GENERAL SCIENCE

 BY

 BERTHA M. CLARK, PH. D. 

 HEAD OF THE SCIENCE DEPARTMENT

 WILLIAM PENN HIGH SCHOOL FOR GIRLS, PHILADELPHIA

 NEW YORK - CINCINNATI - CHICAGO

 AMERICAN BOOK COMPANY

 1912

PREFACE

This book is not intended to prepare for college entranceexaminations; it will not, in fact, prepare for any of the present-daystock examinations in physics, chemistry, or hygiene, but it shouldprepare the thoughtful reader to meet wisely and actively some oflife's important problems, and should enable him to pass muster on theprinciples and theories underlying scientific, and therefore economic, management, whether in the shop or in the home. 

We hear a great deal about the conservation of our natural resources, such as forests and waterways; it is hoped that this book will showthe vital importance of the conservation of human strength and health, and the irreparable loss to society of energy uselessly dissipated, either in idle worry or in aimless activity. Most of us would reproachourselves for lack of shrewdness if we spent for any article more thanit was worth, yet few of us consider that we daily expend on domesticand business tasks an amount of energy far in excess of that actuallyrequired. The farmer who flails his grain instead of threshing itwastes time and energy; the housewife who washes with her hands aloneand does not aid herself by the use of washing machine and properbleaching agents dissipates energy sadly needed for other duties. 

The Chapter on machines is intended not only as a stimulus to theinvention of further labor-saving devices, but also as an eye openerto those who, in the future struggle for existence, must perforce goto the wall unless they understand how to make use of contrivanceswhereby man's limited physical strength is made effective for largertasks. 

The Chapter on musical instruments is more detailed than seemswarranted at first sight; but interest in orchestral instruments isreal and general, and there is a persistent desire for intelligentinformation relative to musical instruments. The child of the laboreras well as the child of the merchant finds it possible to attend someof the weekly orchestral concerts, with their tiers of cheap seats, and nothing adds more to the enjoyment and instruction of such hoursthan an intimate acquaintance with the leading instruments. Unlessthis is given in the public schools, a large percentage of mankind isdeprived of it, and it is for this reason that so large a share of thetreatment of sound has been devoted to musical instruments. 

The treatment of electricity is more theoretical than that used inpreceding Chapters, but the subject does not lend itself readily topopular presentation; and, moreover, it is assumed that theinformation and training acquired in the previous work will give thepupil power to understand the more advanced thought and method. 

The real value of a book depends not so much upon the informationgiven as upon the permanent interest stimulated and the initiativearoused. The youthful mind, and indeed the average adult mind aswell, is singularly non-logical and incapable of continuedconcentration, and loses interest under too consecutive thought andsustained style. For this reason the author has sacrificed at timesdetail to general effect, logical development to present-day interestand facts, and has made use of a popular, light style of writing aswell as of the more formal and logical style common to books ofscience. 

No claim is made to originality in subject matter. The actual facts, theories, and principles used are such as have been presented inprevious textbooks of science, but the manner and sequence ofpresentation are new and, so far as I know, untried elsewhere. Theseare such as in my experience have aroused the greatest interest andinitiative, and such as have at the same time given the maximumbenefit from the informational standpoint. In no case, however, ismental training sacrificed to information; but mental development issought through the student's willing and interested participation inthe actual daily happenings of the home and the shop and the field, rather than through formal recitations and laboratory experiments. 

Practical laboratory work in connection with the study of this book isprovided for in my _Laboratory Manual in General Science_, whichcontains directions for a series of experiments designed to make thepupil familiar with the facts and theories discussed in the textbook. 

I have sought and have gained help from many of the standardtextbooks, new and old. The following firms have kindly placed cutsat my disposal, and have thus materially aided in the preparation ofthe illustrations: American Radiator Company; Commercial Museum, Philadelphia; General Electric Company; Hershey Chocolate Company;_Scientific American_; The Goulds Manufacturing Company; VictorTalking Machine Company. Acknowledgment is also due to Professor AlvinDavison for figures 19, 23, 29, 142, and 161. 

Mr. W. D. Lewis, Principal of the William Penn High School, has readthe manuscript and has given me the benefit of his experience andinterest. Miss. Helen Hill, librarian of the same school, has been ofinvaluable service as regards suggestions and proof reading. Miss. Droege, of the Baldwin School, Bryn Mawr, has also been of very greatservice. Practically all of my assistants have given of their time andskill to the preparation of the work, but the list is too long forindividual mention. 

BERTHA M. CLARK. 

WILLIAM PENN HIGH SCHOOL. 

CONTENTS

 CHAPTER

 I. HEAT

 II. TEMPERATURE AND HEAT

 III. OTHER FACTS ABOUT HEAT

 IV. BURNING OR OXIDATION

 V. FOOD

 VI. WATER

 VII. AIR

 VIII. GENERAL PROPERTIES OF GASES

 IX. INVISIBLE OBJECTS

 X. LIGHT

 XI. REFRACTION

 XII. PHOTOGRAPHY

 XIII. COLOR

 XIV. HEAT AND LIGHT AS COMPANIONS

 XV. ARTIFICIAL LIGHTING

 XVI. MAN'S WAY OF HELPING HIMSELF

 XVII. THE POWER BEHIND THE ENGINE

 XVIII. PUMPS AND THEIR VALUE TO MAN

 XIX. THE WATER PROBLEM OF A LARGE CITY

 XX. MAN'S CONQUEST OF SUBSTANCES

 XXI. FERMENTATION

 XXII. BLEACHING

 XXIII. DYEING

 XXIV. CHEMICALS AS DISINFECTANTS AND PRESERVATIVES

 XXV. DRUGS AND PATENT MEDICINES

 XXVI. NITROGEN AND ITS RELATION TO PLANTS

 XXVII. SOUND

 XXVIII. MUSICAL INSTRUMENTS

 XXIX. SPEAKING AND HEARING

 XXX. ELECTRICITY

 XXXI. SOME USES OF ELECTRICITY

 XXXII. MODERN ELECTRICAL INVENTIONS

 XXXIII. MAGNETS AND CURRENTS

 XXXIV. HOW ELECTRICITY MAY BE MEASURED

 XXXV. HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE

 INDEX

GENERAL SCIENCE

CHAPTER I

HEAT

I. Value of Fire. Every day, uncontrolled fire wipes out humanlives and destroys vast amounts of property; every day, fire, controlled and regulated in stove and furnace, cooks our food andwarms our houses. Fire melts ore and allows of the forging of iron, asin the blacksmith's shop, and of the fashioning of innumerable objectsserviceable to man. Heated boilers change water into the steam whichdrives our engines on land and sea. Heat causes rain and wind, fog andcloud; heat enables vegetation to grow and thus indirectly providesour food. Whether heat comes directly from the sun or from artificialsources such as coal, wood, oil, or electricity, it is vitallyconnected with our daily life, and for this reason the facts andtheories relative to it are among the most important that can bestudied. Heat, if properly regulated and controlled, would never beinjurious to man; hence in the following paragraphs heat will beconsidered merely in its helpful capacity. 

2. General Effect of Heat. _Expansion and Contraction_. One of thebest-known effects of heat is the change which it causes in the sizeof a substance. Every housewife knows that if a kettle is filled withcold water to begin with, there will be an overflow as soon as thewater becomes heated. Heat causes not only water, but all otherliquids, to occupy more space, or to expand, and in some cases theexpansion, or increase in size, is surprisingly large. For example, if100 pints of ice water is heated in a kettle, the 100 pints willsteadily expand until, at the boiling point, it will occupy as muchspace as 104 pints of ice water. 

The expansion of water can be easily shown by heating a flask (Fig. I)filled with water and closed by a cork through which a narrow tubepasses. As the water is heated, it expands and forces its way up thenarrow tube. If the heat is removed, the liquid cools, contracts, andslowly falls in the tube, resuming in time its original size orvolume. A similar observation can be made with alcohol, mercury, orany other convenient liquid. 

[Illustration: FIG. 1. --As the water becomes warmer it expands andrise in the narrow tube. ]

Not only liquids are affected by heat and cold, but solids also aresubject to similar changes. A metal ball which when cool will justslip through a ring (Fig. 2) will, when heated, be too large to slipthrough the ring. Telegraph and telephone wires which in winter arestretched taut from pole to pole, sag in hot weather and are much toolong. In summer they are exposed to the fierce rays of the sun, becomestrongly heated, and expand sufficiently to sag. If the wires werestretched taut in the summer, there would not be sufficient leeway forthe contraction which accompanies cold weather, and in winter theywould snap. 

[Illustration: FIG. 2--When the ball is heated, it become too large toslip through the ring. ]

Air expands greatly when heated (Fig. 3), but since air is practicallyinvisible, we are not ordinarily conscious of any change in it. Theexpansion of air can be readily shown by putting a drop of ink in athin glass tube, inserting the tube in the cork of a flask, andapplying heat to the flask (Fig. 4). The ink is forced up the tube bythe expanding air. Even the warmth of the hand is generally sufficientto cause the drop to rise steadily in the tube. The rise of the dropof ink shows that the air in the flask occupies more space thanformerly, and since the quantity of air has not changed, each cubicinch of space must hold less warm air than| it held of cold air; thatis, one cubic inch of warm air weighs less than one cubic inch of coldair, or warm air is less dense than cold air. All gases, if notconfined, expand when heated and contract as they cool. Heat, ingeneral, causes substances to expand or become less dense. 

[Illustration: FIG. 3--As the air in _A_ is heated, it expands andescapes in the form of bubbles. ]

3. Amount of Expansion and Contraction. While most substances expandwhen heated and contract when cooled, they are not all affectedequally by the same changes in temperature. Alcohol expands more thanwater, and water more than mercury. Steel wire which measures 1/4 mileon a snowy day will gain 25 inches in length on a warm summer day, andan aluminum wire under the same conditions would gain 50 inches inlength. 

[Illustration: FIG. 4. --As the air in _A_ is heated, it expands andforces the drop of ink up the tube. ]

4. Advantages and Disadvantages of Expansion and Contraction. We owethe snug fit of metal tires and bands to the expansion and contractionresulting from heating and cooling. The tire of a wagon wheel is madeslightly smaller than the wheel which it is to protect; it is thenput into a very hot fire and heated until it has expanded sufficientlyto slip on the wheel. As the tire cools it contracts and fits thewheel closely. 

In a railroad, spaces are usually left between consecutive rails inorder to allow for expansion during the summer. 

The unsightly cracks and humps in cement floors are sometimes due tothe expansion resulting from heat (Fig. 5). Cracking from this causecan frequently be avoided by cutting the soft cement into squares, thespaces between them giving opportunity for expansion just as do thespaces between the rails of railroads. 

[Illustration: FIG. 5: A cement walk broken by expansion due to sunheat. ]

In the construction of long wire fences provision must be made fortightening the wire in summer, otherwise great sagging would occur. 

Heat plays an important part in the splitting of rocks and in theformation of débris. Rocks in exposed places are greatly affected bychanges in temperature, and in regions where the changes intemperature are sudden, severe, and frequent, the rocks are not ableto withstand the strain of expansion and contraction, and as a resultcrack and split. In the Sahara Desert much crumbling of the rock intosand has been caused by the intense heat of the day followed by thesharp frost of night. The heat of the day causes the rocks to expand, and the cold of night causes them to contract, and these two forcesconstantly at work loosen the grains of the rock and force them out ofplace, thus producing crumbling. 

[Illustration: FIG. 6. --Splitting and crumbling of rock caused byalternating heat and cold. ]

The surface of the rock is the most exposed part, and during the daythe surface, heated by the sun's rays, expands and becomes too largefor the interior, and crumbling and splitting result from the strain. With the sudden fall of temperature in the late afternoon and night, the surface of the rock becomes greatly chilled and colder than therock beneath; the surface rock therefore contracts and shrinks morethan the underlying rock, and again crumbling results (Fig. 6). 

[Illustration: FIG. 7. --Debris formed from crumbled rock. ]

On bare mountains, the heating and cooling effects of the sun are verystriking(Fig. 7); the surface of many a mountain peak is covered withcracked rock so insecure that a touch or step will dislodge thefragments and start them down the mountain slope. The lower levels ofmountains are frequently buried several feet under débris which hasbeen formed in this way from higher peaks, and which has slowlyaccumulated at the lower levels. 

5. Temperature. When an object feels hot to the touch, we say thatit has a high temperature; when it feels cold to the touch, that ithas a low temperature; but we are not accurate judges of heat. Icewater seems comparatively warm after eating ice cream, and yet we knowthat ice water is by no means warm. A room may seem warm to a personwho has been walking in the cold air, while it may feel decidedly coldto some one who has come from a warmer room. If the hand is cold, lukewarm water feels hot, but if the hand has been in very hot waterand is then transferred to lukewarm water, the latter will seem cold. We see that the sensation or feeling of warmth is not an accurateguide to the temperature of a substance; and yet until 1592, onehundred years after the discovery of America, people relied solelyupon their sensations for the measurement of temperature. Very hotsubstances cannot be touched without injury, and hence inconvenienceas well as the necessity for accuracy led to the invention of thethermometer, an instrument whose operation depends upon the fact thatmost substances expand when heated and contract when cooled. 

[Illustration: FIG. 8. --Making a thermometer. ]

6. The Thermometer. The modern thermometer consists of a glass tubeat the lower end of which is a bulb filled with mercury or coloredalcohol (Fig. 8). After the bulb has been filled with the mercury, itis placed in a beaker of water and the water is heated by a Bunsenburner. As the water becomes warmer and warmer the level of themercury in the tube steadily rises until the water boils, when thelevel remains stationary (Fig. 9). A scratch is made on the tube toindicate the point to which the mercury rises when the bulb is placedin boiling water, and this point is marked 212°. The tube is thenremoved from the boiling water, and after cooling for a few minutes, it is placed in a vessel containing finely chopped ice (Fig. 10). Themercury column falls rapidly, but finally remains stationary, and atthis level another scratch is made on the tube and the point is marked32°. The space between these two points, which represent thetemperatures of boiling water and of melting ice, is divided into 180equal parts called degrees. The thermometer in use in the UnitedStates is marked in this way and is called the Fahrenheit thermometerafter its designer. Before the degrees are etched on the thermometerthe open end of the tube is sealed. 

[Illustration: FIG. 9. --Determining one of the fixed points of athermometer. ]

The Centigrade thermometer, in use in foreign countries and in allscientific work, is similar to the Fahrenheit except that the fixedpoints are marked 100° and 0°, and the interval between the points isdivided into 100 equal parts instead of into 180. 

_The boiling point of water is 212° F. Or 100° C_. 

_The melting point of ice is 32° F. Or 0° C_. 

Glass thermometers of the above type are the ones most generally used, but there are many different types for special purposes. 

[Illustration: FIG. 10. --Determining the lower fixed point of athermometer. ]

7. Some Uses of a Thermometer. One of the chief values of athermometer is the service it has rendered to medicine. If athermometer is held for a few minutes under the tongue of a normal, healthy person, the mercury will rise to about 98. 4° F. If thetemperature of the body registers several degrees above or below thispoint, a physician should be consulted immediately. The temperature ofthe body is a trustworthy indicator of general physical condition;hence in all hospitals the temperature of patients is carefully takenat stated intervals. 

Commercially, temperature readings are extremely important. In sugarrefineries the temperature of the heated liquids is observed mostcarefully, since a difference in temperature, however slight, affectsnot only the general appearance of sugars and sirups, but the qualityas well. The many varieties of steel likewise show the influence whichheat may have on the nature of a substance. By observation and tediousexperimentation it has been found that if hardened steel is heated toabout 450° F. And quickly cooled, it gives the fine cutting edge ofrazors; if it is heated to about 500° F. And then cooled, the metal ismuch coarser and is suitable for shears and farm implements; while ifit is heated but 50° F. Higher, that is, to 550° F. , it gives the fineelastic steel of watch springs. 

[Illustration: FIG. 11. --A well-made commercial thermometer. ]

A thermometer could be put to good use in every kitchen; theinexperienced housekeeper who cannot judge of the "heat" of the ovenwould be saved bad bread, etc. , if the thermometer were a part of herequipment. The thermometer can also be used in detecting adulterants. Butter should melt at 94° F. ; if it does not, you may be sure that itis adulterated with suet or other cheap fat. Olive oil should be aclear liquid above 75° F. ; if, above this temperature, it lookscloudy, you may be sure that it too is adulterated with fat. 

8. Methods of Heating Buildings. _Open Fireplaces and Stoves. _Before the time of stoves and furnaces, man heated his modest dwellingby open fires alone. The burning logs gave warmth to the cabin andserved as a primitive cooking agent; and the smoke which usuallyaccompanies burning bodies was carried away by means of the chimney. But in an open fireplace much heat escapes with the smoke and is lost, and only a small portion streams into the room and gives warmth. 

When fuel is placed in an open fireplace (Fig. 12) and lighted, theair immediately surrounding the fire becomes warmer and, because ofexpansion, becomes lighter than the cold air above. The cold air, being heavier, falls and forces the warmer air upward, and along withthe warm air goes the disagreeable smoke. The fall of the colder andheavier air, and the rise of the warmer and hence lighter air, issimilar to the exchange which takes place when water is poured on oil;the water, being heavier than oil, sinks to the bottom and forces theoil to the surface. The warmer air which escapes up the chimneycarries with it the disagreeable smoke, and when all the smoke is gotrid of in this way, the chimney is said to draw well. 

[Illustration: FIG. 12. --The open fireplace as an early method ofheating. ]

As the air is heated by the fire it expands, and is pushed up thechimney by the cold air which is constantly entering through loosewindows and doors. Open fireplaces are very healthful because the airwhich is driven out is impure, while the air which rushes in is freshand brings oxygen to the human being. 

But open fireplaces, while pleasant to look at, are not efficient foreither heating or cooking. The possibilities for the latter areespecially limited, and the invention of stoves was a great advance inefficiency, economy, and comfort. A stove is a receptacle for fire, provided with a definite inlet for air and a definite outlet forsmoke, and able to radiate into the room most of the heat producedfrom the fire which burns within. The inlet, or draft, admits enoughair to cause the fire to burn brightly or slowly as the case may be. If we wish a hot fire, the draft is opened wide and enough air entersto produce a strong glow. If we wish a low fire, the inlet is onlypartially opened, and just enough air enters to keep the fuelsmoldering. 

When the fire is started, the damper should be opened wide in order toallow the escape of smoke; but after the fire is well started there isless smoke, and the damper may be partly closed. If the damper is keptopen, coal is rapidly consumed, and the additional heat passes outthrough the chimney, and is lost to use. 

9. Furnaces. _Hot Air_. The labor involved in the care of numerousstoves is considerable, and hence the advent of a central heatingstove, or furnace, was a great saving in strength and fuel. A furnaceis a stove arranged as in Figure 13. The stove _S_, like all otherstoves, has an inlet for air and an outlet _C_ for smoke; but inaddition, it has built around it a chamber in which air circulates andis warmed. The air warmed by the stove is forced upward by cold airwhich enters from outside. For example, cold air constantly enteringat _E_ drives the air heated by _S_ through pipes and ducts to therooms to be heated. 

The metal pipes which convey the heated air from the furnace to theducts are sometimes covered with felt, asbestos, or othernon-conducting material in order that heat may not be lost duringtransmission. The ducts which receive the heated air from the pipesare built in the non-conducting walls of the house, and hence losepractically no heat. The air which reaches halls and rooms istherefore warm, in spite of its long journey from the cellar. 

[Illustration: FIG. 13. --A furnace. Pipes conduct hot air to therooms. ]

Not only houses are warmed by a central heating stove, but wholecommunities sometimes depend upon a central heating plant. In thelatter case, pipes closely wrapped with a non-conducting materialcarry steam long distances underground to heat remote buildings. Overbrook and Radnor, Pa. , are towns in which such a system is used. 

10. Hot-water Heating. The heated air which rises from furnaces isseldom hot enough to warm large buildings well; hence furnace heatingis being largely supplanted by hot-water heating. 

The principle of hot-water heating is shown by the following simpleexperiment. Two flasks and two tubes are arranged as in Figure 15, theupper flask containing a colored liquid and the lower flask clearwater. If heat is applied to _B_, one can see at the end of a fewseconds the downward circulation of the colored liquid and the upwardcirculation of the clear water. If we represent a boiler by _B_, aradiator by the coiled tube, and a safety tank by _C_, we shall have avery fair illustration of the principle of a hot-water heating system. The hot water in the radiators cools and, in cooling, gives up itsheat to the rooms and thus warms them. 

