. The insignificant cost of modernlighting may be emphasized in many ways. The interest on the investmentin a picture or a vase which cost $25. 00 will usually cover the cost ofoperating any decorative lamp in the home. A great proportion of theinvestment in personal property in a home is chargeable to an attempt tobeautify the surroundings. The interest on only a small portion of thisinvestment will pay for artistic and utilitarian artificial lighting inthe home. The cost of washing the windows of the average house may be asgreat as the cost of artificial lighting and is usually at least a largefraction of the latter. It would become monotonous to cite the variousexamples of the insignificant cost of artificial light and its highreturn to the user. The example of the home has been chosen because thereader may easily carry the analysis further. The industries where costsare analyzed are now looking upon adequate and proper lighting as anasset which brings in profits by increasing production, by decreasingspoilage, and by decreasing the liability of accidents. 

Inasmuch as daylight saving became an issue during the recent war and islikely to remain a matter of concern, its history is interesting. One ofthe outstanding differences between primitive and civilized beings istheir hours of activities. The former automatically adjusted themselvesto daylight, but as civilization advanced, the span of activities beganto extend more and more beyond the coming of darkness. Finally in manyactivities the work-day was extended to twenty-four hours. There can beno insurmountable objection to working at night with a properarrangement of the periods of work; in fact, the cost of living wouldbe greatly increased if the overhead charges represented by such itemsas machinery and buildings were allowed to be carried by the decreasedproducts of a shortened period of production. There cannot be any basicobjection to artificial lighting, because most factories, for example, may be better illuminated by artificial than by natural light. 

Of course, the lag of comfortable temperature behind daylight isresponsible to some extent for a natural shifting of the ordinaryworking-day somewhat behind the sun. The chill of dawn tends to keepmankind in bed and the cheer of artificial light and the period ofrecreation in the evening tends to keep the civilized races out of bed. There are powerful influences always at work and despite the desirablefeatures of daylight-saving, mankind will always tend to lag. As yearsgo by, doubtless it will be necessary to make the shift again and again. It seems certain that throughout the centuries thoughtful persons haveseen the difficulty of rousing man from his warm bed in the earlymorning and have recognized a simple solution in turning the hands ofthe clock ahead. Among the earliest advocates of daylight saving duringmodern times, when it became important enough to be considered as aneconomic issue, was Benjamin Franklin. In 1784 he wrote a masterfulserio-comic essay entitled "An Economical Project" which was publishedin the _Journal_ of Paris. The article, which appeared in the form of aletter, began thus:

 MESSIEURS: You often entertain us with accounts of new discoveries. Permit me to communicate to the public through your paper one that has lately been made by myself and which I conceive may be of great utility. 

 I was the other evening in a grand company where the new lamp of Messrs. Quinquet and Lange was introduced and much admired for its splendor; but a general inquiry was made whether the oil it consumed was not in exact proportion to the light it afforded, in which case there would be no saving in the use of it. No one present could satisfy us on that point, which all agreed ought to be known, it being a very desirable thing to lessen, if possible, the expense of lighting our apartments, when every other article of family expense was so much augmented. I was pleased to see this general concern for economy, for I love economy exceedingly. 

 I went home, and to bed, three or four hours after midnight, with my head full of the subject. An accidental sudden noise waked me about 6 in the morning, when I was surprised to find my room filled with light, and I imagined at first that a number of those lamps had been brought into it; but, rubbing my eyes, I perceived the light came in at the windows. I got up and looked out to see what might be the occasion of it, when I saw the sun just rising above the horizon, from whence he poured his rays plentifully into my chamber, my domestic having negligently omitted the preceding evening to close the shutters. 

 I looked at my watch, which goes very well, and found that it was but 6 o'clock; and, still thinking it something extraordinary that the sun should rise so early, I looked into the almanac, where I found it to be the hour given for his rising on that day. I looked forward, too, and found he was to rise still earlier every day till toward the end of June, and that at no time in the year he retarded his rising so long as till 8 o'clock. 

 Your readers who, with me, have never seen any signs of sunshine before noon, and seldom regard the astronomical part of the almanac, will be as much astonished as I was when they hear of his rising so early, and especially when I assure them that he gives light as soon as he rises. I am convinced of this. I am certain of my fact. One cannot be more certain of any fact. I saw it with my own eyes. And, having repeated this observation the three following mornings, I found always precisely the same result. 

He then continues in the same vein to show that learned persons did notbelieve him and to point out the difficulties which the pioneerencounters. He brought out the vital point by showing that if he had notbeen awakened so early he would have slept six hours longer by the lightof the sun and in exchange he would have lived six hours the followingnight by candle-light. He then mustered "the little arithmetic" he wasmaster of and made some serious computations. He assumed as the basis ofhis computations that a hundred thousand families lived in Paris andeach used a half-pound of candles nightly. He showed that between March20th and September 20th, 64, 000, 000 pounds of wax and tallow could besaved, which was equivalent to $18, 000, 000. 

After these serious computations he amusingly proposed the means forenforcing the daylight saving. Obviously, it was necessary to arouse thesluggards and his proposals included the use of cannons and bells. Besides, he proposed that each family be restricted to one pound ofcandles per week, that coaches would not be allowed to pass after sunsetexcept those of physicians, etc. , and that a tax be placed upon everywindow which had shutters. His closing paragraph was as follows:

 For the great benefit of this discovery, thus freely communicated and bestowed by me on the public, I demand neither place, pension, exclusive privilege, nor any other regard whatever. I expect only to have the honor of it. And yet I know there are little, envious minds who will, as usual, deny me this and say that my invention was known to the ancients, and perhaps they may bring passages out of the old books in proof of it. I will not dispute with these people that the ancients knew not the sun would rise at certain hours; they possibly had, as we have, almanacs that predicted it; but it does not follow thence that they knew he gave light as soon as he rose. That is what I claim as my discovery. If the ancients knew it, it might have been long since forgotten; for it certainly was unknown to the moderns, at least to the Parisians, which to prove I need use but one plain simple argument. They are as well instructed, judicious and prudent a people as exist anywhere in the world, all professing, like myself, to be lovers of economy, and, for the many heavy taxes required from them by the necessities of the State have surely an abundant reason to be economical. I say it is impossible that so sensible a people, under such circumstances, should have lived so long by the smoky, unwholesome and enormously expensive light of candles, if they had really known that they might have had as much pure light of the sun for nothing. 

Franklin's amusing letter had a serious aim, for in 1784 family expenseswere much augmented and adequate lighting by means of candles was verycostly in those days. However, conditions have changed enormously in thepast hundred and thirty-five years. A great proportion of the populationlives in the darker cities. The wheels of progress must be kept goingcontinuously in order to curb the cost of living, which is constantlymounting higher owing to the addition of conveniences and luxuries. Furthermore, the cost of light has so diminished that it is not only aminor factor at present but in many cases is actually paying dividendsin commerce and industry. It is paying dividends of another kind in thesocial and educational aspects of the home, library, church, and artmuseum. Daylight saving has much to commend it, but the cost of daylightand the value of artificial light are important considerations. 

The cost of fuels for lighting purposes cannot be thoroughly comparedthroughout a span of years without regard to the fluctuating purchasingpower of money, which would be too involved for consideration here. However, it is interesting to make a brief survey throughout the pastcentury. From 1800 until 1845 whale-oil sold for about $. 80 per gallon, but after this period it increased in value, owing apparently to itsgrowing scarcity, until it reached a price of $1. 75 per gallon in 1855. Fortunately, petroleum was discovered about this time, so that theoil-lamp did not become a luxury. From 1800 to 1850 tallow-candles soldat approximately 20 cents a pound. There being six candles to the pound, and inasmuch as each candle burned about seven hours, the light from acandle cost about 1/2 cent per hour. From 1850 to 1875 tallow-candlessold at an average price of approximately 25 cents a pound. It may beinteresting to know that a large match emits about as much light as aburning candle and a so-called safety match about one third as much. 

A candle-hour is the total amount of light emitted by a standard candlein one hour, and candle-hours in any case are obtained by multiplyingthe candle-power of the source by the hours of burning. In a similarmanner, lumens output multiplied by hours of operation give thelumen-hours. A standard candle may be considered to emit an amount oflight approximately equal to 10 lumens. A wax-candle will emit about asmuch light as a sperm candle but will consume about 10 per cent. Lessweight of material. A tallow candle will emit about the same amount oflight with a consumption about 50 per cent. Greater. The tallow-candlehas disappeared from use. 

With the appearance of kerosene distilled from petroleum the camphenelamp came into use. The kerosene cost about 80 cents per gallon duringthe first few years of its introduction. The price of kerosene averagedabout 55 cents a gallon between 1865 and 1875. During the next decade itdropped to about 22 cents a gallon and between 1885 and 1895 it sold aslow as 13 cents. 

Artificial gas in 1865 sold approximately at $2. 50 per thousand cubicfeet; between 1875 and 1885 at $2. 00; between 1885 and 1895 at $1. 50. 

The combined effect of decreasing cost of fuel or electrical energy forlight-sources and of the great improvements in light-production gave tothe householder, for example, a constantly increasing amount of lightfor the same expenditure. For example, the family which a century agospent two or three hours in the light of a single candle now enjoys manytimes more light in the same room for the same price. It is interestingto trace the influence of this greatly diminishing cost of light in thehome. For the sake of simplicity the light of a candle will be retainedas the unit and the cost of light for the home will be considered toremain approximately the same throughout the period to be considered. Infact, the amount of money that an average householder spends forlighting has remained fairly constant throughout the past century, buthe has enjoyed a longer period of artificial light and a greater amountof light as the years advanced. The following is a table of approximatevalues which shows the lighting obtainable for $20. 00 per yearthroughout the past century exclusive of electricity:

 Hours Equivalent of Candle-hours Year per night light in candles per night per year 1800 3 5 15 5, 500 1850 3 8 24 8, 700 1860 3 11 33 12, 000 1870 3 22 66 24, 000 1880 3. 5 36 126 46, 000 1890 4 50 200 73, 000 1900 5 154 770 280, 000

It is seen from the foregoing that in a century the candle-equivalentobtainable for the same cost to the householder increased at leastthirty times, while the hours during which this light is used havenearly doubled. In other words, in the nineteenth century thecandle-hours obtainable for $20. 00 per year increased about fiftytimes. Stated in another manner, the cost of light at the end of thecentury was about one fiftieth that of candle light at the beginning ofthe century. One authority in computing the expense of lighting to thehouseholder in a large city of this country has stated that

 coincident with an increase of 1700 per cent. In the amount of night lighting of an American family, in average circumstances, using gas for light, there has come a reduction in the cost of the year's lighting of 34 per cent. Or approximately $7. 50 per year; and that the cost of lighting per unit of light--the candle-hour--is now but 2. 8 per cent. Of what it was in the first half of the nineteenth century. No other necessity of household use has been so cheapened and improved during the last century. 

In general, the light-user has taken advantage of the decrease byincreasing the amount of light used and the period during which it isused. In this manner the greatly diminished cost of light has been amarked sociological and economic influence. 

After Murdock made his first installation of gas-lighting in anindustrial plant early in the nineteenth century, he published acomparison of the expense of operation with that of candle-lighting. Hearrived at the costs of light equivalent to 1000 candle-hours asfollows:

 1000 candle-hours Gas-lighting at a rate of two hours per day $1. 95 " " " " " three " " " 1. 40 Candle-lighting 6. 50

It is seen that the longer hours of burning reduce the cost ofgas-lighting by reducing the percentage of overhead charges. There areno such factors in lighting by candles because the whole "installation"is consumed. This is an early example of which an authentic record isavailable. At the present time a certain amount of light obtained for$1. 00 with efficient tungsten filament lamps, costs $2. 00 if obtainedfrom kerosene flames and about $50. 00 if obtained by burning candles. 

In order to obtain the cost of an equivalent amount of light throughoutthe past century a great many factors must be considered. Obviously, theresults obtained by various persons will differ owing to the unavoidablefactor of judgment; however, the following list of approximate valueswill at least indicate the trend of the price of light throughout thecentury or more of rapid developments in light-production. A fairaverage of the retail values of fuels and of electrical energy and anaverage luminous efficiency of the light-sources involved have been usedin making the computations. The figures apply particularly to thiscountry. 

TABLE SHOWING THE APPROXIMATE TOTAL COST OF 1000 CANDLE-HOURS FORVARIOUS PERIODS

 Per 1000 candle-hours 1800 to 1850, sperm-oil $2. 40 tallow candle 5. 00 1850 to 1865, kerosene 1. 65 tallow candle 6. 85 1865 to 1875, kerosene . 75 tallow candle 6. 25 gas, open-flame . 90 1875 to 1885, kerosene . 25 gas, open-flame . 60 1885 to 1895, kerosene . 15 gas, open-flame . 40 1895 to 1915, gas mantle . 07 carbon filament . 38 metallized filament . 28 tungsten filament (vacuum) . 12 tungsten filament (gas-filled) . 07

In these days the cost of living has claimed considerable attention andit is interesting to compare that of lighting. In the following tablethe price of food and of electric lighting are compared for twenty yearspreceding the recent war. The great disturbance due to the war isthereby eliminated from consideration, but it should be noted that since1914 the price of food has greatly increased but that of electriclighting has not changed materially. The cost of each commodity is takenas one hundred units for the year 1894 but, of course, the actual costof living for the householder is perhaps a hundred times greater thanthe cost of electric lighting. 

 Year Food Electric lighting 1894 100 100 1896 80 92 1898 92 90 1900 100 85 1902 113 77 1904 110 77 1906 115 57 1908 128 30 1910 138 28 1912 144 23 1914 145 17

One feature of electric lighting which puzzles the consumer and whichgives the politicians an opportunity for crying "discrimination" and"injustice" at the public-service company is the great variation inrates. There is no discrimination or injustice when the householder, forexample, must pay more for his lighting than a factory pays. The ratesare not only affected by "demand" but by the period in which the demandcomes. Residence lighting is chiefly confined to certain hours from 5 to9 P. M. And there is a great "peak" of demand at this time. Thecentral-stations must have equipment available for this short-timedemand and much of the capacity of the equipment is unused during theremainder of the day. The factory which uses electricity throughout theday or night or both is helping to keep the central-station operatingefficiently. The equipment necessary to supply electricity to thefactory is operating long hours. Not only is this overhead charge muchless for factories and many other consumers than for the householder, but the expense of accounting, of reading meters, etc. , is about thesame for all classes of consumers. Therefore, this is an appreciableitem on the bill of the small consumer. 

Doubtless, the public does not realize that the enormous decrease in thecost of lighting during the past century is due largely to the fact thatthe lighting industry has grown large. Increased production isresponsible for some of this decrease and science for much of it. Thelatter, having been called to the aid of the manufacturers, who arebetter able by virtue of their magnitude to spend time and resourcesupon scientific developments, has responded with many improvements whichhave increased the efficiency of light-production. Some figures of theCensus Bureau may be of interest. These are given for 1914 in order thatthe abnormal conditions due to the recent war may be avoided. Thefigures pertaining to the manufacture of gas for sale which do notinclude private plants are as follows for the year 1914 for thiscountry:

 Number of establishments 1, 284 Capital $1, 252, 421, 584 Value of products (gas, coke, tar, etc. ) $220, 237, 790 Cost of materials $76, 779, 288 Value added by manufacture $143, 458, 502 Value of gas $175, 065, 920 Coal used (tons) 6, 116, 672 Coke used (tons) 964, 851 Oil used (gallons) 715, 418, 623 Length of gas mains (miles) 58, 727 Manufactured products sold Total gas (cubic feet) 203, 639, 260, 000 Straight coal gas (cubic feet) 10, 509, 946, 000 Carbureted water gas (cubic feet) 90, 017, 725, 000 Mixed coal- and water-gas (cubic feet) 86, 281, 339, 000 Oil gas (cubic feet) 16, 512, 274, 000 Acetylene (cubic feet) 136, 564, 000 Other gas, chiefly gasolene (cubic feet) 181, 412, 000 Coke (bushels) 114, 091, 753 Tar (gallons) 125, 938, 607 Ammonia liquors (gallons) 50, 737, 762 Ammonia, sulphate (pounds) 6, 216, 618

Of course, only a small fraction of the total gas manufactured is usedfor lighting. 

According to the U. S. Geological Survey, the quantities of gas sold inthis country in the year 1917 were as follows:

 Coal-gas 42, 927, 728, 000 cubic feet Water-gas 153, 457, 318, 000 " " Oil-gas 14, 739, 508, 000 " " Byproduct gas 131, 026, 575, 000 " " Natural gas 795, 110, 376, 000 " "

In 1914 there were 38, 705, 496 barrels (each fifty gallons) ofilluminating oils refined in this country and the value was $96, 806, 452. About half of this quantity was exported. In 1914 the value of allcandles manufactured in this country was about $2, 000, 000, which wasabout half that of the candles manufactured in 1909 and in 1904. In 1914the value of the matches manufactured in this country was $12, 556, 000. This has increased steadily from $429, 000 in 1849. In 1914 the glassindustries in this country made 7, 000, 000 lamps, 70, 000, 000 chimneys, 16, 300, 000 lantern globes, 24, 000, 000 shades, globes, and other gasgoods. Many millions of other lighting accessories were made, butunfortunately they are not classified. 

Some figures pertaining to public electric light and power stations ofthe United States for the years 1907 and 1917 are as follows:

 1917 1907 Number of establishments 6, 541 4, 714 Commercial 4, 224 3, 462 Municipal 2, 317 1, 562 Income $526, 886, 408 $175, 642, 338 Total horse-power of plants 12, 857, 998 4, 098, 188 Steam engines 8, 389, 389 2, 693, 273 Internal combustion engines 217, 186 55, 828 Water-wheels 4, 251, 423 1, 349, 087 Kilowatt capacity of generators 9, 001, 872 2, 709, 225 Output in millions of kilowatt-hours 25, 438 5, 863 Motors served (horse-power) 9, 216, 323 1, 649, 026 Electric-arc street-lamps served 256, 838 . .. . Electric-filament street-lamps served 1, 389, 382 . .. . 

In general, there is a large increase in the various items during thedecade represented. The output of the central stations doubled in thefive years from 1907 to 1912, and doubled again in the next five yearsfrom 1912 to 1917. Street lamps were not reported in 1907, but in 1912there were 348, 643 arc-lamps served by the public companies. The numberof arc-lamps decreased to 256, 838 in 1917. On the other hand, there were681, 957 electric filament street lamps served in 1912, which doubled innumber to 1, 389, 382 in 1917. The cost of construction and equipment ofthese central stations totaled more than $3, 000, 000, 000 in 1917. 

Although there is no immediate prospect of the failure of the coal andoil supplies, exhaustion is surely approaching. And as the supplies offuel for the production of gas and electricity diminish, the cost oflighting may advance. The total amount of oil available in the knownoil-fields of this country at the present time has been estimated byvarious experts between 5, 000, 000, 000 and 20, 000, 000, 000 barrels, thebest estimate being about 7, 000, 000, 000. The annual consumption is nowabout 400, 000, 000 barrels. These figures do not take into account theoil which may be distilled from the rich shale deposits. Apparently thissource will yield a hundred billion barrels of oil. In a similar mannerthe coal-supply is diminishing and the consumption is increasing. In1918 more than a half-billion tons of coal were shipped from the mines. The production of natural gas perhaps has reached its peak, and, owingto its relation to the coal and oil deposits, its supply is limited. 

Although only a fraction of the total production of gas, oil, and coalis used in lighting, the limited supply of these products emphasizes thedesirability of developing the enormous water-power resources of thiscountry. The present generation will not be hard pressed by thediminution of the supply of gas, oil, and coal, but it can profit byencouraging and even demanding the development of water-power. Furthermore, it is an obligation to succeeding generations to harnessthe rivers and even the tides and waves in order that the otherresources will be conserved as long as possible. Science will continueto produce more efficient light-sources, but the cost of light finallyis dependent upon the cost of the energy supplied to these lamps. At thepresent time water-power is the anchor to the windward. 

XVII

LIGHT AND SAFETY

It is established that outdoors life and property are at night saferunder adequate lighting than they are under inadequate lighting. Policedepartments in the large cities will testify that street-lighting is apowerful ally and that crime is fostered by darkness. But in reckoningthe cost of street-lighting to-day how many take into account the valueof safety to life and property and the saving occasioned by thereduction in the police-force necessary to patrol the cities and towns?Owing to the necessity of darkening the streets in order to reduce thehazards of air-raids, London experienced a great increase in accidentson the streets, which demonstrated the practical value ofstreet-lighting from the standpoint of accident prevention. 

During the war, when dastardly traitors and agents of the enemy werestriking at industry, the value of lighting was further recognized bythe industries, with the result that flood-lighting was installed toprotect them. By common consent this new phase was termed "protectivelighting. " Soon after the entrance of this country into the recent war, the U. S. Military Intelligence established a Section of Plant Protectionwhich had thirty-three district offices during the war and gaveattention to thirty-five thousand industrial plants engaged inproduction of war materials. Protective lighting was early recognized bythis section as a very potential agency for defense, and extensive usewas made of it. For example, Edmund Leigh, chief of the section, indiscussing the value of outdoor lighting stated:

 An illustration of our work in this connection is the case of an $80, 000, 000 powder plant of recent construction. We arranged to have all wires buried. In addition to the ordinary lighting on an adjacent hill there is a large searchlight which will command any part of the buildings and grounds. Every three hundred yards there is a watch-tower with a searchlight on top. These searchlights are for use only in emergency. Each tower has a telephone service, one connected with the other. The men in the towers have a view of the building exteriors, which are all well lighted, and the men in the buildings look across the yard to the lighted fence line and so get a silhouette of persons or objects in between. The most vital parts of the buildings are surrounded by three fences. In the near-by woods the underbrush has been cleared out and destroyed. The trunks and limbs of trees have been whitewashed. No one can walk among these trees or between the trees and the plant without being seen in silhouette. .. . I say flatly that I know nothing that is so potential for good defense as good illumination and at the same time so little understood. 

Without such protective lighting an army of men would have been requiredto insure the safety of this one vital plant; still it is obvious thatthe cost of the protective lighting was an insignificant part of thevalue of the plant which it insured against damage and destruction. 

The United States participated for nineteen months in the recent war andduring that time about 400, 000 casualties were suffered by its forces. This was at the rate of about 250, 000 per year, which includedcasualties in battle, at sea, and from sickness, wounds, and accidents. Every one has felt the magnitude of this rate of casualties becauseeither his home or that of a friend was blighted by one or more of thesetragedies in the nineteen months. However, R. E. Simpson of the TravelersInsurance Company has stated that:

 During a one-year period in this country the number of accidents due to inadequate or improper lighting exceeds the yearly rate of our war casualties. 

This is a startling comparison, which emphasizes a phase of lightingthat has long been recognized by experts but has been generally ignoredby the industries and by the public. The condition doubtless is duelargely to a lag in the proper utilization of artificial lighting behindthe rapid increase in congestion in the industries and in public places. 

Accident prevention is an important phase of modern life which mustreceive more attention. From published statistics and conservativeestimates it has been concluded that there are approximately 25, 000persons killed or permanently disabled, 500, 000 seriously injured, and1, 000, 000 slightly injured each year in this country. Translating thesefigures by means of the accident severity rates, Mr. Simpson has foundthat there is a total of 180, 000, 000 days of time lost per year. This isequivalent to the loss of services of 600, 000 men for a full year of 300work-days. This loss is distributed over the entire country andconsequently its magnitude is not demonstrated excepting by statistics. Of course, the causes of the accidents are numerous, but, among themeans of prevention, proper lighting is important. 

According to some authorities at least 18 per cent. Of these accidentsare due to defects in lighting. On this basis the services of 108, 000men as producers and wage-earners are continually lost at the presenttime because the lighting is not sufficient or proper for the safety ofworkers. If the full year's labor of 108, 000 men could be applied to themining of coal, 130, 000, 000 million tons of coal would be added to theyearly output; and only 10, 000 tons would be necessary to supplyadequate lighting for this army of men working for a full year for tenhours each day. 

Statistics obtained under the British workmen's compensation system showthat 25 per cent. Of the accidents were caused by inadequate lighting ofindustrial plants. 

Much has been said and actually done regarding the saving of fuel bycurtailing lighting, but the saving may easily be converted into a greatloss. For example, a 25-watt electric lamp may be operated ten hours aday for a whole year at the expense of one eighth of a ton of coal. Suppose this lamp to be over a stairway or at any vital point and thatby extinguishing it there occurs a single accident which involves theloss of only one day's work on the part of the worker. If this one day'stime could have produced coal, there would have been enough coal minedin the ten hours to operate the lamp for thirty-two years. Theinsignificant cost of lighting is also shown by the distribution of theconsumption of fuel for heating, cooking, and lighting in the home. Ofthe total amount of fuel consumed in the home for these purposes, 87 percent. Is for heating, 11 per cent. For cooking and 2 per cent. Forlighting. The amount of coal used for lighting purposes in general isabout 2. 5 per cent. Of the total consumption of coal, so it is seen thatthe curtailment of lighting at best cannot save much fuel; and it mayactually result in a great economic loss. By replacing inefficient lampsand accessories with efficient lighting-equipment and by washing windowsand artificial lighting devices, a real saving can be realized. 

Improper lighting may be as productive of accidents as inadequatelighting, and throughout the industries and upon the streets the misuseof light is in evidence. The blinding effect of a brilliant light-sourceis easily proved by looking at the sun. After a few moments greatdiscomfort is experienced, and on looking away from this brilliantsource the eyes are temporarily blinded by the after-images. When thishappens in a factory as the result of gazing into an unshieldedlight-source, the workman may be injured by moving machinery, bystumbling over objects, and in many other ways. Unshaded light-sourcesare too prevalent in the industries. Improper lighting is likely tocause deep shadows wherein many dangers may be hidden. On the street theglare from automobile head-lamps is very prevalent and nearly everybodymay testify from experience to the dangers of glare. Even the glaringlocomotive head-lamp has been responsible for many casualties. 