[Illustration: FIG. 14. --Hot-water heating. ]

In hot-water heating systems, fresh air is not brought to the rooms, for the radiators are closed pipes containing hot water. It is largelyfor this reason that thoughtful people are careful to raise windows atintervals. Some systems of hot-water heating secure ventilation byconfining the radiators to the basement, to which cold air fromoutside is constantly admitted in such a way that it circulates overthe radiators and becomes strongly heated. This warm fresh air thenpasses through ordinary flues to the rooms above. 

[Illustration: FIG. 15. --The principle of hot-water heating. ]

In Figure 16, a radiator is shown in a boxlike structure in thecellar. Fresh air from outside enters a flue at the right, passes theradiator, where it is warmed, and then makes its way to the roomthrough a flue at the left. The warm air which thus enters the room isthoroughly fresh. The actual labor involved in furnace heating and inhot-water heating is practically the same, since coal must be fed tothe fire, and ashes must be removed; but the hot-water system has theadvantage of economy and cleanliness. 

[Illustration: FIG. 16. --Fresh air from outside circulates over theradiators and then rises into the rooms to be heated. ]

11. Fresh Air. Fresh air is essential to normal healthy living, and2000 cubic feet of air per hour is desirable for each individual. If agentle breeze is blowing, a barely perceptible opening of a windowwill give the needed amount, even if there are no additional drafts offresh air into the room through cracks. Most houses are so looselyconstructed that fresh air enters imperceptibly in many ways, andwhether we will or no, we receive some fresh air. The supply is, however, never sufficient in itself and should not be depended uponalone. At night, or at any other time when gas lights are required, the need for ventilation increases, because every gas light in a roomuses up the same amount of air as four people. 

[Illustration: FIG. 17. --The air which goes to the schoolrooms iswarmed by passage over the radiators. ]

In the preceding Section, we learned that many houses heated by hotwater are supplied with fresh-air pipes which admit fresh air intoseparate rooms or into suites of rooms. In some cases the amount whichenters is so great that the air in a room is changed three or fourtimes an hour. The constant inflow of cold air and exit of warm airnecessitates larger radiators and more hot water and hence more coalto heat the larger quantity of water, but the additional expense ismore than compensated by the gain in health. 

12. Winds and Currents. The gentlest summer breezes and the fiercestblasts of winter are produced by the unequal heating of air. We haveseen that the air nearest to a stove or hot object becomes hotter thanthe adjacent air, that it tends to expand and is replaced and pushedupward and outward by colder, heavier air falling downward. We havelearned also that the moving liquid or gas carries with it heat whichit gradually gives out to surrounding bodies. 

When a liquid or a gas moves away from a hot object, carrying heatwith it, the process is called _convection_. 

Convection is responsible for winds and ocean currents, for land andsea breezes, and other daily phenomena. 

The Gulf Stream illustrates the transference of heat by convection. Alarge body of water is strongly heated at the equator, and then movesaway, carrying heat with it to distant regions, such as England andNorway. 

Owing to the shape of the earth and its position with respect to thesun, different portions of the earth are unequally heated. In thoseportions where the earth is greatly heated, the air likewise will beheated; there will be a tendency for the air to rise, and for the coldair from surrounding regions to rush in to fill its place. In this waywinds are produced. There are many circumstances which modify windsand currents, and it is not always easy to explain their directionand velocity, but one very definite cause is the unequal heating ofthe surface of the earth. 

13. Conduction. A poker used in stirring a fire becomes hot andheats the hand grasping the poker, although only the opposite end ofthe poker has actually been in the fire. Heat from the fire passedinto the poker, traveled along it, and warmed it. When heat flows inthis way from a warm part of a body to a colder part, the process iscalled _conduction_. A flatiron is heated by conduction, the heat fromthe warm stove passing into the cold flatiron and gradually heatingit. 

In convection, air and water circulate freely, carrying heat withthem; in conduction, heat flows from a warm region toward a coldregion, but there is no apparent motion of any kind. 

Heat travels more readily through some substances than through others. All metals conduct heat well; irons placed on the fire become heatedthroughout and cannot be grasped with the bare hand; iron utensils arefrequently made with wooden handles, because wood is a poor conductorand does not allow heat from the iron to pass through it to the hand. For the same reason a burning match may be held without discomfortuntil the flame almost reaches the hand. 

Stoves and radiators are made of metal, because metals conduct heatreadily, and as fast as heat is generated within the stove by theburning of fuel, or introduced into the radiator by the hot water, theheat is conducted through the metal and escapes into the room. 

Hot-water pipes and steam pipes are usually wrapped with anon-conducting substance, or insulator, such as asbestos, in orderthat the heat may not escape, but shall be retained within the pipesuntil it reaches the radiators within the rooms. 

The invention of the "Fireless Cooker" depended in part upon theprinciple of non-conduction. Two vessels, one inside the other, areseparated by sawdust, asbestos, or other poor conducting material(Fig. 18). Foods are heated in the usual way to the boiling point orto a high temperature, and are then placed in the inner vessel. Theheat of the food cannot escape through the non-conducting materialwhich surrounds it, and hence remains in the food and slowly cooks it. 

[Illustration: FIG. 18. --A fireless cooker. ]

A very interesting experiment for the testing of the efficacy ofnon-conductors may be easily performed. Place hot water in a metalvessel, and note by means of a thermometer the _rapidity_ with whichthe water cools; then place water of the same temperature in a secondmetal vessel similar to the first, but surrounded by asbestos or othernon-conducting material, and note the _slowness_ with which thetemperature falls. 

Chemical Change, an Effect of Heat. This effect of heat has a vitalinfluence on our lives, because the changes which take place when foodis cooked are due to it. The doughy mass which goes into the oven, comes out a light spongy loaf; the small indigestible rice grain comesout the swollen, fluffy, digestible grain. Were it not for thechemical changes brought about by heat, many of our present foodswould be useless to man. Hundreds of common materials like glass, rubber, iron, aluminum, etc. , are manufactured by processes whichinvolve chemical action caused by heat. 

CHAPTER II

TEMPERATURE AND HEAT

14. Temperature not a Measure of the Amount of Heat Present. If twosimilar basins containing unequal quantities of water are placed inthe sunshine on a summer day, the smaller quantity of water willbecome quite warm in a short period of time, while the larger quantitywill become only lukewarm. Both vessels receive the same amount ofheat from the sun, but in one case the heat is utilized in heating toa high temperature a small quantity of water, while in the second casethe heat is utilized in warming to a lower degree a larger quantity ofwater. Equal amounts of heat do not necessarily produce equivalenttemperatures, and equal temperatures do not necessarily indicate equalamounts of heat. It takes more heat to raise a gallon of water to theboiling point than it does to raise a pint of water to the boilingpoint, but a thermometer would register the same temperature in thetwo cases. The temperature of boiling water is 100° C. Whether thereis a pint of it or a gallon. Temperature is independent of thequantity of matter present; but the amount of heat contained in asubstance at any temperature is not independent of quantity, beinggreater in the larger quantity. 

15. The Unit of Heat. It is necessary to have a unit of heat just aswe have a unit of length, or a unit of mass, or a unit of time. Oneunit of heat is called a _calorie_, and is the amount of heat whichwill change the temperature of 1 gram of water 1° C. It is the amountof heat given out by 1 gram of water when its temperature falls 1° C. , or the amount of heat absorbed by 1 gram of water when its temperaturerises 1° C. If 400 grams of water are heated from 0° to 5° C. , theamount of heat which has entered the water is equivalent to 5 × 400 or2000 calories; if 200 grams of water cool from 25° to 20° C. , the heatgiven out by the water is equivalent to 5 × 200 or 1000 calories. 

16. Some Substances Heat more readily than Others. If two equalquantities of water at the same temperature are exposed to the sun forthe same length of time, their final temperatures will be the same. If, however, equal quantities of different substances are exposed, thetemperatures resulting from the heating will not necessarily be thesame. If a basin containing 1 lb. Of mercury is put on the fire, sideby side with a basin containing an equal quantity of water, thetemperatures of the two substances will vary greatly at the end of ashort time. The mercury will have a far higher temperature than thewater, in spite of the fact that the amount of mercury is as great asthe amount of water and that the heat received from the fire has beenthe same in each case. Mercury is not so difficult to heat as water;less heat being required to raise its temperature 1° than is requiredto raise the temperature of an equal quantity of water 1°. In fact, mercury is 30 times as easy to heat as water, and it requires only onethirtieth as much fire to heat a given quantity of mercury 1° as toheat the same quantity of water 1°. 

17. Specific Heat. We know that different substances are differentlyaffected by heat. Some substances, like water, change theirtemperature slowly when heated; others, like mercury, change theirtemperature very rapidly when heated. The number of calories needed by1 gram of a substance in order that its temperature may be increased1° C. Is called the _specific heat_ of a substance; or, specific heatis the number of calories given out by 1 gram of a substance when itstemperature falls 1° C. For experiments on the determination ofspecific heat, see Laboratory Manual. 

Water has the highest specific heat of any known substance excepthydrogen; that is, it requires more heat to raise the temperature ofwater a definite number of degrees than it does to raise thetemperature of an equal amount of any other substance the same numberof degrees. Practically this same thing can be stated in another way:Water in cooling gives out more heat than any other substance incooling through the same number of degrees. For this reason water isused in foot warmers and in hot-water bags. If a copper lid were usedas a foot warmer, it would give the feet only . 095 as much heat as anequal weight of water; a lead weight only . 031 as much heat as water. Flatirons are made of iron because of the relatively high specificheat of iron. The flatiron heats slowly and cools slowly, and, becauseof its high specific heat, not only supplies the laundress withconsiderable heat, but eliminates for her the frequent changing of theflatiron. 

18. Water and Weather. About four times as much heat is required toheat a given quantity of water one degree as to heat an equal quantityof earth. In summer, when the rocks and the sand along the shore areburning hot, the ocean and lakes are pleasantly cool, although theamount of heat present in the water is as great as that present in theearth. In winter, long after the rocks and sand have given out theirheat and have become cold, the water continues to give out the vaststore of heat accumulated during the summer. This explains why landssituated on or near large bodies of water usually have less variationin temperature than inland regions. In the summer the water cools theregion; in the winter, on the contrary, the water heats the region, and hence extremes of temperature are practically unknown. 

19. Sources of Heat. Most of the heat which we enjoy and use we oweto the sun. The wood which blazes on the hearth, the coal which glowsin the furnace, and the oil which burns in the stove owe theirexistence to the sun. 

Without the warmth of the sun seeds could not sprout and develop intothe mighty trees which yield firewood. Even coal, which lies buriedthousands of feet below the earth's surface, owes its existence inpart to the sun. Coal is simply buried vegetation, --vegetation whichsprouted and grew under the influence of the sun's warm rays. Ages agotrees and bushes grew "thick and fast, " and the ground was alwayscovered with a deep layer of decaying vegetable matter. In time someof this vast supply sank into the moist soil and became covered withmud. Then rock formed, and the rock pressed down upon the sunkenvegetation. The constant pressure, the moisture in the ground, andheat affected the underground vegetable mass, and slowly changed itinto coal. 

The buried forest and thickets were not all changed into coal. Somewere changed into oil and gas. Decaying animal matter was often mixedwith the vegetable mass. When the mingled animal and vegetable mattersank into moist earth and came under the influence of pressure, it wasslowly changed into oil and gas. 

The heat of our bodies comes from the foods which we eat. Fruits, grain, etc. , could not grow without the warmth and the light of thesun. The animals which supply our meats likewise depend upon the sunfor light and warmth. 

The sun, therefore, is the great source of heat; whether it is theheat which comes directly from the sun and warms the atmosphere, orthe heat which comes from burning coal, wood, and oil. 

CHAPTER III

OTHER FACTS ABOUT HEAT

20. Boiling. _Heat absorbed in Boiling_. If a kettle of water isplaced above a flame, the temperature of the water graduallyincreases, and soon small bubbles form at the bottom of the kettle andbegin to rise through the water. At first the bubbles do not get farin their ascent, but disappear before they reach the surface; later, as the water gets hotter and hotter, the bubbles become larger andmore numerous, rise higher and higher, and finally reach the surfaceand pass from the water into the air; steam comes from the vessel, andthe water is said to _boil_. The temperature at which a liquid boilsis called the boiling point. 

While the water is heating, the temperature steadily rises, but assoon as the water begins to boil the thermometer reading becomesstationary and does not change, no matter how hard the water boils andin spite of the fact that heat from the flame is constantly passinginto the water. 

If the flame is removed from the boiling water for but a second, theboiling ceases; if the flame is replaced, the boiling begins againimmediately. Unless heat is constantly supplied, water at the boilingpoint cannot be transformed into steam. 

_The number of calories which must be supplied to 1 gram of water atthe boiling point in order to change it into steam at the sametemperature is called the heat of vaporization_; it is the heatnecessary to change 1 gram of water at the boiling point into steam ofthe same temperature. 

21. The Amount of Heat Absorbed. The amount of heat which must beconstantly supplied to water at the boiling point in order to changeit into steam is far greater than we realize. If we put a beaker ofice water (water at 0° C. ) over a steady flame, and note (1) the timewhich elapses before the water begins to boil, and (2) the time whichelapses before the boiling water completely boils away, we shall seethat it takes about 5-1/4 times as long to change water into steam asit does to change its temperature from 0° C. To 100° C. Since, with asteady flame, it takes 5-1/4 times as long to change water into steamas it does to change its temperature from 0° C. To the boiling point, we conclude that it takes 5-1/4 times as much heat to convert water atthe boiling point into steam as it does to raise it from thetemperature of ice water to that of boiling water. 

The amount of heat necessary to raise the temperature of 1 gram ofwater 1° C. Is equal to 1 calorie, and the amount necessary to raisethe temperature 100° C. Is equal to 100 calories; hence the amount ofheat necessary to convert 1 gram of water at the boiling point intosteam at that same temperature is equal to approximately 525 calories. Very careful experiments show the exact heat of vaporization to be536. 1 calories. (See Laboratory Manual. )

22. General Truths. Statements similar to the above hold for otherliquids and for solutions. If milk is placed upon a stove, thetemperature rises steadily until the boiling point is reached; furtherheating produces, not a change in temperature, but a change of thewater of the milk into steam. As soon as the milk, or any other liquidfood, comes to a boil, the gas flame should be lowered until only anoccasional bubble forms, because so long as any bubbles form thetemperature is that of the boiling point, and further heat merelyresults in waste of fuel. 

We find by experiment that every liquid has its own specific boilingpoint; for example, alcohol boils at 78° C. And brine at 103° C. Bothspecific heat and the heat of vaporization vary with the liquid used. 

23. Condensation. If one holds a cold lid in the steam of boilingwater, drops of water gather on the lid; the steam is cooled bycontact with the cold lid and _condenses_ into water. Bottles of waterbrought from a cold cellar into a warm room become covered with a mistof fine drops of water, because the moisture in the air, chilled bycontact with the cold bottles, immediately condenses into drops ofwater. Glasses filled with ice water show a similar mist. 

In Section 21, we saw that 536 calories are required to change 1 gramof water into steam; if, now, the steam in turn condenses into water, it is natural to expect a release of the heat used in transformingwater into steam. Experiment shows not only that vapor gives out heatduring condensation, but that the amount of heat thus set free isexactly equal to the amount absorbed during vaporization. (SeeLaboratory Manual. )

We learn that the heat of vaporization is the same whether it isconsidered as the heat absorbed by 1 gram of water in its change tosteam, or as the heat given out by 1 gram of steam during itscondensation into water. 

24. Practical Application. We understand now the value of steam as aheating agent. Water is heated in a boiler in the cellar, and thesteam passes through pipes which run to the various rooms; there thesteam condenses into water in the radiators, each gram of steamsetting free 536 calories of heat. When we consider the size of theradiators and the large number of grams of steam which they contain, and consider further that each gram in condensing sets free 536calories, we understand the ease with which buildings are heated bysteam. 

Most of us have at times profited by the heat of condensation. In coldweather, when there is a roaring fire in the range, the waterfrequently becomes so hot that it "steams" out of open faucets. If, atsuch times, the hot water is turned on in a small cold bathroom, andis allowed to run until the tub is well filled, vapor condenses onwindows, mirrors, and walls, and the cold room becomes perceptiblywarmer. The heat given out by the condensing steam passes into thesurrounding air and warms the room. 

There is, however, another reason for the rise in temperature. If alarge pail of hot soup is placed in a larger pail of cold water, thesoup will gradually cool and the cold water will gradually becomewarmer. A red-hot iron placed on a stand gradually cools, but warmsthe stand. A hot body loses heat so long as a cooler body is near it;the cold object is heated at the expense of the warmer object, and oneloses heat and the other gains heat until the temperature of both isthe same. Now the hot water in the tub gradually loses heat and thecold air of the room gradually gains heat by convection, but theamount given the room by convection is relatively small compared withthe large amount set free by the condensing steam. 

25. Distillation. If impure, muddy water is boiled, drops of waterwill collect on a cold plate held in the path of the steam, but thedrops will be clear and pure. When impure water is boiled, the steamfrom it does not contain any of the impurities because these are leftbehind in the vessel. If all the water were allowed to boil away, alayer of mud or of other impurities would be found at the bottom ofthe vessel. Because of this fact, it is possible to purify water in avery simple way. Place over a fire a large kettle closed except for aspout which is long enough to reach across the stove and dip into abottle. As the liquid boils, steam escapes through the spout, and onreaching the cold bottle condenses and drops into the bottle as purewater. The impurities remain behind in the kettle. Water freed fromimpurities in this way is called _distilled water_, and the process iscalled _distillation_ (Fig. 19). By this method, the salt water of theocean may be separated into pure drinking water and salt, and many ofthe large ocean liners distill from the briny deep all the drinkingwater used on their ocean voyages. 

[Illustration: FIG. 19. --In order that the steam which passes throughthe coiled tube may be quickly cooled and condensed, cold water ismade to circulate around the coil. The condensed steam escapes at_w_. ]

Commercially, distillation is a very important process. Turpentine, for example, is made by distilling the sap of pine trees. Incisionsare cut in the bark of the long-leaf pine trees, and these serve aschannels for the escape of crude resin. This crude liquid is collectedin barrels and taken to a distillery, where it is distilled intoturpentine and rosin. The turpentine is the product which passes offas vapor, and the rosin is the mass left in the boiler after thedistillation of the turpentine. 

26. Evaporation. If a stopper is left off a cologne bottle, thecontents of the bottle will slowly evaporate; if a dish of water isplaced out of doors on a hot day, evaporation occurs very rapidly. Theliquids which have disappeared from the bottle and the dish havepassed into the surrounding air in the form of vapor. In Section 20, we saw that water could not pass into vapor without the addition ofheat; now the heat necessary for the evaporation of the cologne andwater was taken from the air, leaving it slightly cooler. If wet handsare not dried with a towel, but are left to dry by evaporation, heatis taken from the hand in the process, leaving a sensation ofcoolness. Damp clothing should never be worn, because the moisture init tends to evaporate at the expense of the bodily heat, and thisundue loss of heat from the body produces chills. After a bath thebody should be well rubbed, otherwise evaporation occurs at theexpense of heat which the body cannot ordinarily afford to lose. 

Evaporation is a slow process occurring at all times; it is hastenedduring the summer, because of the large amount of heat present in theatmosphere. Many large cities make use of the cooling effect ofevaporation to lower the temperature of the air in summer; streets aresprinkled not only to lay the dust, but in order that the surroundingair may be cooled by the evaporation of the water. 

Some thrifty housewives economize by utilizing the cooling effects ofevaporation. Butter, cheese, and other foods sensitive to heat areplaced in porous vessels wrapped in wet cloths. Rapid evaporation ofthe water from the wet cloths keeps the contents of the jars cool, andthat without expense other than the muscular energy needed for wettingthe cloths frequently. 

27. Rain, Snow, Frost, Dew. The heat of the sun causes constantevaporation of the waters of oceans, rivers, streams, and marshes, andthe water vapor set free by evaporation passes into the air, whichbecomes charged with vapor or is said to be humid. Constant, unceasingevaporation of our lakes, streams, and pools would mean a steadydecrease in the supply of water available for daily use, if theescaped water were all retained by the atmosphere and lost to theearth. But although the escaped vapor mingles with the atmosphere, hovering near the earth's surface, or rising far above the level ofthe mountains, it does not remain there permanently. When this vapormeets a cold wind or is chilled in any way, condensation takes place, and a mass of tiny drops of water or of small particles of snow isformed. When these drops or particles become large enough, they fallto the earth as rain or snow, and in this way the earth is compensatedfor the great loss of moisture due to evaporation. Fog is formed whenvapor condenses near the surface of the earth, and when the drops areso small that they do not fall but hover in the air, the fog is said"not to lift" or "not to clear. "

If ice water is poured into a glass, a mist will form on the outsideof the glass. This is because the water vapor in the air becomeschilled by contact with the glass and condenses. Often leaves andgrass and sidewalks are so cold that the water vapor in the atmospherecondenses on them, and we say a heavy dew has formed. If thetemperature of the air falls to the freezing point while the dew isforming, the vapor is frozen and frost is seen instead of dew. 

The daily evaporation of moisture into the atmosphere keeps theatmosphere more or less full of water vapor; but the atmosphere canhold only a definite amount of vapor at a given temperature, and assoon as it contains the maximum amount for that temperature, furtherevaporation ceases. If clothes are hung out on a damp, murky day theydo not dry, because the air contains all the moisture it can hold, andthe moisture in the clothes has no chance to evaporate. When the aircontains all the moisture it can hold, it is said to be saturated, andif a slight fall in temperature occurs when the air is saturated, condensation immediately begins in the form of rain, snow, or fog. If, however, the air is not saturated, a fall in temperature may occurwithout producing precipitation. The temperature at which air issaturated and condensation begins is called the _dew point_. 

28. How Chills are Caused. The discomfort we feel in an overcrowdedroom is partly due to an excess of moisture in the air, resulting fromthe breathing and perspiration of many persons. The air soon becomessaturated with vapor and cannot take away the perspiration from ourbodies, and our clothing becomes moist and our skin tender. When weleave the crowded "tea" or lecture and pass into the colder, drier, outside air, clothes and skin give up their load of moisture throughsudden evaporation. But evaporation requires heat, and this heat istaken from our bodies, and a chill results. 

Proper ventilation would eliminate much of the physical danger ofsocial events; fresh, dry air should be constantly admitted to crowdedrooms in order to replace the air saturated by the breath andperspiration of the occupants. 

29. Weather Forecasts. When the air is near the saturation point, the weather is oppressive and is said to be very humid. For comfortand health, the air should be about two thirds saturated. The presenceof some water vapor in the air is absolutely necessary to animal andplant life. In desert regions where vapor is scarce the air is so drythat throat trouble accompanied by disagreeable tickling is prevalent;fallen leaves become so dry that they crumble to dust; plants losetheir freshness and beauty. 

The likelihood of rain or frost is often determined by temperature andhumidity. If the air is near saturation and the temperature isfalling, it is safe to predict bad weather, because the fall oftemperature will probably cause rapid condensation, and hence rain. If, however, the air is not near the saturation point, a fall intemperature will not necessarily produce bad weather. 

The measurement of humidity is of far wider importance than the mereforecasting of local weather conditions. The close relation betweenhumidity and health has led many institutions, such as hospitals, schools, and factories, to regulate the humidity of the atmosphere ascarefully as they do the temperature. Too great humidity isenervating, and not conducive to either mental or physical exertion;on the other hand, too dry air is equally harmful. In summer thehumidity conditions cannot be well regulated, but in winter, whenhouses are artificially heated, the humidity of a room can beincreased by placing pans of water near the registers or on radiators. 

30. Heat Needed to Melt Substances. If a spoon is placed in a vesselof hot water for a few seconds and then removed, it will be warmerthan before it was placed in the hot water. If a lump of melting iceis placed in the vessel of hot water and then removed, the ice willnot be warmer than before, but there will be less of it. The heat ofthe water has been used in melting the ice, not in changing itstemperature. 

If, on a bitter cold day, a pail of snow is brought into a warm roomand a thermometer is placed in the snow, the temperature risesgradually until 32° F. Is reached, when it becomes stationary, and thesnow begins to melt. If the pail is put on the fire, the temperaturestill remains 32°F. , but the snow melts more rapidly. As soon as allthe snow is completely melted, however, the temperature begins to riseand rises steadily until the water boils, when it again becomesstationary and remains so during the passage of water into vapor. 

We see that heat must be supplied to ice at 0° C. Or 32° F. In orderto change it into water, and further, that the temperature of themixture does not rise so long as any ice is present, no matter howmuch heat is supplied. The amount of heat necessary to melt 1 gram ofice is easily calculated. (See Laboratory Manual. )

Heat must be supplied to ice to melt it. On the other hand, water, infreezing, loses heat, and the amount of heat lost by freezing water isexactly equal to the amount of heat absorbed by melting ice. 

The number of units of heat required to melt a unit mass of ice iscalled the _heat of fusion_ of water. 

31. Climate. Water, in freezing, loses heat, even though itstemperature remains at 0° C. Because water loses heat when it freezes, the presence of large streams of water greatly influences the climateof a region. In winter the heat from the freezing water keeps thetemperature of the surrounding higher than it would naturally be, andconsequently the cold weather is less severe. In summer waterevaporates, heat is taken from the air, and consequently the warmweather is less intense. 

32. Molding of Glass and Forging of Iron. The fire which is hotenough to melt a lump of ice may not be hot enough to melt an ironpoker; on the other hand, it may be sufficiently hot to melt a tinspoon. Different substances melt, or liquefy, at differenttemperatures; for example, ice melts at 0° C. , and tin at 233° C. , while iron requires the relatively high temperature of 1200° C. Mostsubstances have a definite melting or freezing point which neverchanges so long as the surrounding conditions remain the same. 

But while most substances have a definite melting point, somesubstances do not. If a glass rod is held in a Bunsen burner, it willgradually grow softer and softer, and finally a drop of molten glasswill fall from the end of the rod into the fire. The glass did notsuddenly become a liquid at a definite temperature; instead itsoftened gradually, and then melted. While glass is in the soft, yielding, pliable state, it is molded into dishes, bottles, and otheruseful objects, such as lamp shades, globes, etc. (Fig. 20). If glassmelted at a definite temperature, it could not be molded in this way. Iron acts in a similar manner, and because of this property theblacksmith can shape his horseshoes, and the workman can make hisengines and other articles of daily service to man. 

[Illustration: FIG. 20. --Molten glass being rolled into a formsuitable for window panes. ]

33. Strange Behavior of Water. One has but to remember that bottlesof water burst when they freeze, and that ice floats on water likewood, to know that water expands on freezing or on solidifying. Aquantity of water which occupies 100 cubic feet of space will, onbecoming ice, need 109 cubic feet of space. On a cold winter night thewater sometimes freezes in the water pipes, and the pipes burst. Wateris very peculiar in expanding on solidification, because mostsubstances contract on solidifying; gelatin and jelly, for example, contract so much that they shrink from the sides of the dish whichcontains them. 

If water contracted in freezing, ice would be heavier than water andwould sink in ponds and lakes as fast as it formed, and our streamsand ponds would become masses of solid ice, killing all animal andplant life. But the ice is lighter than water and floats on top, andanimals in the water beneath are as free to live and swim as they werein the warm sunny days of summer. The most severe winter cannot freezea deep lake solid, and in the coldest weather a hole made in the icewill show water beneath the surface. Our ice boats cut and break theice of the river, and through the water beneath our boats daily plytheir way to and fro, independent of winter and its blighting blasts. 

While most of us are familiar with the bursting of water pipes on acold night, few of us realize the influence which freezing waterexerts on the character of the land around us. 

Water sinks into the ground and, on the approach of winter, freezes, expanding about one tenth of its volume; the expanding ice pushes theearth aside, the force in some cases being sufficient to dislodge evenhuge rocks. In the early days in New England it was said by thefarmers that "rocks grew, " because fields cleared of stones in thefall became rock covered with the approach of spring; the rocks andstones hidden underground and unseen in the fall were forced to thesurface by the winter's expansion. We have all seen fence posts andbricks pushed out of place because of the heaving of the soil beneaththem. Often householders must relay their pavements and walks becauseof the damage done by freezing water. 

The most conspicuous effect of the expansive power of freezing wateris seen in rocky or mountainous regions (Fig. 21). Water easily findsentrance into the cracks and crevices of the rocks, where it lodgesuntil frozen; then it expands and acts like a wedge, widening cracks, chiseling off edges, and even breaking rocks asunder. In regions wherefrequent frosts occur, the destructive action of water works constantchanges in the appearance of the land; small cracks and crevices areenlarged, massive rocks are pried up out of position, huge slabs aresplit off, and particles large and small are forced from the parentrock. The greater part of the debris and rubbish brought down from themountain slopes by the spring rains owes its origin to the fact thatwater expands when it freezes. 

[Illustration: FIG. 21. --The destruction caused by freezing water. ]

34. Heat Necessary to Dissolve a Substance. It requires heat todissolve any substance, just as it requires heat to change ice towater. If a handful of common salt is placed in a small cup of waterand stirred with a thermometer, the temperature of the mixture fallsseveral degrees. This is just what one would expect, because the heatneeded to liquefy the salt must come from somewhere, and naturally itcomes from the water, thereby lowering the temperature of the water. We know very well that potatoes cease boiling if a pinch of salt isput in the water; this is because the temperature of the water hasbeen lowered by the amount of heat necessary to dissolve the salt. 

Let some snow or chopped ice be placed in a vessel and mixed with onethird its weight of coarse salt; if then a small tube of cold water isplaced in this mixture, the water in the test tube will soon freezesolid. As soon as the snow and salt are mixed they melt. The heatnecessary for this comes in part from the air and in part from thewater in the test tube, and the water in the tube becomes inconsequence cold enough to freeze. But the salt mixture does notfreeze because its freezing point is far below that of pure water. Theuse of salt and ice in ice-cream freezers is a practical applicationof this principle. The heat necessary for melting the mixture of saltand ice is taken from the cream which thus becomes cold enough tofreeze. 

CHAPTER IV

BURNING OR OXIDATION

35. Why Things Burn. The heat of our bodies comes from the food weeat; the heat for cooking and for warming our houses comes from coal. The production of heat through the burning of coal, or oil, or gas, orwood, is called combustion. Combustion cannot occur without thepresence of a substance called oxygen, which exists rather abundantlyin the air; that is, one fifth of our atmosphere consists of thissubstance which we call oxygen. We throw open our windows to allowfresh air to enter, and we take walks in order to breathe the pure airinto our lungs. What we need for the energy and warmth of our bodiesis the oxygen in the air. Whether we burn gas or wood or coal, theheat which is produced comes from the power which these varioussubstances possess to combine with oxygen. We open the draft of astove that it may "draw well": that it may secure oxygen for burning. We throw a blanket over burning material to smother the fire: to keepoxygen away from it. Burning, or oxidation, is combining with oxygen, and the more oxygen you add to a fire, the hotter the fire will burn, and the faster. The effect of oxygen on combustion may be clearly seenby thrusting a smoldering splinter into a jar containing oxygen; thesmoldering splinter will instantly flare and blaze, while if it isremoved from the jar, it loses its flame and again burns quietly. Oxygen for this experiment can be produced in the following way. 

[Illustration: FIG. 22. --Preparing oxygen from potassium chlorate andmanganese dioxide. ]

36. How to Prepare Oxygen. Mix a small quantity of potassiumchlorate with an equal amount of manganese dioxide and place themixture in a strong test tube. Close the mouth of the tube with aone-hole rubber stopper in which is fitted a long, narrow tube, andclamp the test tube to an iron support, as shown in Figure 22. Fillthe trough with water until the shelf is just covered and allow theend of the delivery tube to rest just beneath the hole in the shelf. Fill a medium-sized bottle with water, cover it with a glass plate, invert the bottle in the trough, and then remove the glass plate. Heatthe test tube very gently, and when gas bubbles out of the tube, slipthe bottle over the opening in the shelf, so that the tube runs intothe bottle. The gas will force out the water and will finally fill thebottle. When all the water has been forced out, slip the glass plateunder the mouth of the bottle and remove the bottle from the trough. The gas in the bottle is oxygen. 

Everywhere in a large city or in a small village, smoke is seen, indicating the presence of fire; hence there must exist a large supplyof oxygen to keep all the fires alive. The supply of oxygen neededfor the fires of the world comes largely from the atmosphere. 

37. Matches. The burning material is ordinarily set on fire bymatches, thin strips of wood tipped with sulphur or phosphorus, orboth. Phosphorus can unite with oxygen at a fairly low temperature, and if phosphorus is rubbed against a rough surface, the frictionproduced will raise the temperature of the phosphorus to a point whereit can combine with oxygen. The burning phosphorus kindles the wood ofthe match, and from the burning match the fire is kindled. If you wantto convince yourself that friction produces heat, rub a centvigorously against your coat and note that the cent becomes warm. Matches have been in use less than a hundred years. Primitive mankindled his camp fire by rubbing pieces of dry wood together untilthey took fire, and this method is said to be used among some isolateddistant tribes at the present time. A later and easier way was tostrike flint and steel together and to catch the spark thus producedon tinder or dry fungus. Within the memory of some persons now living, the tinder box was a valuable asset to the home, particularly in thepioneer regions of the West. 

38. Safety Matches. Ordinary phosphorus, while excellent as afire-producing material, is dangerously poisonous, and those to whomthe dipping of wooden strips into phosphorus is a daily occupationsuffer with a terrible disease which usually attacks the teeth andbones of the jaw. The teeth rot and fall out, abscesses form, andbones and flesh begin to decay; the only way to prevent the spread ofthe disease is to remove the affected bone, and in some instances ithas been necessary to remove the entire jaw. Then, too, matches madeof yellow or white phosphorus ignite easily, and, when rubbed againstany rough surface, are apt to take fire. Many destructive fires havebeen started by the accidental friction of such matches against roughsurfaces. 

For these reasons the introduction of the so-called safety match wasan important event. When common phosphorus, in the dangerous andeasily ignited form, is heated in a closed vessel to about 250° C. , itgradually changes to a harmless red mass. The red phosphorus is notonly harmless, but it is difficult to ignite, and, in order to beignited by friction, must be rubbed on a surface rich in oxygen. Thehead of a safety match is coated with a mixture of glue andoxygen-containing compounds; the surface on which the match is to berubbed is coated with a mixture of red phosphorus and glue, to whichfinely powdered glass is sometimes added in order to increase thefriction. Unless the head of the match is rubbed on the preparedphosphorus coating, ignition does not occur, and accidental fires areavoided. 

Various kinds of safety matches have been manufactured in the last fewyears, but they are somewhat more expensive than the ordinary form, and hence manufacturers are reluctant to substitute them for thecheaper matches. Some foreign countries, such as Switzerland, prohibitthe sale of the dangerous type, and it is hoped that the United Stateswill soon follow the lead of these countries in demanding the sale ofsafety matches only. 

39. Some Unfamiliar Forms of Burning. While most of us think ofburning as a process in which flames and smoke occur, there are inreality many modes of burning accompanied by neither flame nor smoke. Iron, for example, burns when it rusts, because it slowly combineswith the oxygen of the air and is transformed into new substances. When the air is dry, iron does not unite with oxygen, but whenmoisture is present in the air, the iron unites with the oxygen andturns into iron rust. The burning is slow and unaccompanied by thefire and smoke so familiar to us, but the process is none the lessburning, or combination with oxygen. Burning which is not accompaniedby any of the appearances of ordinary burning is known as oxidation. 

The tendency of iron to rust lessens its efficiency and value, andmany devices have been introduced to prevent rusting. A coating ofpaint or varnish is sometimes applied to iron in order to preventcontact with air. The galvanizing of iron is another attempt to securethe same result; in this process iron is dipped into molten zinc, thereby acquiring a coating of zinc, and forming what is known asgalvanized iron. Zinc does not combine with oxygen under ordinarycircumstances, and hence galvanized iron is immune from rust. 

Decay is a process of oxidation; the tree which rots slowly away isundergoing oxidation, and the result of the slow burning is thedecomposed matter which we see and the invisible gases which pass intothe atmosphere. The log which blazes on our hearth gives outsufficient heat to warm us; the log which decays in the forest givesout an equivalent amount of heat, but the heat is evolved so slowlythat we are not conscious of it. Burning accompanied by a blaze andintense heat is a rapid process; burning unaccompanied by fire andappreciable heat is a slow, gradual process, requiring days, weeks, and even long years for its completion. 

Another form of oxidation occurs daily in the human body. In Section35 we saw that the human body is an engine whose fuel is food; theburning of that food in the body furnishes the heat necessary forbodily warmth and the energy required for thought and action. Oxygenis essential to burning, and the food fires within the body are keptalive by the oxygen taken into the body at every breath by the lungs. We see now one reason for an abundance of fresh air in daily life. 

40. How to Breathe. Air, which is essential to life and health, should enter the body through the nose and _not through the mouth_. The peculiar nature and arrangement of the membranes of the noseenable the nostrils to clean, and warm, and moisten the air whichpasses through them to the lungs. Floating around in the atmosphereare dust particles which ought not to get into the lungs. The nose isprovided with small hairs and a moist inner membrane which serve asfilters in removing solid particles from the air, and in thuspurifying it before its entrance into the lungs. 

In the immediate neighborhood of three Philadelphia high schools, having an approximate enrollment of over 8000 pupils, is a hugemanufacturing plant which day and night pours forth grimy smoke andsoot into the atmosphere which must supply oxygen to this vast groupof young lives. If the vital importance of nose breathing is impressedupon these young people, the harmful effect of the foul air may begreatly lessened, the smoke particles and germs being held back by thenose filters and never reaching the lungs. If, however, this principleof hygiene is not brought to their attention, the dangerous habit ofbreathing through the open, or at least partially open, mouth willcontinue, and objectionable matter will pass through the mouth andfind a lodging place in the lungs. 

There is another very important reason why nose breathing ispreferable to mouth breathing. The temperature of the human body isapproximately 98° F. , and the air which enters the lungs should not befar below this temperature. If air reaches the lungs through the nose, its journey is relatively long and slow, and there is opportunity forit to be warmed before it reaches the lungs. If, on the other hand, air passes to the lungs by way of the mouth, the warming process isbrief and insufficient, and the lungs suffer in consequence. Naturally, the gravest danger is in winter. 

41. Cause of Mouth Breathing. Some people find it difficult tobreathe through the nostrils on account of growths, called adenoids, in the nose. If you have a tendency toward mouth breathing, let aphysician examine your nose and throat. 

Adenoids not only obstruct breathing and weaken the whole systemthrough lack of adequate air, but they also press upon the bloodvessels and nerves of the head and interfere with normal braindevelopment. Moreover, they interfere in many cases with the hearing, and in general hinder activity and growth. The removal of adenoids issimple, and carries with it only temporary pain and no danger. Somephysicians claim that the growths disappear in later years, but evenif that is true, the physical and mental development of earlier yearsis lost, and the person is backward in the struggle for life andachievement. 

[Illustration: FIG. 23. --Intelligent expression is often lacking inchildren with adenoid growths. ]

42. How to Build a Fire. Substances differ greatly as to the easewith which they may be made to burn or, in technical terms, with whichthey may be made to unite with oxygen. For this reason, we put lightmaterials, like shavings, chips, and paper, on the grate, twisting thelatter and arranging it so that air (oxygen in the air) can reach alarge surface; upon this we place small sticks of wood, piling themacross each other so as to allow entrance for the oxygen; and finallyupon this we place our hard wood or coal. 

The coal and the large sticks cannot be kindled with a match, but thepaper and shavings can, and these in burning will heat the largesticks until they take fire and in turn kindle the coal. 

43. Spontaneous Combustion. We often hear of fires "startingthemselves, " and sometimes the statement is true. If a pile of oilyrags is allowed to stand for a time, the oily matter will begin tocombine slowly with oxygen and as a result will give off heat. Theheat thus given off is at first insufficient to kindle a fire; but asthe heat is retained and accumulated, the temperature rises, andfinally the kindling point is reached and the whole mass bursts intoflames. For safety's sake, all oily cloths should be burned or kept inmetal vessels. 

44. The Treatment of Burns. In spite of great caution, burns fromfires, steam, or hot water do sometimes occur, and it is well to knowhow to relieve the suffering caused by them and how to treat theinjury in order to insure rapid healing. 