Unfortunately, natural lighting outdoors has not been under the controlof man and he has accepted it as it is. The sky is a harmless source oflight when viewed outdoors and the sun is in such a position that it isusually easy to avoid looking at it. It is so intensely glaring that manunconsciously avoids looking directly at it. These conditions areresponsible to an extent for man's indifference and even ignorance ofthe rudiments of safe lighting. When he has artificial light, over whichhe may exercise control, he either ignores it or owing to the lessstriking glare he misuses it and his eyesight without realizing it. Agreat deal of eye-strain and permanent eye trouble arises from the abuseof the eyes by improper lighting. For example, near-sightedness is oftendue to inadequate illumination, which makes it necessary for the eyes tobe near the work or the reading-page. Improper or inadequate lightingespecially influences eyes that are immature in growth and in function, and it has been shown that with improvements in lighting the percentageof short-sightedness has decreased in the schools. Furthermore, it hasbeen shown that where no particular attention has been given to lightingand vision, the percentage of short-sightedness has increased with thegrade. There are twenty million school children in this country whosefuture eyesight is in the hands of those who have jurisdiction overlighting and vision. There are more than a hundred million persons inthis country whose eyes are daily subjected to improperlighting-conditions, either through their own indifference or throughthe negligence of others. 

Of a certain group of 91, 000 purely industrial accidents in the year1910, Mr. Simpson has stated that 23. 8 per cent. Were due, directly orindirectly, to the lack of proper illumination. These may be furtherdivided into two approximately equal groups, one of which comprises theaccidents due to inadequate illumination and the other to those towardwhich improper lighting was a contributing cause. The seasonal variationof these accidents is given in the following table, both for those duedirectly or indirectly to inadequate and improper lighting and those dueto other causes. 

SEASONAL DISTRIBUTION OF INDUSTRIAL ACCIDENTS DUE TO LIGHTINGCONDITIONS AND TO OTHER CAUSES

 Percentage due to Lighting conditions Other causes

 July 4. 8 5. 9 August 5. 2 6. 2 September 6. 1 6. 9 October 8. 6 8. 5 November 10. 9 10. 5 December 15. 6 12. 2 January 16. 1 11. 9 February 10. 0 10. 5 March 7. 6 8. 8 April 6. 1 6. 9 May 5. 2 5. 8 June 3. 8 5. 9

The figures in one column have no direct relation to those in the other;that is, each column must be considered by itself. It is seen from theforegoing that about half the number of the accidents due to poorillumination occurred in the months of November, December, January, andFebruary. These are the months of inadequate illumination unlessartificial lighting has been given special attention. The same generaltype of seasonal distribution of accidents due to other causes is seento exist but not so prominently. The greatest monthly rate of accidentsduring the winter season is nearly four times the minimum monthly rateduring the summer for those accidents due to lighting conditions. Thisratio reduces to about twice in the case of accidents due to othercauses. Looking at the data from another angle, it may be consideredthat the likelihood of an accident being caused by lighting conditionsis about twice as great in any of the four "winter" months as in any ofthe remaining eight months. Doubtless, this may be explained largelyupon the basis of morale. The winter months are more dreary than thoseof summer and the workman's general outlook is different in winter thanin summer. In the former season he goes back and forth to work in thedark, or at best, in the cold twilight. He is not only more depressedbut he is clumsier in his heavier clothing. If the enervating influenceof these factors is combined with a greater clumsiness due to cold andperhaps to colds, it is not difficult to account for this type ofseasonal distribution of accidents. A study of the accidents of 1917indicated that 13 per cent. Occurred between 5 and 6 P. M. Whenartificial lighting is generally in use to help out the failingdaylight. Only 7. 3 per cent. Occurred between 12 M. And 1 P. M. 

[Illustration: SIGNAL-LIGHT FOR AIRPLANE]

[Illustration: TRENCH LIGHT-SIGNALING OUTFIT]

[Illustration: AVIATION FIELD LIGHT-SIGNAL PROJECTOR]

[Illustration: SIGNAL SEARCH-LIGHT FOR AIRPLANE]

[Illustration: UNSAFE, UNPRODUCTIVE LIGHTING WORTHY OF THE DARK AGES]

[Illustration: THE SAME FACTORY MADE SAFE, CHEERFUL, AND MORE PRODUCTIVEBY MODERN LIGHTING]

There is another aspect of the subject which deals particularly with thesafety of the light-source or method of lighting. As each innovationin lighting appeared during the past century there immediately arose thequestion of safety. The fire-hazard of open flames received attention inearly days, and when gas-lighting appeared it was condemned as a poisonand an explosive. Mineral-oil lamps introduced the danger of explosionsof the vapors produced by evaporation. When electric lighting appearedit was investigated thoroughly. The result of all this has been aneffort to make lamps and methods safe. Insurance companies have therelative safety of these systems established to their satisfaction andto-day little fire-hazard is attached to the present modes of generallighting if proper precautions have been taken. 

When electric lighting was first introduced the public looked uponelectricity as dangerous and naturally many questions pertaining tohazards arose. The distribution of electricity has been so highlyperfected that little is heard of the hazards which were so magnified inthe early years. Data gathered between 1884 and 1889 showed that about13, 000 fires took place in a certain district. Of these, 42 wereattributed to electric wires; 22 times as many to breakage and explosionof kerosene lamps; and ten times as many through carelessness withmatches. These figures cannot be taken at their face value because ofthe absence of data showing the relative amount of electric and kerosenelighting; nevertheless they are interesting because they represent theearly period. 

There are industries where unusual care must be exercised in regard tothe lighting. In certain chemical industries no lamps are used exceptingthe incandescent lamp and this is enclosed in an air-tight glass globe. Even a public-service gas company cautions its employees and patronsthus: "_Do not look for a gas-leak with a naked light! Use electriclight. _" The coal-mine offers an interesting example of the precautionsnecessary because the same type of problems are found in it as inindustries in general, with the additional difficulties attending thepresence or possible presence of explosive gas. The surroundings in acoal-mine reflect a small percentage of the light, so that much light iswasted unless the walls are whitewashed. This is a practical method forincreasing safety in coal-mines. However, the most dangerous feature isthe light-source itself. According to the Bureau of Mines during theyears 1916 and 1917 about 60 per cent. Of the fatalities due to gas andcoal-dust explosions were directly traceable to the use of defectivesafety lamps and to open flames. 

In the early days of coal-mining it was found that the flame of a candleoccasionally caused explosions in the mines. It was also found thatsparks of flint and steel would not readily ignite the gas or coal-dustand this primitive device was used as a light-source. Of course, statistics are unavailable concerning the casualties in coal-minesthroughout the past centuries, but with the accidents not uncommon inthis scientific age, with its elaborate organizations striving to stampout such casualties, there is good reason to believe that previous to acentury or two ago the risks of coal-mining must have been great. Openflames have been widely used in this industry, but there has always beenthe risk of the presence or the appearance of gas or explosive dust. 

The early open-flame lamps not only were sources of danger but theirfeeble varying intensity caused serious damage to the eyesight ofminers. This factor is always present in inadequate and improperlighting, but its influence is noticeable in coal-mining in the nervousdisease affecting the eyes which is known as nystagmus. The symptoms ofthe disease are inability to see at night and the dazzling effect ofordinary lamps. Finally objects appear to the sufferer to dance aboutand his vision is generally very much disturbed. 

The oil-lamps used in coal-mining have a luminous intensity equivalentto about one to four candles, but owing to the atmospheric conditions inthe mines a flame does not burn as brightly as in the fresh air. Thepossibility of explosion due to the open flame was eliminated bysurrounding it with a metal gauze. Davy was the inventor of this deviceand his safety lamp introduced about a hundred years ago has been a boonto the coal-miner. Various improvements have been devised, but Davy'slamp contained the essentials of a safety device. The flame issurrounded by a cylinder of metal gauze which by forming a much coolerboundary prevents the mine-gas from becoming heated locally by the lampflame to a sufficient temperature to ignite and consequently to explode. This device not only keeps the flame from igniting the gas but it alsoserves as an indicator of the amount of gas present, by the variation inthe size and appearance of the tip of the flame. However, the gauzereduces the luminous output, and as it accumulates soot and dust thelight is greatly diminished. One of these lamps is about as luminous asa candle, initially, but its intensity is often reduced by accumulationsupon the gauze to only one fifth of the initial value. 

The acetylene lamp is the best open-flame light-source available to theminer, for several reasons. It is of a higher candle-power than theothers and as it is a burning gas, there is not the danger of flyingsparks as in the case of burning wicks. The greater intensity ofillumination affords a greater safety to the miner by enabling him todetect loose rock which may be ready to fall upon him. However, thislamp may be a source of danger, owing to the fact that it will burn morebrilliantly in a vitiated atmosphere than other flame-lamps. Anotherdisadvantage is the possibility of calcium carbide accidentally spiltcoming in contact with water and thereby causing the generation ofacetylene gas. If this is produced in the mine in sufficient quantitiesit is a danger which may not be suspected. If ignited it will explodeand may also cause severe burns. 

The electric lamp, being an enclosed light-source capable of beingsubdivided and fed by a small portable battery, early gave promise ofsolving the problem of a safe mine-lamp of adequate candle-power. Muchingenuity has been applied to the development of a portable electricsafety mine-lamp, and several such lamps are now approved by the Bureauof Mines. Two general types are being manufactured, the cap outfit andthe hand outfit. They consist essentially of a lamp in a reflector whoseaperture is closed with a sheet or a lens of clear glass. The batterymay be of the "dry" or "storage" type and in the case of the cap outfitthe battery is carried on the back. The specifications for these lampsdemand that a luminous intensity averaging at least 0. 4 candle bemaintained throughout twelve consecutive hours of operation. At no timeduring this period shall the output of light fall below 1. 25 lumens fora cap-lamp and below 3 lumens for a hand-lamp. Inasmuch as these areequipped with reflectors, the specifications insist that a circle oflight at least seven feet in diameter shall be cast on a wall twentyinches away. It appears that a portable lamp is an economic necessity inthe coal-mines, on account of the expense, inconvenience, and possibledangers introduced by distribution systems such as are used in mostplaces. 

Although the major defects in lighting are due to absence of light indangerous places, to glare, and to other factors of improper lighting, there are many minor details which may contribute to safety. Forexample, low lamps are useful in making steps in theaters and in otherplaces, in drawing attention to entrances of elevators, in lighting theaisles of Pullman cars, under hand-rails on stairways, and in many othervital places. A study of accidents indicates that simple expedients areeffective preventives. 

XVIII

THE COST OF LIVING

A comparison of the civilization of the present with that of a centuryago reveals a startling difference in the standards of living. To-daymankind enjoys conveniences and luxuries that were undreamed of by thepast generations. For example, a certain town in Iowa, a score of yearsago, was appraised for a bond-issue and it was necessary to extend itslimits considerably in order to include a valuation of one half milliondollars required by the underwriters. On a summer's evening at thepresent time a thousand "pleasure" automobiles may be found parked alongits streets and these exceed in valuation that of the entire town onlytwenty years ago and equal it to-day. There are economists who wouldargue that the automobile has paid for itself by its usefulness, but thefact still exists that a great amount of labor has been diverted fromproducing food, clothing, and fuel to the production of "pleasure"automobiles. And this is the case with many other conveniences andluxuries. It is admitted that mankind deserves these refinements ofmodern civilization, but he must expect the cost of living to increaseunless counteracting measures are taken. 

The economics of the increasing cost of living and the analysis of therelations of necessities, conveniences, and luxuries are too complex tobe thoroughly discussed here. In fact, the most expert economists woulddisagree on many points. However, it is certain that the cost of livinghas steadily increased during the past century and it is reasonablycertain that the standards of the present civilization are responsiblefor some if not all of the increase. Increased production is an anchorto the windward. It may drag and give way to some extent, but it willalways oppose the course of the cost of living. 

When the first industrial plant was lighted by gas, early in thenineteenth century, the aim was merely to reinforce daylight toward theend of the day. Continuous operation of industrial plants was notpractised in those days, excepting in a very few cases where it wasessential. To-day some industries operate continuously, but most of themdo not. In the latter case the consumer pays more for the productbecause the percentage of fixed or overhead charge is greater. Investment in ground, buildings, and equipment exacts its tollcontinuously and it is obvious that three successive shifts producingthree times as much as a single day shift, or as much as a trebled dayshift, will produce the less costly product. In the former case thefixed charge is distributed over the production of continuous operation, but in the latter case the production of a single day shift assumes theentire burden. Of course, there are many factors which enter into such aconsideration and an important one is the desirability of working atnight. It is not the intention to touch upon the psychological andsociological aspects but merely to look coldly upon the facts pertainingto artificial light and production. 

In the first place, it has been proved that in factories proper lightingas obtained by artificial means is generally more satisfactory than thenatural lighting. Of course, a narrow building with windows on two sidesor a one-story building with a saw-tooth roof of best design may beadequately illuminated by natural light, but these buildings are theexception and they will grow rarer as industrial districts become morecongested. Artificial light may be controlled so that light of asatisfactory quality is properly directed and diffused. Sufficientintensities of illumination may be obtained and the failure ofartificial light is a remote possibility as compared with the dailyfailure of natural light. With increasing cost of ground space, factories are built of several stories and with less space given tolight courts, with the result that the ratio of window area to that ofthe floor is reduced. These tendencies militate against satisfactorydaylighting. In the smoky congested industrial districts the period ofeffective daylight is gradually diminishing and artificial lighting isalways essential at least as a reinforcement for daylight. It has beenproved that proper artificial lighting--and there is no excuse forimproper artificial lighting--is superior to most interior daylightingconditions. 

[Illustration: LOCOMOTIVE ELECTRIC HEADLIGHT]

[Illustration: SEARCH-LIGHT ON A FIRE-BOAT]

[Illustration: BUILDING SHIPS UNDER ARTIFICIAL LIGHT AT HOG ISLANDSHIPYARD]

Although it is difficult to present figures in a brief discussion ofthis character, it may be stated that, in general, the cost of adequateartificial light is about 2 per cent. Of the pay-roll of the workers;about 10 per cent. Of the rental charges; and only a fraction of 1 percent. Of the cost of the manufactured products. These figures varyconsiderably, but they represent conservative average estimates. Fromthese it is seen that artificial lighting is a small factor in adding tothe cost of the product. But does artificial lighting add to the cost ofa product? Many examples could be cited to prove that proper artificiallighting may be responsible for an actual reduction in the cost of theproduct. 

In a certain plant it was determined that the workmen each lost anappreciable part of an hour per day because of inadequate lighting. Aproperly designed and maintained lighting-system was installed and thesaving in the wages previously lost, more than covered theoperating-expense of the artificial lighting. Besides really costing themanufacturer less than nothing, the new artificial lighting system wasresponsible for better products, decreased spoilage, minimizedaccidents, and generally elevated spirits of the workmen. In some casesit is only necessary to save one minute per hour per workman to offsetentirely the cost of lighting. The foregoing and many other examplesillustrate the insignificance of the cost of lighting. 

The effectiveness of artificial lighting in reducing the cost of livingis easily demonstrated by comparing the output of a factory operating onone and two shifts per day respectively. In a well-lighted factory whichoperated day and night shifts, the cost of adequate lighting was 7 centsper square foot per year. If this factory, operating only in thedaytime, were to maintain the same output, it would be necessary todouble its size. In order to show the economic value of artificiallighting it is only necessary to compare the cost of lighting with therental charge of the addition and of its equipment. A fair rental valuefor plant and equipment is 50 cents per square foot per year; but ofcourse this varies considerably, depending upon the type of plant andthe character of the equipment. An investigation showed that this valuevaries usually between 30 to 70 cents per square foot per year. Usingthe mean value, 50 cents, it is seen that the rental charge is aboutseven times the cost of lighting. Furthermore, there is a saving of 43cents per square foot per year during the night operation by operatingthe night shift. Of course, this is not strictly true because adepreciation of machinery during the night shift should be allowed for. These fixed charges would average slightly more than half as much in thecase of the two-shift factory as in the case of the same output from afactory twice as large but operating only a day shift. Incidentally, thetwo-shift factory need not be a hardship for the workers, for, if theeight-hour shifts are properly arranged, the worker on the night shiftmay be in bed by midnight and the objection to a disturbance of ordinaryhours of sleep is virtually eliminated. 

In a discussion of light and safety presented in another chapter thestartling industrial losses due to accidents are shown to be duepartially to inadequate or improper lighting. About one fourth of thetotal number of accidents may be charged to defective lighting. Theconsumer bears the burden of the support of an unproducing army of idlemen. According to some experts an average of about 150, 000 men arecontinuously idle in this country owing to inadequate and improperlighting. 

This is an appreciable factor in the cost of living, but the greatesteffectiveness of artificial lighting in curtailing costs is to be foundin reducing the fixed charges borne by the product through the operationof two shifts and by directly increasing production owing to improvedlighting. The standard of artificial-lighting intensity possessed by theaverage person at the present time is an inheritance from the past. Inthose days when artificial light was much more costly than at presentthe tendency naturally was to use just as little light as necessary. That attitude could not have been severely criticized in those earlydays of artificial lighting, but it is inexcusable to-day. Eyesight andgreater safety from accidents are in themselves valuable enough towarrant adequate lighting, but besides these there is the appeal ofincreased production. 

Outdoors on a clear summer day at noon the intensity of daylightillumination at the earth's surface is about 10, 000 foot-candles; inother words, it is equal to the illumination on a surface produced by alight-source equivalent to 10, 000 candles at a distance of one foot fromthe surface. This will be recognized as an enormous intensity ofillumination. On a cloudy day the intensity of illumination at theearth's surface may be as high as 3000 foot-candles and on a "gloomy"day the illumination at the earth's surface may be 1000 foot-candles. When it is considered that mankind works under artificial light with anintensity of only a few foot-candles, the marvels of the visualapparatus are apparent. But it should be noted that the eyes of thehuman race evolved under natural light. They have been used to greatintensities when called upon for their greatest efforts. The human beingis wonderfully adaptive, but it could scarcely be hoped that the eyescould readjust themselves in a few generations to the changed conditionsof low-intensity artificial lighting. There is no complaint against therange of intensities to which the eye responds, for in range ofsensibility it is superior to any man-made device. 

For extremely low brightnesses another set of physiological processescome into play. Based purely upon the physiological laws of vision itseems reasonable to conclude that mankind should not work underartificial illumination as low as has been considered necessary owing tothe cost in the past. With this principle of vision as a foundation, experiments have been made with greater intensities of illumination inthe industries and elsewhere and increased production has been theresult. In a test in a factory where an adequate record of productionwas in effect it was found that an increase in the intensity ofillumination from 4 to 12 foot-candles increased the production invarious operations. The lowest increase in production was 8 per cent. , the highest was 27 per cent. , and the average was 15 per cent. Theoriginal lighting in this case was better than that of the typicalindustrial conditions, so that it seems reasonable to expect a greaterincrease in production when a change is made from the average inadequatelighting of a factory to a well-designed lighting-system giving a highintensity of illumination. 

In another test the production under a poor system of lighting by meansof bare lamps on drop-cords was compared with that of an excellentsystem in which well-designed reflectors were used. The intensity ofillumination in the latter case was twenty-five times that of the formerand the production was increased in various operations from 30 per cent. For the least increase to 100 per cent. For the greatest increase. Inasmuch as the energy consumption in the latter case was increasedseven times and the illumination twenty-five times, it is seen that theincrease in intensity of illumination was due largely to the use ofproper reflectors and to the general layout of the new lighting-system. 

In another case a 10 per cent. Increase in production was obtained byincreasing the intensity of illumination from 3 foot-candles to about 12foot-candles. This increase of four times in the intensity ofillumination involved an increase in consumption of electrical energy ofthree times the original amount at an increase in cost equal to 1. 2 percent. Of the pay-roll. In another test an increase of 10 per cent. Inproduction was obtained at an increase in cost equal to less than 1 percent. Of the payroll. The efficiency of well-designed lightinginstallations is illustrated in this case, for the illuminationintensity was increased six times by doubling the consumption ofelectrical energy. 

Various other tests could be cited, but these would merely emphasize thesame results. However, it may be stated that the factorysuperintendents involved are convinced that adequate and properartificial lighting is a great factor in increasing production. Mr. W. A. Durgin, who conducted the tests, has stated that the average result ofincreasing the intensity of illumination and of properly designing thelighting installations in factories will be at least a 15 per cent. Increase in production at an increased cost of not more than 5 per cent. Of the pay-roll. This is apparently a conservative statement. When it isconsidered that generally the cost of lighting is only a fraction of 1per cent. Of the cost of products to the consumer, it is seen that theadditional cost of obtaining an increase of 15 per cent. In productionis inappreciable. 

Industrial superintendents are just beginning to see the advantage ofadequate artificial lighting, but the low standards of lighting whichwere inaugurated when artificial light was much more costly than it isto-day persist tenaciously. When high intensities of proper illuminationare once tried, they invariably prove successful in the industries. Notonly does the worker see all his operations better, but there appears tobe an enlivening effect upon individuals under the higher intensities ofillumination. Mankind chooses a dimly lighted room in which to rest andto dream. A room intensely lighted by means of well-designed units whichare not glaring is comfortable but not conducive to quiet contemplation. It is a place in which to be active. This is perhaps one of the factorswhich makes for increased production under adequate lighting. 

Civilization has just passed the threshold of the age of adequateartificial lighting and only a small percentage of the industries haveincreased their lighting standards commensurately to the possibilitiesof the present time. If high-intensity artificial lighting was installedin all the industries and a 15 per cent. Increase in productionresulted, as tests appear to indicate, the increased production would beequal to that of nearly two million workers. This great increase inoutput is brought about by lighting at an insignificant increase in costbut without the additional consumption of food or clothing. Besides thisincrease in production there is the decrease in spoilage. The savingpossible in this respect through adequate lighting has been estimatedfor the industries of this country at $100, 000, 000. If mankind is tohave conveniences and luxuries, efficiency in production must bepractised to the utmost and in the foregoing a proved means has beendiscussed. 

There are many other ways in which artificial light may serve inincreasing production. Man has found that eight hours of sleep issufficient to keep him fit for work if he has a sufficient amount ofrecreation. Before the advent of artificial light the activities of theprimitive savage were halted by darkness. This may have been Nature'sintention, but civilized man has adapted himself to the changedconditions brought about by efficient and adequate artificial light. There appears to be no fundamental reason for not imposing an artificialday upon plants, animals, chemical processes, etc. ; and, in fact, experiments are being prosecuted in these directions. 

The hen, when permitted to follow her natural course, rises with thesun and goes to roost at sunset. During the winter months she puts inshort days off the roost. It has been shown that an artificial day, madeby piecing out daylight by means of artificial light, might keep the henscratching and feeding longer, with an increased production of eggs as aresult. Many experiments of this character have been carried out, andthere appears to be a general conclusion that the use of artificiallight for this purpose is profitable. 

Experiments conducted recently by the agricultural department of a largeuniversity indicate that in poultry husbandry, when artificial light isapplied to the right kind of stock with correct methods of feeding, thedistribution of egg-production throughout the whole year can beradically changed. The supply of eggs may be increased in autumn andwinter and decreased in spring and summer. Data on the amount ofillumination have not been published, but it is said that the mostsatisfactory results have been obtained when the artificial illuminationis used from sunset until about 9 P. M. Throughout the year. 

An increase of 30 to 40 per cent. In the number of eggs laid on apoultry-farm in England as the result of installing electric lamps inthe hen-houses was reported in 1913. On this farm there were nearly 200yards of hen-houses containing about 6000 hens, and the runs werelighted on dark mornings and early nights of the year preceding thereport. About 300 small lamps varying from 8 to 32 candle-power wereused in the houses. It was found that an imitation of sunset wasnecessary by switching off the 32 candle-power lamps at 6 P. M. And the 16 candle-power lamps at 9:30. This left only the 8candle-power lamps burning, and in the faint illumination the henssought the roosting-places. At 10 P. M. The remaining lightswere extinguished. It was found that if all the lights were extinguishedsuddenly the fowls went to sleep on the ground and thus became a prey toparasites. The increase in production of eggs is brought about merely bykeeping the fowls awake longer. On the same farm the growth of chicksincubated during the winter months increased by one third through theuse of electric light which kept them feeding longer. 

Many fishermen will testify that artificial light seems to attract fish, and various reports have been circulated regarding the efficacy of usingartificial light for this purpose on a commercial scale. One reportwhich bears the earmarks of authenticity is from Italy, where it is saidthat electric lights were successfully used as "bait" to augment thesupply of fish during the war. The lamps were submerged to aconsiderable depth and the fish were attracted in such large numbersthat the use of artificial light was profitable. The claims made werethat the supply of fish was not only increased by night fishing but thata number of fishermen were thereby released for national service duringthe war. An interesting incident pertaining to fish, but perhaps not animportant factor in production, is the use of electric lights in thesummer over the reservoirs of a fish hatchery. These lights, which hanglow, attract myriads of bugs, many of which fall in the water andfurnish natural and inexpensive food for the fish. 

Many experiments have been carried out in the forcing of plants bymeans of artificial light. Some of these were conducted forty years ago, when artificial light was more costly than at the present time. Ofcourse, it is well known that light is essential to plant life and ingeneral it is reasonable to believe that daylight is the most desirablequality of light for plants. In greenhouses the forcing of plants isdesirable, owing to the restricted area for cultivation. It has beenestablished that some of the ultra-violet rays which are absorbed or nottransmitted by glass are harmful to growing plants. For this reason anarc-lamp designed for forcing purposes should be equipped with a glassglobe. F. W. Rane reported in 1894 upon some experiments with electriccarbon-filament lamps in greenhouses in which satisfactory results wereobtained by using the artificial light several hours each night. Prof. L. H. Bailey also conducted experiments with the arc-lamp and concludedthat there were beneficial results if the light was filtered throughclear glass. Without considering the details of the experiment, we findsome of Rane's conclusions of interest, especially when it is rememberedthat the carbon-filament lamps used at that time were of very lowefficiency compared with the filament lamps at the present time. Some ofhis conclusions were as follows:

 The incandescent electric light has a marked effect upon greenhouse plants. 