Burns are dangerous because they destroy skin and thus open up anentrance into the body for disease germs, and in addition because theylay bare nerve tissue which thereby becomes irritated and causes ashock to the entire system. 

In mild burns, where the skin is not broken but is merely reddened, anapplication of moist baking soda brings immediate relief. If thissubstance is not available, flour paste, lard, sweet oil, or vaselinemay be used. 

In more severe burns, where blisters are formed, the blisters shouldbe punctured with a sharp, sterilized needle and allowed to dischargetheir watery contents before the above remedies are applied. 

In burns severe enough to destroy the skin, disinfection of the openwound with weak carbolic acid or hydrogen peroxide is very necessary. After this has been done, a soft cloth soaked in a solution of linseedoil and limewater should be applied and the whole bandaged. In such acase, it is important not to use cotton batting, since this sticks tothe rough surface and causes pain when removed. 

45. Carbon Dioxide. _A Product of Burning. _ When any fuel, such ascoal, gas, oil, or wood, burns, it sends forth gases into thesurrounding atmosphere. These gases, like air, are invisible, and wereunknown to us for a long time. The chief gas formed by a burningsubstance is called carbon dioxide (CO_2) because it is composed ofone part of carbon and two parts of oxygen. This gas has thedistinction of being the most widely distributed gaseous compound ofthe entire world; it is found in the ocean depths and on the mountainheights, in brilliantly lighted rooms, and most abundantly inmanufacturing towns where factory chimneys constantly pour forth hotgases and smoke. 

Wood and coal, and in fact all animal and vegetable matter, containcarbon, and when these substances burn or decay, the carbon in themunites with oxygen and forms carbon dioxide. 

The food which we eat is either animal or vegetable, and it is madeready for bodily use by a slow process of burning within the body;carbon dioxide accompanies this bodily burning of food just as itaccompanies the fires with which we are more familiar. The carbondioxide thus produced within the body escapes into the atmosphere withthe breath. 

We see that the source of carbon dioxide is practically inexhaustible, coming as it does from every stove, furnace, and candle, and furtherwith every breath of a living organism. 

46. Danger of Carbon Dioxide. When carbon dioxide occurs in largequantities, it is dangerous to health, because it interferes withnormal breathing, lessening the escape of waste matter through thebreath and preventing the access to the lungs of the oxygen necessaryfor life. Carbon dioxide is not poisonous, but it cuts off the supplyof oxygen, just as water cuts it off from a drowning man. 

Since every man, woman, and child constantly breathes forth carbondioxide, the danger in overcrowded rooms is great, and properventilation is of vital importance. 

47. Ventilation. In estimating the quantity of air necessary to keepa room well aired, we must take into account the number of lights(electric lights do not count) to be used, and the number of people tooccupy the room. The average house should provide at the _minimum_ 600cubic feet of space for each person, and in addition, arrangements forallowing at least 300 cubic feet of fresh air per person to enterevery hour. 

In houses which have not a ventilating system, the air should be keptfresh by intelligent action in the opening of doors and windows; andsince relatively few houses are equipped with a satisfactory system, the following suggestions relative to intelligent ventilation areoffered. 

1. Avoid drafts in ventilation. 

2. Ventilate on the sheltered side of the house. If the wind isblowing from the north, open south windows. 

48. What Becomes of the Carbon Dioxide. When we reflect that carbondioxide is constantly being supplied to the atmosphere and that it isinjurious to health, the question naturally arises as to how the airremains free enough of the gas to support life. This is largelybecause carbon dioxide is an essential food of plants. Through theirleaves plants absorb it from the atmosphere, and by a wonderfulprocess break it up into its component parts, oxygen and carbon. Theyreject the oxygen, which passes back to the air, but they retain thecarbon, which becomes a part of the plant structure. Plants thus serveto keep the atmosphere free from an excess of carbon dioxide and, inaddition, furnish oxygen to the atmosphere. 

[Illustration: FIG. 24. --Making carbon dioxide from marble andhydrochloric acid. ]

49. How to Obtain Carbon Dioxide. There are several ways in whichcarbon dioxide can be produced commercially, but for laboratory usethe simplest is to mix in a test tube powdered marble, or chalk, andhydrochloric acid, and to collect the effervescing gas as shown inFigure 24. The substance which remains in the test tube after the gashas passed off is a solution of a salt and water. From a mixture ofhydrochloric acid (HCl) and marble are obtained a salt, water, andcarbon dioxide, the desired gas. 

50. A Commercial Use of Carbon Dioxide. If a lighted splinter isthrust into a test tube containing carbon dioxide, it is promptlyextinguished, because carbon dioxide cannot support combustion; if astream of carbon dioxide and water falls upon a fire, it acts like ablanket, covering the flames and extinguishing them. The value of afire extinguisher depends upon the amount of carbon dioxide and waterwhich it can furnish. A fire extinguisher is a metal case containing asolution of bicarbonate of soda, and a glass vessel full of strongsulphuric acid. As long as the extinguisher is in an upright position, these substances are kept separate, but when the extinguisher isinverted, the acid escapes from the bottle, and mixes with the sodasolution. The mingling liquids interact and liberate carbon dioxide. A part of the gas thus liberated dissolves in the water of the sodasolution and escapes from the tube with the outflowing liquid, while aportion remains undissolved and escapes as a stream of gas. The fireextinguisher is therefore the source of a liquid containing thefire-extinguishing substance and further the source of a stream ofcarbon dioxide gas. 

[Illustration: FIG. 25. --Inside view of a fire extinguisher. ]

51. Carbon. Although carbon dioxide is very injurious to health, both of the substances of which it is composed are necessary to life. We ourselves, our bones and flesh in particular, are partly carbon, and every animal, no matter how small or insignificant, contains somecarbon; while the plants around us, the trees, the grass, the flowers, contain a by no means meager quantity of carbon. 

Carbon plays an important and varied role in our life, and, in someone of its many forms, enters into the composition of most of thesubstances which are of service and value to man. The food we eat, theclothes we wear, the wood and coal we burn, the marble we employ inbuilding, the indispensable soap, and the ornamental diamond, allcontain carbon in some form. 

52. Charcoal. One of the most valuable forms of carbon is charcoal;valuable not in the sense that it costs hundreds of dollars, but inthe more vital sense, that its use adds to the cleanliness, comfort, and health of man. 

The foul, bad-smelling gases which arise from sewers can be preventedfrom escaping and passing to streets and buildings by placing charcoalfilters at the sewer exits. Charcoal is porous and absorbs foul gases, and thus keeps the region surrounding sewers sweet and clean and freeof odor. Good housekeepers drop small bits of charcoal into vases offlowers to prevent discoloration of the water and the odor of decayingstems. 

If impure water filters through charcoal, it emerges pure, having leftits impurities in the pores of the charcoal. Practically all householdfilters of drinking water are made of charcoal. But such a device maybe a source of disease instead of a prevention of disease, unless thefilter is regularly cleaned or renewed. This is because the pores soonbecome clogged with the impurities, and unless they are cleaned, thewater which flows through the filter passes through a bed ofimpurities and becomes contaminated rather than purified. Frequentcleansing or renewal of the filter removes this difficulty. 

Commercially, charcoal is used on a large scale in the refining ofsugars, sirups, and oils. Sugar, whether it comes from the maple tree, or the sugar cane, or the beet, is dark colored. It is whitened bypassage through filters of finely pulverized charcoal. Cider andvinegar are likewise cleared by passage through charcoal. 

The value of carbon, in the form of charcoal, as a purifier is verygreat, whether we consider it a deodorizer, as in the case of thesewage, or a decolorizer, as in the case of the refineries, or whetherwe consider the service it has rendered man in the elimination ofdanger from drinking water. 

53. How Charcoal is Made. Charcoal may be made by heating wood in anoven to which air does not have free access. The absence of airprevents ordinary combustion, nevertheless the intense heat affectsthe wood and changes it into new substances, one of which is charcoal. 

The wood which smolders on the hearth and in the stove is charcoal inthe making. Formerly wood was piled in heaps, covered with sod or sandto prevent access of oxygen, and then was set fire to; the smolderingwood, cut off from an adequate supply of air, was slowly transformedinto charcoal. Scattered over the country one still finds isolatedcharcoal kilns, crude earthen receptacles, in which wood thus deprivedof air was allowed to smolder and form charcoal. To-day charcoal ismade commercially by piling wood on steel cars and then pushing thecars into strong walled chambers. The chambers are closed to preventaccess of air, and heated to a high temperature. The intense heattransforms the wood into charcoal in a few hours. A student can makein the laboratory sufficient charcoal for art lessons by heating in anearthen vessel wood buried in sand. The process will be slow, however, because the heat furnished by a Bunsen burner is not great, and thewood is transformed slowly. 

A form of charcoal known as animal charcoal, or bone black, isobtained from the charred remains of animals rather than plants, andmay be prepared by burning bones and animal refuse as in the case ofthe wood. 

Destructive Distillation. When wood is burned without sufficientair, it is changed into soft brittle charcoal, which is very differentfrom wood. It weighs only one fourth as much as the original wood. Itis evident that much matter must leave the wood during the process ofcharcoal making. We can prove this by putting some dry shavings in astrong test tube fitted with a delivery tube. When the wood is heateda gas passes off which we may collect and burn. Other substances alsocome off in gaseous form, but they condense in the water. Among theseare wood alcohol, wood tar, and acetic acid. In the older method ofcharcoal making all these products were lost. Can you give any uses ofthese substances?

54. Matter and Energy. When wood is burned, a small pile of ashes isleft, and we think of the bulk of the wood as destroyed. It is true wehave less matter that is available for use or that is visible tosight, but, nevertheless, no matter has been destroyed. The matter ofwhich the wood is composed has merely changed its character, some ofit is in the condition of ashes, and some in the condition ofinvisible gases, such as carbon dioxide, but none of it has beendestroyed. It is a principle of science that matter can neither bedestroyed nor created; it can only be changed, or transformed, and itis our business to see that we do not heedlessly transform it intosubstances which are valueless to us and our descendants; as, forexample, when our magnificent forests are recklessly wasted. Thesmoke, gases, and ashes left in the path of a raging forest fire areno compensation to us for the valuable timber destroyed. The sum totalof matter has not been changed, but the amount of matter which man canuse has been greatly lessened. 

The principle just stated embodies one of the fundamental laws ofscience, called the law of the _conservation of matter_. 

A similar law holds for energy as well. We can transform electricenergy into the motion of trolley cars, or we can make use of theenergy of streams to turn the wheels of our mills, but in all thesecases we are transforming, not creating, energy. 

When a ball is fired from a rifle, most of the energy of the gunpowderis utilized in motion, but some is dissipated in producing a flash anda report, and in heat. The energy of the gunpowder has been scattered, but the sum of the various forms of energy is equal to the energyoriginally stored away in the powder. The better the gun is, the lesswill be the energy dissipated in smoke and heat and noise. 

CHAPTER V

FOOD

55. The Body as a Machine. Wholesome food and fresh air arenecessary for a healthy body. Many housewives, through ignorance, supply to their hard-working husbands and their growing sons anddaughters food which satisfies the appetite, but which does not giveto the body the elements needed for daily work and growth. Some foods, such as lettuce, cucumbers, and watermelons, make proper andsatisfactory changes in diet, but are not strength giving. Otherfoods, like peas and beans, not only satisfy the appetite, but supplyto the body abundant nourishment. Many immigrants live cheaply andwell with beans and bread as their main diet. 

It is of vital importance that the relative value of different foodsas heat producers be known definitely; and just as the yard measureslength and the pound measures weight the calorie is used to measurethe amount of heat which a food is capable of furnishing to the body. Our bodies are human machines, and, like all other machines, requirefuel for their maintenance. The fuel supplied to an engine is not allavailable for pulling the cars; a large portion of the fuel is lost insmoke, and another portion is wasted as ashes. So it is with the fuelthat runs the body. The food we eat is not all available fornourishment, much of it being as useless to us as are smoke and ashesto an engine. The best foods are those which do the most for us withthe least possible waste. 

56. Fuel Value. By fuel value is meant the capacity foods have foryielding heat to the body. The fuel value of the foods we eat daily isso important a factor in life that physicians, dietitians, nurses, and those having the care of institutional cooking acquaint themselveswith the relative fuel values of practically all of the important foodsubstances. The life or death of a patient may be determined by thepatient's diet, and the working and earning capacity of a fatherdepends largely upon his prosaic three meals. An ounce of fat, whetherit is the fat of meat or the fat of olive oil or the fat of any otherfood, produces in the body two and a quarter times as much heat as anounce of starch. Of the vegetables, beans provide the greatestnourishment at the least cost, and to a large extent may besubstituted for meat. It is not uncommon to find an outdoor laborerconsuming one pound of beans per day, and taking meat only on "highdays and holidays. "

[Illustration: FIG. 26. --The bomb calorimeter from which the fuelvalue of food can be estimated. ]

The fuel value of a food is determined by means of the _bombcalorimeter_ (Fig. 26). The food substance is put into a chamber _A_and ignited, and the heat of the burning substance raises thetemperature of the water in the surrounding vessel. If 1000 grams ofwater are in the vessel, and the temperature of the water is raised 2°C. , the number of calories produced by the substance would be 2000, and the fuel value would be 2000 calories. [A] From this the fuel valueof one quart or one pound of the substance can be determined, and thefood substance will be said to furnish the body with that number ofheat units, providing all of the pound of food were properly digested. 

[Footnote A: As applied to food, the calorie is greater than that usedin the ordinary laboratory work, being the amount of heat necessary toraise the temperature of 1000 grams of water 1° C. , rather than 1 gram1° C. ]

 TABLE SHOWING THE NUMBER OF CALORIES FURNISHED BY ONE POUND OF VARIOUS FOODS ---------------------------------------------------- |FOOD |CALORIES|FOOD |CALORIES| ---------------------------------------------------- |Leg of lean mutton | 790|Carrots | 210| ---------------------------------------------------- |Rib of beef | 1150|Lettuce | 90| ---------------------------------------------------- |Shad | 380|Onion | 225| ---------------------------------------------------- |Chicken | 505|Cucumber | 80| ---------------------------------------------------- |Apples | 290|Almonds | 3030| ---------------------------------------------------- |Bananas | 460|Walnuts | 3306| ---------------------------------------------------- |Prunes | 370|Peanuts | 2560| ---------------------------------------------------- |Watermelons | 140|Oatmeal | 4673| ---------------------------------------------------- |Lima beans | 570|Rolled wheat | 4175| ---------------------------------------------------- |Beets | 215|Macaroni | 1665| ----------------------------------------------------

57. Varied Diet. The human body is a much more varied and complexmachine than any ever devised by man; personal peculiarities, as wellas fuel values, influence very largely the diet of an individual. Strawberries are excluded from some diets because of a rash which isproduced on the skin, pork is excluded from other diets for a likereason; cauliflower is absolutely indigestible to some and is readilydigested by others. From practically every diet some foods must beexcluded, no matter what the fuel value of the substance may be. 

Then, too, there are more uses for food than the production of heat. Teeth and bones and nails need a constant supply of mineral matter, and mineral matter is frequently found in greatest abundance in foodsof low fuel value, such as lettuce, watercress, etc. , thoughpractically all foods yield at least a small mineral constituent. Whenfuel values alone are considered, fruits have a low value, but becauseof the flavor they impart to other foods, and because of the healthfulinfluence they exercise in digestion, they cannot be excluded from thediet. 

Care should be constantly exercised to provide substantial foods ofhigh fuel value. But the nutritive foods should be wisely supplementedby such foods as fruits, whose real value is one of indirect ratherthen direct service. 

58. Our Bodies. Somewhat as a house is composed of a group ofbricks, or a sand heap of grains of sand, the human body is composedof small divisions called cells. Ordinarily we cannot see these cellsbecause of their minuteness, but if we examine a piece of skin, or ahair of the head, or a tiny sliver of bone under the microscope, wesee that each of these is composed of a group of different cells. Amerchant, watchful about the fineness of the wool which he ispurchasing, counts with his lens the number of threads to the inch; aphysician, when he wishes, can, with the aid of the microscope, examine the cells in a muscle, or in a piece of fat, or in a nervefiber. Not only is the human body composed of cells, but so also arethe bodies of all animals from the tiny gnat which annoys us, and thefly which buzzes around us, to the mammoth creatures of the tropics. These cells do the work of the body, the bone cells build up theskeleton, the nail cells form the finger and toe nails, the lung cellstake care of breathing, the muscle cells control motion, and the braincells are responsible for thought. 

59. Why we eat so Much. The cells of the body are constantly, day byday, minute by minute, breaking down and needing repair, areconstantly requiring replacement by new cells, and, in the case of thechild, are continually increasing in number. The repair of an ordinarymachine, an engine, for example, is made at the expense of money, butthe repair and replacement of our human cell machinery areaccomplished at the expense of food. More than one third of all thefood we eat goes to maintain the body cells, and to keep them in goodorder. It is for this reason that we consume a large quantity of food. If all the food we eat were utilized for energy, the housewife couldcook less, and the housefather could save money on grocer's andbutcher's bills. If you put a ton of coal in an engine, its availableenergy is used to run the engine, but if the engine were like thehuman body, one third of the ton would be used up by the engine inkeeping walls, shafts, wheels, belts, etc. , in order, and only twothirds would go towards running the engine. When an engine is notworking, fuel is not consumed, but the body requires food for mereexistence, regardless of whether it does active work or not. When wework, the cells break down more quickly, and the repair is greaterthan when we are at rest, and hence there is need of a larger amountof food; but whether we work or not, food is necessary. 

60. The Different Foods. The body is very exacting in its demands, requiring certain definite foods for the formation and maintenance ofits cells, and other foods, equally definite, but of differentcharacter, for heat; our diet therefore must contain foods of highfuel value, and likewise foods of cell-forming power. 

Although the foods which we eat are of widely different character, such as fruits, vegetables, cereals, oils, meats, eggs, milk, cheese, etc. , they can be put into three great classes: the carbohydrates, thefats, and the proteids. 

61. The Carbohydrates. Corn, wheat, rye, in fact all cereals andgrains, potatoes, and most vegetables are rich in carbohydrates; asare also sugar, molasses, honey, and maple sirup. The foods of thefirst group are valuable because of the starch they contain; forexample, corn starch, wheat starch, potato starch. The substances ofthe second group are valuable because of the sugar they contain; sugarcontains the maximum amount of carbohydrate. In the sirups there is aconsiderable quantity of sugar, while in some fruits it is present inmore or less dilute form. Sweet peaches, apples, grapes, contain amoderate amount of sugar; watermelons, pears, etc. , contain less. Mostof our carbohydrates are of plant origin, being found in vegetables, fruits, cereals, and sirups. 

Carbohydrates, whether of the starch group or the sugar group, arecomposed chiefly of three elements: carbon, hydrogen, and oxygen; theyare therefore combustible, and are great energy producers. On theother hand, they are worthless for cell growth and repair, and if welimited our diet to carbohydrates, we should be like a man who hadfuel but no engine capable of using it. 

62. The Fats. The best-known fats are butter, lard, olive oil, andthe fats of meats, cheese, and chocolate. When we test fats for fuelvalues by means of a calorimeter (Fig. 26), we find that they yieldtwice as much heat as the carbohydrates, but that they burn out morequickly. Dwellers in cold climates must constantly eat largequantities of fatty foods if they are to keep their bodies warm andsurvive the extreme cold. Cod liver oil is an excellent food medicine, and if taken in winter serves to warm the body and to protect itagainst the rigors of cold weather. The average person avoids fattyfoods in summer, knowing from experience that rich foods make him warmand uncomfortable. The harder we work and the colder the weather, themore food of that kind do we require; it is said that a lumbermandoing heavy out-of-door work in cold climates needs three times asmuch food as a city clerk. Most of our fats, like lard and butter, areof animal origin; some of them, however, like olive oil, peanutbutter, and coconut oil, are of plant origin. 