 The light appears to be beneficial to some plants grown for foliage, such as lettuce. The lettuce was earlier, weighed more and stood more erect. 

 Flowering plants blossomed earlier and continued to bloom longer under the light. The light influences some plants, such as spinach and endive, to quickly run to seed, which is objectionable in forcing these plants for sale. 

 The stronger the candle-power the more marked the results, other conditions being the same. 

 Most plants tended toward a taller growth under the light. 

 It is doubtful whether the incandescent light can be used in the greenhouse from a practical and economic standpoint on other plants than lettuce and perhaps flowering plants; and at present prices (1894) it is a question if it will pay to employ it even for these. 

 There are many points about the incandescent electric light that appear to make it preferable to the arc light for greenhouse use. 

 Although we have not yet thoroughly established the economy and practicability of the electric light upon plant growth, still I am convinced that there is a future in it. 

These are encouraging conclusions, considering the fact that the cost oflight from incandescent lamps at the present time is only a smallfraction of its cost at that time. 

In an experiment conducted in England in 1913 mercury glass-tube arcswere used in one part of a hothouse and the other part was reserved fora control test. The same kind of seeds were planted in the two parts ofthe hothouse and all conditions were maintained the same, excepting thata mercury-vapor lamp was operated a few hours in the evening in one ofthem. Miss Dudgeon, who conducted the test, was enthusiastic over theresults obtained. Ordinary vegetable seeds and grains germinated ineight to thirteen days in the hothouse in which the artificial light wasused to lengthen the day. In the other, germination took place in fromtwelve to fifty-seven days. In all cases at least several days weresaved in germination and in some cases several weeks. Flowers alsoincreased in foliage, and a 25 per cent. Increase in the crop ofstrawberries was noted. Seedlings produced under the forcing byartificial light needed virtually no hardening before being planted inthe open. Professor Priestley of Bristol University said of this work:

 The light seems to have been extraordinarily efficacious, producing accelerated germination, increased growth, greater depth of color, and more important still, no signs of lanky, unnatural extension of plant usually associated with forcing. Rather the plants exposed to the radiation seem to have grown if anything more sturdy than the control plants. A structural examination of the experimental and control plants carried out by means of the microscope fully confirmed Miss Dudgeon's statements both as to depth of color and greater sturdiness of the treated plants. 

Unfortunately there is much confusion amid the results of experimentspertaining to the effects of different rays, including ultra-violet, visible and infra-red, upon plant growth. If this aspect was thoroughlyestablished, investigations could be outlined to greater advantage andefficient light-sources could be chosen with certainty. There is thediscouraging feature that the average intensity of daylight illuminationfrom sunrise to sunset in the summer-time is several thousandfoot-candles. The cost of obtaining this great intensity by means ofartificial light would be prohibitive. However, the daylightillumination in a greenhouse in winter is very much less than theintensity outdoors in summer. Indeed, this intensity perhaps averagesonly a few hundred foot-candles in winter. There is encouragement inthis fact and there is hope that a little light is relatively much moreeffective than a great amount. Expressed in another manner, it ispossible that a little light is much more effective than no light atall. Experiments with artificial light indicate very generally anincreased growth. 

Recently Hayden and Steinmetz experimented with a plot of ground 5 feetby 9 feet, over which were hung five 500-watt gas-filled tungsten lamps3 feet above the ground and 17 inches apart. The lamps were equippedwith reflectors and the resulting illumination was 700 foot-candles. This is an extremely high intensity of artificial illumination and iscomparable with daylight in greenhouses. The only seeds planted werethose of string beans and two beds were carried through to maturity, onelighted by daylight only and the other by daylight and artificial light, the latter being in operation twenty-fours hours per day. The plantsunder the additional artificial light grew more rapidly than the others, and of the various records kept the gain in time was in all cases about50 per cent. From the standpoint of profitableness the artificiallighting was not justified. However, there are several points to bebrought out before considering this conclusion too seriously. First, itappears unwise to use the artificial light during the day; second, itappears possible that a few hours of artificial light in the eveningwould suffice for considerable forcing; third, it is possible that amuch lower intensity of artificial light might be more effective perlumen than the great intensity used; fourth, it is quite possible thatsome other efficient light-source may be more effective in forcing thegrowth of plants. These and many other factors must be carefullydetermined before judgment can be passed on the efficacy of artificiallight in reducing the cost of living in this direction. Certainly, artificial light has been shown to increase the growth of plants and itappears probable that future generations at least will find itprofitable to use the efficient light-producers of the coming ages inthis manner. 

Many other instances could be cited in which artificial light is veryclosely associated with the cost of living. Overseas shipment of fruitfrom the Canadian Northwest is responsible for a decided innovation infruit-picking. In searching for a cause of rotting during shipment itwas finally concluded that the temperature at the time of picking wasthe controlling factor. As a consequence, daytime was consideredundesirable for picking and an electric company supplied electriclighting for the orchards in order that the picking might be done duringthe cool of night. This change is said to have remedied the situation. Cases of threshing and other agricultural operations being carried on atnight are becoming more numerous. These are just the beginnings ofartificial light in a new field or in a new relation to civilization. Its economic value has been demonstrated in the ordinary fields oflighting and these new applications are merely the initial skirmisheswhich precede the conquest of new territory. The modern illuminants havebeen developed so recently that the new possibilities have not yet beenestablished. However, artificial light is already a factor on the sideof the people in the struggle against the increasing cost of living, andits future in this direction is still more promising. 

XIX

ARTIFICIAL LIGHT AND CHEMISTRY

Some one in an early century was the first to notice that the sun's raystanned the skin, and this unknown individual made the initial discoveryin what is now an extensive branch of science known as photo-chemistry. The fading of dyes, the bleaching of textiles, the darkening of silversalts, the synthesis and decomposition of compounds are common examplesof chemical reactions induced by light. There are thousands of otherexamples of the chemical effects of light some of which have beenutilized by mankind. Others await the development of more efficientlight-sources emitting greater quantities of active rays, and many stillremain interesting scientific facts without any apparent practicalapplications at the present time. Visible and ultra-violet rays are theradiations almost entirely responsible for photochemical reactions, butthe most active of these are the blue, violet, and ultra-violet rays. These are often designated chemical or actinic rays in order todistinguish the group as a whole from other groups such as ultra-violet, visible, and infra-red. Light is a unique agent in chemical reactionsbecause it is not a material substance. It neither contaminates norleaves a residue. Although much information pertaining to photochemistryhas been available for years, the absence of powerful light-sourcesemitting so-called chemical rays in large quantities inhibited thepractical development of the science of photochemistry. Even to-day, with vast applications of light in this manner, mankind is onlybeginning to utilize its chemical powers. 

[Illustration: In a moving-picture studio

In a portrait studio

ARTIFICIAL LIGHT IN PHOTOGRAPHY]

[Illustration: Swimming pool

City waterworks

STERILIZING WATER WITH RADIANT ENERGY FROM QUARTZ MERCURY-ARCS]

Although it appears that the chemical action of light was known to theancients, the earliest photochemical investigations which could beconsidered scientific and systematic were those of K. W. Scheele in 1777on silver salts. An extract from his own account is as follows:

 I precipitated a solution of silver by sal-ammoniac; then I edulcorated (washed) it and dried the precipitate and exposed it to the beams of the sun for two weeks; after which I stirred the powder and repeated the same several times. Hereupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstruum (solvent) dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's light, and likewise the solution of silver poured on chalk, is _silver by reduction_. I mixed so much of distilled water with the well-washed horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver. 

This extract shows that Scheele dealt with the reducing action of light. He found that silver chloride was decomposed by light and that there wasa liberation of chlorine. However, it was learned later that driedsilver chloride sealed in a tube from which the air was exhausted is notdiscolored by light and that substances must be present to absorb thechlorine. Scheele's work aroused much interest in photochemical effectsand many investigations followed. In many of these the superiority ofblue, violet, and ultra-violet rays was demonstrated. In 1802 the firstphotograph was made by Wedgwood, who copied paintings upon glass andmade profiles by casting shadows upon a sensitive chemical compound. However, he was not able to fix the image. Much study andexperimentation were expended upon photochemical effects, especiallywith silver compounds, before Niepce developed a method of producingpictures which were subsequently unaffected by light. Later Daguerrebecame associated with Niepce and the famous daguerreotype was theresult. Apparently the latter was chiefly responsible for thedevelopment of this first commercial process, the products of which arestill to be found in the family album. A century has elapsed since thisearliest period of commercial photography, and during each year progresshas been made, until at the present time photography is thoroughly woveninto the activities of civilized mankind. 

In those earliest years a person was obliged to sit motionless in thesun for minutes in order to have his picture taken. The development of acentury is exemplified in the "snapshot" of the present time. Photographic exposures outdoors at present are commonly one thousandthof a second, and indoors under modern artificial light miles of"moving-picture" film are made daily in which the individual exposuresare very small fractions of a second. Artificial light is playing agreat part in this branch of photochemistry, and the development ofartificial light for the various photographic needs is best emphasizedby reminding the reader that the sources must be generally comparablewith the sun in actinic or chemical power. The intensity of illuminationdue to sunlight on a clear day when the sun is near the zenith iscommonly 10, 000 foot-candles on a surface perpendicular to the directrays. This is equivalent to the illumination due to a source 90, 000candle-power at a distance of three feet. The sun delivers about200, 000, 000, 000 horse-power to the earth continuously, which isestimated to be about one million times the amount of power generatedartificially on the earth. Of this inconceivable quantity of energy asmall part is absorbed by vegetation, some is reflected and radiatedback into space, and the balance heats the earth. To store some of thisenergy so that it may be utilized at will in any desired form is one ofthe dreams of science. However, artificial light-sources are dependedupon at present in many photographic and other chemical processes. 

Although two illuminants may be of the same luminous intensity, they maydiffer widely in actinic value. It is impossible to rate the differentilluminants in a general manner as to actinic value because the variousphotochemical reactions are not affected to the same extent by rays of agiven wave-length. Nearly all human eyes see visible rays inapproximately the same manner, but the multitude of chemical reactionsshow a wide variation in sensitivity to the various rays. For example, one photographic emulsion may be sensitive only to ultra-violet, violet, and blue rays and another to all these rays and also to the green, yellow, and red. Therefore, one illuminant may be superior to anotherfor one photochemical reaction, while the reverse may be true in thecase of another reaction. In general, it may be said that the arc-lampsincluding the mercury-arcs provide the most active illuminants forphotochemical processes; however, a large number of electricincandescent filament lamps are used in photographic work. 

The photo-engraver has been independent of sunlight since the practicaldevelopment of his art. In fact, the printer could not depend uponsunlight for making the engravings which are used to illustrate themagazines and newspapers. The newspaper photographer may make a"flashlight" exposure, develop his negative, and make a print from itunder artificial light. He may turn this over to the photo-engraver whocarries out his work by means of powerful arc-lamps and in an hour ortwo after the original exposure was made the newspaper containing theillustration is being sold on the streets. 

The moving-picture studio is independent of daylight in indoor settingsand there is a tendency toward the exclusive use of artificial light. In this field mercury-vapor lamps, arc-lamps, and tungsten photographiclamps are used. Similarly, in the portrait studio there is a tendencyfor the photographer to leave the skylighted upper floors and to utilizeartificial light. In this field the tungsten photographic lamp isgaining in popularity, owing to its simplicity and to other advantages. Artificial light in general is more satisfactory than natural light formany kinds of photographic work because through the ease of controllingit a greater variety of more artistic effects may be obtained. Inordinary photographic printing tungsten lamps are widely used, but inblue-printing the white flame-arc and the mercury-vapor lamp aregenerally employed. Not many years ago the blue-printer waited for thesun to appear in order to make his prints, but to-day large machinesoperate continuously under the light of powerful artificial sources. Howmany realize that the blue-print is almost universally at the foundationof everything at the present time? Not only are products made fromblue-prints but the machinery which makes the products is built fromblue-prints. Even the building which houses the machinery is firstconstructed from blue-prints. They form an endless chain in theactivities of present civilization. 

Artificial light has been a great factor in the practical development ofphotography and it is looked upon for aid in many other directions. Although there is a multitude of reactions in photographic processeswhich are brought about by exposure to light, these represent relativelyfew of the photochemical reactions. In general, it may be stated thatlight is capable of causing nearly every type of reaction. The chemicalcompounds which are photo-sensitive are very numerous. Many of thecompounds of silver, gold, platinum, mercury, iron, copper, manganese, lead, nickel, and tin are photo-sensitive and these have been widelyinvestigated. Light and oxygen cause many oxidation reactions and, onthe other hand, light reduces many compounds such as silver salts, evento the extent of liberating the metal. Oxygen is converted partiallyinto ozone under the influence of certain rays and there are manyexamples of polymerization caused by light. 

Various allotropic changes of the elements are due to the influence oflight; for example, a sulphur soluble in carbon disulphide is convertedinto sulphur which is insoluble, and the rate of change of yellowphosphorus into the red variety is greatly accelerated by light. Hydrogen and chlorine combine under the action of light with explosiverapidity to form hydrochloric acid and there are many other examples ofthe synthesizing action of light. Carbon monoxide and chlorine combineto form phosgene and the combination of chlorine, bromine, and iodine, with organic compounds, is much hastened by exposing the mixture tolight. In a similar manner many decompositions are due to light; forexample, hydrogen peroxide is decomposed into water and oxygen. Thissuggests the reason for the use of brown bottles as containers for manychemical compounds. Such glass does not transmit appreciably theso-called actinic or chemical rays. 

There is a large number of reactions due to light in organic chemistryand one of fundamental importance to mankind is the effect of light onthe chlorophyll, the green coloring matter in vegetation. No permanentchange takes place in the chlorophyll, but by the action of light itenables the plant to absorb oxygen, carbon dioxide, and water and to usethese to build up the complex organic substances which are found inplants. Radiant energy or light is absorbed and converted into chemicalenergy. This use of radiant energy occurs only in those parts of theplant in which chlorophyll is present, that is, in the leaves and stems. These parts absorb the radiant energy and take carbon dioxide from theair through breathing openings. They convert the radiant energy intochemical energy and use this energy in decomposing the carbon dioxide. The oxygen is exhausted and the carbon enters into the structure of theplant. The energy of plant life thus comes from radiant energy and withthis aid the simple compounds, such as the carbon dioxide of the air andthe phosphates and nitrates of the soil, are built into complexstructures. Thus plants are constructive and synthetic in operation. Itis interesting to note that the animal organism converts complexcompounds into mechanical and heat energy. The animal organism dependsupon the synthetic work of plants, consuming as food the complexstructures built by them under the action of light. For example, plantsinhale carbon dioxide, liberate the oxygen, and store the carbon incomplex compounds, while the animal uses oxygen to burn up the complexcompounds derived from plants and exhales carbon dioxide. It is abeautiful cycle, which shows that ultimately all life on earth dependsupon light and other radiant energy associated with it. Contrary to mostphotochemical reactions, it appears that plant life utilize yellow, red, and infra-red energy more than the blue, violet, and ultra-violet. 

In general, great intensities of blue light and of the closelyassociated rays are necessary for most photochemical reactions withwhich man is industrially interested. It has been found that the whiteflame-arc excels other artificial light-sources in hastening thechlorination of natural gas in the production of chloroform. Oneadvantage of the radiation from this light-source is that it does notextend far into the ultra-violet, for the ultra-violet rays of shortwave-lengths decompose some compounds. In other words, it is necessaryto choose radiation which is effective but which does not have raysassociated with it that destroy the desired products of the reaction. Bythe use of a shunt across the arc the light can be gradually varied overa considerable range of intensity. Another advantage of the flame-arc inphotochemistry is the ease with which the quality or spectral characterof the radiant energy may be altered by varying the chemical salts usedin the carbons. For example, strontium fluoride is used in the redflame-arc whose radiant energy is rich in red and yellow. Calciumfluoride is used in the carbons of the yellow flame-arc which emitsexcessive red and green rays causing by visual synthesis the yellowcolor. The radiant energy emitted by the snow-white flame-arc is a closeapproximation to average daylight both as to visible and to ultra-violetrays. Its carbons contain rare-earths. The uses of the flame-arcs arecontinually being extended because they are of high intensity andefficiency and they afford a variety of color or spectral quality. Amillion white flame-carbons are being used annually in this country forvarious photochemical processes. 

Of the hundreds of dyes and pigments available many are not permanentand until recent years sunlight was depended upon for testing thepermanency of coloring materials. As a consequence such tests could notbe carried out very systematically until a powerful artificial source oflight resembling daylight was available. It appears that the whiteflame-arc is quite satisfactory in this field, for tests indicate thatthe chemical effect of this arc in causing dye-fading is four or fivetimes as great as that of the best June sunlight if the materials areplaced within ten inches of a 28-ampere arc. It has been computed thatin several days of continuous operation of this arc the same fadingresults can be obtained as in a year's exposure to daylight in thenorthern part of this country. Inasmuch as the fastness of colors indaylight is usually of interest, the artificial illuminant used forcolor-fading should be spectrally similar to daylight. Apparently thewhite flame-arc fulfils this requirement as well as being a powerfulsource. 

Lithopone, a white pigment consisting of zinc sulphide and bariumsulphate, sometimes exhibits the peculiar property of darkening onexposure to sunlight. This property is due to an impurity and apparentlycannot be predicted by chemical analysis. During the cloudy days andwinter months when powerful sunlight is unavailable, the manufacturer isin doubt as to the quality of his product and he needs an artificiallight-source for testing it. In such a case the white flame-arc isserving satisfactorily, but it is not difficult to obtain effects withother light-sources in a short time if an image of the light-source isfocused upon the material by means of a lens. In fact, a darkening oflithopone may be obtained in a minute by focusing upon it the image of aquartz mercury-arc by means of a quartz lens. In special cases of thissort the use of a focused image is far superior to the ordinaryillumination from the light-source, but, of course, this isimpracticable when testing a large number of samples simultaneously. Incidentally, lithopone which turns gray or nearly black in the sunlightregains its whiteness during the night. 

An amusing incident is told of a young man who painted his boat onenight with a white paint in which lithopone was the pigment. Onreturning home the next afternoon after the boat had been exposed tosunlight all day, he was astonished to see that it was black. Being verymuch perturbed, he telephoned to the paint store, but the proprietorescaped a scathing lecture by having closed his shop at the usual hour. The young man telephoned in the morning and told the proprietor what hadhappened, but on being asked to make certain of the facts he went to thewindow and looked at his boat and behold! it was white. It had regainedwhiteness during the night but would turn black again during the day. Although pigments and dyes are not generally as peculiar as lithopone, much uncertainty is eliminated by systematic tests under constant, continuous, and controllable artificial light. 

The sources of so-called chemical rays are numerous for laboratory work, but there is a need for highly efficient powerful producers of this kindof energy. In general the flame-arcs perhaps are foremost sources at thepresent time, with other kinds of carbon arcs and the quartz mercury-arcranking next. One advantage of the mercury-arc is its constancy. Furthermore, for work with a single wave-length it is easy to isolateone of the spectral lines. The regular glass-tube mercury-arc is anefficient producer of the actinic rays and as a consequence has beenextensively used in photographic work and in other photochemicalprocesses. An excellent source for experimental work can be made easilyby producing an arc between two small iron rods. The electric spark hasserved in much experimental work, but the total radiant energy from itis small. By varying the metals used for electrodes a considerablevariety in the radiant energy is possible. This is also true of theelectric arcs, and the flame-arcs may be varied widely by usingdifferent chemical compounds in the carbons. 

There are other effects of light which have found applications but notin chemical reactions. For example, selenium changes its electricalresistance under the influence of light and many applications of thisphenomenon have been made. Another group of light-effects forms a branchof science known as photo-electricity. If a spark-gap is illuminated byultra-violet rays, the resistance of the gap is diminished. If aninsulated zinc plate is illuminated by ultra-violet or violet rays, itwill gradually become positively charged. These effects are due to theemission of electrons from the metal. Violet and ultra-violet rays willcause a colorless glass containing manganese to assume a pinkish color. The latter is the color which manganese imparts to glass and under theinfluence of these rays the color is augmented. Certain ultra-violetrays also ionize the air and cause the formation of ozone. This can bedetected near a quartz mercury-arc, for example, by the characteristicodor. 

The foregoing are only a few of the multitude of photochemical reactionsand other effects of radiant energy. The development of this fieldawaits to some extent the production of so-called actinic rays moreefficiently and in greater quantities, but there are now many practicalapplications of artificial light for these purposes. In the extensivefields of photography various artificial light-sources have served formany years and they are constantly finding more applications. Artificiallight is now used to a considerable extent in the industries inconnection with chemical processes, but little information is available, owing to the secrecy attending these new developments in industrialprocesses. However, this brief chapter has been introduced in order toindicate another field of activity in which artificial light is serving. It is agreed by scientists that photochemistry has a promising future. Mankind harnesses nature's forces and produces light and this light isput to work to exert its influence for the further benefit of mankind. Science has been at work systematically for only a century, but theaccomplishments have been so wonderful that the imagination dares notattempt to prophesy the achievements of the next century. 

XX

LIGHT AND HEALTH

The human being evolved without clothing and the body was bathed withlight throughout the day, but civilization has gone to the other extremeof covering the body with clothing which keeps most of it in darkness. Inasmuch as light and the invisible radiant energy which is associatedwith it are known to be very influential agencies in a multitude ofways, the question arises: Has this shielding of the body had any markedinfluence upon the human organism? Although there is a vast literatureupon the subject of light-therapy, the question remains unanswered, owing to the conflicting results and the absence of standardization ofexperimental details. In fact, most investigations are subject to thecriticism that the data are inadequate. Throughout many centuries lighthas been credited with various influences upon physiological processesand upon the mind. But most of the early applications had no foundationof scientific facts. Unfortunately, many of the claims pertaining to thephysiological and psychological effects of light at the present time areconflicting and they do not rest upon an established scientificfoundation. Furthermore some of them are at variance with thepossibilities and an unprejudiced observer must conclude that muchsystematic work must be done before order may arise from the presentchaos. This does not mean that many of the effects are not real, forradiant energy is known to cause certain effects, and viewing thesubject broadly it appears that light is already serving humanity inthis field and that its future is promising. 

The present lack of definite data pertaining to the effects of radiationis due to the failure of most investigators to determine accurately thequantities and wave-lengths of the rays involved. For example, it iseasy to err by attributing an effect to visible rays when the effect maybe caused by accompanying invisible rays. Furthermore, it may bepossible that certain rays counteract or aid the effective rays withoutbeing effective alone. In other words, the physical measurements havebeen neglected notwithstanding the fact that they are generally moreeasily made than the determinations of curative effects or of germicidalaction. Radiant energy of all kinds and wave-lengths has played a partin therapeutics, so it is of interest to indicate them according towave-length or frequency. These groups vary in range of wave-length, butthe actual intervals are not particularly of interest here. Beginningwith radiant energy of highest frequencies of vibration and shortestwave-lengths, the following groups and subgroups are given in theirorder of increasing wave-length:

 Röntgen or X-rays, which pass readily through many substances opaque to ordinary light-rays. 

 Ultra-violet rays, which are divided empirically into three groups, designated as "extreme, " "middle, " and "near" in accordance with their location in respect to the visible region. 

 Visible rays producing various sensations of color, such as violet, blue, green, yellow, orange, and red. 

 Infra-red or the invisible rays bordering on the red rays. 

 An unknown, unmeasured, or unfilled region between the infra-red and the "electric" waves. 

 Electric waves, which include a class of electromagnetic radiant energy of long wave-length. Of these the Herzian waves are of the shortest wave-length and these are followed by "wireless" waves. Electric waves of still greater wave-length are due to the slower oscillations in certain electric circuits caused by lightning discharges, etc. 

The Röntgen rays were discovered by Röntgen in 1896 and they have beenstudied and applied very widely ever since. Their great use has been inX-ray photography, but they are also being used in therapeutics. Theextreme ultra-violet rays are not available in sunlight and areavailable only near a source rich in ultra-violet rays, such as thearc-lamps. They are absorbed by air, so that they are studied in avacuum. These are the rays which convert oxygen into ozone because theformer strongly absorbs them. The middle ultra-violet rays are not foundin sunlight, because they are absorbed by the atmosphere. They are alsoabsorbed by ordinary glass but are freely transmitted by quartz. Thenearer ultra-violet rays are found in sunlight and in most artificialilluminants and are transmitted by ordinary glass. Next to this regionis the visible spectrum with the various colors, from violet to red, induced by radiant energy of increasing wave-length. The infra-red raysare sometimes called heat-rays, but all radiant energy may be convertedinto heat. Various substances transmit and absorb these rays in generalquite differently from the visible rays. Water is opaque to most of theinfra-red rays. Next there is a region of wave-lengths or frequenciesfor which no radiant energy has been found. The so-called electric wavesvary in wave-length over a great range and they include those employedin wireless telegraphy. All these radiations are of the same generalcharacter, consisting of electromagnetic energy, but differing inwave-length or frequency of vibration and also in their effects. Ineffect they may overlap in many cases and the whole is a chaos if thephysical details of quantity and wave-length are not specified inexperimental work. 

[Illustration: In art work

In a haberdashery

JUDGING COLOR UNDER ARTIFICIAL DAYLIGHT]

[Illustration: In an underground tunnel

In an art gallery

ARTIFICIAL DAYLIGHT]

It has been conclusively shown that radiant energy kills bacteria. Theearly experiments were made with sunlight and the destruction ofmicro-organisms is generally attributed to the so-called chemical rays, namely, the blue, violet, and ultra-violet rays. It appears in generalthat the middle ultra-violet rays are the most powerful destroyers. Itis certainly established that sunlight sterilizes water, for example, and the quartz mercury-lamp is in daily use for this purpose on apracticable scale. However, there still appears to be a difference ofopinion as to the destructive effect of radiant energy upon bacteria inliving tissue. It has been shown that the middle ultra-violet raysdestroy animal tissue and, for example, cause eye-cataracts. It appearspossible from some experiments that ultra-violet rays destroy bacteriain water and on culture plates more effectively in the absence ofvisible rays than when these attend the ultra-violet rays as in the caseof sunlight. This is one of the reasons for the use of blue glass inlight-therapy, which isolates the blue, violet, and near ultra-violetrays from the other visible rays. If the infra-red rays are notdesired they can be readily eliminated by the use of a water-cell. 

There is a vast amount of testimony which proves the bactericidal actionof light. Bacteria on the surface of the body are destroyed byultra-violet rays. Typhus and tubercle bacilli are destroyed equallywell by the direct rays from the sun and from the electric arcs. Cultures of diphtheria develop in diffused daylight but are destroyed bydirect sunlight. Lower organisms in water are readily killed by theradiation from any light-source emitting ultra-violet rays comparablewith those in direct sunlight. From the great amount of data availableit appears reasonable to conclude that radiant energy is a powerfulbactericidal agency but that the action is due chiefly to ultra-violetrays. It appears also that no bacteria can resist these rays if they areintense enough and are permitted to play upon the bacteria long enough. The destruction of these organisms appears to be a phenomenon ofoxidation, for the presence of oxygen appears to be necessary. 

The foregoing remarks about the bactericidal action of radiant energyapply only to bacteria in water, in cultures, and on the surface of thebody. There is much uncertainty as to the ability of radiant energy todestroy bacteria within living tissue. The active rays cannot penetrateappreciably into such tissue and many authorities are convinced that nodirect destruction takes place. In fact, it has been stated that theso-called chemical rays are more destructive to the tissue cells than tobacteria. Finsen, a pioneer in the use of radiant energy in thetreatment of disease, effected many wonderful cures and believed thatthe bacteria were directly destroyed by the ultra-violet rays. However, many have since come to the conclusion that the beneficent action of therays is due to the irritation which causes an outflow of serum, thusbringing more antibodies in contact with the bacilli, and causing thedestruction of the latter. Hot applications appear to work in the samemanner. 

Primitive beings of the tropics are known to treat open wounds byexposing them to the direct rays of the sun without dressings of anykind. These wounds are usually infected and the sun's rays render themaseptic and they heal readily. Many cases of sores and surgical woundshave been quickly healed by exposure to sunlight. Even red light hasbeen effective, so it has been concluded by some that rays of almost anywave-length, if intense enough, will effect a cure of this character bycausing an effusion of serum. It has also been stated that the chemicalrays have anæsthetic powers and have been used in this rôle for manyminor operations. 

It is said that the Chinese have used red light for centuries in thetreatment of smallpox and throughout the Middle Ages this practice wasnot uncommon. In the oldest book on medicine written in English there isan account of a successful treatment of the son of Edward I for smallpoxby means of red light. It is also stated that this treatment wasadministered throughout the reigns of Elizabeth and of Charles II. Another account states that a few soldiers confined in dark dungeonsrecovered from smallpox without pitting. Finsen also obtained excellentresults in the treatment of this disease by means of red light. However, in this case it appears that the exclusion of the so-calledchemical rays favors healing of the postules of smallpox and that theuse of red light is therefore a negative application of light-therapy. In other words, the red light plays no part except in furnishing a lightwhich does not inhibit healing. 

Although the so-called actinic rays have curative value in certaincases, there are some instances where light-baths are claimed to beharmful. It is said that sun-baths to the naked body are not so popularas they were formerly, except for obesity, gout, rheumatism, andsluggish metabolism, because it is felt that the shorter ultra-violetrays may be harmful. These rays are said to increase the pulse, respiration, temperature, and blood-pressure and may even starthemorrhages and in excessive amounts cause headache, palpitation, insomnia, and anemia. These same authorities condemn sun-baths to thenaked body of the tuberculous, claiming that any cures effected areconsummated despite the injury done by the energy of short wave-length. There is no doubt that these rays are beneficial in local lesions, butit is believed that the cure is due to the irritation caused by the raysand the consequent bactericidal action of the increased flow of serum, and not to any direct beneficial result on the tissue-cells. Othersclaim to cure tuberculosis by means of powerful quartz mercury-arcsequipped with a glass which absorbs the ultra-violet rays of shorterwave-lengths. These conclusions by a few authorities are submitted forwhat they are worth and to show that this phase of light-therapy is alsounsettled. 

Any one who has been in touch with light-therapy in a scientific rôle isbound to note that much ignorance is displayed in the use of light inthis manner. In fact, it appears safe to state that light-therapy oftensmacks of quackery. Very mysterious effects are sometimes attributed toradiant energy, which occasionally border upon superstition. Nevertheless, this kind of energy has value, and notwithstanding thechaos which still exists, it is of interest to note some of theequipment which has been used. Some practitioners have great confidencein the electric bath, and elaborate light-baths have been devised. Inthe earlier years of this kind of treatment the electric arc wasconspicuous. Electrodes of carbon, carbon and iron, and iron have beenused when intense ultra-violet rays were desired. The quartz mercury-arcof later years supplies this need admirably. Dr. Cleaves, after manyyears of experience with the electric-arc bath, has stated:

 From the administration of an electric-arc bath there is obtained an action upon the skin, the patient experiences a pleasant and slightly prickly sensation. There is produced, even from a short exposure, upon the skin of some patients a slight erythema, while with others there is but little such effect even from long exposures. The face assumes a normal rosy coloring and an appearance of refreshment and repose on emerging from the bath is always observed. From the administration of the electric-arc bath there is also noted the establishment of circulatory changes with a uniform regulation of the heart's action, as evidenced by improved volume and slower pulse rate, the augmentation of the temperature, increased activity of the skin, fuller and slower respiration, gradually increased respiratory capacity, and diminished irritability of the mucous membrane in tubercular, bronchitic, or asthmatic patients. There is also lessened discharge in those patients suffering from catarrhal conditions of the nasal passages. In diseases of the respiratory system, a soothing effect upon the mucous membranes is always experienced, while cough and expectoration are diminished. 

The cabinet used by Dr. Cleaves was large enough to contain a cot uponwhich the patient reclined. An arc-lamp was suspended at each of the twoends of the cabinet and a flood of light was obtained directly and byreflection from the white inside surfaces of the cabinet. By means ofmirrors the light from the arcs could be concentrated upon any desiredpart of the patient. 

Finsen, who in 1895 published his observations upon the stimulatingaction of light, is considered the pioneer in the use of so-calledchemical rays in the treatment of disease. He had a circular room aboutthirty-seven feet in diameter, in which two powerful 100-amperearc-lamps about six feet from the floor were suspended from the ceiling. Low partitions extended radially from the center, so that a number ofpatients could be treated simultaneously. The temperature of the roomwas normal, so that the treatment was essentially by radiant energy andnot by heat. The chemical action upon the skin was said to be quite asstrong as under sunlight. The exposures varied from ten minutes to anhour. 

Light-baths containing incandescent filament lamps are also used. Insome cases the lamp, sometimes having a blue bulb, is merely containedas a reflector and the light is applied locally as desired. Light-cabinets are also used, but in these there is considerable effectdue to heat. The ultra-violet rays emitted by the small electricfilament lamps used in these cabinets are of very low intensity and thebactericidal action of the light must be feeble. The glass bulbs do nottransmit the extreme ultra-violet rays responsible for the production ofozone, or the middle ultra-violet rays which are effective in destroyinganimal tissue. The cabinets contain from twenty to one hundredincandescent filament lamps of the ordinary sizes, from 25 to 60 watts. In the days of the carbon filament lamp the 16-candle-power lamp wasused. Certainly the heating effect has advantages in some cases overother methods of heating. The light-rays penetrate the tissue and areabsorbed and transformed into heat. Other methods involve conduction ofheat from the hot air or other hot applications. Of course, it is alsocontended that the light-rays are directly beneficial. 

Light is also concentrated upon the body by means of lenses and mirrors. For this purpose the sun, the arc, the quartz mercury-arc, and theincandescent lamp have been used. Besides these, vacuum-tube dischargesand sparks have been utilized as sources for radiant energy and"electrical" treatment. Röntgen rays and radium have also figured inrecent years in the treatment of disease. 

The quartz mercury-arc has been extensively used in the past decade forthe treatment of skin diseases and there appears to be less uncertaintyabout the efficacy of radiant energy for the treatment of surfacediseases than of others. Herod related that the Egyptians treatedpatients by exposure to direct sunlight and throughout the centuries andamong all types of civilization sunlight has been recognized as havingcertain valuable healing or purifying properties. Finsen in his earlyexperiments cured a case of lupus, a tuberculous skin disease, by meansof the visible and near ultra-violet rays in sunlight. He demonstratedthat these were the effective rays by using only the radiant energywhich passed through a water-cell made by using a convex lens for eachend of the cell and filling the intervening space with water. This wasreally a lens made of glass and water. The glass absorbed theultra-violet rays of shorter wave-length and the water absorbed theinfra-red rays. Thus he was able to concentrate upon the diseased skinradiant energy consisting of visible and near ultra-violet rays. 

The encouraging results which Finsen obtained in the treatment of skindiseases led him to become independent of sunlight by equipping aspecial arc-lamp with quartz lenses. This gave him a powerful source ofso-called chemical rays, which could be concentrated wherever desired. However, when science contributed the mercury-vapor arc, developmentswere immediately begun which aimed to utilize this artificial source ofsteady powerful ultra-violet rays in light-therapy. As a consequence, there are now available very compact quartz mercury-arcs designedespecially for this purpose. Apparently their use has been veryeffective in curing many skin diseases. Certainly if radiant energy iseffective, it has a great advantage over drugs. An authority has statedin regard to skin diseases that, treatment with the ultra-violet rays, especially in conjunction with the Röntgen rays, radium and mesothorium is that treatment which in most instances holds rank as the first, and in many as the only and often enough the most effective mode of handling the disease. 

Sterilization by means of the radiation from the quartz mercury-arc hasbeen practised successfully for several years. Compact apparatus is inuse for the sterilization of water for drinking, for surgical purposes, and for swimming-pools, and the claims made by the manufacturers of theapparatus apparently are substantiated. One type of apparatus withstandsa pressure of one hundred pounds per square inch and may be connected inseries with the water-main. The water supplied to the sterilizer shouldbe clear and free of suspended matter, in order that the radiant energymay be effective. Such apparatus is capable of sterilizing any quantityof water up to a thousand gallons an hour, and the lamp is kept burningonly when the water is flowing. It is especially useful in hotels, stores, factories, on ships, and in many industries where sterile wateris needed. 

Water is a vital necessity in every-day life, whether for drinking, cooking, or industrial purposes. It is recognized as a carrier ofdisease and the purification of water-supply in large cities is animportant problem. Chlorination processes are in use which render thetreated water disagreeable to the taste and filtration alone is lookedupon with suspicion. The use of chemicals requires constant analysis, but it is contended that the bactericidal action of ultra-violet rays isso certain and complete that there is never any doubt as to thesterilization of the water if it is clear, or if it has been properlyfiltered before treating. The system of sterilization by ultra-violetrays is the natural way, for the sun's rays perform this function innature. Apparatus for sterilization of water by means of ultra-violetrays is built for public plants in capacities up to ten million gallonsper day and these units may be multiplied to meet the needs of thelargest cities. Large mechanical filters are used in conjunction withthese sterilizers, and thus mankind copies nature's way, for naturalsupplies of pure water have been filtered through sand and have beenexposed to the rays of the sun which free it from germ life. 

Some sterilizers of this character are used at the place where a supplyof pure water is desired or at a point where water is bottled for use invarious parts of a factory, hospital, store, or office building. Thesewere used in some American hospitals during the recent war, where theysupplied sterilized water for drinking and for the antiseptic bathing ofwounds. In warfare the water supply is exceedingly important. Forexample, the Japanese in their campaign in Manchuria boiled the water tobe used for drinking purposes. The mortality of armies in many previouswars was often much greater from preventable diseases than from bullets, but the Japanese in their war with Russia reversed the mortalitystatistics. Of a total mortality of 81, 000 more than 60, 000 died ofcasualties in battle. 

The sterilization of water for swimming-pools is coming into vogue. Heretofore it was the common practice to circulate the water through afilter, in order to remove the impurities imparted to it by the bathersand to return it to the pool. It is insisted by the adherents ofsterilization that filtration of this sort is likely to leave harmfulbacteria in the water. Sterilizers in which ultra-violet rays are theactive rays are now in use for this purpose, being connected beyond theoutflow from the filter. The effectiveness of the apparatus has beenestablished by the usual method of counting the bacteria. Near theoutlet of the ordinary filter a count revealed many thousand bacteriaper cubic inch of water and among these there were bacteria ofintestinal origin. Then a sterilizer was installed in which theeffective elements were two quartz mercury-lamps which consumed 2. 2amperes each at 220 volts. A count of bacteria in the water leaving thesterilizer showed that these organisms had been reduced to 5 per cent. And finally to a smaller percentage of their original value, and thatall those of intestinal origin had been destroyed. In fact, the waterwhich was returned to the pool was better than that which most personsdrink. Radiant energy possesses advantages which are unequaled by otherbactericidal agents, in that it does not contaminate or change theproperties of the water in any way. It does its work of destroyingbacteria and leaves the water otherwise unchanged. 

These glimpses of the use of the radiant energy as a means of regainingand retaining good health suggest greater possibilities when the factsbecome thoroughly established and correlated. The sun is of primaryimportance to mankind, but it serves in so many ways that it isnaturally a compromise. It cannot supply just the desired radiantenergy for one purpose and at the same time serve for another purpose inthe best manner. It is obscured on cloudy days and disappears nightly. These absences are beneficial to some processes, but man in the highlyorganized activity of present civilization desires radiant energy ofvarious qualities available at any time. In this respect artificiallight is superior to the sun and is being improved continually. 

XXI

MODIFYING ARTIFICIAL LIGHT

In a single century science has converted the dimly lighted nights withtheir feeble flickering flames into artificial daytime. In this briefspan of years the production of light has advanced far from theprimitive flames in use at the beginning of the nineteenth century, but, as has been noted in another chapter, great improvements inlight-production are still possible. Nevertheless, the wonderfuldevelopments in the last four decades, which created the arc-lamps, thegas-mantle, the mercury-vapor lamps, and the series of electricincandescent-filament lamps, have contributed much to the efficiency, safety, health, and happiness of mankind. 

A hundred years ago civilization was more easily satisfied and animprovement which furnished more light at the same cost was all thatcould be desired. To-day light alone is not sufficient. Certain kinds ofradiant energy are required for photography and other photochemicalprocesses and a vast array of colored light is demanded for displays andfor effects upon the stage. Man now desires lights of various colors fortheir expressive effects. He is no longer satisfied with mere light inadequate quantities; he desires certain qualities. Furthermore, he nolonger finds it sufficient to be independent of daylight merely inquantity of light. In fact, he has demanded artificial daylight. 

Doubtless the future will see the production of efficient light of manyqualities or colors, but to-day many of the demands must be met bymodifying the artificial illuminants which are available. Vision isaccomplished entirely by the distinction of brightness and color. Animage of any scene or any object is focused upon the retina as aminiature map in light, shade, and color. Although the distinction ofbrightness is a more important function in vision than the ability todistinguish colors, color-vision is far more important in daily lifethan is ordinarily appreciated. One may go through life color-blindwithout suffering any great inconvenience, but the divine gift ofcolor-vision casts a magical drapery over all creation. Relatively feware conscious of the wonderful drapery of color, except for occasionalmoments when the display is unusual. Nevertheless a study of vision innearly all crafts reveals the fact that the distinction of colors playsan important part. 

In the purchase of food and wearing-apparel, in the decoration of homesand throughout the arts and industries, mankind depends a great dealupon the appearance of colors. He depends upon daylight in this respectand unconsciously often, when daylight fails, ceases work which dependsupon the accurate distinction of colors. His color-vision evolved underdaylight; arts and industries developed under daylight; and all hisassociations of color are based primarily upon daylight. For thesereasons, adequate artificial illumination does not make mankindindependent of daylight in the practice of arts and crafts and in manyminor activities. In quality or spectral character, the unmodifiedilluminants used for general lighting purposes differ from daylight andtherefore do not fully replace it. Noon sunlight contains all thespectral colors in approximately the same proportions, but this is nottrue of these artificial illuminants. For these reasons there is ademand for artificial daylight. 

The "vacuum" tube affords a possibility of an extensive variety ofilluminants differing widely in spectral character or color. Every gaswhen excited to luminescence by an electric discharge in the "vacuum"tube (containing the gas at a low pressure) emits light of acharacteristic quality or color. By varying the gas a variety ofilluminants can be obtained, but this means of light-production has notbeen developed to a sufficiently practicable state to be satisfactoryfor general lighting. Nitrogen yields a pinkish light and the nitrogentube as developed by Dr. Moore was installed to some extent a few yearsago. Neon yields an orange light and has been used in a few cases fordisplays. Carbon dioxide furnishes a white light similar to daylight andsmall tubes containing this gas are in use to-day where accuratediscrimination of color is essential. 

The flame-arcs afford a means of obtaining a variety of illuminantsdiffering in spectral character or color. By impregnating the carbonswith various chemical compounds the color of the flame can be widelyaltered. The white flame-arc obtained by the use of rare-earth compoundsin the carbons provides an illuminant closely approximating averagedaylight. By using various substances besides carbon for theelectrodes, illuminants differing in spectral character can beobtained. These are usually rich in ultra-violet rays and therefore havetheir best applications in processes demanding this kind of radiantenergy. The arc-lamp is limited in its application by its unsteadiness, its bulkiness, and the impracticability of subdividing it intolight-sources of a great range of luminous intensities. 

The most extensive applications of artificial daylight have been made bymeans of the electric incandescent filament lamp, equipped with acolored glass which alters the light to the same quality as daylight. The light from the electric filament lamp is richer in yellow, orange, and red rays than daylight, and by knowing the spectral character of thetwo illuminants and the spectral characteristics of colored glasses inwhich various chemicals have been incorporated, it is possible todevelop a colored glass which will filter out of the excess of yellow, orange, and red rays so that the transmitted light is of the samespectral character as daylight. Thousands of such artificial daylightunits are now in use in the industries, in stores, in laboratories, indye-works, in print-shops, and in many other places. Currency andLiberty Bonds have been made under artificial daylight and such unitsare in use in banks for the detection of counterfeit currency. Thediamond expert detects the color of jewels and the microscopist iscertain of the colors of his stains under artificial daylight. The dyermixes his dyes for the coloring of tons of valuable silk and the artistpaints under this artificial light. These are only a few of a vastnumber of applications of artificial daylight, but they illustrate thatmankind is independent of natural light in another respect. 

There are various kinds of daylight, two of which are fairly constant inspectral character. These are noon sunlight and north skylight. Theformer may be said to be white light and its spectrum indicates thepresence of visible radiant energy of all wave-lengths in approximatelyequal proportions. North skylight contains an excess of violet, blue, and blue-green rays and as a consequence is a bluish white. Noonsunlight on a clear day is fairly constant in spectral character, butnorth skylight varies somewhat depending upon the absence or presence ofclouds and upon the character of the clouds. If large areas of sunlitclouds are present, the light is largely reflected sunlight. If the skyis overcast, the north skylight is a result of a mixture of sunlight andblue skylight filtered through the clouds and is slightly bluish. If thesky is clear, the light varies from light blue to deep blue. 

[Illustration: FIREWORKS AND ILLUMINATED BATTLE-FLEET AT HUDSON-FULTONCELEBRATION]

[Illustration: FIREWORKS EXHIBITION ON MAY DAY AT PANAMA-PACIFICEXPOSITION]

The daylight which enters buildings is often considerably altered incolor by reflection from other buildings and from vegetation, and afterit enters a room it is sometimes modified by reflection from coloredsurroundings. It may be commonly noted that the light reflected fromgreen grass through a window to the upper part of a room is very muchtinted with green and the light reflected from a yellow brick buildingis tinted yellow. Besides these alterations, sunlight varies in colorfrom the yellow or red of dawn through white at noon to orange or red atsunset. Throughout the day the amount of light from the sky does notchange nearly as much as the amount of sunlight, so there is acontinual variation in the proportion of direct sunlight and skylightreaching the earth. This is further varied by the changing position ofthe sun. For example, at a north window in which the direct sunlight maynot enter throughout the day, the amount of sunlight which enters byreflection from adjacent buildings and other objects may vary greatly. Thus it is seen that daylight not only varies in quantity but also inquality, and an artificial daylight, which is based upon an extensiveanalysis, has the advantage of being constant in quantity and quality aswell as correct in quality. Modern artificial-daylight units which havebeen scientifically developed not only make mankind independent ofdaylight in the discrimination of colors but they are superior todaylight. 

Although there are many expert colorists who require an accurateartificial daylight, there are vast fields of lighting where a lessaccurate daylight quality is necessary. The average eyes are notsufficiently skilled for the finest discrimination of colors andtherefore the Mazda "daylight" lamp supplies the less exactingrequirements of color matching. It is a compromise between quality andefficiency of light and serves the purpose so well that millions ofthese lamps have found applications in stores, offices, and industries. In order to make an accurate artificial north skylight for color-work bymeans of colored glass, from 75 to 85 per cent. Of the light from atungsten lamp must be filtered out. This absorption in a broad senseincreases the efficiency of the light, for the fraction that remains isnow satisfactory, whereas the original light is virtually useless foraccurate color-discrimination. About one third of the original light isabsorbed by the bulb of the tungsten "daylight" lamp, with a resultantlight which is an approximation to average daylight. 

Old illuminants such as that emitted by the candle and oil-lamp wereused for centuries in interiors. All these illuminants were of a warmyellow color. Even the earlier modern illuminants were not verydifferent in color, so it is not surprising that there is a deeplyrooted desire for artificial light in the home and in similar interiorsof a warm yellow color simulating that of old illuminants. Thepsychological effect of warmth and cheerfulness due to such illuminantsor colors is well established. Artificial light in the home symbolizesindependence of nature and protection from the elements and there is afirm desire to counteract the increasing whiteness of modern illuminantsby means of shades of a warm tint. The white light is excellent for thekitchen, laundry, and bath-room, and for reading-lamps, but the warmyellow light is best suited for making cozy and cheerful the environmentof the interiors in which mankind relaxes. An illuminant of thischaracter can be obtained efficiently by using a properly tinted bulb ontungsten filament lamps. By absorbing about one fourth to one third ofthe light (depending upon the temperature of the filament) the color ofthe candle flame may be simulated by means of a tungsten filament lamp. Some persons are still using the carbon-filament lamp despite its lowefficiency, because they desire to retain the warmth of tint of theolder illuminants. However, light from a tungsten lamp may be filteredto obtain the same quality of light as is emitted by the carbonfilament lamp by absorbing from one fifth to one fourth of the light. The luminous efficiency of the tungsten lamp equipped with such a tintedbulb is still about twice as great as that of the carbon-filament lamp. Thus the high efficiency of the modern illuminants is utilized toadvantage even though their color is maintained the same as the oldilluminants. 

All modern illuminants emit radiant energy, which does not affect theordinary photographic plate. This superfluous visible energy merelycontributes toward glare or a superabundance of light in photographicstudios. A glass has been developed which transmits virtually all therays that affect the ordinary photographic plate and greatly reduces theaccompanying inactive rays. Such a glass is naturally blue in color, because it must transmit the blue, violet, and near ultra-violet rays. Its density has been so determined for use in bulbs for thehigh-efficiency tungsten lamps that the resultant light appearsapproximately the color of skylight without sacrificing an appreciableamount of the value of the radiant energy for ordinary photography. Thisglass, it is seen, transmits the so-called chemical rays and is usefulin other activities where these rays alone are desired. It is used inlight-therapy and in some other activities in which the chemical effectsof these rays are utilized. 

In the photographic dark-room a deep red light is safe for all emulsionsexcepting the panchromatic, and lamps of this character are standardproducts. An orange light is safe for many printing papers. Panchromaticplates and films are usually developed in the dark where extreme safetyis desired, but a very weak deep red light is not unsafe if usedcautiously. However, many photographic emulsions of this character arenot very sensitive to green rays, so a green light has been used forthis purpose. 

A variety of colored lights are in demand for theatrical effects, displays, spectacular lighting, signaling, etc. , and there are manysuperficial colorings available for this purpose. Few of these show anyappreciable degree of permanency. Permanent superficial colorings haverecently been developed, but these are secret processes unavailable forthe market. For this reason colored glass is the only medium generallyavailable where permanency is desired. For permanent lighting effects, signal glasses, colored caps, and sheets of colored glass may be used. Tints may be obtained by means of colored reflectors. Other coloredmedia are dyes in lacquers and in varnishes, colored inks, coloredtextiles, and colored pigments. 

Inasmuch as colored glass enters into the development of permanentdevices, it may be of interest to discuss briefly the effects of variousmetallic compounds which are used in glass. The exact color produced bythese compounds, which are often oxides, varies slightly with thecomposition of the glass and method of manufacture, but this phase isonly of technical interest. The coloring substances in glass may bedivided into two groups. The first and largest group consists of thosein which the coloring matter is in true solution; that is, the coloringis produced in the same manner as the coloring of water in which achemical salt is dissolved. In the second group the coloring substancesare present in a finely divided or colloidal state; that is, thecoloring is due to the presence of particles in mechanical suspension. In general, the lighter elements do not tend to produce colored glasses, but the heavier elements in so far as they can be incorporated intoglass tend to produce intense colors. Of course, there are exceptions tothis general statement. 