[Illustration: FIG. 27. --_a_ is the amount of fat necessary to makeone calorie; _b_ is the amount of sugar or proteid necessary to makeone calorie. ]

63. The Proteids. The proteids are the building foods, furnishingmuscle, bone, skin cells, etc. , and supplying blood and other bodilyfluids. The best-known proteids are white of egg, curd of milk, andlean of fish and meat; peas and beans have an abundant supply of thissubstance, and nuts are rich in it. Most of our proteids are of animalorigin, but some protein material is also found in the vegetableworld. This class of foods contains carbon, oxygen, and hydrogen, andin addition, two substances not found in carbohydrates orfats--namely, sulphur and nitrogen. Proteids always contain nitrogen, and hence they are frequently spoken of as nitrogenous foods. Sincethe proteids contain all the elements found in the two other classesof foods, they are able to contribute, if necessary, to the store ofbodily energy; but their main function is upbuilding, and the dietshould be chosen so that the proteids do not have a double task. 

For an average man four ounces of dry proteid matter daily willsuffice to keep the body cells in normal condition. 

It has been estimated that 300, 000, 000 blood cells alone need dailyrepair or renewal. When we consider that the blood is but one part ofthe body, and that all organs and fluids have correspondingrequirements, we realize how vast is the work to be done by the foodwhich we eat. 

64. Mistakes in Buying. The body demands a daily ration of the threeclasses of food stuffs, but it is for us to determine from whatmeats, vegetables, fruits, cereals, etc. , this supply shall beobtained (Figs. 28 and 29). 

[Illustration: FIG. 28. --Table of food values. ]

[Illustration: FIG. 29. --Diagram showing the difference in the cost ofthree foods which give about the same amount of nutrition each. ]

Generally speaking, meats are the most expensive foods we canpurchase, and hence should be bought seldom and in small quantities. Their place can be taken by beans, peas, potatoes, etc. , and at lessthan a quarter of the cost. The average American family eats meatthree times a day, while the average family of the more conservativeand older countries rarely eats meat more than once a day. Thefollowing tables indicate the financial loss arising from an unwiseselection of foods:--

 FOOD CONSUMED--ONE WEEK|===========================|=======================================|| FAMILY No. 1 | || FAMILY No. 2 ||---------------------------|---------------------------------------||20 loaves of bread | $1. 00 ||15 lb. Flour, bread ||10 to 12 lb. Loin steak | || home made (skim milk used) | $. 45| or meat of similar cost | 2. 00 ||Yeast, shortening, and ||20 to 25 lb. Rib roast | || skim milk | . 10| or similar meat | 4. 40 ||10 lb. Steak (round, Hamburger||4 lb. High-priced cereal | || and some loin) | 1. 50| breakfast food, 20¢ | . 80 ||10 lb. Other meats, boiling ||Cake and pastry purchased | 3. 00 || pieces, rump roast, etc. | 1. 00|8 lb. Butter, 30¢ | 2. 40 || 5 lb. Cheese, 16¢ | . 80|Tea, coffee, spices, etc. | . 75 || 5 lb. Oatmeal (bulk) | . 15|Mushrooms | . 75 || 5 lb. Beans | . 25|Celery | 1. 00 ||Home-made cake and pastry | 1. 00|Oranges | 2. 00 || 6 lb. Butter, 30¢ | 1. 80|Potatoes | . 25 || 3 lb. Home-made shortening | . 25|Miscellaneous canned goods | 2. 00 ||Tea, coffee, and spices | . 40|Milk | . 50 ||Apples | . 50|Miscellaneous foods | 2. 00 ||Prunes | . 25|3 doz. Eggs | . 60 ||Potatoes | . 25| |-------||Milk | 1. 00| |$23. 45 ||Miscellaneous foods | 1. 00| | || 3 doz. Eggs | . 60| | || -|-----| | || $|11. 30| -----------------------|-----------------------------------------|---| -----------------------|-----------------------------------------|---

"The tables show that one family spends over twice as much in thepurchase of foods as the other family, and yet the one whose foodcosts the less actually secures the larger amount of nutritivematerial and is better fed than the family where more money isexpended. "--From _Human Foods_, Snyder. 

The Source of the Different Foods. All of our food comes from eitherthe plant world or the animal world. Broadly speaking, plants furnishthe carbohydrates, that is, starch and sugar; animals furnish the fatsand proteids. But although vegetable foods yield carbohydrates mainly, some of them, like beans and peas, contain large quantities of proteinand can be substituted for meat without disadvantage to the body. Other plant products, such as nuts, have fat as their most abundantfood constituent. The peanut, for example, contains 43% of fat, 30% ofproteids, and only 17% of carbohydrates; the Brazil nut has 65% offat, 17% of proteids, and only 9% of carbohydrates. Nuts make a goodmeat substitute, and since they contain a fair amount of carbohydratesbesides the fats and proteins, they supply all of the essential foodconstituents and form a well-balanced food. 

CHAPTER VI

WATER

65. Destructive Action of Water. The action of water in stream andsea, in springs and wells, is evident to all; but the activity ofground water--that is, rain water which sinks into the soil andremains there--is little known in general. The real activity of groundwater is due to its great solvent power; every time we put sugar intotea or soap into water we are using water as a solvent. When rainfalls, it dissolves substances floating in the atmosphere, and when itsinks into the ground and becomes ground water, it dissolves materialout of the rock which it encounters (Fig. 30). We know that watercontains some mineral matter, because kettles in which water is boiledacquire in a short time a crust or coating on the inside. This crustis due to the accumulation in the kettle of mineral matter which wasin solution in the water, but which was left behind when the waterevaporated. (See Section 25. )

[Illustration: FIG. 30. --Showing how caves and holes are formed by thesolvent action of water. ]

The amount of dissolved mineral matter present in some wells andsprings is surprisingly great; the famous springs of Bath, England, contain so much mineral matter in solution, that a column 9 feet indiameter and 140 feet high could be built out of the mineral mattercontained in the water consumed yearly by the townspeople. 

[Illustration: FIG. 31. --The work of water as a solvent. ]

Rocks and minerals are not all equally soluble in water; some are solittle soluble that it is years before any change becomes apparent, and the substances are said to be insoluble, yet in reality they areslowly dissolving. Other rocks, like limestone, are so readily solublein water that from the small pores and cavities eaten out by thewater, there may develop in long centuries, caves and caverns (Fig. 30). Most rock, like granite, contains several substances, some ofwhich are readily soluble and others of which are not readily soluble;in such rocks a peculiar appearance is presented, due to the rapiddisappearance of the soluble substance, and the persistence of themore resistant substance (Fig. 31). 

We see that the solvent power of water is constantly causing changes, dissolving some mineral substances, and leaving others practicallyuntouched; eating out crevices of various shapes and sizes, and bygradual solution through unnumbered years enlarging these crevicesinto wonderful caves, such as the Mammoth Cave of Kentucky. 

66. Constructive Action of Water. Water does not always act as adestructive agent; what it breaks down in one place it builds up inanother. It does this by means of precipitation. Water dissolves salt, and also dissolves lead nitrate, but if a salt solution is mixed witha lead nitrate solution, a solid white substance is formed in thewater (Fig. 32). This formation of a solid substance from the minglingof two liquids is called precipitation; such a process occurs daily inthe rocks beneath the surface of the earth. (See Laboratory Manual. )

[Illustration: FIG. 32. --From the mingling of two liquids a solid issometimes formed. ]

Suppose water from different sources enters a crack in a rock, bringing different substances in solution; then the mingling of thewaters may cause precipitation, and the solid thus formed will bedeposited in the crack and fill it up. Hence, while ground water tendsto make rock porous and weak by dissolving out of it large quantitiesof mineral matter, it also tends under other conditions to make itmore compact because it deposits in cracks, crevices, and pores themineral matter precipitated from solution. 

These two forces are constantly at work; in some places thedestructive action is more prominent, in other places the constructiveaction; but always the result is to change the character of theoriginal substance. When the mineral matter precipitated from thesolutions is deposited in cracks, _veins_ are formed (Fig. 33), whichmay consist of the ore of different metals, such as gold, silver, copper, lead, etc. Man is almost entirely dependent upon these veinsfor the supply of metal needed in the various industries, because inthe original condition of the rocks, the metallic substances are soscattered that they cannot be profitably extracted. 

[Illustration: FIG. 33. --Mineral matter precipitated from solution isdeposited in crevices and forms veins. ]

Naturally, the veins themselves are not composed of one substancealone, because several different precipitates may be formed. But thereis a decided grouping of valuable metals, and these can then bereadily separated by means of electricity. 

67. Streams. Streams usually carry mud and sand along with them;this is particularly well seen after a storm when rivers and brooksare muddy. The puddles which collect at the foot of a hill after astorm are muddy because of the particles of soil gathered by the wateras it runs down the hill. The particles are not dissolved in thewater, but are held there in suspension, as we call it technically. The river made muddy after a storm by suspended particles usuallybecomes clear and transparent after it has traveled onward for miles, because, as it travels, the particles drop to the bottom and aredeposited there. Hence, materials suspended in the water are bornealong and deposited at various places (Fig. 34). The amount ofdeposition by large rivers is so great that in some places channelsfill up and must be dredged annually, and vessels are sometimes caughtin the deposit and have to be towed away. 

Running water in the form of streams and rivers, by carrying sandparticles, stones, and rocks from high slopes and depositing them atlower levels, wears away land at one place and builds it up atanother, and never ceases in its work of changing the nature of theearth's surface (Fig. 35). 

[Illustration: FIG. 34. --Deposit left by running water. ]

[Illustration: FIG. 35. --Water by its action constantly changes thecharacter of the land. ]

68. Relation of Water to Human Life. Water is one of the mostessential of food materials, and whether we drink much or littlewater, we nevertheless get a great deal of it. The larger part of manyof our foods is composed of water; more than half of the weight of themeat we eat is made up of water; and vegetables are often more thannine tenths water. (See Laboratory Manual. ) Asparagus and tomatoeshave over 90 per cent. Of water, and most fruits are more than threefourths water; even bread, which contains as little water as any ofour common foods, is about one third water (Fig. 36). 

[Illustration: FIG. 36. --Diagram of the composition of a loaf of breadand of a potato: 1. Ash; 2, food; 3, water. ]

Without water, solid food material, although present in the body, would not be in a condition suitable for bodily use. An abundantsupply of water enables the food to be dissolved or suspended in it, and in solution the food material is easily distributed to all partsof the body. 

Further, water assists in the removal of the daily bodily wastes, andthus rids the system of foul and poisonous substances. 

The human body itself consists largely of water; indeed, about twothirds of our own weight is water. The constant replenishing of thislarge quantity is necessary to life, and a considerable amount of thenecessary supply is furnished by foods, particularly the fruits andvegetables. 

But while the supply furnished by the daily food is considerable, itis by no means sufficient, and should be supplemented by good drinkingwater. 

69. Water and its Dangers. Our drinking water comes from far andnear, and as it moves from place to place, it carries with it insolution or suspension anything which it can find, whether it beanimal, vegetable, or mineral matter. The power of water to gather upmatter is so great that the average drinking water contains 20 to 90grains of solid matter per gallon; that is, if a gallon of ordinarydrinking water is left to evaporate, a residue of 20 to 90 grains willbe left. (See Laboratory Manual. ) As water runs down a hill slope(Fig. 37), it carries with it the filth gathered from acres of land;carries with it the refuse of stable, barn, and kitchen; and too oftenthis impure surface water joins the streams which supply our cities. Lakes and rivers which furnish drinking water should be carefullyprotected from surface draining; that is, from water which has flowedover the land and has thus accumulated the waste of pasture andstable and, it may be, of dumping ground. 

[Illustration: FIG. 37. --As water flows over the land, it gathersfilth and disease germs. ]

It is not necessary that water should be absolutely free from allforeign substances in order to be safe for daily use in drinking; alimited amount of mineral matter is not injurious and may sometimes bereally beneficial. It is the presence of animal and vegetable matterthat causes real danger, and it is known that typhoid fever is duelargely to such impurities present in the drinking water. 

70. Methods of Purification. Water is improved by any of thefollowing methods:--

(_a_) _Boiling_. The heat of boiling destroys animal and vegetablegerms. Hence water that has been boiled a few minutes is safe to use. This is the most practical method of purification in the home, and isvery efficient. The boiled water should be kept in clean, corkedbottles; otherwise foreign substances from the atmosphere reënter thewater, and the advantage gained from boiling is lost. 

(_b_) _Distillation_. By this method pure water is obtained, but thismethod of purification cannot be used conveniently in the home(Section 25). 

(_c_) _Filtration_. In filtration, the water is forced throughporcelain or other porous substances which allow the passage of water, but which hold back the minute foreign particles suspended in thewater. (See Laboratory Manual. ) The filters used in ordinary dwellingsare of stone, asbestos, or charcoal. They are often valueless, becausethey soon become choked and cannot be properly cleaned. 

The filtration plants owned and operated by large cities are usuallysafe; there is careful supervision of the filters, and frequent andeffective cleanings are made. In many cities the filtration system isso good that private care of the water supply is unnecessary. 

71. The Source of Water. In the beginning, the earth was stored withwater just as it was with metal, rock, etc. Some of the watergradually took the form of rivers, lakes, streams, and wells, as now, and it is this original supply of water which furnishes us all that wehave to-day. We quarry to obtain stone and marble for building, and wefashion the earth's treasures into forms of our own, but we cannotcreate these things. We bore into the ground and drill wells in orderto obtain water from hidden sources; we utilize rapidly flowingstreams to drive the wheels of commerce, but the total amount of waterremains practically unchanged. 

The water which flows on the earth is constantly changing its form;the heat of the sun causes it to evaporate, or to become vapor, and tomingle with the atmosphere. In time, the vapor cools, condenses, andfalls as snow or rain; the water which is thus returned to the earthfeeds our rivers, lakes, springs, and wells, and these in turn supplywater to man. When water falls upon a field, it soaks into the ground, or collects in puddles which slowly evaporate, or it runs off anddrains into small streams or into rivers. That which soaks into theground is the most valuable because it remains on the earth longestand is the purest. 

[Illustration: FIG. 38. --How springs are formed. _A_, porous layer;_B_, non-porous layer; _C_, spring. ]

Water which soaks into the ground moves slowly downward and after alonger or shorter journey, meets with a non-porous layer of rockthrough which it cannot pass, and which effectually hinders itsdownward passage. In such regions, there is an accumulation of water, and a well dug there would have an abundant supply of water. Thenon-porous layer is rarely level, and hence the water whose verticalpath is obstructed does not "back up" on the soil, but flows down hillparallel with the obstructing non-porous layer, and in some distantregion makes an outlet for itself, forming a spring (Fig. 38). Thestreams originating in the springs flow through the land andeventually join larger streams or rivers; from the surface of streamsand rivers evaporation occurs, the water once more becomes vapor andpasses into the atmosphere, where it is condensed and again falls tothe earth. 

Water which has filtered through many feet of earth is far purer andsafer than that which fell directly into the rivers, or which ran offfrom the land and joined the surface streams without passing throughthe soil. 

72. The Composition of Water. Water was long thought to be a simplesubstance, but toward the end of the eighteenth century it was foundto consist of two quite different substances, oxygen (O) and hydrogen(H. )

[Illustration: FIG. 39. --The decomposition of water. ]

If we send an electric current through water (acidulated to make it agood conductor), as shown in Figure 39, we see bubbles of gas risingfrom the end of the wire by which the current enters the water, andother bubbles of gas rising from the end of the wire by which thecurrent leaves the water. These gases have evidently come from thewater and are the substances of which it is composed, because thewater begins to disappear as the gases are formed. If we place overeach end of the wire an inverted jar filled with water, the gases areeasily collected. The first thing we notice is that there is alwaystwice as much of one gas as of the other; that is, water is composedof two substances, one of which is always present in twice as largequantities as the other. 

73. The Composition of Water. On testing the gases into which wateris broken up by an electric current, we find them to be quitedifferent. One proves to be oxygen, a substance with which we arealready familiar. The other gas, hydrogen, is new to us and isinteresting as being the lightest known substance, being even "lighterthan a feather. "

An important fact about hydrogen is that in burning it gives as muchheat as five times its weight of coal. Its flame is blue and almostinvisible by daylight, but intensely hot. If fine platinum wire isplaced in an ordinary gas flame, it does not melt, but if placed in aflame of burning hydrogen, it melts very quickly. 

74. How to prepare Hydrogen. There are many different methods ofpreparing hydrogen, but the easiest laboratory method is to poursulphuric acid, or hydrochloric acid, on zinc shavings and to collectin a bottle the gas which is given off. This gas proves to becolorless, tasteless, and odorless. (See Laboratory Manual. )

CHAPTER VII

AIR

75. The Instability of the Air. We are usually not conscious of theair around us, but sometimes we realize that the air is heavy, whileat other times we feel the bracing effect of the atmosphere. We livein an ocean of air as truly as fish inhabit an ocean of water. If youhave ever been at the seashore you know that the ocean is never stillfor a second; sometimes the waves surge back and forth in angry fury, at other times the waves glide gently in to the shore and the surfaceis as smooth as glass; but we know that there is perpetual motion ofthe water even when the ocean is in its gentlest moods. Generally ouratmosphere is quiet, and we are utterly unconscious of it; at othertimes we are painfully aware of it, because of its furious winds. Thenagain we are oppressed by it because of the vast quantity of vaporwhich it holds in the form of fog, or mist. The atmosphere around usis as restless and varying as is the water of the sea. The air at thetop of a high tower is very different from the air at the base of thetower. Not only does the atmosphere vary greatly at differentaltitudes, but it varies at the same place from time to time, at oneperiod being heavy and raw, at another being fresh and invigorating. 

Winds, temperature, and humidity all have a share in determiningatmospheric conditions, and no one of these plays a small part. 

76. The Character of the Air. The atmosphere which envelops us atall times extends more than fifty miles above us, its height being fargreater than the greatest depths of the sea. This atmosphere variesfrom place to place; at the sea level it is heavy, on the mountain topless heavy, and far above the earth it is so light that it does notcontain enough oxygen to permit man to live. Figure 40 illustrates bya pile of pillows how the pressure of the air varies from level tolevel. 

[Illustration: FIG. 40. --To illustrate the decrease in pressure withheight. ]

Sea level is a low portion of the earth's surface, hence at sea levelthere is a high column of air, and a heavy air pressure. As one passesfrom sea level to mountain top a gradual but steady decrease in theheight of the air column occurs, and hence a gradual but definitelessening of the air pressure. 

[Illustration: FIG. 41. --The water in the tube is at the same level asthat in the glass. ]

77. Air Pressure. If an empty tube (Fig. 41) is placed upright inwater, the water will not rise in the tube, but if the tube is put inwater and the air is then drawn out of the tube by the mouth, thewater will rise in the tube (Fig. 42). This is what happens when wetake lemonade through a straw. When the air is withdrawn from thestraw by the mouth, the pressure within the straw is reduced, and theliquid is forced up the straw by the air pressure on the surface ofthe liquid in the glass. Even the ancient Greeks and Romans knew thatwater would rise in a tube when the pressure within the tube wasreduced, and hence they tried to obtain water from wells in thisfashion, but the water could never be raised higher than 34 feet. Letus see why water could rise 34 feet and no more. If an empty pipe isplaced in a cistern of water, the water in the pipe does not riseabove the level of the water in the cistern. If, however, the pressurein the tube is removed, the water in the tube will rise to a height of34 feet approximately. If now the air pressure in the tube isrestored, the water in the tube sinks again to the level of that inthe cistern. The air pressing on the liquid in the cistern tends topush some liquid up the tube, but the air pressing on the water in thetube pushes downwards, and tends to keep the liquid from rising, andthese two pressures balance each other. When, however, the pressurewithin the tube is reduced, the liquid rises because of the unbalancedpressure which acts on the water in the cistern. 

[Illustration: FIG. 42. --Water rises in the tube when the air iswithdrawn. ]

[Illustration: FIG. 43. --The air supports a column of mercury 30inches high. ]

The column of water which can be raised this way is approximately 34feet, sometimes a trifle more, sometimes a trifle less. If water weretwice as heavy, just half as high a column could be supported by theatmosphere. Mercury is about thirteen times as heavy as water and, therefore, the column of mercury supported by the atmosphere is aboutone thirteenth as high as the column of water supported by theatmosphere. This can easily be demonstrated. Fill a glass tube about ayard long with mercury, close the open end with a finger, and quicklyinsert the end of the inverted tube in a dish of mercury (Fig. 43). When the finger is removed, the mercury falls somewhat, leaving anempty space in the top of the tube. If we measure the column in thetube, we find its height is about one thirteenth of 34 feet or 30inches, exactly what we should expect. Since there is no air pressurewithin the tube, the atmospheric pressure on the mercury in the dishis balanced solely by the mercury within the tube, that is, by acolumn of mercury 30 inches high. The shortness of the mercury columnas compared with that of water makes the mercury more convenient forboth experimental and practical purposes. (See Laboratory Manual. )

78. The Barometer. Since the pressure of the air changes from timeto time, the height of the mercury will change from day to day, andhour to hour. When the air pressure is heavy, the mercury will tend tobe high; when the air pressure is low, the mercury will show a shortercolumn; and by reading the level of the mercury one can learn thepressure of the atmosphere. If a glass tube and dish of mercury areattached to a board and the dish of mercury is inclosed in a case forprotection from moisture and dirt, and further if a scale of inches orcentimeters is made on the upper portion of the board, we have amercurial barometer (Fig. 44). 