The alkali metals, such as sodium, potassium, and lithium, do not colorglass appreciably, but they have indirect effects upon the colorsproduced by manganese, nickel, selenium, and some other elements. Goldin sufficient amounts produces a red in glass and in low concentration abeautiful rose. It is present in the colloidal state. In the manufactureof "gold" red glass, the glass when first cooled shows no color, but onreheating the rich ruby color develops. The glass is then cooled slowly. The gold is left in a colloidal state. Copper when added to a glassproduces two colors, blue-green and red. The blue-green color, whichvaries in different kinds of glasses, results when the copper is fullyoxidized, and the red by preventing oxidation by the presence of areducing agent. This red may be developed by reheating as in the case ofmaking gold ruby glass. Selenium produces orange and red colors inglass. 

Silver when applied to the surface of glass produces a beautiful yellowcolor and it has been widely used in this manner. It has little coloringeffect in glass, because it is so readily reduced, resulting in ametallic black. Uranium produces a canary yellow in soda and potash-limeglasses, which fluoresce, and these glasses may be used in thedetection of ultra-violet rays. The color is topaz in lead glass. Bothsulphur and carbon are used in the manufacture of pale yellow glasses. Antimony has a weak effect, but in the presence of much lead it is usedfor making opaque or translucent yellow glasses. Chromium produces agreen color, which is reddish in lead glass, and yellowish in soda, andpotash-lime glasses. 

Iron imparts a green or bluish green color to glass. It is usuallypresent as an impurity in the ingredients of glass and its color isneutralized by adding some manganese, which produces a purple colorcomplementary to the bluish green. This accounts for the manganesepurple which develops from colorless glass exposed to ultra-violet rays. Iron is used in "bottle green" glass. Its color is greenish blue inpotash-lime glass, bluish green in soda-lime glass, and yellowish greenin lead glass. 

Cobalt is widely used in the production of blue glasses. It produces aviolet-blue in potash-lime and soda-lime glasses and a blue in leadglasses. It appears blue, but it transmits deep red rays. For thisreason when used in conjunction with a deep red glass, a filter for onlythe deepest red rays is obtained. Nickel produces an amethyst color inpotash-lime glass, a reddish brown in soda-lime glass, and a purple inlead glass. Manganese is used largely as a "decolorizing" agent incounteracting the blue-green of iron. It produces an amethyst color inpotash-lime glass and reddish violet in soda-lime and lead glasses. 

These are the principal coloring ingredients used in the manufacture ofcolored glass. The staining of glass is done under lower temperatures, so that a greater variety of chemical compounds may be used. Theresulting colors of metals and metallic oxides dissolved in glass dependnot only upon the nature of the metal used, but also partly upon thestage of oxidation, the composition of the glass and even upon thetemperature of the fusion. 

In developing a glass filter the effects of the various coloringelements are determined spectrally and the various elements are variedin proper proportions until the glass of desired spectral transmissionis obtained. It is seen that the coloring elements are limited and thecombination of these is further limited by chemical considerations. Incombining various colored glasses or various coloring elements in thesame glass the "subtractive" method of color-mixture is utilized. Forexample, if a green glass is desired, yellowish green chromium glass maybe used as a basis. By the addition of some blue-green due to copper, the yellow rays may be further subdued so that the resulting color isgreen. 

The primary colors for this method of color-mixture are the same asthose of the painter in mixing pigments--namely, purple, yellow, andblue-green. Various colors may be obtained by superposing or intimatelymixing the colors. The resulting transmission (reflection in the case ofreflecting media such as pigments) are those colors commonly transmittedby all the components of a mixture. Thus, Purple and yellow = red Yellow and blue-green = green Blue-green and purple = blue

The colors produced by adding lights are based not on the "subtractive"method but on the actual addition of colors. These primaries are red, green, and blue and it will be noted that they are the complementariesof the "subtractive" primaries. By the use of red, green, and bluelights in various proportions, all colors may be obtained in varyingdegrees of purity. The chief mixtures of two of the "additive" primariesproduce the "subtractive" primaries. Thus, Red and blue = purple Red and green = yellow Green and blue = blue-green

Although the coloring media which are permanent under the action oflight, heat, and moisture are relatively few, by a knowledge of theirspectral characteristics and other principles of color the expert isable to produce many permanent colors for lighting effects. The additiveand subtractive methods are chiefly involved, but there is anothermethod which is an "averaging" additive one. For example, if a warm tintof yellow is desired and only a dense yellow glass is available, theyellow glass may be cut into small pieces and arranged upon a colorlessglass in checker-board fashion. Thus a great deal of uncolored lightwhich is transmitted by the filter is slightly tinted by the yellowlight passing through the pieces of yellow glass. If this light isproperly mixed by a diffusing glass the effect is satisfactory. Theseare the principal means of obtaining colored light by means of filtersand by mixing colored lights. By using these in conjunction with thearray of light-sources available it is possible to meet most of thegrowing demands. Of course, the ideal solution is to make the coloredlight directly at the light-source, and doubtless future developmentswhich now appear remote or even impossible will supply such coloredilluminants. In the meantime, much is being accomplished with the meansavailable. 

XXII

SPECTACULAR LIGHTING

Artificial light is a natural agency for producing spectacular effects. It is readily controlled and altered in color and the brightness whichit lends to displays outdoors at night renders them extremelyconspicuous against the darkness of the sky. It surpasses otherdecorative media by the extreme range of values which may be obtained. The decorator and painter are limited by a range of values from black towhite pigments, which ordinarily represents an extreme contrast of aboutone to thirty. The brightnesses due to light may vary from darkness tothose of the light-sources themselves. The decorator deals withsecondary light--that is, light reflected by more or less diffuselyreflecting objects. The lighting expert has at his command not only thissecondary light but the primary light of the sources. Lighting effectseverywhere attract attention and even the modern merchant testifies thatadequate lighting in his store is of advertising value. In all the fieldof spectacular lighting the superiority of artificial light over naturallight is demonstrated. 

Light is a universal medium with which to attract attention and toenthrall mankind. The civilizations of all ages have realized thisnatural power of light. It has played a part in the festivals andtriumphal processions from time immemorial and is still the mostimportant feature of many celebrations. In the early festivals fires, candles, and oil-lamps were used and fireworks were invented for thepurpose. Even to-day the pyrotechnical displays against the dark depthsof the night sky hold mankind spellbound. But these evanescent notes oflight have been improved upon by more permanent displays on a hugescale. Thirty years before the first practical installation ofgas-lighting an exhibition of "Philosophical Fireworks" produced by thecombustion of inflammable gases was given in several cities of England. 

It is a long step from the array of flickering gas-flames with which thefronts of the buildings of the Soho works were illuminated a century agoto the wonderful lighting effects a century later at the Panama-PacificExposition. Some who saw that original display of gas-jets totaling afew hundred candle-power described it as an "occasion of extraordinarysplendour. " What would they have said of the modern spectacular lightingat the Exposition where Ryan used in a single effect forty-eight largesearch-lights aggregating 2, 600, 000, 000 beam candle-power! No othercomparison exemplifies more strikingly the progress of artificiallighting in the hundred years which have elapsed since it began to bedeveloped. 

The nature of the light-sources in the first half of the nineteenthcentury did not encourage spectacular or display lighting. In fact, thisphase of lighting chiefly developed along with electric lamps. Ofcourse, occasionally some temporary effect was attempted as in the caseof illuminating the dome of St. Paul's Cathedral in London in 1872, butcontinued operation of the display was not entertained. In the case oflighting this dome a large number of ship's lanterns were used, but theresult was unsatisfactory. After this unsuccessful attempt at lightingSt. Paul's, a suggestion was made of "flooding it with electric lightprojected from various quarters. " Spectacular lighting outdoors reallybegan in earnest in the dawn of the twentieth century. 

Although some of the first attempts at spectacular lighting outdoorswere made with search-lights, spectacular lighting did not becomegenerally popular until the appearance of incandescent filament lamps ofreasonable efficiency and cost. The effects were obtained primarily bythe use of small electric filament lamps draped in festoons or installedalong the outlines and other principal lines of buildings and monuments. The effect was almost wholly that of light, for the glare from thevisible lamps obscured the buildings or other objects. The method isstill used because it is simple and the effects may be permanentlyinstalled without requiring any attention excepting to replaceburned-out lamps. However, the method has limitations from an artisticpoint of view because the artistic effects of painting, sculpture, andarchitecture cannot be combined with it very effectively. For example, the details of a monument or of a building cannot be seen distinctlyenough to be appreciated. The effect is merely that of outlines or linesand patterns of points of light and is usually glaring. 

The next step was to conceal these lamps behind the cornices or otherprojections or in nooks constructed the purpose. Light now began tomold and to paint the objects. The structures began to be visible; atleast the important cornices and other details were no longer mereoutlines. The introduction of the drawn-wire tungsten lamp isresponsible for an innovation in spectacular lighting of this sort, fornow it became possible to make concentrated light-sources so essentialto projectors. Furthermore, these lighting units require very littleattention after once being located. With the introduction ofelectric-filament lamps of this character small projectors came intouse, and by means of concentrated beams of light whole buildings andmonuments could be flooded with light from remote positions. The effectsobtained by concealing lamps behind cornices had demonstrated that thelighting of the surfaces was the object to be realized in most cases, and when small projectors not requiring constant attention becameavailable, a great impetus was given to flood-lighting. 

When France gave to this country the Bartholdi Statue of Liberty therewas no thought of having this emblem visible at night excepting for thetorch held the hand of Liberty. This torch was modified at the time ofthe erection of the statue to accommodate the lamps available, with theresult that it was merely a lantern containing a number of electriclamps. At night it was a speck of light more feeble than manysurrounding shore lights. The statue had been lighted during festivalswith festoons and outlines of lamps, but in 1915, when the freedom ofthe generous donor of the statue appeared to be at stake, a movement wasbegun which culminated in a fund for flood-lighting Liberty. The broadfoundation of the statue made the lighting comparatively easy by meansof banks of incandescent filament search-lights. About 225 of theseunits were used with a total beam candle-power of about 20, 000, 000. Theoriginal idea of an imitation flame for the torch was restored bybuilding this from pieces of yellow cathedral glass of three densities. About six hundred pieces of glass were used, the upper ones beinggenerally of the lighter tints and the lower ones of the darker tints. Alighthouse lens was placed in this lantern so that an intense beam oflight would radiate from it. The flood-lighted Statue of Liberty is nowvisible by night as well as by day and it has a double significance atnight, for light also symbolizes independence. 

Just as the Statue of Liberty stands alone in the New York Harbor sodoes the Woolworth Building reign supreme on lower Manhattan. Libertyproclaims independence from the bondage of man and the Woolworth Towerstands majestically in defiance of the elements as a symbol of man'sgrowing independence of nature. This building with its cream terra-cottasurface and intricate architectural details touched here and there withbuff, blue, green, red, and gold, rises 792 feet or sixty stories abovethe street and typifies the American spirit of conceiving and ofexecuting great undertakings. In it are blended art, utility, andmajesty. Viewed by multitudes during the day, it is a valuableadvertisement for the name which stands for a national institution. Butby day it shares attention with its surroundings. If lighted at night itwould stand virtually alone against the dark sky and the investmentwould not be wholly idle during the evening hours. 

Mr. H. H. Magdsick, who designed the lighting for Liberty, planned thelighting for the Woolworth Tower, which rises 407 feet or thirty-onestories above the main building. Five hundred and fifty projectorscontaining tungsten filament lamps were distributed about the base ofthe tower and among some of the architectural details. The mainarchitectural features of the mansard roof extending from thefifty-third to the fifty-seventh floor, the observation balcony at thefifty-eighth and the lantern structures at the fifty-ninth and sixtiethfloors are covered with gold-leaf. By proper placing of the projectors aglittering effect is obtained from these gold surfaces. The crowningfeatures of the lighting effect are the lanterns in the crest of thespire. Twenty-four 1000-watt tungsten lamps were placed behind crystaldiffusing glass, which transmits the light predominantly in a horizontaldirection. Thus at long distances, from which the architectural detailscannot be distinguished, the brilliant crowning light is visible. Anautomatic dimmer was devised so that the effect of a huge varying flamewas obtained. At close range, owing to the nature of the glass panels, this portion is not much brighter than the remainder of the surfaces. When the artificial lighting is in operation the tower becomes amajestic spire of light and this magnificent Gothic structure projectingdefiantly into the depths of darkness is in more than one sense a torchof modern civilization. 

Many prominent buildings and monuments have burst forth in a flood oflight, and their beauty and symbolism have been appreciated at night bymany persons who do not notice them by day. Not only are the beautifulstructures of man lighted permanently but many temporary effects aredevised. Artificial lighting effects have become a prominent part inoutdoor festivals, pageants, and theatricals. Candles have beenassociated with Christmas trees ever since the latter came into use andnaturally artificial light has been a feature in the community Christmastrees which have come into vogue in recent years. The MunicipalChristmas Tree in Chicago in 1916 was ninety feet high and was lightedwith projectors. Thousands of gems taken from the Tower of Jewels at theSan Francisco Exposition added life and sparkle to that of the otherdecorations. 

[Illustration: The Capitol flooded with light

Luna Park, Coney Island, studded with 60, 000 incandescentfilament lamps

THE NEW FLOOD LIGHTING CONTRASTED WITH THE OLD OUTLINE LIGHTING]

[Illustration: NIAGARA FALLS FLOODED WITH LIGHT]

After the close of the recent war artificial light played a prominentpart throughout the country in the joyful festivals. A jeweled archerected in New York in honor of the returning soldiers rivaled some ofthe spectacles of the Panama-Pacific Exposition. The arch hung like agigantic curtain of jewels between two obelisks, which rose to a heightof eighty feet and were surmounted by jeweled forms in the shape ofsunbursts. Approximately thirty thousand jewels glittered in the beamsof batteries of arc-projectors. Many of the signs and devices whichplayed a part in the "Welcome Home" movement were of striking nature andof a character to indicate permanency. The equipment of a large buildingconsisted of more than five thousand 10-watt lamps, the entire buildingbeing outlined with stars consisting of eleven lamps each. The "BrightenUp" campaign spread throughout the country. The lighting andinstallation of signs and special patriotic displays, the flooding ofstreets and shop-windows with light without stint, produced an inspiringand uplifting effect which did much to restore cheerfulness andoptimism. A glowing example was set in Washington, where theflood-lighting of the Capitol, discontinued shortly after our entranceinto the war, was resumed. 

In Chicago a "Victory Way" was established, with street-lighting postson both sides of the street equipped with red, white, and blue globessurmounted by a golden goddess of Victory. One hundred and seventy-fiveprojectors were installed along the way on the roofs and in the windowsof office buildings. A brilliant, scintillating "Altar of Victory" waserected at the center of the Way. It was composed of two enormouscandelabra erected one on each side of a platform ninety feet high. These were studded with jewels and supported a curtain of jewelssuspended from the altar. In the center of the curtain was a hugejeweled eagle bearing the Allied flags. This was illuminated byarc-projectors which delivered 200, 000, 000 beam candle-power. Inaddition to these there were many smaller projectors. In the top of eachcandelabra six large red-and-orange lamps were installed in reflectors. These illuminated live steam which issued from the top. Surmounting thewhole was a huge luminous fan formed by beams from large arcsearch-lights. These are only a few of the many lighting effects whichwelcomed the returning soldiers, but they illustrate how much moderncivilization depends upon artificial light for expressing its feelingsand emotions. Throughout all these festivals light silently symbolizedhappiness, freedom, and advancement. 

Projectors were used on a large scale in several cases before the adventof the concentrated filament lamp. W. D'A. Ryan, the leader inspectacular lighting, lighted the Niagara Falls in 1907 with batteriesof arc-projectors aggregating 1, 115, 000, 000-beam candle-power. In 1908he used thirty arc-projectors to flood the Singer Tower in New York withlight and projected light to the flag on top by means of a search-lightthirty inches in diameter. Many flags waved throughout the war in thebeams of search-lights, symbolizing a patriotism fully aroused. Thesearch-light beam as it bores through the atmosphere at night is usuallyfaintly bright, owing to the small amount of fog, dust, and smoke in theair. By providing more "substance" in the atmosphere, the beams are madeto appear brighter. Following this reasoning, Ryan developed hisscintillator consisting of a battery of search-light beams projectedupward through clouds of steam which provided an artificial fog. Thiswas first displayed at the Hudson-Fulton celebration with a battery ofarc search-lights totaling 1, 000, 000, 000-candle-power. 

All these effects despite their magnitude were dwarfed by those at thePanama-Pacific Exposition, and inasmuch as this up to the present timerepresents the crowning achievement in spectacular lighting, some of thedetails worked out by Ryan may be of interest. In general, the lightingeffects departed from the bizarre outline lighting in which glaringlight-sources studded the structures. The radiant grandeur and beautyof flood-lighting from concealed light-sources was the key-note of thelighting. In this manner wonderful effects were obtained, which not onlyappealed to the eye and to the artistic sensibility but which were freefrom glare. By means of flood-lighting and relief-lighting fromconcealed light-sources the third dimension or depth was obtained andthe architectural details and colorings were preserved. A great manydifferent kinds of devices and lamps were used to make the night effectssuperior in grandeur to those of daytime. The Zone or amusement sectionwas lighted with bare lamps in the older manner and the glaring bizarreeffects contrasted the spectacular lighting of the past with theillumination of the future. 

In another section the visitor was greeted with a gorgeous display ofcarnival spirit. Beautifully colored heraldic shields on which werewritten the early history of the Pacific coast were illuminated bygroups of luminous arc-lamps on standards varying from twenty-five tofifty-five feet in height. The Tower of Jewels with more than a hundredthousand dangling gems was flood-lighted, and the myriads of minutereflected images of light-sources glittering against the dark skyproduced an effect surpassing the dreams of imagination. Shadows andhigh-lights of striking contrasts or of elusive colors greeted thevisitor on every hand. Individual isolated effects of light were to befound here and there. Fire hissed from the mouths of serpents and castthe spell of mobile light over the composite Spanish-Gothic-Orientalsetting. A colored beam of a search-light played here and there. Mysterious vapors rising from caldrons were in reality illuminatedsteam. Symbolic fountain groups did not escape the magic touch of thelighting wizard. 

In the Court of the Universe great areas were illuminated by twofountains rising about a hundred feet above the sunken gardens. One ofthese symbolized the setting sun, the other the rising sun. The shaftand ball at the crest of each fountain were glazed with heavy opal glassimitating travertine marble and in these were installed incandescentlamps of a total candle-power of 500, 000. The balustrade seventy feetabove the sunken gardens was surmounted by nearly two hundredincandescent filament search-lights. Light was everywhere, eithervarying in color into a harmonious scene or changing in light and shadowto mold the architecture and sculpture. The enormous glass dome of thePalace of Horticulture was converted into an astronomical sphere byprojecting images upon it in such a manner that spots of light revolved;rings and comets which appeared at the horizon passed on their waythrough the heavens, changing in color and disappearing again at thehorizon. All these effects and many more were mirrored in the waters ofthe lagoons and the whole was a Wonderland indeed. 

The scintillator consisted of 48 arc search-lights three feet indiameter totaling 2, 600, 000, 000 beam candle-power. The lighting unitswere equipped with colored screens and the beams which radiated upwardwere supplied with an artificial fog by means of steam generated by amodern express locomotive. The latter was so arranged that the wheelscould be driven at a speed of sixty miles per hour under brake, therebyemitting great volumes of steam and smoke, which when illuminated withvarious colors produced a magnificent spectacle. Over three hundredscintillator effects were worked out and this feature of firelessfireworks was widely varied. The aurora borealis and other effectscreated by this battery of search-lights extended for many miles. Themany effects regularly available were augmented on special occasions andit is safe to state that this apparatus built upon a huge scale provideda flexibility of fireless fireworks never attained even with small-scaledevices. 

The lighting of the exposition can barely be touched upon in a fewparagraphs and it would be difficult to describe in words even if spacewere unlimited. It represented the power of light to beautify and toawe. It showed the feebleness of the decorator's media in comparisonwith light pulsating with life. It consisted of a great variety ofdirect, masked, concealed, and projected effects, but these were blendedharmoniously with one another and with the decorative and architecturaldetails of the structures. It was a crowning achievement of a century ofpublic lighting which began with Murdock's initial display of a hundredflickering gas-jets. It demonstrated the powers of science in theproduction of light and of genius and imagination in the utilization oflight. It was a silent but pulsating display of grandeur dwarfing intoinsignificance the aurora borealis in its most resplendent moments. 

XXIII

THE EXPRESSIVENESS OF LIGHT

From an esthetic or, more broadly, a psychological point of view nomedium rivals light in expressiveness. Not only is light allied withman's most important sense but throughout long ages of associations anduses mankind has bestowed upon it many attributes. In fact, it ispossible that light, color, and darkness possess certain fundamentallyinnate powers; at least, they have acquired expressive and impressivepowers through the many associations in mythology, religion, nature, andcommon usage. Besides these attributes, light possesses a greatadvantage over the media of decoration in obtaining brightness and coloreffects. For example, the landscape artist cannot reproduce the range ofvalues or brightnesses in most of nature's scenes, for if black is usedto represent a deep shadow, white is not bright enough to represent thevalue of the sky. In fact, the range of brightnesses represented by thedeep shadow and the sky extends far beyond the range represented byblack and white pigments. The extreme contrast ordinarily available bymeans of artist's colors is about thirty to one, but the sky is athousand times brighter than a shadow, a sunlit cloud is thousands oftimes brighter than the deep shadows of woods, and the sun is millionsof times brighter than the shadows in a landscape. 

The range of brightnesses obtainable by means of light extends fromdarkness or black throughout the range represented by pigments underequal illumination and beyond these through the enormous rangeobtainable by unequal illumination of surfaces to the brightnesses ofthe light-sources themselves. In the matter of purity of colors, lightsurpasses reflecting media, for it is easy to obtain approximately purehues by means of light and to obtain pure spectral hues by resorting tothe spectrum of light. It is impossible to obtain pure hues by means ofpigments or of other reflecting media. These advantages of light arevery evident on turning to spectacular lighting effects, and even thelighting of interiors illustrates a potentiality in light superior toother media. For example, in a modern interior in which concealedlighting produces brilliantly illuminated areas above a cornice and darkshadows on the under side, the range in values is often much greaterthan that represented by black and white, and still there remains thepossibility of employing the light-sources themselves in extending thescale of brightness. Superposing color upon the whole it is obvious thatthe combination of "primary" light with reflected light possesses muchgreater potentiality than the latter alone. This potentiality of lightis best realized if lighting is regarded as "painting with light" in amanner analogous to the decorator's painting with pigments, etc. 

The expressive possibilities of lighting find extensive applications inrelation to painting, sculpture, and architecture. A painting is anexpression of light and the sculptor's product finally depends uponlighting for its effectiveness. Lighting is the master painter andsculptor. It may affect the values of a painting to some extent and itis a great influence upon the colors. It molds the model from which thesculptor works and it molds the completed work. The direction, distribution, and quality of light influence the appearance of allobjects and groups of them. Aside from the modeling of ornament, thelight and shade effects of relatively large areas in an interior such aswalls and ceiling, the contrasts in the brightnesses of alcoves withthat of the main interior, and the shadows under cornices, beams, andarches are expressions of light. 

The decorator is able to produce a certain mood in a given interior byvarying the distribution of values and the choice of colors and thelighting artist is able to do likewise, but the latter is even able toalter the mood produced by the decorator. For example, a large interiorflooded with light from concealed sources has the airiness andextensiveness of outdoors. If lighted solely by means of sourcesconcealed in an upper cornice, the ceiling may be bright and the wallsmay be relatively dark by contrast. Such a lighting effect may produce afeeling of being hemmed in by the walls without a roof. If the room islighted by means of chandeliers hung low and equipped with shades insuch a manner that the lower portions of the walls may be light whilethe upper portions of the interior may be ill defined, the feelingproduced may be that of being hemmed in by crowding darkness. Thuslighting is productive of moods and illusions ranging from the mysteryof crowding darkness to the extensiveness of outdoors. 

Future lighting of interiors doubtless will provide an adequacy oflighting effects which will meet the respective requirements of variousoccasions. A decorative scheme in which light and medium grays areemployed produces an interior which is very sensitive to lightingeffects. To these light-and-shade effects colored light may add itscharming effectiveness. Not only are colored lighting effects able toadd much to the beauty of the setting but they possess certain otherpowers. Blue tints produce a "cold" effect and the yellow and orangetints a "warm" effect. For example, a room will appear cooler in thesummer when illuminated by means of bluish light and a practicalapplication of this effect is in the theater which must attractaudiences in the summer. How tinted illuminants fit the spirit of anoccasion or the mood of a room may be fully appreciated only throughexperiments, but these are so effective that the future of lighting willwitness the application of the idea of "painting with light" to itsfullest extent. Color is demanded in other fields, and, considering itseffectiveness and superiority in lighting, it will certainly be demandedin lighting when its potentiality becomes appreciated and readilyutilized. 

The expressiveness of light is always evident in a landscape. On a sunnyday the mood of a scene varies throughout the day and it grows moreenticing and agreeable as the shadows lengthen toward evening. Theartist in painting a desert scene employs short harsh shadows if hedesires to suggest the excessive heat. These shadows suggest therelentless noonday sun. The overcast sky is universally depressing andit has been found that on a sunny day most persons experience a slightdepression when a cloud obscures the sun. Nature's lighting varies frommoment to moment, from day to day, and from season to season. Itpresents the extremes of variation in distributions of light fromovercast to sunny days and in the latter cases the shadows arecontinually shifting with the sun's altitude. They are harshest at noonand gradually fade as they lengthen, until at sunset they disappear. Thecolors of sunlit surfaces and of shadows vary from sunrise to sunset. These are the fundamental variations in the lighting, but in the variousscenes the lighting effects are further modified by clouds and by localconditions or environment. The vast outdoors provides a fruitful fieldfor the study of the expressiveness of light. 