[Illustration: FIG. 44. --A simple barometer. ]

If the barometer is taken to the mountain top, the column of mercuryfalls gradually during the ascent, showing that as one ascends, thepressure decreases in agreement with the statement in Section 76. Observations similar to these were made by Torricelli as early as thesixteenth century. Taking a barometric reading consists in measuringthe height of the mercury column. 

79. A Portable Barometer. The mercury barometer is large andinconvenient to carry from place to place, and a more portable formhas been devised, known as the aneroid barometer (Fig. 45). This formof barometer is extremely sensitive; indeed, it is so delicate thatit shows the slight difference between the pressure at the table topand the pressure at the floor level, whereas the mercury barometerwould indicate only a much greater variation in atmospheric pressure. The aneroid barometers are frequently made no larger than a watch andcan be carried conveniently in the pocket, but they get out of ordereasily and must be frequently readjusted. The aneroid barometer is anair-tight box whose top is made of a thin metallic disk which bendsinward or outward according to the pressure of the atmosphere. If theatmospheric pressure increases, the thin disk is pushed slightlyinward; if, on the other hand, the atmospheric pressure decreases, thepressure on the metallic disk decreases and the disk is not pressed sofar inward. The motion of the disk is small, and it would beimpossible to calculate changes in atmospheric pressure from themotion of the disk, without some mechanical device to make the slightchanges in motion perceptible. 

[Illustration: FIG. 45. --Aneroid barometer. ]

In order to magnify the slight changes in the position of the disk, the thin face is connected with a system of levers, or wheels, whichmultiplies the changes in motion and communicates them to a pointerwhich moves around a graduated circular face. In Figure 45 the realbarometer is scarcely visible, being securely inclosed in a metal casefor protection; the principle, however, can be understood by referenceto Figure 46. 

[Illustration: FIG. 46. --Principle of the aneroid barometer. ]

80. The Weight of the Air. We have seen that the pressure of theatmosphere at any point is due to the weight of the air column whichstretches from that point far up into the sky above. This weightvaries slightly from time to time and from place to place, but it isequal to about 15 pounds to the square inch as shown by actualmeasurement. It comes to us as a surprise sometimes that air actuallyhas weight; for example, a mass of 12 cubic feet of air at averagepressure weighs 1 pound, and the air in a large assembly hall weighsmore than 1 ton. 

We are practically never conscious of this really enormous pressure ofthe atmosphere, which is exerted over every inch of our bodies, because the pressure is exerted equally over the outside and theinside of our bodies; the cells and tissues of our bodies containinggases under atmospheric pressure. If, however, the finger is placedover the open end of a tube and the air is sucked out of the tube bythe mouth, the flesh of the finger bulges into the tube because thepressure within the finger is no longer equalized by the usualatmospheric pressure (Fig. 47). 

[Illustration: FIG. 47. --The flesh bulges out. ]

Aëronauts have never ascended much higher than 7 miles; at that heightthe barometer stands at 7 inches instead of at 30 inches, and theinternal pressure in cells and tissues is not balanced by an equalexternal pressure. The unequalized internal pressure forces the bloodto the surface of the body and causes rupture of blood vessels andother physical difficulties. 

81. Use of the Barometer. Changes in air pressure are very closelyconnected with changes in the weather. The barometer does not directlyforetell the weather, but a low or falling pressure, accompanied by asimultaneous fall of the mercury, usually precedes foul weather, whilea rising pressure, accompanied by a simultaneous rise in the mercury, usually precedes fair weather. The barometer is not an infallibleprophet, but it is of great assistance in predicting the general trendof the weather. There are certain changes in the barometer whichfollow no known laws, and which allow of no safe predictions, but onthe other hand, general future conditions for a few days ahead can befairly accurately determined. Figure 48 shows a barograph orself-registering barometer which automatically registers air pressure. 

[Illustration: FIG. 48. --Barograph. ]

Seaport towns in particular, but all cities, large or small, andvillages too, are on request notified by the United States WeatherBureau ten hours or more in advance, of probable weather conditions, and in this way precautions are taken which annually save millions ofdollars and hundreds of lives. 

I recollect a summer spent on a New Hampshire farm, and know that anold farmer started his farm hands haying by moonlight at two o'clockin the morning, because the Special Farmer's Weather Forecast of thepreceding evening had predicted rain for the following day. Hisreliance on the weather report was not misplaced, since the storm camewith full force at noon. Sailing vessels, yachts, and fishing doriesremain within reach of port if the barometer foretells storms. 

[Illustration: FIG. 49. --Isotherms. ]

82. Isobaric and Isothermal Lines. If a line were drawn through allpoints on the surface of the earth having an equal barometric pressureat the same time, such a line would be called an isobar. For example, if the height of barometers in different localities is observed atexactly the same time, and if all the cities and towns which have thesame pressure are connected by a line, the curved lines will be calledisobars. By the aid of these lines the barometric conditions over alarge area can be studied. The Weather Bureau at Washington reliesgreatly on these isobars for statements concerning local and distantweather forecasts, any shift in isobaric lines showing change inatmospheric pressure. 

If a line is drawn through all points on the surface of the earthhaving the same _temperature_ at the same instant, such a line iscalled an isotherm (Fig. 49). 

83. Weather Maps. Scattered over the United States are about 125Government Weather Stations, at each of which three times a day, atthe same instant, accurate observations of the weather are made. Theseobservations, which consist of the reading of barometer andthermometer, the determination of the velocity and direction of thewind, the determination of the humidity and of the amount of rain orsnow, are telegraphed to the chief weather official at Washington. From the reports of wind storms, excessive rainfall, hot waves, clearing weather, etc. , and their rate of travel, the chief officialspredict where the storms, etc. , will be at a definite future time. Inthe United States, the _general_ movement of weather conditions, asindicated by the barometer, is from west to east, and if a certainweather condition prevails in the west, it is probable that it willadvance eastward, although with decided modifications. So manyinfluences modify atmospheric conditions that unfailing predictionsare impossible, but the Weather Bureau predictions prove true in abouteight cases out of ten. 

The reports made out at Washington are telegraphed on request tocities in this country, and are frequently published in the dailypapers, along with the forecast of the local office. A careful studyof these reports enables one to forecast to some extent the probableweather conditions of the day. 

The first impression of a weather map (Fig. 50) with its various linesand signals is apt to be one of confusion, and the temptation comes toabandon the task of finding an underlying plan of the weather. If onewill bear in mind a few simple rules, the complexity of the weathermap will disappear and a glance at the map will give one informationconcerning general weather conditions just as a glance at thethermometer in the morning will give some indication of the probabletemperature of the day. (See Laboratory Manual. )

[Illustration: FIG. 50. Weather Map]

On the weather map solid lines represent isobars and dotted linesrepresent isotherms. The direction of the wind at any point isindicated by an arrow which flies with the wind; and the state of theweather--clear, partly cloudy, cloudy, rain, snow, etc. --is indicatedby symbols. 

84. Components of the Air. The best known constituent of the air isoxygen, already familiar to us as the feeder of the fire without andwithin the body. Almost one fifth of the air which envelops us is madeup of the life-giving oxygen. This supply of oxygen in the air isconstantly being used up by breathing animals and glowing fires, andunless there were some constant source of additional supply, thequantity of oxygen in the air would soon become insufficient tosupport animal life. The unfailing constant source of atmosphericoxygen is plant life (Section 48). The leaves of plants absorb carbondioxide from the air, and break it up into oxygen and carbon. Theplant makes use of the carbon but it rejects the oxygen, which passesback into the atmosphere through the pores of the leaves. 

Although oxygen constitutes only one fifth of the atmosphere, it isone of the most abundant and widely scattered of all substances. Almost the whole earth, whether it be rich loam, barren clay, orgranite boulder, contains oxygen in some form or other; that is, incombination with other substances. But nowhere, except in the airaround us, do we find oxygen free and uncombined with othersubstances. 

A less familiar but more abundant constituent of the atmosphere is thenitrogen. Almost four fifths of the air around us is made up ofnitrogen. If the atmosphere were composed of oxygen alone, the merestflicker of a match would set the whole world ablaze. The fact that theoxygen of the air is diluted as it were with so large a proportion ofnitrogen, prevents fires from sweeping over the world and destroyingeverything in their path. Nitrogen does not support combustion, and aburning match placed in a corked bottle goes out as soon as it hasused up the oxygen in the bottle. The nitrogen in the bottle, not onlydoes not assist the burning of the match, but it acts as a damper tothe burning. 

Free nitrogen, like oxygen, is a colorless, odorless gas. It is notpoisonous; but one would die if surrounded by nitrogen alone, just asone would die if surrounded by water. The vast supply of nitrogen inthe atmosphere would be useless if the smaller amount of oxygen werenot present to keep the body alive. Nitrogen is so important a factorin daily life that an entire chapter will be devoted to it later. 

Another constituent of the air with which we are familiar is carbondioxide. In pure air, carbon dioxide is present in very smallproportion, being continually taken from the air by plants in themanufacture of their food. 

Various other substances are present in the air in very minuteproportions, but of all the substances in the air, oxygen, nitrogen, and carbon dioxide are the most important. 

CHAPTER VIII

GENERAL PROPERTIES OF GASES

85. Bicycle Tires. We know very well that we cannot put more than acertain amount of water in a tube, but we know equally well that theamount of air which can be pumped into a bicycle or automobile tiredepends largely upon our muscular energy. A gallon of water remains agallon of water and requires a perfectly definite amount of space, butair can be compressed and compressed, and made to occupy less and lessspace. While it is true that air is easily compressed, it is also truethat air is elastic and capable of very rapid and easy expansion. If apuncture occurs in a tire, the compressed air escapes very quickly;that is, the compressed air within the tube has taken the firstopportunity offered for expansion. 

[Illustration: FIG. 51. --By squeezing the bulb, air is forced out ofthe nozzle. ]

The fact that air is elastic has added materially to the comfort ofthe world. Transportation by bicycles and automobiles has been greatlyfacilitated by the use of air tires. In many hospitals, air mattressesare used in place of hair, feather, or cotton mattresses, and in thisway the bed is kept fresher and cleaner, and can be moved with lessdanger of discomfort to the patient. Every time we squeeze the bulb ofan atomizer, we force compressed or condensed air through theatomizer, and the condensed air pushes the liquid out of the nozzle(Fig. 51). Thus we see that in the necessities and conveniences oflife compressed air plays an important part. 

86. The Danger of Compression. Air under ordinary atmosphericconditions exerts a pressure of 15 pounds to the square inch. If, now, large quantities of air are compressed into a small space, thepressure exerted becomes correspondingly greater. If too much air isblown into a toy balloon, the balloon bursts because it cannot supportthe great pressure exerted by the compressed air within. What is trueof air is true of all gases. Dangerous boiler explosions have occurredbecause the boiler walls were not strong enough to withstand thepressure of the steam (which is water in the form of gas). Thepressure within the boilers of engines is frequently several hundredpounds to the square inch, and such a pressure needs a strong boiler. 

87. How Pressure is Measured in Buildings. In the preceding Sectionwe saw that undue pressure of a gas may cause explosion. It isimportant, therefore, that authorities keep strict watch on gasesconfined within pipes and reservoirs, never allowing the pressure toexceed that which the walls of the reservoir will safely bear. 

[Illustration: FIG. 52. --A pressure gauge. ]

Pressure in a gas pipe may be measured by a simple instrument calledthe pressure gauge: The gauge consists of a bent glass tube containingmercury, and so made that one end can be fitted to a gas jet (Fig. 52). When the gas cock is closed, the mercury stands at the same levelin both arms, but when the cock is opened, the gas whose pressure isbeing measured forces the mercury up the opposite arm. If the pressureof the gas is small, the mercury changes its level but very little. Itis clear that the height of a column of mercury is a measure of thegas pressure. Now it is known that one cubic inch of mercury weighsabout half a pound. Hence a column of mercury one inch high indicatesa pressure of about one half pound to the square inch; a column twoinches high indicates a pressure of about one pound to the squareinch, and so on. 

This is a very convenient way to measure the pressure of theilluminating gas in our homes and offices. The gauge is attached tothe gas burner and the pressure is read by means of a scale attachedto the gauge. (See Laboratory Manual. )

In order to have satisfactory illumination, the pressure must bestrong enough to give a steady, broad flame. If the flame from any gasjet is flickering and weak, it is usually an indication ofinsufficient pressure and the gas company should investigateconditions and see to it that the consumer receives his proper value. 

87. The Gas Meter. Most householders are deeply interested in theactual amount of gas which they consume (gas is charged for accordingto the number of cubic feet used), and therefore they should be ableto read the gas meter which indicates their consumption of gas. Suchgas meters are furnished by the companies, and can be read easily. 

[Illustration: FIG. 53. --The gas meter indicates the number of cubicfeet of gas consumed. ]

The instrument itself is somewhat complex. It will suffice to say thatwithin the meter box are thin disks which are moved by the stream ofgas that passes them. This movement of the disks is recorded byclockwork devices on a dial face. In this way, the number of cubicfeet of gas which pass through the meter is automatically registered. 

89. The Relation between Pressure and Volume. It was long known thatas the pressure of a gas increases, that is, as it becomes compressed, its volume decreases, but Robert Boyle was the first to determine theexact relation between the volume and the pressure of a gas. He didthis in a very simple manner. 

Pour mercury into a U-shaped tube until the level of the mercury inthe closed end of the tube is the same as the level in the open end. The air in the long arm is pressing upon the mercury in that arm, andis tending to force it up the short arm. The air in the short closedarm is pressing down upon the mercury in that arm and tending to sendit up the long arm. Since the mercury is at the same level in the twoarms, the pressure in the long arm must be equal to the pressure inthe short arm. But the long arm is open, and the pressure in that armis the pressure of the atmosphere. Therefore the pressure in the shortarm must be one atmosphere. Measure the distance _bc_ between the topof the mercury and the closed end of the tube. 

[Illustration: FIGS. 54, 55. --As the pressure on the gas increases, its volume decreases. ]

Pour more mercury into the open end of the tube, and as the mercuryrises higher and higher in the long arm, note carefully the decreasein the volume of the air in the short arm. Pour mercury into the tubeuntil the difference in level _bd_ is just equal to the barometricheight, approximately 32 inches. The pressure of the air in the closedend now supports the pressure of one atmosphere, and in addition, acolumn of mercury equal to another atmosphere. If now the air columnin the closed end is measured, its volume will be only one half of itsformer volume. By doubling the pressure we have reduced the volume onehalf. Similarly, if the pressure is increased threefold, the volumewill be reduced to one third of the original volume. 

90. Heat due to Compression. We saw in Section 89 that whenever thepressure exerted upon a gas is increased, the volume of the gas isdecreased; and that whenever the pressure upon a gas is decreased, thevolume of the gas is increased. If the pressure is changed veryslowly, the change in the temperature of the gas is imperceptible; if, however, the pressure is removed suddenly, the temperature fallsrapidly, or if the pressure is applied suddenly, the temperature risesrapidly. When bicycle tires are being inflated, the pump becomes hotbecause of the compression of the air. 

The amount of heat resulting from compression is surprisingly large;for example, if a mass of gas at 0° C. Is suddenly compressed to onehalf its original volume, its temperature rises 87° C. 

91. Cooling by Expansion. If a gas expands suddenly, its temperaturefalls; for example, if a mass of gas at 87° C. Is allowed to expandrapidly to twice its original volume, its temperature falls to 0° C. If the compressed air of a bicycle tire is allowed to expand and asensitive thermometer is held in the path of the escaping air, thethermometer will show a decided drop in temperature. 

The low temperature obtained by the expansion of air or other gases isutilized commercially on a large scale. By means of powerful pistonsair is compressed to one third or one fourth its original volume, ispassed through a coil of pipe surrounded with cold water, and is thenallowed to escape into large refrigerating vaults, which thereby havetheir temperatures noticeably lowered, and can be used for thepermanent storage of meats, fruits, and other perishable material. Insummer, when the atmospheric temperature is high, the storage andpreservation of foods is of vital importance to factories and coldstorage houses, and but for the low temperature obtainable by theexpansion of compressed gases, much of our food supply would be lostto use. 

92. Unexpected Transformations. If the pressure on a gas is greatlyincreased, a sudden transformation sometimes occurs and the gasbecomes a liquid. Then, if the pressure is reduced, a secondtransformation occurs, and the liquid evaporates or returns to itsoriginal form as a gas. 

In Section 23 we saw that a fall of temperature caused water vapor tocondense or liquefy. If temperature alone were considered, most gasescould not be liquefied, because the temperature at which the averagegas liquefies is so low as to be out of the range of possibility; ithas been calculated, for example, that a temperature of 252° C. Belowzero would have to be obtained in order to liquefy hydrogen. 

Some gases can be easily transformed into liquids by pressure alone, some gases can be easily transformed into liquids by cooling alone; onthe other hand, many gases are so difficult to liquefy that bothpressure and low temperature are needed to produce the desired result. If a gas is cooled and compressed at the same time, liquefactionoccurs much more surely and easily than though either factor alonewere depended upon. The air which surrounds us, and of whose existencewe are scarcely aware, can be reduced to the form of a liquid, but thepressure exerted upon the portion to be liquefied must be thirty-ninetimes as great as the atmospheric pressure, and the temperature musthave been reduced to a very low point. 

93. Artificial Ice. Ammonia gas is liquefied by strong pressure andlow temperature and is then allowed to flow into pipes which runthrough tanks containing salt water. The reduction of pressure causesthe liquid to evaporate or turn to a gas, and the fall of temperaturewhich always accompanies evaporation means a lowering of thetemperature of the salt water to 16° or 18° below zero. But immersedin the salt water are molds containing pure water, and since thefreezing point of water is 0° C, the water in the molds freezes andcan be drawn from the mold as solid cakes of ice. 

[Illustration: FIG. 56. --Apparatus for making artificial ice. ]

Ammonia gas is driven by the pump _C_ into the coil _D_ (Fig. 56)under a pressure strong enough to liquefy it, the heat generated bythis compression being carried off by cold water which constantlycirculates through _B_. The liquid ammonia flows through theregulating valve _V_ into the coil _E_, in which the pressure is keptlow by the pump _C_. The accompanying expansion reduces thetemperature to a very low degree, and the brine which circulatesaround the coil _E_ acquires a temperature below the freezing point ofpure water. The cold brine passes from _A_ to a tank in which areimmersed cans filled with water, and within a short time the water inthe cans is frozen into solid cakes of ice. 

CHAPTER IX

INVISIBLE OBJECTS

94. Very Small Objects. We saw in Section 84 that gases have atendency to expand, but that they can be compressed by the applicationof force. This observation has led scientists to suppose thatsubstances are composed of very minute particles called molecules, separated by small spaces called pores; and that when a gas iscondensed, the pores become smaller, and that when a gas expands, thepores become larger. 

The fact that certain substances are soluble, like sugar in water, shows that the molecules of sugar find a lodging place in the spacesor pores between the molecules of water, in much the same way thatpebbles find lodgment in the chinks of the coal in a coal scuttle. Anindefinite quantity of sugar cannot be dissolved in a given quantityof liquid, because after a certain amount of sugar has been dissolvedall the pores become filled, and there is no available molecularspace. The remainder of the sugar settles at the bottom of the vessel, and cannot be dissolved by any amount of stirring. 

If a piece of potassium permanganate about the size of a grain of sandis put into a quart of water, the solid disappears and the waterbecomes a deep rich red. The solid evidently has dissolved and hasbroken up into minute particles which are too small to be seen, butwhich have scattered themselves and lodged in the pores of the water, thus giving the water its rich color. 