Having become convinced of this power of light, the lighting expert mayturn to artificial light, which is so easily controlled in direction, distribution, and color, and draw upon its potentiality. Not only is iteasy to provide a lighting suitable to the mood or to the function of aninterior but it is possible to obtain some variety in effect so that thelighting may always suit the occasion. A study of nature's lightingreveals one great principle, namely, variety. Mankind demands variety inmost of his activities. Work is varied and alternated with recreation. Meals are not always the same. Clothing, decorations, and furnishingsare relieved of monotony. One of the most potent features of artificiallight is the ease with which variety may be obtained. In obtainingrelief from the monotony of decorations and furnishings, considerableexpense and inconvenience are inevitably encountered. With an adequatesupply of outlets, circuits, and controls a wide variety of lightingeffects may be obtained with perhaps an insignificant increase in theinitial investment. Variety is the spice of lighting as well as of life. 

These various principles of lighting are readily exemplified in thelighting of the home, which is discussed in another chapter. The churchis even a better example of the expressive possibilities of lighting. The architectural features are generally of a certain period and firstof all it is essential to harmonize the lighting effect with that of thearchitectural and decorative scheme. Obviously, the dark-stained ceilingof a certain type of church would not be flooded with light. The factthat it is made dark by staining precludes such a procedure in lighting. The characteristics of creeds are distinctly different and these are tosome extent exemplified by the lines of the architecture of theirchurches. In the same way the lighting effect may be harmonized with thecreed and the spirit of the interior. The lighting may always bedignified, impressive, and congruous. Few churches are properly lightedwith a high intensity of illumination; moderate lighting is moreappropriate, for it is conducive to the spirit of worship. In somecreeds a dominant note is extreme penitence and severity. Thearchitecture may possess harsh outlines, and this severity or extremesolemnity may be expressed in lighting by harsher contrasts, althoughthis does not mean that the lighting must be glaring. On the other hand, in a certain modern creed the dominant note appears to be cheerfulness. The spacious interiors of the churches of this creed are lacking insevere lines and the walls and ceilings are highly reflecting. Adequateillumination by means of diffused light without the production of severecontrasts expresses the creed, modernity, and enlightenment. On thealtar of certain churches the expressiveness of light is utilized in theceremonial uses which vary with the creed. Even the symbolism of colormay be appropriately woven into the lighting of the church. 

The expressiveness of light and color originated through the contact ofprimitive man with nature. Sunlight meant warmth and a bountifulvegetation, but darkness restricted his activities and harbored manifolddangers. Many associations thus originated and they were extendedthrough ignorance and superstition. Yellow is naturally emblematical ofthe sun and it became the symbol of warmth. Brown as the predominantcolor of the autumn foliage became tinctured with sadness because thedecay of the vegetation presaged the death of the year and the colddreary months of winter. The first signs of green vegetation in thespring were welcomed as an end of winter and a beginning of anotherbountiful summer; hence green symbolized youth and hope. It becameassociated with the springtime of life and thus signified inexperience, but as the color of vegetation it also meant life itself and became asymbol of immortality. Blue acquired certain divine attributes because, as the color of the sky, it was associated with the abode of the gods orheaven. Also a blue sky is the acme of serenity and this color acquiredcertain appropriate attributes. 

Associations of this character became woven into mythology and thusbecame firmly established. Poets have felt these influences of lightand color in nature and have given expression to them in words. Theyalso have entwined much of the mythology of past civilizations and theserepetitions have helped to establish the expressiveness of light andcolor. Early ecclesiasts employed these symbolisms in religiousceremonies and dictated the garbs of saints and other religiouspersonages in the paintings which decorated their edifices. Thus therewere many influences at work during the early centuries when intellectswere particularly susceptible through superstition and lack ofknowledge. The result has been an extensive symbolism of light, color, and darkness. 

At the present time it is difficult to separate the innate appeal oflight, color, and darkness from those attributes which have beenacquired through associations. Possibly light and color have no innatepowers but merely appear to have because the acquired attributes havebeen so thoroughly established through usage and common consent. Spacedoes not permit a discussion of this point, but the chief aim isconsummated if the existence of an expressiveness and impressiveness oflight is established. There are many other symbolisms of color and lightwhich have arisen in various ways but it is far beyond the scope of thisbook to discuss them. 

Psychological investigations reveal many interesting facts pertaining tothe influence of light and color upon mankind. When choosing color forcolor's sake alone, that is, divorced from any associations of usage, mankind prefers the pure colors to the tints and shades. It isinteresting to note that this is in accord with the preferenceexhibited by uncivilized beings in their use of colors for decoratingthemselves and their surroundings. Civilized mankind chooses tints andshades predominantly to live with, that is, for the decoration of hissurroundings. However, civilized man and the savage appear to have thesame fundamental preference for pure colors and apparently culture andrefinement are responsible for their difference in choice of colors tolive with. This is an interesting discovery and it has its applicationsin lighting, especially in spectacular and stage-lighting. 

It appears to be further established that when civilized man choosescolor for color's sake alone he not only prefers the pure colors butamong these he prefers those near the ends of the spectrum, such as redand blue. Red is favored by women, with blue a close second, but thereverse is true for men. It is also thoroughly established that red, orange, and yellow exert an exciting influence; yellow-green, green, andblue-green, a tranquilizing influence, and blue and violet a subduinginfluence upon mankind. All these results were obtained with colorsdivorced from surroundings and actual usage. In the use of light andcolor the laws of harmony and esthetics must be obeyed, but thesensibility of the lighting artist is a satisfactory guide. Harmoniesare of many varieties, but they may be generally grouped into twoclasses, those of analogy and those of contrast. The former includescolors closely associated in hue and the latter includes complementarycolors. No rules in simplified form can be presented for the productionof harmonies in light and color. These simplifications are made only bythose who have not looked deeply enough into the subject throughobservation and experiment to see its complexity. 

The expressiveness of light finds applications throughout the vast fieldof lighting, but the stage offers great opportunities which have beenbarely drawn upon. When one has awakened to the vast possibilities oflight, shade, and color as a means of expression it is difficult tosuppress a critical attitude toward the crudity of lighting effects onthe present stage, the lack of knowledge pertaining to the latentpossibilities of light, and the superficial use of this potentialmedium. The crude realism and the almost total absence of deep insightinto the attributes of light and color are the chief defects ofstage-lighting to-day. One turns hopefully toward the gallant thoughsmall band of stage artists who are striving to realize a harmony oflighting, setting, and drama in the so-called modern theater. Unappreciated by a public which flocks to the melodramatic movie, whosescenarios produced upon the legitimate stage would be jeered by the samepublic, the modern stage artist is striving to utilize the potentialityof light. But even among these there are impostors who have neverachieved anything worth while and have not the perseverance to learn toextract some of the power of light and to apply it effectively. Lightingsuffers in the hands of the artist owing to the absence of scientificknowledge and it is misused by the engineer who does not possess anesthetic sensibility. Science and art must be linked in lighting. 

The worthy efforts of stage artists in some of the modern theaters lackthe support of the producers, who cater to the taste of the public whichpays the admission fees. Apparently the modern theater must first passthrough a period in which financial support must be obtained from thosewho are able to give it, just as the symphony orchestra has beensupported for the sake of art. Certainly the time is at hand forphilanthropy to come to the aid of worthy and capable stage artists whohope to rescue theatrical production from the mire of commercialism. 

Those who have not viewed stage-lighting from behind the scenes wouldoften be surprised at the crudity of the equipment, and especially atthe superficial intellects which are responsible for some of therealistic effects obtained. But these are the result usually ofexperiment, not of directed knowledge. Furthermore, little thought isgiven to the emotional value of light, shade, and color. The flood oflight and the spot of light are varied with gaudy color-effects, but howseldom is it possible to distinguish a deep relation between thelighting and the dramatic incidents!

[Illustration: Soldiers' and Sailors' Monument

Jeweled portal welcoming returned soldiers

ARTIFICIAL LIGHT HONORING THOSE WHO FELL AND THOSE WHO RETURNED]

[Illustration]

[Illustration: THE EXPRESSIVENESS OF LIGHT IN CHURCHES]

In much of the foregoing discussion the present predominating theatricalproductions are not considered, for the lighting effects are good enoughfor them. Many ingenious tricks and devices are resorted to in theseproductions, and as a whole lighting is serving effectively enough. Butin considering the expressiveness of light the deeper play is the mediumnecessary for utilizing the potentiality of light. These are rare andunfortunately the stage artist appreciative of the significations andemotional value of light and color is still rarer. 

The equipment of the present stage consists of footlights, side-lights, border-lights, flood-lights, spot-lights, and much special apparatus. One of the severest criticisms of stage-lighting from an artistic pointof view may be directed against the use of footlights for obtaining thedominant light. This is directed upward and the effect is an unnaturaland even a grotesque modeling of the actors' features. The shadowsproduced are incongruous, for they are opposed to the other real andpainted effects of light and shade. The only excuse for such lighting isthat it is easily done and that proper lighting is difficult to obtain, owing to the fact that it involves a change in construction. By no meansshould the footlights be abandoned, for they would still be invaluablein obtaining diffused light even when the dominant light is directedfrom above the horizontal. In the present stage-lighting, in which thefootlights generally predominate, the expressiveness of light is notsatisfactory. Perhaps they are a necessary compromise, but inasmuch astheir effect is unnatural they should not be accepted until it isthoroughly proved that ingenuity cannot eliminate the present defects. 

The stage as a whole is a mobile picture in light, shade, and color withthe addition of words and music. Excepting the latter, it is anexpression of light worthy of the same care and consideration that thepainting, which is also an expression of light, receives from theartist. The scenery and costumes should be considered in terms of thelighting effects because they are affected by changes in the color ofthe light. In fact, the author showed a number of years ago that bycarefully relating the colors of the light with the colors used inpainting the scenery, a complete change of scene can be obtained bymerely changing the color of the light. Rather wonderful dissolvingeffects can be produced in this manner without shifting scenery. Forexample, a warm summer scene with trees in full foliage under a yellowlight may be changed under a bluish light to a winter scene with groundcovered with snow and trees barren of leaves. But before suchaccomplishments can be realized upon the stage, scientific knowledgemust be available behind the scenes. 

The art museum affords a multitude of opportunities for utilizing theexpressiveness of light. This is more generally true of sculpturedobjects than of paintings because the latter may be treated as a whole. The artist almost invariably paints a picture by daylight and unless itis illuminated by daylight it is altered in appearance, that is, itbecomes another picture. The great difference in the appearance of apainting under daylight and ordinary artificial light is quitestartling, when demonstrated by means of apparatus in which the twoeffects may be rapidly alternated. Art museums are supposed to exhibitthe works of artists and, therefore, no changes in these works should betolerated if they can be avoided. The modern artificial-daylight lampsmake it possible to illuminate galleries with light at night whichapproximates daylight. A further advantage of artificial light is thatit may be easily controlled and a more satisfactory lighting may beobtained than with natural light. Considering the cost of daylight inmuseums and its disadvantages it appears possible that artificialdaylight with its advantages may replace it eventually in the largegalleries. If the works of artists are really prized for theirappearance, the lighting of them is very important. 

Sculpture is modeled by light and although it is impossible to ascertainthe lighting under which the sculptor viewed his completed work withpride and satisfaction, it is possible to give the best consideration toits lighting in its final place of exhibition. The appearance of asculpture depends upon the dominant direction of the light, thesolid-angle subtended by the light-source (skylight, area of sky, etc. )and the amount of scattered light. The direction of dominant lightdetermines the general direction of the shadows; the solid-angle of thelight-source affects the character of the edges of the shadows; and thescattered light accounts for the brightness of the shadows. It should beobvious that variations of these factors affect the appearance orexpression of three-dimensional objects. Therefore the position of asculptured object with respect to the window or other skylight and theamount of light reflected from the surroundings are important. Visits toart museums with these factors in mind reveal a gross neglect in thelighting of objects of art which are supposed to appeal by virtue oftheir appearances, for they can arouse the emotions only through thedoorway of vision. 

A century ago mankind gave no thought to utilizing the expressive andimpressive powers of light except in religious ceremonies. It was notpracticable to utilize light from the feeble flames of those days in theelaborate manner necessary to draw upon these powers. Man was concernedwith the more pressing needs. He wanted enough light to make the winterevenings endurable and the streets reasonably safe. The artists of thosedays saw the wonderful expressions of light exhibited by Nature, butthey dared not dream of rivaling these with artificial light. To-dayNature surpasses man in the production of lighting effects only inmagnitude. Man surpasses her artistically. In fact, the artist becomes amaster only when he can improve upon her settings; when he is able byrare judgment in choosing and in eliminating and by skill and ingenuityto substitute a complete harmony for her incomplete and unsatisfactoryreality. But everywhere Nature is the great teacher, for her world isfull of an everchanging infinitude of expressions of light. Mankindneeds only to study these with an attuned sensibility to be ableeventually to play the music of light for those who are blessed with anesthetic sense. 

XXIV

LIGHTING THE HOME

In the home artificial light exerts its influence upon every one. Without artificial lighting the family circle may not have become theimportant civilizing influence that it is to-day. Certainly civilizedman now shudders at the thought of spending his evenings in the light ofthe fire upon the hearth or of a burning splinter. 

The importance of artificial light is emphatically impressed upon thehouseholder when he is forced temporarily to depend upon the primitivecandle through the failure of the modern system of lighting. He fleesfrom his home to that of his more fortunate neighbor, or he retires inhis helplessness to awaken in the morning with a blessing for daylight. He cannot conceive of happiness and recreation in the homes of a centuryor two ago, when a few candles or an oil-lamp or two were the solesources of light. But when the electric or gas service is again restoredhe relapses shortly into his former placid indifference toward thewonderfully efficient and adequate artificial light of the present age. 

Until recently artificial light was costly and the householder in commonwith other users of light did not concern himself with the question ofadequate and artistic lighting. His chief aim was to utilize as littleas possible, for cost was always foremost in his mind. The developmentof the science of light-production has been so rapid during the pastgeneration that adequate, efficient, and cheap artificial light findsmankind unconsciously viewing lighting with the same attitude as hedisplays toward his food and fuel bills. Another consequence of thisrapid development is that mankind does not know how to extract the joyfrom modern artificial light. This is readily demonstrated by analyzingthe lighting of middle-class homes. 

The cost of light has been discussed in another chapter and it has beenshown that it has decreased enormously in a century. It is now the mostpotential agency in the home when viewed from the standpoint of cost. The average householder pays less than twenty dollars per year forever-ready light throughout his home. For about five cents per day theaverage family enjoys all the blessings of modern lighting, which issufficient proof that cost is an insignificant item. 

In order to simplify the discussion of lighting the home the terminologyof electric-lighting will be used. The principles expounded apply aswell to gas as to electricity, and owing to the ingenuity of thegas-lighting experts, the possibilities of gas-lighting are extensivedespite its handicaps. There are some places in the home, such as thekitchen and basement, where lighting is purely utilitarian in the narrowsense, but in most of the rooms the esthetic or, more broadly, thepsychological aspects of lighting should dominate. Pure utility isalways a by-product of artistic lighting and furthermore, the lightingeffects will be without glare when they satisfy all the demands ofesthetics. 

In dealing with lighting in the home the householder should concentratehis attention upon lighting effects. Unfortunately, he is not taught todo so, for everywhere he turns for help he finds the discussion directedtoward fixtures and lamps instead of toward lighting effects. However, these are merely links in the chain from the meter to the eye. Lamps areof interest from the standpoint of quantity and quality of light, andfixtures are of importance chiefly as distributers of light. Thesedetails are merely means to an end and the end is the lighting effect. Of course, the fixtures are more important as objects than the wiresbecause they are visible and should harmonize with the generaldecorative and architectural scheme. 

The home is the theater of life full of various moods and occasions;hence the lighting of a home should be flexible. A degree of varietyshould be possible. Controls, wiring, outlets, and fixtures shouldconspire to provide this variety. At the present time the averagehouseholder does not give much attention to lighting until he purchasesfixtures. It is probable that he thought of it when he laid out orapproved the wiring, but usually he does not consider it seriously untilhe visits the fixture-dealer to purchase fixtures. And thenunfortunately the fixture-dealer does not light his home; he does notsell the householder lighting-effects designed to meet the requirementsof the particular home; he sells merely fixtures. 

Unfortunately there are few fixtures available which have definite aimsin lighting as demanded by the home. Of the great variety of fixturesavailable there are many artistic objects, but it is obvious that littleattention is given to their design from the standpoint of lighting. That the fixture-dealer usually thinks of fixtures as objects and giveslittle or no thought to lighting effects is apparent from hisconversation and from his display. He exhibits fixtures usually en masseand seldom attempts to illustrate the lighting effects produced in theroom. 

The foregoing criticisms are presented to emphasize the fact thatthroughout the field of lighting the great possibilities which have beenopened by modern light-sources are not fully appreciated. The point atwhich to begin to design the lighting for a home is the wiring. Unfortunately this is too often done by a contractor who has given nospecial thought to the possibilities of lighting and to the requirementsin wiring and switches necessary in order to realize them. At this pointthe householder should attempt to form an opinion as to the relativevalues. Is artificial lighting important enough to warrant anexpenditure of two per cent. Of the total investment in the home and itsfurnishings? The answer will depend upon the extent to which artificiallight is appreciated. It appears that four or five per cent. Is not toomuch if it is admitted that the artificial lighting system ranks next tothe heating plant in importance and that these two are the mostimportant features of an interior of a residence. A switch or abaseboard outlet costs an insignificant sum but either may pay foritself many times in the course of a few years through its utility orconvenience. 

It appears best to take up this subject room by room because therequirements vary considerably, but in order to be specific in thediscussions, a middle-class home will be chosen. The more importantrooms will be treated first and various simple details will be touchedupon because, after all, the proper lighting of a home is realized byattention to small details. 

The living-room is the scene of many functions. It serves at times forthe quiet gathering of the family, each member devoted to reading. Atanother time it may contain a happy company engaged at cards or inconversation. The lighting requirements vary from a spot or two of lightto a flood of light. Excepting in the small living-rooms there does notappear to be a single good reason for a ceiling fixture. It is nearlyalways in the field of vision when occupants are engaged inconversation, and for reading purposes the portable lamp of satisfactorydesign has no rival. Wall brackets cannot supply general lightingwithout being too bright for comfort. If they are heavily shaded theymay still emit plenty of light upward, but the adjacent spots on thewalls or ceiling will generally be too bright. Wall brackets may bebeautiful ornaments and decorative spots of light and have a right toexist as such, but they cannot be safely depended upon for adequategeneral lighting on those occasions which demand such lighting. 

As a general principle, it is well to visualize the furniture in theroom when looking at the architect's drawings and it is advantageouseven to cut out pieces of paper representing the furniture in scale. Byplacing these on the drawings the furnished room is readily visualizedand the locations of baseboard outlets become evident. It appears thatthe best method of lighting a living-room is by means of decorativeportable lamps. Such lamps are really lighting-furniture, for they aidin decorating and in furnishing the room at all times. A number of theselamps in the living-room insures great flexibility in the lighting, andthe light may be kept localized if desired so that the room is restful. A room whose ceiling and walls are brilliantly illuminated is not socomfortable for long periods as one in which these areas are dimlylighted. Furthermore, the latter is more conducive to reading and toother efforts at concentration. The furniture may be readily shifted asdesired and the portable lamps may be rearranged. 

Such lighting serves all the purposes of the living-room excepting thoserequiring a flood of light, but it is easy to conceal elaborate lightingmechanisms underneath the shades of portable lamps. Several types ofportable lamps are available which supply an indirect component as wellas direct light. The former illuminates the ceiling with a flood oflight without any discomforting glare. Such a lighting-unit is one ofthe most satisfactory for the home, for two distinct effects and acombination of these introduce a desirable element of variety into thelighting. Not less than four and preferably six baseboard outlets shouldbe provided in a living-room of moderate size. One outlet on the mantelis also to be desired for connecting decorative candlesticks, andbrackets above the fireplace are of ornamental value. Although theabsence of ceiling fixtures improves the appearance of the room, wiringmay be provided for ceiling outlets in new houses as a matter ofinsurance against the possible needs of the future. When ceilingfixtures are not used, switches may be provided for the mantel bracketsor certain baseboard outlets in order that light may be had uponentering the room. 

The merits of a portable lamp may be ascertained before purchasing byactual demonstration. Some of them are not satisfactory forreading-lamps, owing to the shape of the shade or to the position of thelamps. The utility of a table lamp may be determined by placing it upona table and noting the spread of light while seated in a chair besideit. A floor lamp may also be tested very easily. A miniature floor lampabout four feet in height with an appropriate shade provides anexcellent lamp for reading purposes because it may be placed by the sideof a chair or moved about independent of other furniture. A tall floorlamp often serves for lighting the piano, but small piano lamps may befound which are decorative as well as serviceable in illuminating themusic without glare. 

The dining-room presents an entirely different problem for the settingis very definite. The dining-table is the most important area in theroom and it should be the most brilliantly illuminated area in the room. A demonstration of this point is thoroughly convincing. The decoratorwho designs wall brackets for the dining-room is interested in beautifulobjects of art and not in a proper lighting effect. The fixture-dealer, having fixtures to sell and not recognizing that he could fill a cryingneed as a lighting specialist, is as likely to sell a semi-indirect oran indirect lighting fixture as he is to provide a properly balancedlighting effect with the table brightly illuminated. The indirect andsemi-indirect units illuminate the ceiling predominantly with theresult that this bright area distracts attention from the table. Abrightly illuminated table holds the attention of the diners. Lightattracts and a semi-darkness over the remainder of the room crowds inwith a result that is far more satisfactory than that of a dining-roomflooded with light. 

The old-fashioned dome which hung over the dining-table has served well, for it illuminated the table and left the remainder of the room dimlylighted. But its wide aperture made it necessary to suspend it ratherlow in order that the lamps within should not be visible. It is anobtrusive fixture and despite its excellent lighting effect, it went outof style. But satisfactory lighting principles never become antiquated, and as taste in fixtures changes the principles may be retained in newfixtures. Modern domes are available which are excellent for thedining-room if the lamps are well concealed. The so-called showers aresatisfactory if the shades are dense and of such shape as to conceal thelamps from the eyes. Various modifications readily suggest themselves tothe alert fixture-designer. Even the housewife can do much with silkshades when the principle of lighting the dining-table is understood. The so-called candelabra have been sold extensively for dining-rooms andthey are fairly satisfactory if equipped with shades which reflect muchof the light downward. Semi-indirect and indirect fixtures have manyapplications in lighting, but they do not provide the proper effect fora dining-room. 

It is easy to make a special fixture which will send a component oflight downward to the table and will permit a small amount of diffusedlight to the ceiling and walls. If a daylight lamp is used for thedirect component, the table will appear very beautiful. Under this lightthe linen and china are white, flowers and decorations on the chinaappear in their full colors, the silver is attractive, and the variouscolor-harmonies such as butter, paprika, and baked potato are enticing. This is an excellent place for a daylight lamp if diffused lightilluminating the remainder of the room and the faces of the diners is ofa warm tone obtained by warm yellow lamps or by filtering thesecomponents of the light through orange shades. The ceiling fixtureshould be provided with two circuits and switches. In some cases it iseasy to provide a dangling plug for connecting such electric equipmentas a toaster, percolator, or candlesticks. Two candlesticks areeffective on the buffet, but usually the smallest normal-voltage lampsavailable give too much light. Miniature lamps may be used with a smalltransformer, or two regular lamps may be connected in series. At leasttwo baseboard outlets are convenient. 

The foregoing deals with the more or less essential lighting of adining-room, but there are various practicable additional lightingeffects which add much charm to certain occasions. Colored light of lowintensity obtained from a cove or from "flower-boxes" fastened upon thewall is very pleasing. If a cove is provided around the room, twocircuits containing orange and blue lamps respectively will supply twocolors widely differing in effect. By mixing the two a beautiful rosetint may be obtained. This equipment has been installed with muchsatisfaction. A simpler method of obtaining a similar effect is to useimitation flower-boxes plugged into wall outlets. Artificial foliageadds to the charm of these boxes. The colored light is merely to addanother effect on special occasions and its intensity should never behigh. In the dining-room such unusual effects are not out of place andthey need not be garish. 

The sun-room partakes of the characteristics of the living-room to someextent, but, it being smaller, a semi-indirect fixture may besatisfactory for general illumination. However, a portable lamp whichsupplies an indirect component of light besides the direct light servesadmirably for reading as well as for flooding the room with light whennecessary. Two or three baseboard outlets are desirable for attachingdecorative or even purely utilitarian lamps. The sun-room is anexcellent place for utilizing "flower-box" fixtures decorated withartificial foliage. In fact, a central fixture may assume the appearanceof a "hanging basket" of foliage. The library and den offer no problemsdiffering from those already discussed in the living-room. A carefulconsideration of the disposition of the furniture will reveal the bestpositions for the outlets. In a small library wall brackets may serve asdecorative spots of light and if the shades are pendent they may serveas lamps for reading purposes. In both these rooms an excellentreading-lamp is desired, but it may be decorative as well. Wall outletsmay be desired for decorative portable lamps upon the bookcases. 

The sleeping-room, which commonly is also a dressing-room, oftenexhibits the errors of a lack of foresight in lighting. In most rooms ofthis character there is one best arrangement of furniture and if thisis determined it is easy to ascertain where the windows and outletsshould be located. The windows may usually be arranged for twin beds aswell as for a single one with obvious advantages of flexibility inarrangement. With the position of the bureau determined it is easy tolocate outlets for two wall brackets, one on each side, about sixty-sixinches above the floor and about five feet apart. When the brackets areequipped with dense upright shades, the figure before the mirror is wellilluminated without glare and sufficient light reaches the ceiling toilluminate the whole room. 

A baseboard outlet should be available for small portable lamps whichmay be used upon the bureau or for electric heating devices. The same istrue for the dressing-table; indeed, two small decorative lamps on thetable serve better than high wall brackets owing to the fact that theuser is seated. A baseboard outlet near the head of the bed or betweenthe beds is convenient for a reading-lamp and for other purposes. Anoutlet in the center of the ceiling controlled by a convenient switchmay be installed on building, as insurance against future needs ordesires. But a single lighting-unit in the center of the ceiling doesnot serve adequately the needs at the bureau and dressing-table. Infact, two wall brackets properly located with respect to the bureauafford a lighting much superior for all purposes in the bedroom to thatproduced by a ceiling fixture. 

In the bath-room the principal problem is to illuminate the person, especially the face, before the mirror. Many mistakes are made at thispoint, despite the simplicity of the solution. In order to see theimage of an object in a mirror, the object must be illuminated. It isbest to do this in a straightforward manner by means of a smalllighting-unit on each side of the mirror at a height of five feet. Bothsides of the face will be well illuminated and the light-sources are lowenough to eliminate objectionable shadows. The units may be merelypull-chain sockets containing frosted or opal lamps. A center bracket ora single unit suspended from the ceiling is not as satisfactory as thetwo brackets. These afford enough light for the entire bath-room. Abaseboard or wall outlet is convenient for connecting a heater, curling-iron, and other electrically heated devices. 

The sewing-room, which in the middle-class home is usually a small room, is sometimes used as a bedroom. A ceiling fixture will supply adequategeneral lighting, but a baseboard outlet should be available for a shortfloor lamp or a table lamp for sewing purposes. An intense local lightis necessary for this occupation, which severely taxes the eyes. Aso-called daylight lamp serves very well in this case. 

[Illustration: OBTAINING TWO DIFFERENT MOODS IN A ROOM BY A PORTABLELAMP WHICH SUPPLIES DIRECT AND INDIRECT COMPONENTS OF LIGHT]

[Illustration: THE LIGHTS OF NEW YORK CITY

Towering shafts of light defy the darkness and thousands of lightedwindows symbolize man's successful struggle against nature]

In the kitchen the wall brackets are easily located after the positionsof the range, work-table, sink, etc. , are determined. A bracket for eachis advisable unless they are so located that one will serve twopurposes. It is customary to have a combination fixture for gas andelectricity. This is often suspended from the center of the ceiling, butinasmuch as the gas-light cannot be close to the ceiling, the fixtureextends so far downward as to become a nuisance. Furthermore, alight-source hung low from the center of the ceiling is in such aposition that the worker in the kitchen usually works in his shadow. Ifa ceiling outlet is used it should be an electrical socket at theceiling. The combination fixture is best placed on the wall as abracket. The so-called daylight lamps are valuable in the kitchen. 

In the basement a generous supply of ceiling outlets adds much to thesatisfaction of a basement. One in each locker, one before the furnace, and a large daylight lamp above but to one side of the laundry trays areworth many times their cost. Furthermore, a wall socket for the electriciron and washing-machine is a convenience very much appreciated. 

In the stairways convenient three-way switches for each of the ceilingfixtures represents the best practice. A baseboard outlet in the upperhall affords a connection for a decorative lamp and pays for itself manytimes as a place to attach the vacuum-cleaner from which all the roomson that floor may be served. In vestibules and on porches ceilingfixtures controlled by means of convenient switches are satisfactory. The entrance hall may be made to express hospitality by means oflighting which should be adequate and artistic. 

An adequate supply of outlets and wall switches is not costly and theypay generous dividends. With a scanty supply of these, the possibilitiesof lighting are very much curtailed. There is nothing intricate aboutlocating switches and outlets, so the householder may do this himself, or he may view critically the plans as submitted. The chief difficultiesare to throw aside his indifference and to readjust his ideas andvalues. It may be confidently stated that the possibilities of lightingfar outrank most of the features which contribute to the cost of a houseand of its furnishings. 

After considering the requirements and decorative schemes of the variousrooms the householder should be competent to judge the appropriatenessof the lighting effects obtained from fixtures which the dealerdisplays, but he should insist upon a demonstration. If the dealer isnot equipped with a room for this purpose, he should be asked todemonstrate in the rooms to be lighted. If the fixture-dealer does notrealize that he should be selling lighting effects, the householdershould make him understand that lighting effects are of primaryimportance and the fixtures themselves are of secondary interest in mostcases. The unused outlets that have been installed for possible futureneeds may be sealed in plastering if the positions are marked so thatthey may be found when desired. 

An advantage of portable lamps is that they may be taken away on moving. In fact, when lighting is eventually considered a powerful decorativemedium, as it should be, it is probable that fixtures will be personalproperty attached to ceiling, wall, and floor outlets by means of plugs. 

A variety of incandescent lamps are available. For the home, opal, frosted, or bowl-frosted lamps are usually more satisfactory than clearlamps. Bare filaments should not be visible, for they not only causediscomfort and eye-strain but they spoil what might otherwise be anartistic effect. Lamps with diffusing bulbs do much toward eliminatingharsh shadows cast by the edges of the shades, by the chains of thefixtures, etc. These lamps are available in many shapes and sizes andthe householder should make a record of voltage, wattage, and shape ofthe lamps which he finds satisfactory in the various fixtures. The Mazdadaylight lamp has several places in the home and the Mazda white-glassand other high-efficiency lamps supply many needs better than the vacuumlamps. In brackets and other purely decorative lighting-units smallfrosted lamps are usually the most satisfactory. There is a generaldesire for the warm yellowish light of the candle-flame, and this may beobtained by a tinted shade but usually more satisfactorily by means of atinted lamp. 

The householder will find it interesting to become intimate withlighting, for it can serve him well. The housewife will often find muchinterest in making shades of textiles and of parchment. Charmingglassware in appropriate tints and painted designs is available for allrooms. In the bedchamber and the nursery some of these painted designsare exceedingly effective. Fixtures should shield the lamps from theeyes, and the diffusing media whether glass or textile should be denseenough to prevent glare. No fixture can be beautiful and no lightingeffect can be artistic if glare is present. If the architect and thehouseholder will realize that light is a medium comparable with thedecorator's media, better lighting will result. Light has the greatadvantage of being mobile and with adequate outlets and controlssupplemented by fixtures from which different effects are available, thehouseholder will find in lighting one of the most fruitful sources ofinterest and pleasure. It can do much toward expressing the taste ofthe householder or if neglected it can undo much of the effect ofexcellent decoration and furnishing. Artificial lighting, softlydiffused and properly localized, is one of the most important factors inmaking a house a home. 

XXV

LIGHTING--A FINE ART?

In the preceding chapters the progress of light has been sketched fromits obscure infancy to its vigorous youth of the present time. It hasbeen seen that progress was slow until the beginning of the nineteenthcentury, after which it began to gain momentum until the present centuryhas witnessed tremendous advances. Until the latter part of thenineteenth century artificial light was considered an expensive utility, but as modern lamps appeared which supplied adequate light at reasonablecost attention began to be centered upon utilization, and the lightingengineer was born. Gradually it is being realized that artificial lightis no longer a luxury, that it may be used in great quantity, and thatit may be directed, diffused, and altered in color as desired. Althoughthe potentiality of light has been barely drawn upon, the present usagessurpass the most extravagant dreams of civilized beings a half-centuryago. Mere light of that time was changed into more light as gas-lightingdeveloped, and more light has increased to adequate light of the presenttime through the work of scientists. 

It is apparent that a sudden enforced reversion to the primitive flamesof fifty years ago would paralyze many activities. Much of interest andbeauty would be blotted out of this brilliant, pulsating, productiveage. It is startling to note that almost the entire progress inartificial lighting has taken place during the past hundred years andthat most of it has been crowded into the latter part of this period. Infact, its development since it began in earnest has gone forward withever-increasing momentum. On viewing the wonders of modern artificiallighting on every hand it is not difficult to muster the couragenecessary to venture into its future. 

The lighting engineer has been a natural evolution of the present age, for the economic aspects of lighting have demanded attention. He isincreasing the safety, efficiency, and happiness of mankind andcivilization is beginning to feel his influence economically. However, with the advent of adequate, efficient, and controllable light, thepotentiality of light as an artistic medium may be drawn upon and thelighting artist with a deep insight into the possibilities of artificiallight now has his opportunity. But the artist who believes that a newart may be evolved to perfection in a few years is doomed todisappointment, for it is necessary only to view retrospectively sucharts as painting and music to be convinced that understanding andappreciation develop slowly through centuries of experiment and contact. 

Will lighting ever become a fine art? Will it ever be able alone toarouse emotional man as do the fine arts? Are the powers of lightsufficiently great to enthrall mankind without the aid of form, music, action, or spoken words? It is safer to answer "yes" than "no" to thesequestions. Painting has reached a high place as an art and this art isthe expressiveness of secondary or reflected light reinforced byimitation forms, which by a combination of light and drawing comprisethe "subjects. " A painting is a momentary expression of light, across-section of something mobile, such as nature, thought, or action. Light has the essential qualifications of painting with the advantagesof a greater range of brightness, of greater purity of colors, and thegreat potentiality of mobility. If lighting becomes a fine art it willdoubtless be related to painting somewhat in the same manner thatarchitecture is akin to sculpture. With the introduction of mobility itwill borrow something from the arts of succession and especially frommusic. 

The art of lighting in its present infancy is leaning upon establishedarts, just as the infant learns to walk alone by first depending uponsupport. The use of color in painting developed slowly, being supportedfor centuries by the strength of drawing or subject. The landscapes of acentury ago were dull, for color was employed hesitatingly andsparingly. The colors in the portraits of the past merely representedthe gorgeous dress of bygone days. But the painter of the present showsthat color is beginning to be used for itself and that the painter is nolonger hesitant concerning its power to go hand in hand with drawing. Drafting and coloring are now in partnership, the former having given upguardianship when the latter reached maturity. 

Lighting is now an accompaniment of the drama, of the dance, ofarchitecture, of decoration, and of music. It has been a background or apart of the "atmosphere" excepting occasionally when some one withimagination and daring has given it the leading rôle. Even in itsinfancy it has on occasions performed admirably almost without any aid. The bursting rocket, the marvelous effects at the Panama-PacificExposition, and some of the exhibitions on the theatrical stage areglimpses of the potentiality of light. To fall back upon the terminologyof music, these may be glimmerings of light-symphonies. 

Harmony is simultaneity and a painting in this respect is a chord--amomentary expression fixed in material media. A melody of light requiressuccession just as the melody in music. The restless colors of the opalcomprise a light melody like the songs of birds. The gorgeous splendorof the sunset compares in magnitude and in its various moods with thesymphony orchestra and its powers. Throughout nature are to be foundgentle chords, beautiful melodies and powerful symphonies of light andthis music of light exhibits the complexity and structure analogous tomusic. There is no physical relation between music, poetry, and light, but it is easy to lean upon the established terminology for purposes ofdiscussion. Those who would build color-music identical to sound musicare making the mistake of starting with a physical foundation instead ofbasing the art of light-expression upon psychological effects of light. In other words, a relation between light and music can exist only in thepsychological realm. 

These melodies and symphonies of light in nature are admittedly pleasingor impressive as the case may be, but are they as appealing as music, poetry, painting, or sculpture? The consensus of opinion of a largegroup of average persons might indicate a negative reply, but thecombined opinion of this group is not so valuable as the opinion of acolorist or of an artist who has sensed the wonders of light. Theunprejudiced opinion of artists is that light is a powerfully expressiveand impressive medium. The psychologist will likely state that theemotive value of light or color is not comparable to the appeal of anexcellent dinner or of many other commonplace things. But he hasexperimented only with single colors or with simple patterns and hissubjects are selected more or less at random from the multitude. Whatwould be his conclusion if he examined painters and others who havedeveloped their sensibilities to a deep appreciation of light and color?It is certain that the painter who picks up a purple petal fallen from arose and places it upon a green leaf is as thrilled by the powerfulvibrant color-chord as the musician who hears an exquisite harmony ofsounds. 

Music has been presented to civilized mankind in an organized manner forages and the fundamental physical basis of modern music is a thousandyears old. Would the primitive savage appreciate the modern symphonyorchestra? Even the majority of civilized beings prefer the modernragtime or jazz to the exquisite art of the symphony. An appreciation ofthe opera and the symphony is reached by educational methods extendingover long periods. An appreciation of the expressiveness of light cannotbe expected to be realized by any short-cut. Most persons to-day enjoythe melodramatic "movie" more than the drama and relatively fewexperience the deep appeal of the fine arts. Surely the symphony oflight cannot be justly condemned because of a lack of appreciation andunderstanding of it, for it has not been introduced to the public. Furthermore, the expressiveness of music is still indefinite at bestdespite the many centuries of experimenting on the part of musicians. 

If poetry is to be believed, the symphonies of light as rendered bynature in the sunsets, in the aurora borealis, and in other sky-effectsof great magnitude have deeply impressed the poet. If his descriptionsare to be accepted at their face-value, the melodies of light renderedin the precious stone, in the ice-crystal, and in the iridescence ofbird-plumage please his finer sensibilities. If he is sincere, mobilelight is a seductive agency. 

The painter has contributed little of direct value in developing themusic of light. He is concerned with an instantaneous expression. Hewaits for it patiently and, while waiting, learns to appreciate thefickleness of mood in nature, but when he fixes one of these moods hehas contributed very little to the art of mobile light. Unfortunatelythe art schools teach the student little or nothing pertaining to colorfor color's sake. When the student is capable of drawing fairly well andis acquainted with a few stereotyped principles of color-harmony he issent forth to follow in the footsteps of past masters. He may be seen atthe art museum faithfully copying a famous painting or out in the fieldsstalking a tree with the hopes of an embryo Corot. The world moves andhas only a position in the rank and file for imitators. Occasionally anartist goes to work with a vim and indulges in research, therebydemonstrating originality in two respects. Painting is just as much afield for research as light-production. 

Recently experiments are being made in the production of color-harmoniesdevoid of form. Surely there is a field for pure color-composition andthis the field of the painter which leads toward the art of mobilelight. Many of the formless paintings of the present day which passunder the banner of this _ism_ or that are merely experiments in theexpressiveness of light. Being formless, they are devoid of subject inthe ordinary sense and cannot be more or less than a fixed expression oflight. Naturally they have received much criticism and have beenridiculed, but they can expect nothing else until they are understood. They cannot be understood until mankind learns their language and thenthey must be understandable. In other words, there are impostorsgathered around the sincere research-artist because the former haveneither the ability to paint for a living nor the inclination to forsakethe comparative safety of the mystery of art for the practical worldwhere their measure would be quickly taken. This army of camp-followerswill not advance the art of mobile light, but the sincere seekers afterthe principles of light-expression who form the foundation of thevarious _isms_ may contribute much. 

The painter will always be available with his finer sensibility toappreciate and to aid in developing the art of mobile light, but hisdirect contribution appears most likely to come from the present chaosof experiments in pure color-composition, in the psychology of light, or, more broadly, in the expressiveness of light. The decorator and thedesigner of gowns and costumes do not arrogate to themselves the name"artist, " but they are daily creating something which is leading towarda fuller appreciation of the expressiveness of light. If they do notcontribute directly to the development of the art of mobile light, theyare at least aiding in developing what may eventually be an appreciativepublic. 

The artist paints a "still-life, " the decorator creates a color-harmonyof abstract or conventional forms, and the costumer produces acolor-composition in textiles. The decorator and costumer approachcloser to pure color-composition than the artist in his still-life. Thelatter is a grouping of objects primarily for their color-notes. Whybother with a banana when a yellow-note is desired? Why utilize theabstract or conventional forms of the decorator? Why not follow thislead further to the less definite forms employed by the costumer? Whynot eliminate form even more completely? This is an important point andan interesting lead, for to become rid of form has been one of theperplexing problems encountered by those who have dreamed of an art ofmobile light. 

The painter who uses line and color imitatively has perhaps acquiredskill in depicting objects and more or less appreciation of thebeautiful. But if he is to be creative and to produce a higher art hemust be able to use line and color without reference to objects. He thusmay aid in the development of an abstract art which is the higher artand at the same time aid in educating the public to appreciate purecolor-harmonies. From these momentary expressions of light and from theexperience gained, the mobile colorist would receive material aid andhis productions would be viewed by a more receptive audience or rather"optience" as it may be called. The development of taste for abstractart is needed in order that the art of mobile light may develop and, incidentally, an appreciation of the abstract in art is needed in allarts. 

Science has contributed much by way of clearing the decks. It hasproduced the light-sources and the apparatus for controlling light. Ithas analyzed the physical aspects of color-mixture and has accumulatedextensive data pertaining to color-vision. It has pointed out pitfallsand during recent years has been delving further by investigating thepsychology of light and color. The latter field is looked to forvaluable information, but, after all, there is one way of makingprogress in the absence of data and that is to make attempts at theproduction of impressive effects of mobile light. Some of these havebeen made, but unfortunately they have been heralded as finishedproducts. 

Perhaps the most general mistake made is in relating sounds and colorsby stressing a mere analogy too far. Notwithstanding the vibratorynature of the propagation of sound and light, this is no reason forstressing a helpful analogy. After all it is the psychological effectthat is of importance and it is absurd to attribute any connectionbetween light-waves and sound-waves based upon a relation of physicalquantities. No space will be given to such a relation because it is soabsurdly superficial; however, the language of music will be borrowedwith the understanding that no relation is assumed. 

A few facts pertaining to vision will indicate the trend of developmentsnecessary in the presentation of mobile light. The visual processsynthesizes colors and at this point departs widely from the auditoryprocess. The sensation of white may be due to the synthesis of all thespectral colors in the proportions in which they exist in noon sunlightor it may be due to the synthesis of proper proportions of yellow andblue, of red, green, and blue, of purple and green, and a vast array ofother combinations. A mixture of red and green lights may produce anexact match for a pure yellow. Thus it is seen that the mixture oflights will cause some difficulty. For example, the components of amusical chord may be picked out one by one by the trained ear, but iftwo or more colored lights are mixed they are merged completely and theresultant color is generally quite different from any of the components. In music of light, the components of color-chords must be keptseparated, for if they are intermingled like those of musical chordsthey are indistinguishable. Therefore, the elements of harmony in mobilelight must be introduced by giving the components different spatialpositions. 

The visual process is more sluggish than the auditory process; that is, lights must succeed each other less rapidly than musical notes if theyare to be distinguished separately. The ear can follow the most rapidexecution of musical passages, but there is a tendency for colors toblend if they follow one another rapidly. This critical frequency orrate at which successive colors blend decreases with the brightness ofthe components. If red and green are alternated at a rate exceeding thecritical frequency, a sensation of yellow will result; that is, neithercomponent will be distinguishable and a steady yellow or a yellow offlickering brightness will be seen. The hues blend at a lower frequencythan the brightness components of colors; hence there may be a blend ofcolor which still flickers in brightness. Many weird results may beobtained by varying the rate of succession of colors. If this rate is solow that the colors do not tend to merge, they are much enriched bysuccessive contrast. It is known that juxtaposed colors generally enrichone another and this phenomenon is known as simultaneous contrast. Successive contrast causes a similar effect of heightened color. 

An effect analogous to dynamic contrast in music may be obtained withmobile light by varying the intensity of the light or possibly the area. Melody may be simply obtained by mere succession of lights. Tone-qualityhas an analogy in the variation of the purity of color. For example, agiven spectral hue may be converted into a large family of tints by theaddition of various amounts of white light. Rhythm is as easily appliedto light as to music, to poetry, to pattern, or to the dance, but inmobile lights its limitations already have been suggested. However, itis bound to play an important part in the art of mobile light becauserhythmic experiences are much more agreeable than those which arenon-rhythmic. Rhythm abounds everywhere and nothing so stirs mankindfrom the lowliest savage to the highly cultivated being as rhythmicsequences. 

Many psychological effects of light have been recorded from experimentand observation and affective values of light have been established invarious other byways. It is possible that the degree of pleasureexperienced by most persons on viewing a color-harmony or the delightfulcolor-melody of a sunlit opal may be less than that experienced onlistening to the rendition of music. However, if this were true it wouldoffer no discouragement, because absolute values play a small part inlife. Two events when directly compared apparently may differ enormouslyin their ability to arouse emotions, but the human organism is soadaptive that each in its proper environment may powerfully affect theemotions. For example, those who have sported in aërial antics in theheights of cloudland or have stormed the enemy's trench are stillcapable of enjoying a sunset or the call of a bird to its mate at dusk. The wonderful adaptability of the inner being is the salvation of art aswell as of life. 

In the rendition of mobile light it is fair to give the medium everyadvantage. Sometimes this means to eliminate competitors and sometimesit means to remove handicaps. On the stage light has had competitorswhich are better understood. For example, in the drama words and actionare easily understood, and regardless of the effectiveness of light itwould not receive much credit for the emotive value of the production. In the wonderful harmony of music, dance, and light in certain recentexhibitions, the dance and music overpowered the effects of lightsbecause they speak familiar languages. 

[Illustration: A community Christmas tree

A community song-festival

ARTIFICIAL LIGHT IN COMMUNITY AFFAIRS]

[Illustration: PANAMA-PACIFIC EXPOSITION

Artificial light not only reveals the beauty of decoration andarchitecture but enthralls mankind with its own unlimited powers]

A number of attempts have been made to utilize light as an accompanimentof music and some of them on a small scale have been sincere andcreditable, but a much-heralded exhibition on a large scale a fewyears ago was not the product of deep thought and sincere effort. Forexample, colored lights thrown upon a screen having an area of perhapstwenty square feet were expected to compete with a symphony orchestra inCarnegie Hall. The music reached the most distant auditor in sufficientvolume, but the lighting effect dwindled to insignificance. Withoutentering into certain details which condemned the exhibition in advance, the method of rendition of the light-accompaniment revealed a lack ofappreciation of the problems involved on the part of those responsible. 

Incidentally, it has been shown that the composer of this particularmusical selection with its light accompaniment was psychologicallyabnormal; that is, he was affected with colored audition. It is not yetestablished to what extent normal persons are similarly affected bylight and color. Certainly there is no similarity among the abnormal andnone between the abnormal and normal. 

If light is to be used as an accompaniment to music, it must be given anopportunity to supply "atmosphere. " This it cannot do if confined to aninsignificant spot; it must be given extensity. Furthermore, by the useof diaphanous hangings, form will be minimized and the evanescenteffects surely can be charming. But finally the lighting effects mustfill the field of vision just as the music "fills the field of audition"in order to be effective. There are fundamental objections to the use ofmobile light as an accompaniment to music and therefore the future ofthe art of mobile light must not be allowed to rest upon its successwith music. If it progresses through its relation with music, so muchis gained; if not, the relation may be broken for music is quite capableof standing alone. 

There is a tendency on the part of some revolutionary stage artists togive to lighting an emotional part in the play, or, in other words, toutilize lighting in obtaining the proper mood for the action of theplay. Color and purely pictorial effect are the dominant notes of someof them. All of these modern stage-artists are abandoning theintricately realistic setting, and, as a consequence, light is enjoyinga greater opportunity. In the more common and shallow theatricalproduction, lighting and color effects have many times saved the day, and, although these effects are not of the deeper emotional type, theymay add a spectacular beauty which brings applause where the singing ismediocre and the comedy isn't comedy. The potentiality of lightingeffects for the stage has been barely drawn upon, but as theexpressiveness of light is more and more utilized on the stage, the artof mobile light will be advanced just so much more. Light, color, anddarkness have many emotional suggestions which are easily understood andutilized, but the blending of mobile light with the action is difficultbecause its language is only faintly understood. 

It is futile to attempt to describe a future composition of mobilelight. Certainly there is an extensive variety of possibilities. Asunset may be compressed into minutes or an opalescent sky may be amotif. Varying intensities of a single hue or of allied hues may serveas a gentle melody. Realistic effects may be introduced. Theexpressiveness of individual colors may be taken as a basis forconstructing the various motifs. These may be woven into melody in whichrhythm both in time and in intensity may be introduced. Action may beeasily suggested and the number of different colors, in a broad sense, which are visible is comparable to the audible tones. Shading is aseasily accomplished as in music and the development of this art need notbe inhibited by a lack of mechanical devices and light-sources. Thetools will be forthcoming if the conscientious artist requests them. 

Whatever the future of the art of mobile light may be, it is certainthat the utilization of the expressiveness of light has barely begun. Itmay be that light-music must pass through the "ragtime" stage offireworks and musical-revue color-effects. If so, it is gratifying toknow that it is on its way. Certainly it has already served on a higherlevel in some of the artistic lighting effects in which mobility hasfeatured to some extent. 

If the art does not develop rapidly it will be merely following thecourse of other arts. A vast amount of experimenting will be necessaryand artists and public alike must learn. But if it ever does develop tothe level of a fine art its only rival will be music, because the latteris the only other abstract art. Material civilization has progressed farand artificial light has been a powerful influence. May it not be truethat artificial light will be responsible for the development ofspiritual civilization to its highest level? If mobile light becomes afine art, it will be man's most abstract achievement in art and it maybe incomparably finer and more ethereal than music. If this is realized, artificial light in every sense may well deserve to be known as thetorch of civilization. 