There is no visible proof of the existence of molecules and molecularspaces, because not only are our eyes unable to see them directly, buteven the most powerful microscope cannot make them visible to us. Theyare so small that if one thousand of them were laid side by side, theywould make a speck too small to be seen by the eye and too small to bevisible under the most powerful microscope. 

We cannot see molecules or molecular pores, but the phenomena ofcompression and expansion, solubility and other equally convincingfacts, have led us to conclude that all substances are composed ofvery minute particles or molecules separated by spaces called pores. 

95. Journeys Made by Molecules. If a gas jet is turned on and notlighted, an odor of gas soon becomes perceptible, not only throughoutthe room, but in adjacent halls and even in distant rooms. An uncorkedbottle of cologne scents an entire room, the odor of a rose or violetpermeates the atmosphere near and far. These simple everydayoccurrences seem to show that the molecules of a gas must be in astate of continual and rapid motion. In the case of the cologne, somemolecules must have escaped from the liquid by the process ofevaporation and traveled through the air to the nose. We know that themolecules of a liquid are in motion and are continually passing intothe air because in time the vessel becomes empty. The only way inwhich this could happen would be for the molecules of the liquid topass from the liquid into the surrounding medium; but this is reallysaying that the molecules are in motion. 

From these phenomena and others it is reasonably clear that substancesare composed of molecules, and that molecules are not inert, quietparticles, but that they are in incessant motion, moving rapidlyhither and thither, sometimes traveling far, sometimes near. Even thelog of wood which lies heavy and motionless on our woodpile is madeup of countless billions of molecules each in rapid incessant motion. The molecules of solid bodies cannot escape so readily as those ofliquids and gases, and do not travel far. The log lies year after yearin an apparently motionless condition, but if one's eyes were keenenough, the molecules would be seen moving among themselves, eventhough they cannot escape into the surrounding medium and make longjourneys as do the molecules of liquids and gases. 

96. The Companions of Molecules. Common sense tells us that amolecule of water is not the same as a molecule of vinegar; themolecules of each are extremely small and in rapid motion, but theydiffer essentially, otherwise one substance would be like every othersubstance. What is it that makes a molecule of water differ from amolecule of vinegar, and each differ from all other molecules? Strangeto say, a molecule is not a simple object, but is quite complex, beingcomposed of one or more smaller particles, called atoms, and thenumber and kind of atoms in a molecule determine the type of themolecule, and the type of the molecule determines the substance. Forexample, a glass of water is composed of untold millions of molecules, and each molecule is a company of three still smaller particles, oneof which is called the oxygen atom and two of which are alike in everyparticular and are called hydrogen atoms. 

97. Simple Molecules. Generally molecules are composed of atomswhich are different in kind. For example, the molecule of water hastwo different atoms, the oxygen atom and the hydrogen atoms; alcoholhas three different kinds of atoms, oxygen, hydrogen, and carbon. Sometimes, however, molecules are composed of a group of atoms all ofwhich are alike. Now there are but seventy or eighty different kindsof atoms, and hence there can be but seventy or eighty differentsubstances whose molecules are composed of atoms which are alike. Whenthe atoms comprising a molecule are all alike, the substance is calledan element, and is said to be a simple substance. Throughout thelength and breadth of this vast world of ours there are only abouteighty known elements. An element is the simplest substanceconceivable, because it has not been separated into anything simpler. Water is a compound substance. It can be separated into oxygen andhydrogen. 

Gold, silver, and lead are examples of elements, and water, alcohol, cider, sand, and marble are complex substances, or compounds, as weare apt to call them. Everything, no matter what its size or shape orcharacter, is formed from the various combinations into molecules of afew simple atoms, of which there exist about eighty known differentkinds. But few of the eighty known elements play an important part inour everyday life. The elements in which we are most interested aregiven in the following table, and the symbols by which they are knownare placed in columns to the right:

 |Oxygen |O |Copper |Cu |Phosphorus |P | |Hydrogen |H |Iodine |I |Potassium |K | |Carbon |C |Iron |Fe |Silver |Ag | |Aluminium Al |Lead |Pb |Sodium |Na | | |Calcium |Ca |Nickel |Ni |Sulphur |S | |Chlorine |Cl |Nitrogen |N |Tin |Sn |

We have seen in an earlier experiment that twice as much hydrogen asoxygen can be obtained from water. Two atoms of the element hydrogenunite with one atom of the element oxygen to make one molecule ofwater. In symbols we express this H_2O. A group of symbols, such asthis, expressing a molecule of a compound is called a _formula_. NaClis the formula for sodium chloride, which is the chemical name ofcommon salt. 

CHAPTER X

LIGHT

98. What Light Does for Us. Heat keeps us warm, cooks our food, drives our engines, and in a thousand ways makes life comfortable andpleasant, but what should we do without light? How many of us could behappy even though warm and well fed if we were forced to live in thedark where the sunbeams never flickered, where the shadows never stoleacross the floor, and where the soft twilight could not tell us thatthe day was done? Heat and light are the two most important physicalfactors in life; we cannot say which is the more necessary, because inthe extreme cold or arctic regions man cannot live, and in the darkplaces where the light never penetrates man sickens and dies. Bothheat and light are essential to life, and each has its own part toplay in the varied existence of man and plant and animal. 

Light enables us to see the world around us, makes the beautifulcolors of the trees and flowers, enables us to read, is essential tothe taking of photographs, gives us our moving pictures and our magiclanterns, produces the exquisite tints of stained-glass windows, andbrings us the joy of the rainbow. We do not always realize that lightis beneficial, because sometimes it fades our clothing and ourcarpets, and burns our skin and makes it sore. But we shall see thateven these apparently harmful effects of light are in reality of greatvalue in man's constant battle against disease. 

99. The Candle. Natural heat and light are furnished by the sun, butthe absence of the sun during the evening makes artificial lightnecessary, and even during the day artificial light is needed inbuildings whose structure excludes the natural light of the sun. Artificial light is furnished by electricity, by gas, by oil in lamps, and in numerous other ways. Until modern times candles were the mainsource of light, and indeed to-day the intensity, or power, of anylight is measured in candle power units, just as length is measured inyards; for example, an average gas jet gives a 10 candle power light, or is ten times as bright as a candle; an ordinary incandescentelectric light gives a 16 candle power light, or furnishes sixteentimes as much light as a candle. Very strong large oil lamps can attimes yield a light of 60 candle power, while the large arc lampswhich flash out on the street corners are said to furnish 1200 timesas much light as a single candle. Naturally all candles do not givethe same amount of light, nor are all candles alike in size. Thecandles which decorate our tea tables are of wax, while those whichserve for general use are of paraffin and tallow. 

[Illustration: FIG. 57. --A photograph at _a_ receives four times asmuch light as when held at _b_. ]

100. Fading Illumination. The farther we move from a light, the lessstrong, or intense, is the illumination which reaches us; the light ofthe street lamp on the corner fades and becomes dim before the middleof the block is reached, so that we look eagerly for the next lamp. The light diminishes in brightness much more rapidly than we realize, as the following simple experiment will show. Let a single candle(Fig. 57) serve as our light, and at a distance of one foot from thecandle place a photograph. In this position the photograph receives adefinite amount of light from the candle and has a certain brightness. 

If now we place a similar photograph directly behind the firstphotograph and at a distance of two feet from the candle, the secondphotograph receives no light because the first one cuts off all thelight. If, however, the first photograph is removed, the light whichfell on it passes outward and spreads itself over a larger area, untilat the distance of the second photograph the light spreads itself overfour times as large an area as formerly. At this distance, then, theillumination on the second photograph is only one fourth as strong asit was on a similar photograph held at a distance of one foot from thecandle. 

The photograph or object placed at a distance of one foot from a lightis well illuminated; if it is placed at a distance of two feet, theillumination is only one fourth as strong, and if the object is placedthree feet away, the illumination is only one ninth as strong. Thisfact should make us have thought and care in the use of our eyes. Wethink we are sixteen times as well off with our incandescent lights asour ancestors were with simple candles, but we must reflect that ourancestors kept the candle near them, "at their elbow, " so to speak, while we sit at some distance from the light and unconcernedly readand sew. 

As an object recedes from a light the illumination which it receivesdiminishes rapidly, for the strength of the illumination is inverselyproportional to the square of distance of the object from the light. Our ancestors with a candle at a distance of one foot from a book wereas well off as we are with an incandescent light four feet away. 

101. Money Value of Light. Light is bought and sold almost asreadily as are the products of farm and dairy; many factories, churches, and apartments pay a definite sum for electric light of astandard strength, and naturally full value is desired. An instrumentfor measuring the strength of a light is called a photometer, andthere are many different varieties, just as there are varieties ofscales which measure household articles. One light-measuring scaledepends upon the law that the intensity of illumination decreases withthe square of the distance of the object from the light. Suppose wewish to measure the strength of the electric light bulbs in our homes, in order to see whether we are getting the specified illumination. Infront of a screen place a black rod (Fig. 58) which is illuminated bytwo different lights; namely, a standard candle and an incandescentbulb whose strength is to be measured. Two shadows of the rod willfall on the screen, one caused by the candle and the other caused bythe incandescent light. The shadow due to the latter source is not sodark as that due to the candle. Now let the incandescent light bemoved away from the screen until the two shadows are of equaldarkness. If the incandescent light is four times as far away from thescreen as the candle, and the shadows are equal, we know, by Section100, that its strength is sixteen candle power. If the incandescentlight is four times as far away from the screen as the candle is, itspower must be sixteen times as great, and we know the company isfurnishing the standard amount of light for a sixteen candle powerelectric bulb. If, however, the bulb must be moved nearer to the rodin order that the two shadows may be similar then the light given bythe bulb is less than sixteen candle power, and less than that due theconsumer. 

[Illustration: FIG. 58. --The two shadows are equally dark. ]

102. How Light Travels. We never expect to see around a corner, andif we wish to see through pinholes in three separate pieces ofcardboard, we place the cardboards so that the three holes are in astraight line. When sunlight enters a dark room through a smallopening, the dust particles dancing in the sun show a straight ray. Ifa hole is made in a card, and the card is held in front of a light, the card casts a shadow, in the center of which is a bright spot. Thelight, the hole, and the bright spot are all in the same straightline. These simple observations lead us to think that light travels ina straight line. 

[Illustration: FIG. 59. --The candle cannot be seen unless the threepinholes are in a strait line. ]

We can always tell the direction from which light comes, either by theshadow cast or by the bright spot formed when an opening occurs in theopaque object casting the shadow. If the shadow of a tree fallstowards the west, we know the sun must be in the cast; if a brightspot is on the floor, we can easily locate the light whose rays streamthrough an opening and form the bright spot. We know that lighttravels in a straight line, and following the path of the beam whichcomes to our eyes, we are sure to locate the light. 

103. Good and Bad Mirrors. As we walk along the street, wefrequently see ourselves reflected in the shop windows, in polishedmetal signboards, in the metal trimmings of wagons and automobiles;but in mirrors we get the best image of ourselves. We resent the imagegiven by a piece of tin, because the reflection is distorted and doesnot picture us as we really are; a rough surface does not give a fairrepresentation; if we want a true image of ourselves, we must use asmooth surface like a mirror as a reflector. If the water in a pondis absolutely still, we get a clear, true image of the trees, but ifthere are ripples on the surface, the reflection is blurred anddistorted. A metal roof reflects so much light that the eyes aredazzled by it, and a whitewashed fence injures the eyes because of theglare which comes from the reflected light. Neither of these could becalled mirrors, however, because although they reflect light, theyreflect it so irregularly that not even a suggestion of an image canbe obtained. 

Most of us are sufficiently familiar with mirrors to know that theimage is a duplicate of ourselves with regard to size, shape, color, and expression, but that it appears to be back of the mirror, while weare actually in front of the mirror. The image appears not only behindthe mirror, but it is also exactly as far back of the mirror as we arein front of it; if we approach the mirror, the image also drawsnearer; if we withdraw, it likewise recedes. 

104. The Path of Light. If a mirror or any other polished surface isheld in the path of a sunbeam, some of the light is reflected, and byrotating the mirror the reflected sunbeam may be made to take anypath. School children amuse themselves by reflecting sunbeams from amirror into their companions' faces. If the companion moves his headin order to avoid the reflected beam, his tormentor moves or inclinesthe mirror and flashes the beam back to his victim's face. 

If a mirror is held so that a ray of light strikes it in aperpendicular direction, the light is reflected backward along thepath by which it came. If, however, the light makes an angle with themirror, its direction is changed, and it leaves the mirror along a newpath. By observation we learn that when a beam strikes the mirror andmakes an angle of 30° with the perpendicular, the beam is reflected insuch a way that its new path also makes an angle of 30° with theperpendicular. If the sunbeam strikes the mirror at an angle of 32°with the perpendicular, the path of the reflected ray also makes anangle of 32° with the perpendicular. The ray (_AC_, Fig. 60) whichfalls upon the mirror is called the incident ray, and the angle whichthe incident ray (_AC_) makes with the perpendicular (_BC_) to themirror, at the point where the ray strikes the mirror, is called theangle of incidence. The angle formed by the reflected ray (_CD_) andthis same perpendicular is called the angle of reflection. Observationand experiment have taught us that light is always reflected in such away that the angle of reflection equals the angle of incidence. Lightis not the only illustration we have of the law of reflection. Everychild who bounces a ball makes use of this law, but he uses itunconsciously. If an elastic ball is thrown perpendicularly againstthe floor, it returns to the sender; if it is thrown against the floorat an angle (Fig. 61), it rebounds in the opposite direction, butalways in such a way that the angle of reflection equals the angle ofincidence. 

[Illustration: FIG. 60. --The ray _AC_ is reflected as _CD_. ]

[Illustration: FIG. 61. --A bouncing ball illustrates the law ofreflection. ]

105. Why the Image seems to be behind the Mirror. If a candle isplaced in front of a mirror, as in Figure 62, one of the rays of lightwhich leaves the candle will fall upon the mirror as _AB_ and will bereflected as _BC_ (in such a way that the angle of reflection equalsthe angle of incidence). If an observer stands at _C_, he will thinkthat the point _A_ of the candle is somewhere along the line _CB_extended. Such a supposition would be justified from Section 102. Butthe candle sends out light in all directions; one ray therefore willstrike the mirror as _AD_ and will be reflected as _DE_, and anobserver at _E_ will think that the point _A_ of the candle issomewhere along the line _ED_. In order that both observers may becorrect, that is, in order that the light may seem to be in both thesedirections, the image of the point _A_ must seem to be at theintersection of the two lines. In a similar manner it can be shownthat every point of the image of the candle seems to be behind themirror. 

[Illustration: FIG. 62. --The image is a duplicate of the object, butappears to be behind the mirror. ]

It can be shown by experiment that the distance of the image behindthe mirror is equal to the distance of the object in front of themirror. 

106. Why Objects are Visible. If the beam of light falls upon asheet of paper, or upon a photograph, instead of upon a smoothpolished surface, no definite reflected ray will be seen, but a glarewill be produced by the scattering of the beam of light. The surfaceof the paper or photograph is rough, and as a result, it scatters thebeam in every direction. It is hard for us to realize that a smoothsheet of paper is by no means so smooth as it looks. It is roughcompared with a polished mirror. The law of reflection always holds, however, no matter what the reflecting surface is, --the angle ofreflection always equals the angle of incidence. In a smooth body thereflected beams are all parallel; in a rough body, the reflected beamsare inclined to each other in all sorts of ways, and no two beamsleave the paper in exactly the same direction. 

[Illustration: FIG. 63. --The surface of the paper, although smooth inappearance, is in reality rough, and scatters the light in everydirection. ]

Hot coals, red-hot stoves, gas flames, and candles shine by their ownlight, and are self-luminous. Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receivelight from a luminous source and reflect that light. We know thatthese objects are not self-luminous, because they are not visible atnight unless a lamp or gas is burning. When light from any luminousobject falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularlyin all directions. No matter where the eye is, some reflected raysenter it, and the various objects are clearly seen. 

CHAPTER XI

REFRACTION

107. Bent Rays of Light. A straw in a glass of lemonade seems to bebroken at the surface of the liquid, the handle of a teaspoon in a cupof water appears broken, and objects seen through a glass of water mayseem distorted and changed in size. When light passes from air intowater, or from any transparent substance into another of differentdensity, its direction is changed, and it emerges along an entirelynew path (Fig. 64). We know that light rays pass through glass, because we can see through the window panes and through ourspectacles; we know that light rays pass through water, because we cansee through a glass of clear water; on the other hand, light rayscannot pass through wood, leather, metal, etc. 

[Illustration: FIG. 64. --A straw or stick in water seems broken. ]

Whenever light meets a transparent substance obliquely, some of it isreflected, undergoing a change in its direction; and some of it passesonward through the medium, but the latter portion passes onward alonga new path. The ray _RO_ (Fig. 65) passes obliquely through the air tothe surface of the water, but, on entering the water, it is bent orrefracted and takes the new path _OS_. The angle _AOR_ is called theangle of incidence. The angle _POS_ is called the angle of refraction. 

[Illustration: FIG. 65. --When the ray _RO_ enters the water, its pathchanges to _OS_. ]

The angle of refraction is the angle formed by the refracted ray andthe perpendicular to the surface at the point where the light strikesit. 

When light passes from air into water or glass, the refracted ray isbent toward the perpendicular, so that the angle of refraction issmaller than the angle of incidence. When a ray of light passes fromwater or glass into air, the refracted ray is bent away from theperpendicular so that the angle of refraction is greater than theangle of incidence. 

The bending or deviation of light in its passage from one substance toanother is called refraction. 

108. How Refraction Deceives us. Refraction is the source of manyillusions; bent rays of light make objects appear where they reallyare not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of thestick in Figure 64 seems to be nearer the surface of the water than itreally is. 

[Illustration: FIG. 66. --A fish at _A_ seems to be at _B_. ]

The light from the sun, moon, and stars can reach us only by passingthrough the atmosphere, but in Section 76, we learned that theatmosphere varies in density from level to level; hence all the lightwhich travels through the atmosphere is constantly deviated from itsoriginal path, and before the light reaches the eye it has undergonemany changes in direction. Now we learned in Section 102, that thedirection of the rays of light as they enter the eye determines thedirection in which an object is seen; hence the sun, moon, and starsseem to be along the lines which enter the eye, although in realitythey are not. 

109. Uses of Refraction. If it were not for refraction, or thedeviation of light in its passage from medium to medium, the wondersand beauties of the magic lantern and the camera would be unknown tous; sun, moon, and stars could not be made to yield up their distantsecrets to us in photographs; the comfort and help of spectacles wouldbe lacking, spectacles which have helped unfold to many the rarebeauties of nature, such as a clear view of clouds and sunset, ofhumming bee and flying bird. Books with their wealth of entertainmentand information would be sealed to a large part of mankind, if glassesdid not assist weak eyes. 

By refraction the magnifying glass reveals objects hidden because oftheir minuteness, and enlarges for our careful contemplation objectsotherwise barely visible. The watchmaker, unassisted by the magnifyingglass, could not detect the tiny grains of dust or sand which clog thedelicate wheels of our watches. The merchant, with his lens, examinesthe separate threads of woolen and silk fabrics to determine thestrength and value of the material. The physician, with his invaluablemicroscope, counts the number of infinitesimal corpuscles in the bloodand bases his prescription on that count; he examines the sputum of apatient to determine whether tuberculosis wastes the system. Thebacteriologist with the same instrument scrutinizes the drinking waterand learns whether the dangerous typhoid germs are present. Thefuture of medicine will depend somewhat upon the additional secretswhich man is able to force from nature through the use of powerfullenses, because as lenses have, in the past, been the means ofrevealing disease germs, so in the future more powerful lenses mayserve to bring to light germs yet unknown. How refraction accomplishesthese results will be explained in the following Sections. 

110. The Window Pane. We have seen that light is bent when it passesfrom one medium to another of different density, and that objectsviewed by refracted light do not appear in their proper positions. 

When a ray of light passes through a piece of plane glass, such as awindow pane (Fig. 67), it is refracted at the point _B_ toward theperpendicular, and continues its course through the glass in the newdirection _BC_. On emerging from the glass, the light is refractedaway from the perpendicular and takes the direction _CD_, which isclearly parallel to its original direction. Hence, when we viewobjects through the window, we see them slightly displaced inposition, but otherwise unchanged. The deviation or displacementcaused by glass as thin as window panes is too slight to be noticed, and we are not conscious that objects are out of position. 