READING REFERENCES

No attempt will be made to give a pretentious bibliography of theliterature pertaining to the various aspects of artificial lighting, forthere are many articles widely scattered through many journals. _TheTransactions of the Illuminating Engineering Society_ afford the mostfruitful source of further information; the _Illuminating Engineer_(London), contains much of interest; and _Zeitschrift fürBeleuchtungswesen_ deals with lighting in Germany. H. R. D'Allemagne hascompiled an elaborate "Historie du Luminaire" which is profuselyillustrated, and L. Von Benesch in his "Beleuchtungswesen" has presentedmany elaborate charts. In both these volumes lighting devices andfixtures from the early primitive ones to those of the nineteenthcentury are illustrated. A few of the latest books on lighting, in theEnglish language, are "The Art of Illumination, " by Bell; "ModernIlluminants and Illuminating Engineering, " by Gaster and Dow;"Radiation, Light and Illumination, " by Steinmetz; "The Lighting Art, "by Luckiesh; "Illuminating Engineering Practice, " consisting of a courseof lectures presented by various experts under the joint auspices of theUniversity of Pennsylvania and the Illuminating Engineering Society;"Lectures on Illuminating Engineering, " comprising a series of lecturespresented under the joint auspices of Johns Hopkins University and theIlluminating Engineering Society; and "The Range of Electric SearchlightProjectors, " by Rey; "The Electric Arc, " by Mrs. Ayrton; "Electric ArcLamps, " by Zeidler and Lustgarten, and "The Electric Arc, " by Childtreat the scientific and technical aspects of the arc. G. B. Barham hasfurnished a book on "The Development of the Incandescent Electric Lamp. ""Color and Its Applications, " and "Light and Shade and TheirApplications, " are two books by Luckiesh which deal with lighting fromunique points of view. "The Language of Color, " by Luckiesh, aims topresent what is definitely known regarding the expressiveness andimpressiveness of color. W. P. Gerhard has supplied a volume on "TheAmerican Practice of Gaspiping and Gas Lighting in Buildings, " and Leedsand Butterfield one on "Acetylene. " A recent book in French by V. Trudelle treats "Lumière Electrique et ses différentes Applications auThéatre. " Many books treat of photometry, power-plants, etc. , but theseare omitted because they deal with phases of light which have not beendiscussed in the present volume. "Light Energy, " by Cleaves, is a largevolume devoted to light-therapy, germicidal action of radiant energy, etc. References to individual articles will often be found in thevarious indexes of publications. 

THE END

INDEX

Aaron, 43

Accidents: 8; street-lighting in relation to, 225 _et seq. _; percentage (table) of, due to improper lighting, 231

Acetylene: 62; light-yield of, 106, 107, 170, 187, 191

Actinic rays: effect of, upon human organism, 275

Africa, public lighting in ancient, 31

Agni, god of fire, 40

Air-pump, 130

Air-raids, 225

Alaska, 18, 29

Alchemy, 20

Aleutians, 18

Alexandria, 43, 163

Allylene, 106

Aluminum, 108, 179, 180

Amiens, Treaty of, 69

Amylene, 106

Aniline dyes, 106

Animal: distinction between, and human being, 3; 15; production of light, 24 _et seq. _; sources of light, 30, 31; oils, 51

Antimony, 294

Antioch, 153

Arago, 114, 196

Archbishop of Canterbury, 49

Archimedes, 19

Arc: lamps, 69, 89; electric, 111 _et seq. _; distinction between spark and, 112; Davy's notes on electric, 113; formation of, 115, 116; Staite and enclosed, 117, 118; principle of enclosed, 118, 119; types of, 120; flame-, 121, 122; luminous, 122; electric, 127; luminous efficiency of electric (table), 124; 160 _et seq. _; -lamp in lighthouses, 168 _et seq. _; magnetite-, 187; 261

Ardois system of signaling, 199

Argand, Ami: 52; inaugurates new era in artificial lighting, 53, 54; 63, 70, 76, 77, 78, 97, 167, 196

Argon, 137

Aristophanes, "The Clouds, " 19

Art Museums, 9, 13, 322, 323

Asbestos, 170

Asia: public lighting in ancient, 31, 39

Automobiles, 238

Babylon, 39

Bacteria: effect of artificial light upon, 272 _et seq. _; 281, 282

Bailey, Prof. L. H. , 250

Baltimore, 98

Bamboo: carbon filaments, 169

Bartholdi, 302, 303

Beacons. _See_ Lighthouses. 

Beck, 186

Beecher, 72

Beeswax, 35, 51

Benzene, 106

Bible, cited on importance of artificial light, 42-44

"Bluebird, The, " Maeterlinck, 9

Blue-prints, 261

Bollman, 98

Bolton, von, 132, 133

Bombs, illuminating, 182 _et seq. _

Boston Light, 164, 165, 166, 177

Bowditch, production of regenerative lamp by, 78, 79

Boy Scouts, 17

Bremer, 120

Bristol University, 252

Brush, 68, 159

Building, 8

Bunsen, 81, 85, 89, 148, 149

Bureau of Mines: cited on open flames, 234; 236

Burning-glasses, 19, 20. _See also_ Lenses. 

Butylene, 106

Byzantium, 34

Cæsar, 163

Canada, 254

Candle-hour, defined, 215

Candles: progress and, 7; 25, 28, 29, 30, 33; religious uses of, 34, 35; as a modern light-source, 36, 37; ceremonial uses of, 38 _et seq. _; 44, 48, 57, 82, 97, 222, 299, 304

Calcium, 107, 108

Carbolic acid, 106

Carbon: 53, 80, 81; physical characteristics of, 80, 81; 90, 104, 105, 128, 129, 144, 170

Carbon filament: 127 _et seq. _; preparation of, 129, 130, 131; luminous efficiency of, 131, 132; lamps, 161; lamps in greenhouses, 250 _et seq. _

Carbons, formation of, 115, 116

Carbureted hydrogen, 75

Carcel, invention of clockwork lamp by, 54, 55

Cat-gut, 130

Ceria, 85, 101

Charleston, S. C. , 185

Charcoal: 113; uses of, for electrodes, 115

Chartered Gas Light and Coke Co. , London, 74

Chemistry: artificial light and, 256-268

Chicago, 62, 304, 305

Chimneys, 54, 60, 62

China, 19, 31, 32

Chlorate of potash, 22

Christ, 33, 46, 47

Christians, "children of light, " 42

Christmas trees, 43, 304

Chromium, 294

Church of England, 49

Cities: economy of artificial lighting in congested, 13

Civilization: effect of artificial light upon, 4 _et seq. _; fire and, 15

Clark, Parker and, 139

Clayton, Dr. : invention of portable gas-light by, 64; quoted, 64, 65; experiments of, with coal-gas, 67

Claude, 147

Cleaves, Dr. , quoted, 276, 277

Clegg, Samuel: 74; gas-lighting accomplishments of, 75, 76

Cleveland, 159

"Clouds, The, " Aristophanes, 19

Coal: 32; as a light-source, 55; supply, 223; 228

Coal-gas: 63 _et seq. _; public lighting by, developed, 70 _et seq. _; analytical production of, 103, 104; yield of, retort (table), 105; analysis of, 106

Coal-mines, 234 _et seq. _

Cobalt, 294

Coke, 68, 105

Cologne, 157, 158

Colomb, Philip, 197

Color: 9; relation of artificial light to, 284 _et seq. _

Colza, 31, 52, 167

Combustion, 82 _et seq. _

Commerce, 8, 97

Constantine, 42

Copper, 262, 295

Cornwall, 63

Cotton: 101; carbon filaments, 129, 130

Cromartie, 78

Crookes, 90, 146

Crosley, Samuel, improvement of gas-meter by, 76

Crusies, 32

Daguerre, 258

Dancing, 346

Davy, Sir Humphrey: 33, 68, 73; first use by, of charcoal for sparking points, 112; notes of, on electric arc, 113; 114

Daylight, artificial, 12: 284 _et seq. _; application of, 287

Daylighting, 12-14

Dollond, 195

Doty, 61, 167

Drake, Col. E. L. , discovery of oil in Pennsylvania by, 56

Drummond, Thomas: 171, 185, 196; quoted on signaling, 197

Dudgeon, Miss, 251, 252

Dyes, 256, 265

East Indies, 29

Eddystone Light, 166, 167

Edison: and problem of electric incandescent filament lamps, 128 _et seq. _; 129; quoted on birth of incandescent lamp, 130

Edward I, 274

Edward VI, 49

Efficiency, effect of artificial light upon, 14

Eggs: relation of artificial light to production of, 247, 248

Egypt: 31; sacredness of light in ancient, 39; 153, 195

Electric filament: 81, 127 _et seq. _: approximate value of, lamps (table), 138

Electric pile: construction of, 111; 127

Electricity: 13, 22; as a light-source, 57; for home-lighting, 62, 84; 87, 89; ignition of gas by, 102; lighting by, 109 _et seq. _

Electromagnetic waves, 68, 86, 87

Electromagnets, 114, 116

Electrodes, 113, 114, 115 _et seq. _; life of, 122

Elizabeth, Queen, 274

England: 32; petroleum discovered in, 56; gas-lighting in, 63 _et seq. _; 166, 251, 274

Erbia, 85

Esquimaux: 18; use of artificial light by, 31

Ethylene, 106

Factories: 13; artificial light in, 239 _et seq. _

Faraday, 113

Filaments, carbon, 129 _et seq. _

Finsen: 273, 274, 275; on stimulating action of artificial light, 277; 279, 280

Fire: importance of, to man, 5 _et seq. _; man's dependence upon, 15; mythical origin of, 16; making, 17 _et seq. _; production of, in the stone age, 18; in early civilization, 19; ancient worship of, 29, 299

Fireflies: 24, 81, 96, 148, 149, 150

"First Men in the Moon, The, " H. G. Wells, cited, 148

Fish: artificial light as bait for, 249

Flame-arcs, 120, 121, 122, 187

Flames: 86, 88, 89; open, 233, 234 _et seq. _

Flint, 33

Fool's gold, 18

Fort Wagner, 185

France: lamps in, 55; early gas-light in, 72

Franchot, invention of moderator lamps by, 55

Frankland, 77

Franklin, Benjamin: 165; quoted, 210-212; 213

Fresnel, 167, 196

Friction, 16, 17

Gas: 13, 22; discovery of coal, 32, 33; early uses of, as light-source, 63 _et seq. _; installment of, pipes in England, 63, 64; Shirley's report on Natural, 66, 67; first public display of, lighting, 69; cost of, lighting, 71; first attempt at industrial, lighting, 72; first English, company, 74; first, explosion, 75; house, lighting, 76, 77; 80, 82; spectrum of, 90; modern, lighting industry, 97 _et seq. _; origin of lighting by, 98; first, works in America, 98; growth of, consumption in United States, 99; electrical ignition applied to, lighting, 102; pressure, 102, 103; water, 105; carbons in, 106; production of Pintsch, 109, 110; salts applied to, flames, 120; 157; Census Bureau figures on cost of, plants, 221, 222; 224, 341

Gas-burners: 63, 64, 77; candle-power of pioneer (table), 79; improvements in, 84

Gas-mantle: 61, 81; influence of, 99; characteristics of, 100 _et seq. _; 187

Gas-meter, Clegg's, 76

Gasolene: lamps, 55; 57

Gassiot, 114

Gauss, 196

Geissler, 146

General Electric Company, 132, 135, 136

Germany: development of lamps in, 56; early gas-lighting in, 72

Glass, 195, 290 _et seq. _

Glowers, 139

Glow-worms, 24

Glycerides, 52

Gold, 293

Gout, 275

Gramme dynamo, 117

Grass: 18; carbon filaments, 129

Greece: 39; sacred lamps in ancient, 41; 42

Greenhouses, carbon-filament lamps in, 250 _et seq. _

Hall of Fame, 134

Happiness, effect of artificial light upon, 14Hayden and Steinmetz, 253

Health, artificial light in relation to, 269-283

Helium, 89

Hemig, 155

Hemp, 21

Henry, William, 75

Herodotus, 56

Hertz, 68

Hertzian waves, 271

Hewitt, Cooper, produces mercury-arcs, 124, 125

Home: artificial light in relation to, 6; lighting, 325 _et seq. _

Hindu: light in, ceremonials, 40

Hudson-Fulton Celebration, 306

Huygens, 195

Hydrocarbons, 82

Hydrogen, 81

Illiteracy, artificial light and, 9

Invention, 7, 97

Iowa, 238

Iridium, 129

Iron, 18, 262, 294

Iron pyrites, 18

Italy, 249

Jablochkov: electric candle of, 117

Jamaica, 19

Jandus, 118, 122

Japan: 19; use of oil in, 30; 281

Jerusalem, 43

Jews: artificial light among, 40

_Journal_, Paris, quoted, 210-212

Kerosene: 57; weight of, lumens, 60; 62, 187, 233

Kitson, platinum-gauze mantle applied by, 61

Laboratories: achievements of, 137

Lamps: 16, 25; Roman, 30; 31; invention of safety, 33; ancient funereal, 39; sacred, of antiquity, 41; ceremonial, 44; scientific development of oil, 51 _et seq. _; Holliday, 55; Carcel, 54, 55; Franchot's moderator, 55; gasolene, 55; development of, in Germany, 56; air pressure, 61; supremacy of oil, ends, 62; Bowditch's, 77, 78; 80, 97; mercury-arc, 126; electric incandescent filament, 127 _et seq. _; gem, 132; tungsten, 133 _et seq. _; luminous efficiency (table) of incandescent filament, 141; 299; in home, 328-333

Lange, 167

Lard-oil, 51

Lavoisier, 195

Lead, 262, 294

Le Bon, 72

"Legend of Montrose, The, " Scott, cited on primitive lighting, 27

Leigh, Edmund, quoted, 226

Lenses, 20, 171 _et seq. _

Libanius, quoted, 153, 154

Liberty, Statue of, 301, 302, 303

Libraries, 9

Light: relation of artificial, to progress, 3 _et seq. _; as a civilizing agency, 3-14; primitive man and artificial, 4; Milton, quoted on importance of, 5; artificial, and science, 7; artificial, and industrial development, 8; Maeterlinck's tribute to, 9; Lincoln's debt to artificial, 9; symbolism of, 9, 10; therapy, 10; in war, 11; adaptations of, 12; 13; mythical origin of artificial, 16; earliest source of, 16; production of, in stone age, 18; matches as source of, 21; animals as, sources, 24, 25; primitive sources of, 24-37; evolution of artificial, sources, 24-37; development, 28 _et seq. _; early outdoor use of artificial, 28; Roman uses of artificial, 30; beginning of scientific, 33, 34; candles as modern, source, 36, 37; symbolism and religious uses of, 38 _et seq. _; Bible cited on artificial, 42-44; in relation to worship, 43, 45, 46; Argand's contribution to, 53, 54; coal as, source, 55; early uses of gas as, source, 63 _et seq. _; as a public utility, 70; first installation of industrial gas, 72; science of, production, 80 _et seq. _; causes of, radiation, 80, 81; 83; lime, 84; electric, 89 _et seq. _; principle of, production, 90, 91; sources, 93; various gas-burners', supply, 95; relative efficiency of, sources, 95, 96; in the home, 97; influence of, upon science, invention, and commerce, 97 _et seq. _; yield of acetylene, 106, 107; electric, 109; influence of gas upon development of artificial, 110; development of artificial, 111 _et seq. _; efforts to improve color of mercury-arc, 125; electric-incandescent-filament, 127 _et seq. _; effect of tungsten, upon, 133 _et seq. _; of the future, 143-152; in warfare, 178-193; signaling, 194-207; cost of, 208-224; and safety, 225 _et seq. _; improper use of, 229, 230; comparison of daylight and artificial, 240; reducing action of, 258; bactericidal action of, 272 _et seq. _; modifying, 284 _et seq. _; spectacular uses of, 298-309; expressiveness of, 310-324; utility of modern, 325-340; evolution of the art of applying, 341-356; mobile, 347, 348, 349, 350; psychological effect of, 351 _et seq. _; as an accompaniment to music, 352-354

Light-buoys, 10, 169

Lighthouses: 10, 163-177; optical apparatus of, 172 _et seq. _

Light-ships, 10, 169

Lighting-systems: comparison of, 12-14

Lime, 84, 107, 108, 294

Lincoln, Abraham, 9

Linen, 18

Link-buoys, 28

Lithopone, 265, 266

Liverpool, 167

Living: comparison of, standards, 238 _et seq. _

London, 152, 154, 155, 156, 157, 202

London Gas Light and Coke Company, 74

Lucigen, 61

Lumen-hour: defined, 215

Lumens: 60, 94, 215

Lutheran Church, 49

Lyceum Theatre, London, 73

Maeterlinck, Maurice, 9

Magazines, 8

Magdsick, H. H. , 303

Magnesia: 84; Nernst's application of, 138

Magnesium, 179, 180

Magnetite arc, 187

Man: distinction between, and animal, 3; artificial light and early, 4; light-sources of primitive, 25

Manganese, 262, 268, 294

Mangin, 188

Mann, 129

Mantles, 95

Manufacturing, 8

Marconi, 68

Marks, 118

Matches: as light-sources, 21; 22, 82

Maxwell, 68

Mazda lamps, 289, 339

Mecca, 40

Mediterranean Sea, 163

Mercury-arc: Way's, 124; 125, 126; quartz, 125, 126; attempts to improve color of, light, 125

Middle Ages, 46, 47, 474

Milton, quoted, 5

Mirror, 19

Mohammedans, 40

Moore, Dr. McFarlan, 146, 147

Morality, effect of light upon, 9

Morse code: application of, to light-signaling, 198, 199

Moses, 195

Moving-pictures, 9, 260, 261

Munich, 72

Murdock, William: installment of gas-pipes by, 63; 68, 69, 70; quoted on industrial use of artificial, 71; 72, 73, 74, 76, 78, 217, 309

Museums: 13; utilization of artificial light by, 322, 323

Music: light as an accompaniment to, 352-354

Mythology, 16

Nantes, 85

Napoleon, 111

Napthalene, 106

National Heat and Light Co. , 72, 74

Natural gas, 99

Navesink Light, 206

Nernst, 138, 139

Newspapers, 8

Newton, Sir Isaac: 7; quoted on discovery of visible spectrum, 87; 88

New York, 98, 165, 166, 206, 302, 304

Niagara Falls, 108, 306

Nickel, 262

Nielson, 77

Niepce, 258

Niter, 21

Nitrogen, 137

Norfolk, 169

Obesity, 275

Offices, 13

Oil: as a light-source, 29 _et seq. _; development of, lamps, 51 _et seq. _; 155; in lighthouse, 165 _et seq. _; 222, 224, 299

O'Leary, Mrs. , and her lamp, 62

Olive-oil, 51, 52, 167

Orkney Islands, 29, 177

Osmium, 133

Oxygen: relative consumption of, by oil-lamps, 58, 59; 262

Ozone, 262

Painting, 342, 343, 347, 348, 349

Pall Mall, 74

Panama-Pacific Exposition: 304; artificial lighting of, 306, 307, 308, 309

Paper: 18; carbon filaments, 129, 130

Paraffin, 35, 57

Parker and Clark, 139

Paris: experimental gas-lighting in, 83, 84; Volta in, 111; 154, 185, 210, 212, 213

Peckham, John, 195

Pennsylvania: discovery of oil in, 56

Periodic Law, 145

Petroleum: 35, 51, 55; discovery of, 56; constitution of crude, 57; 58, 214

Pharos, 163

Philadelphia, 98, 99, 157

Phillips and Lee, 70, 72

"Philosophical Transactions of the Royal Society of London, " 33; quoted on industrial lighting, 63; Shirley's report on natural gas in, 66, 67; quoted, 87

Phoenicians, 34, 39

Phosphorus, 21

Photo-micrography, 12

Photography: 126; early experiments in, 258; development of, 259; 291, 292

Picric acid, 106

Pigments, 265

Pintsch: production of, gas, 109, 110, 170

Pitch, 106

Plant-growth: artificial light and, 11, 249 _et seq. _

Platinum, 85, 128, 129, 262

Plumbago, 113, 130

Plymouth, 166

Poetry, 346

Police, 162

Potash, chlorate of, 22

Priestley, Professor, quoted, 252

Printing, 8

Progress: influence of fire upon, 15 _et seq. _

Prometheus, 16, 41

Propylene, 106

Ptolemy II, 163

Quartz: 18, 19; mercury-arcs, 125; uses of, 126; in skin diseases, 278, 279

Radiators, energy, 88 _et seq. _

Radium, 150

Railway Signal Association, 205

Railways: light-signaling applied to, 205

Ramie fiber, 101

Rane, 250, 251

Rare-earth oxides: 85; properties of, 88, 99

Recreation, 9

Redruth, 63

Reformation: ceremonial uses of light during the, 48, 49

Rheumatism, 275

Robins, Benjamin, 201

Rome, 30, 32, 34, 39, 41, 42, 44

Röntgen, 270, 280. _See also_ X-ray. 

Royal Society of London: 33, 63, 66, 67, 70, 73; and first gas explosion, 75, 111, 112

Rumford, 167

Rushlights, 28, 33

Russia, 281

Ryan, W. D'A. , 306

Safety: artificial light in relation to, 14, 225 _et seq. _

Salts: chemical, 88, 89; metallic, 120; silver, 257, 258

Sandy Hook Light, 165, 166

San Francisco, 304, 306-309

Savages, 3, 15, 17

Sawyer, 129

Scheele, K. W. , 133; quoted, 257, 258

Schools, 9

Science: light and, 6, 7; 97; systematized, 268

Scotland: 26, 31, 32, 48; oil industry in, 56

Scott, Sir Walter, cited, 27, 98

Sculpture: artificial light in relation to, 184

Search-lights, 11, 169

Section of Plant Protection, 225, 226

Selenium, 267, 293

Semaphore, 199

Shells: illuminating, 179 _et seq. _

Shirley, Thomas: quoted on natural gas, 66, 67

Siemens, 78

Signaling, 194-207

Silicon: filament, 140

Silk: artificial, 101; carbon-filaments, 129

Simpson, R. E. , 227, 231

Silver, 258, 293

Skin diseases: treatment of, 278, 279, 280

Skylights, 13

Sleep, 8

Smallpox, 274, 275

Smeaton, 166

Soho, 69, 72

South Africa, 129

Sparks: 33, 125

Spectrum: visible, 86; Newton quoted on, 87; of elements, 89; of gases, 90; 120, 121; mercury, 124-126

Sperm, 31, 51, 52, 167

Spermaceti, 35, 51

Splinter-holders, 27, 28

Stage: and artificial light, 319 _et seq. _; 343

Staite, 117, 118

Stearine, 35, 52

Stearn, 129

Steel, 18, 33

Steinmetz, Hayden and, 253

Sterilization: quartz-mercury-arc and, 280, 281, 282

Stevenson, Robert Louis, quoted, 177

Stores, 13

St. Paul, 43

St. Paul's Cathedral, 300

Street-lighting: development of, 152-162

Sugar, 22

Sulphide of iron, 18

Sulphur, 18, 21, 179, 180, 294

Sulphuric acid, 21, 22

Sun, 8, 16, 19, 20

Swan, 129

Syracuse, 19

Syria, 153

Tallow, 34, 35, 51, 52

Tantalum: 132; filament lamps, 133

Tar, 68, 106

Telegraphy, 195

Telephony, 194

Textiles, 256

Thames, 169

Theaters, 9, 319 _et seq. _

Thoria, 85

Tin, 262

Tinder-boxes, 18, 19, 22

Travelers Insurance Company, 227

Trees, 26

Troy, 42

Tuberculosis, 273

Tungsten lamp, 161 _et seq. _, 187, 261, 290, 303

Typhus, 273

Ultra-violet rays: 126, 150; in photographic electricity, 267, 268; 270, 272, 294

United States: petroleum in, 57; gas-consumption in, 99; 164, 165, 166

United States Geological Survey, cited on sale of gas, 222

United States Military Intelligence, 225, 226

Vacuum tubes, 81, 286

Venetians, 195

Ventilation, 13

Verne, Jules, 143

Vestal Virgins, 42

Volcanoes, 166

Volta, 111, 112, 127

Voltaic pile: construction of, 111, 127

Von Bolton. _See_ Bolton. 

War: and artificial light, 11, 178-193

Washington, 305

Water: sterilization of, by artificial light, 280 _et seq. _

Watson, Dr. Richard, 67, 68

Watt, 94

Waves: electro-magnetic, 68, 86, 125 _et seq. _

Wax, 34, 46, 51

Way: mercury-arc produced by, 124

Wells, 61

Wells, H. G. , cited, 148

Welsbach, Auer von: 61; invention of mantle by, 99, 100, 133

Wenham, 78

West Indies, 25

Whale-oil, 31

Wicks, 35, 36, 53, 54, 58, 59

Winsor, 72, 73. _See also_ Winzler. 

Winzler. _See_ Winsor. 

_Wolfram. _ _See_ Tungsten. 

Wood, 26, 27, 28

Woolworth Building, 302, 303

Wounds: treatment of, by artificial light, 10

X-ray: production of, tubes during War, 131; 137, 150, 270, 280

Young, James: discovers petroleum, 56

Yttria, 85

_Zeitung_, Cologne: 157; extract from, on street-lighting, 158

Zinc, 125, 130, 267

Zirconia, 84, 85

Transcriber's List of Corrections

LOCATION ORIGINAL CORRECTED

Chapter II and similiar material and similar material

Chapter XIII as a constant level at a constant level

Chapter XIV the carbons to distintegrate the carbons to disintegrate

Chapter XV John Pechham John Peckham coated with an allow coated with an alloy with various billiant with various brilliant key in depressed key is depressed

Chapter XVI has nearly doubled have nearly doubled

Chapter XVII this own indifference their own indifference

Chapter XXIII Nature's lighting varied Nature's lighting varies

Chapter XXIV so-called cadelabra so-called candelabra possibilties possibilities

READING REFERENCES . .. Applications an Théatre. " . .. Applications au Théatre. "

INDEX Photo-micography Photo-micrography Siemans Siemens