[Illustration: FIG. 67. --Objects looked at through a window pane seemto be in their natural place. ]

111. Chandelier Crystals and Prisms. When a ray of light passesthrough plane glass, like a window pane, it is shifted somewhat, butits direction does not change; that is, the emergent ray is parallelto the incident ray. But when a beam of light passes through atriangular glass prism, such as a chandelier crystal, its direction isgreatly changed, and an object viewed through a prism is seen quiteout of its true position. 

Whenever light passes through a prism, it is bent toward the base ofthe prism, or toward the thick portion of the prism, and emerges fromthe prism in quite a different direction from that in which it entered(Fig. 68). Hence, when an object is looked at through a prism, it isseen quite out of place. In Figure 68, the candle seems to be at _S_, while in reality it is at _A_. 

[Illustration: FIG. 68. --When looked at through the prism, _A_ seemsto be at _S_. ]

112. Lenses. If two prisms are arranged as in Figure 69, and twoparallel rays of light fall upon the prisms, the beam _A_ will be bentdownward toward the thickened portion of the prism, and the beam _B_will be bent upward toward the thick portion of the prism, and afterpassing through the prism the two rays will intersect at some point_F_, called a focus. 

[Illustration: FIG. 69. --Rays of light are converged and focused at_F_. ]

If two prisms are arranged as in Figure 70, the ray _A_ will berefracted upward toward the thick end, and the ray _B_ will berefracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect. 

[Illustration: FIG. 70. --Rays of light are diverged and do not come toany real focus. ]

Lenses are very similar to prisms; indeed, two prisms placed as inFigure 69, and rounded off, would make a very good convex lens. A lensis any transparent material, but usually glass, with one or both sidescurved. The various types of lenses are shown in Figure 71. 

[Illustration: FIG. 71. --The different types of lenses. ]

The first three types focus parallel rays at some common point _F_, asin Figure 69. Such lenses are called convex or converging lenses. Thelast three types, called concave lenses, scatter parallel rays so thatthey do not come to a focus, but diverge widely after passage throughthe lens. 

113. The Shape and Material of a Lens. The main or principal focusof a lens, that is, the point at which rays parallel to the base line_AB_ meet (Fig. 71), depends upon the shape of the lens. For example, a thick lens, such as _A_ (Fig. 72), focuses the rays very near to thelens; _B_, which is not so thick, focuses the rays at a greaterdistance from the lens; and _C_, which is a very thin lens, focusesthe rays at a considerable distance from the lens. The distance of theprincipal focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, theshorter the focal length. 

[Illustration: FIG. 72. --The more curved the lens, the shorter thefocal length, and the nearer the focus is to the lens. ]

The position of the principal focus depends not only on the shape ofthe lens, but also on the refractive power of the material composingthe lens. A lens made of ice would not deviate the rays of light somuch as a lens of similar shape composed of glass. The greater therefractive power of the lens, the greater the bending, and the nearerthe principal focus to the lens. 

There are many different kinds of glass, and each kind of glassrefracts the light differently. Flint glass contains lead; the leadmakes the glass dense, and gives it great refractive power, enablingit to bend and separate light in all directions. Cut glass and toiletarticles are made of flint glass because of the brilliant effectscaused by its great refractive power, and imitation gems are commonlynothing more than polished flint glass. 

114. How Lenses Form Images. Suppose we place an arrow, _A_, infront of a convex lens (Fig. 73). The ray _AC_, parallel to theprincipal axis, will pass through the lens and emerge as _DE_. The rayis always bent toward the thick portion of the lens, both at itsentrance into the lens and its emergence from the lens. 

[Illustration: FIG. 73. --The image is larger than the object. By meansof a lens, a watchmaker gets an enlarged image of the dust which clogsthe wheels of his watch. ]

In Section 105, we saw that two rays determine the position of anypoint of our image; hence in order to locate the image of the top ofthe arrow, we need to consider but one more ray from the top of theobject. The most convenient ray to choose would be one passing through_O_, the optical center of the lens, because such a ray passes throughthe lens unchanged in direction, as is clear from Figure 74. The pointwhere _AC_ and _AO_ meet after refraction will be the position of thetop of the arrow. Similarly it can be shown that the center of thearrow will be at the point _T_, and we see that the image is largerthan the object. This can be easily proved experimentally. Let aconvex lens be placed near a candle (Fig. 75); move a paper screenback and forth behind the lens; for some position of the screen aclear, enlarged image of the candle will be made. 

[Illustration: FIG. 74. --Rays above _O_ are bent downward, those below_O_ are bent upward, and rays through _O_ emerge from the lensunchanged in direction. ]

If the candle or arrow is placed in a new position, say at _MA_ (Fig. 76), the image formed is smaller than the object, and is nearer to thelens than it was before. Move the lens so that its distance from thecandle is increased, and then find the image on a piece of paper. Thesize and position of the image depend upon the distance of the objectfrom the lens (Fig. _77_). By means of a lens one can easily get on avisiting card a picture of a distant church steeple. 

[Illustration: FIG. 75. --The lens is held in such a position that theimage of the candle is larger than the object. ]

[Illustration: FIG. 76. --The image is smaller than the object. ]

115. The Value of Lenses. If it were not for the fact that a lenscan be held at such a distance from an object as to make the imagelarger than the object, it would be impossible for the lens to assistthe watchmaker in locating the small particles of dust which clog thewheels of the watch. If it were not for the opposite fact--that a lenscan be held at such a distance from the object as to make an imagesmaller than the object, it would be impossible to have a photographof a tall tree or building unless the photograph were as large as thetree itself. When a photographer takes a photograph of a person or atree, he moves his camera until the image formed by the lens is of thedesired size. By bringing the camera (really the lens of the camera)near, we obtain a large-sized photograph; by increasing the distancebetween the camera and the object, a smaller photograph is obtained. The mountain top may be so far distant that in the photograph it willnot appear to be greater than a small stone. 

[Illustration: FIG. 77. --The lens is placed in such a position thatthe image is about the same size as the object. ]

Many familiar illustrations of lenses, or curved refracting surfaces, and their work, are known to all of us. Fish globes magnify the fishthat swim within. Bottles can be so shaped that they make the olives, pickles, and peaches that they contain appear larger than they reallyare. The fruit in bottles frequently seems too large to have gonethrough the neck of the bottle. The deception is due to refraction, and the material and shape of the bottle furnish a sufficientexplanation. 

By using combinations of two or more lenses of various kinds, it ispossible to have an image of almost any desired size, and inpractically any desired position. 

116. The Human Eye. In Section 114, we obtained on a movable screen, by means of a simple lens, an image of a candle. The human eyepossesses a most wonderful lens and screen (Fig. 78); the lens iscalled the crystalline lens, and the screen is called the retina. Raysof light pass from the object through the pupil _P_, go through thecrystalline lens _L_, where they are refracted, and then pass onwardto the retina _R_, where they form a distinct image of the object. 

[Illustration: FIG. 78. --The eye. ]

We learned in Section 114 that a change in the position of the objectnecessitated a change in the position of the screen, and that everytime the object was moved the position of the screen had to be alteredbefore a clear image of the object could be obtained. The retina ofthe eye cannot be moved backward and forward, as the screen was, andthe crystalline lens is permanently located directly back of the iris. How, then, does it happen that we can see clearly both near anddistant objects; that the printed page which is held in the hand isvisible at one second, and that the church spire on the distanthorizon is visible the instant the eyes are raised from the book? Howis it possible to obtain on an immovable screen by means of a simplelens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changesshape according to need. The lens is attached to the eye by means ofsmall muscles, _m_, and it is by the action of these muscles that thelens is able to become small and thick, or large and thin; that is, tobecome more or less curved. When we look at near objects, the musclesact in such a way that the lens bulges out, and becomes thick in themiddle and of the right curvature to focus the near object upon thescreen. When we look at an object several hundred feet away, themuscles change their pull on the lens and flatten it until it is ofthe proper curvature for the new distance. The adjustment of themuscles is so quick and unconscious that we normally do not experienceany difficulty in changing our range of view. The ability of the eyeto adjust itself to varying distances is called accommodation. Thepower of adjustment in general decreases with age. 

117. Farsightedness and Nearsightedness. A farsighted person is onewho cannot see near objects so distinctly as far objects, and who inmany cases cannot see near objects at all. The eyeball of a farsightedperson is very short, and the retina is too close to the crystallinelens. Near objects are brought to a focus behind the retina instead ofon it, and hence are not visible. Even though the muscles ofaccommodation do their best to bulge and thicken the lens, the rays oflight are not bent sufficiently to focus sharply on the retina. Inconsequence objects look blurred. Farsightedness can be remedied byconvex glasses, since they bend the light and bring it to a closerfocus. Convex glasses, by bending the rays and bringing them to anearer focus, overbalance a short eyeball with its tendency to focusobjects behind the retina. 

[Illustration: FIG. 79. --The farsighted eye. ]

[Illustration: FIG. 80. --The defect is remedied by convex glasses. ]

A nearsighted person is one who cannot see objects unless they areclose to the eye. The eyeball of a nearsighted person is very wide, and the retina is too far away from the crystalline lens. Far objectsare brought to a focus in front of the retina instead of on it, andhence are not visible. Even though the muscles of accommodation dotheir best to pull out and flatten the lens, the rays are notseparated sufficiently to focus as far back as the retina. Inconsequence objects look blurred. Nearsightedness can be remedied bywearing concave glasses, since they separate the light and move thefocus farther away. Concave glasses, by separating the rays and makingthe focus more distant, overbalance a wide eyeball with its tendencyto focus objects in front of the retina. 

[Illustration: FIG. 81. --The nearsighted eye. The defect is remediedby concave glasses. ]

118. Headache and Eyes. Ordinarily the muscles of accommodationadjust themselves easily and quickly; if, however, they do not, frequent and severe headaches occur as a result of too great musculareffort toward accommodation. Among young people headaches arefrequently caused by over-exertion of the crystalline muscles. Glassesrelieve the muscles of the extra adjustment, and hence are effectivein eliminating this cause of headache. 

An exact balance is required between glasses, crystalline lens, andmuscular activity, and only those who have studied the subjectcarefully are competent to treat so sensitive and necessary a part ofthe body as the eye. The least mistake in the curvature of theglasses, the least flaw in the type of glass (for example, the kind ofglass used), means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache andgeneral physical discomfort. 

119. Eye Strain. The extra work which is thrown upon the nervoussystem through seeing, reading, writing, and sewing with defectiveeyes is recognized by all physicians as an important cause of disease. The tax made upon the nervous system by the defective eye lessens thesupply of energy available for other bodily use, and the generalhealth suffers. The health is improved when proper glasses areprescribed. 

Possibly the greatest danger of eye strain is among school children, who are not experienced enough to recognize defects in sight. For thisreason, many schools employ a physician who examines the pupils' eyesat regular intervals. 

The following general precautions are worth observing:--

1. Rest the eyes when they hurt, and as far as possible do close work, such as writing, reading, sewing, wood carving, etc. , by daylight. 

2. Never read in a very bright or a very dim light. 

3. If the light is near, have it shaded. 

4. Do not rub the eyes with the fingers. 

5. If eyes are weak, bathe them in lukewarm water in which a pinch ofborax has been dissolved. 

CHAPTER XII

PHOTOGRAPHY

120. The Magic of the Sun. Ribbons and dresses washed and hung inthe sun fade; when washed and hung in the shade, they are not so aptto lose their color. Clothes are laid away in drawers and hung inclosets not only for protection against dust, but also against thewell-known power of light to weaken color. 

Many housewives lower the window shades that the wall paper may notlose its brilliancy, that the beautiful hues of velvet, satin, andplush tapestry may not be marred by loss in brilliancy and sheen. Bright carpets and rugs are sometimes bought in preference to moredelicately tinted ones, because the purchaser knows that the latterwill fade quickly if used in a sunny room, and will soon acquire adull mellow tone. The bright and gay colors and the dull and sombercolors are all affected by the sun, but why one should be affectedmore than another we do not know. Thousands of brilliant and daintyhues catch our eye in the shop and on the street, but not one of themis absolutely permanent; some may last for years, but there is alwaysmore or less fading in time. 

Sunlight causes many strange, unexplained effects. If the twosubstances, chlorine and hydrogen, are mixed in a dark room, nothingremarkable occurs any more than though water and milk were mixed, butif a mixture of these substances is exposed to sunlight, a violentexplosion occurs and an entirely new substance is formed, a compoundentirely different in character from either of its components. 

By some power not understood by man, the sun is able to form newsubstances. In the dark, chlorine and hydrogen are simply chlorine andhydrogen; in the sunlight they combine as if by magic into a totallydifferent substance. By the same unexplained power, the sun frequentlydoes just the opposite work; instead of combining two substances tomake one new product, the sun may separate or break down someparticular substance into its various elements. For example, if thesun's rays fall upon silver chloride, a chemical action immediatelybegins, and as a result we have two separate substances, chlorine andsilver. The sunlight separates silver chloride into its constituents, silver and chlorine. 

121. The Magic Wand in Photography. Suppose we coat one side of aglass plate with silver chloride, just as we might put a coat ofvarnish on a chair. We must be very careful to coat the plate in thedark room, [B] otherwise the sunlight will separate the silver chlorideand spoil our plan. Then lay a horseshoe on the plate for good luck, and carry the plate out into the light for a second. The light willseparate the silver chloride into chlorine and silver, the latter ofwhich will remain on the plate as a thin film. All of the plate wasaffected by the sun except the portion protected by the horseshoewhich, because it is opaque, would not allow light to pass through andreach the plate. If now the plate is carried back to the dark room andthe horseshoe is removed, one would expect to see on the plate animpression of the horseshoe, because the portion protected by thehorseshoe would be covered by silver chloride and the exposedunprotected portion would be covered by metallic silver. But we aremuch disappointed because the plate, when examined ever so carefully, shows not the slightest change in appearance. The change is there, butthe unaided eye cannot detect the change. Some chemical, theso-called "developer, " must be used to bring out the hidden change andto reveal the image to our unseeing eyes. There are many differentdevelopers in use, any one of which will effect the necessarytransformation. When the plate has been in the developer for a fewseconds, the silver coating gradually darkens, and slowly but surelythe image printed by the sun's rays appears. But we must not take thispicture into the light, because the silver chloride which wasprotected by the horseshoe is still present, and would be stronglyaffected by the first glimmer of light, and, as a result, our entireplate would become similar in character and there would be no contrastto give an image of the horseshoe on the plate. 

[Footnote B: That is, a room from which ordinary daylight isexcluded. ]

But a photograph on glass, which must be carefully shielded from thelight and admired only in the dark room, would be neither pleasurablenor practical. If there were some way by which the hitherto unaffectedsilver chloride could be totally removed, it would be possible to takethe plate into any light without fear. To accomplish this, theunchanged silver chloride is got rid of by the process technicallycalled "fixing"; that is, by washing off the unreduced silver chloridewith a solution such as sodium thiosulphite, commonly known as hypo. After a bath in the hypo the plate is cleansed in clear running waterand left to dry. Such a process gives a clear and permanent picture onthe plate. 

[Illustration: FIG. 82. --A camera. ]

122. The Camera. A camera (Fig. 82) is a light-tight box containinga movable convex lens at one end and a screen at the opposite end. Light from the object to be photographed passes through the lens, falls upon the screen, and forms an image there. If we substitute forthe ordinary screen a plate or film coated with silver chloride or anyother silver salt, the light which falls upon the sensitive plate andforms an image there will change the silver chloride and produce ahidden image. If the plate is then removed from the camera in thedark, and is treated as described in the preceding Section, the imagebecomes visible and permanent. In practice some gelatin is mixed withthe silver salt, and the mixture is then poured over the plate or filmin such a way that a thin, even coating is made. It is the presence ofthe gelatin that gives plates a yellowish hue. The sensitive platesare left to dry in dark rooms, and when the coating has becomeabsolutely firm and dry, the plates are packed in boxes and sent forthfor sale. 

Glass plates are heavy and inconvenient to carry, so that celluloidfilms have almost entirely taken their place, at least for outdoorwork. 

123. Light and Shade. Let us apply the above process to a realphotograph. Suppose we wish to take the photograph of a man sitting ina chair in his library. If the man wore a gray coat, a black tie, anda white collar, these details must be faithfully represented in thephotograph. How can the almost innumerable lights and shades beproduced on the plate?

The white collar would send through the lens the most light to thesensitive plate; hence the silver chloride on the plate would be mostchanged at the place where the lens formed an image of the collar. Thegray coat would not send to the lens so much light as the whitecollar, hence the silver chloride would be less affected by the lightfrom the coat than by that from the collar, and at the place where thelens produced an image of the coat the silver chloride would not bechanged so much as where the collar image is. The light from the facewould produce a still different effect, since the light from the faceis stronger than the light from the gray coat, but less than that froma white collar. The face in the image would show less changed silverchloride than the collar, but more than the coat, because the face islighter than the coat, but not so light as the collar. Finally, thesilver chloride would be least affected by the dark tie. The wallpaper in the background would affect the plate according to thebrightness of the light which fell directly upon it and whichreflected to the camera. When such a plate has been developed andfixed, as described in Section 121, we have the so-called negative(Fig. 83). The collar is very dark, the black tie and gray coat white, and the white tidy very dark. 

[Illustration: FIG. 83. --A negative. ]

The lighter the object, such as tidy or collar, the more salt ischanged, or, in other words, the greater the portion of the silversalt that is affected, and hence the darker the stain on the plate atthat particular spot. The plate shows all gradations of intensity--thetidy is dark, the black tie is light. The photograph is true as far asposition, form, and expression are concerned, but the actualintensities are just reversed. How this plate can be transformed intoa photograph true in every detail will be seen in the followingSection. 

124. The Perfect Photograph. Bright objects, such as the sky or awhite waist, change much of the silver chloride, and hence appeardark on the negative. Dark objects, such as furniture or a black coat, change little of the chloride, and hence appear light on the negative. To obtain a true photograph, the negative is placed on a piece ofsensitive photographic paper, or paper coated with a silver salt inthe same manner as the plate and films. The combination is exposed tothe light. The dark portions of the negative will act as obstructionsto the passage of light, and but little light will pass through thatpart of the negative to the photographic paper, and consequently butlittle of the silver salt on the paper will be changed. On the otherhand, the light portion of the negative will allow free and easypassage of the light rays, which will fall upon the photographic paperand will change much more of the silver. Thus it is that dark placesin the negative produce light places in the positive or realphotograph (Fig. 84), and that light places in the negative producedark places in the positive; all intermediate grades are likewiserepresented with their proper gradations of intensity. 

[Illustration: FIG. 84. --A positive or true photograph. ]

If properly treated, a negative remains good for years, and will servefor an indefinite number of positives or true photographs. 

125. Light and Disease. The far-reaching effect which light has uponsome inanimate objects, such as photographic films and clothes, leadsus to inquire into the relation which exists between light and livingthings. We know from daily observation that plants must have light inorder to thrive and grow. A healthy plant brought into a dark roomsoon loses its vigor and freshness, and becomes yellow and drooping. Plants do not all agree as to the amount of light they require, forsome, like the violet and the arbutus, grow best in moderate light, while others, like the willows, need the strong, full beams of thesun. But nearly all common plants, whatever they are, sicken and dieif deprived of sunlight for a long time. This is likewise true in theanimal world. During long transportation, animals are sometimesnecessarily confined in dark cars, with the result that many deathsoccur, even though the car is well aired and ventilated and the foodsupply good. Light and fresh air put color into pale cheeks, just aslight and air transform sickly, yellowish plants into hardy greenones. Plenty of fresh air, light, and pure water are the watchwordsagainst disease. 

[Illustration: FIG. 85--Stems and leaves of oxalis growing toward thelight. ]

In addition to the plants and animals which we see, there are manystrange unseen ones floating in the atmosphere around us, lying in thedust of corner and closet, growing in the water we drink, andthronging decayed vegetable and animal matter. Everyone knows thatmildew and vermin do damage in the home and in the field, but very fewunderstand that, in addition to these visible enemies of man, thereare swarms of invisible plants and animals some of which do far moredamage, both directly and indirectly, than the seen and familiarenemies. All such very small plants and animals are known as_microorganisms_. 

Not all microörganisms are harmful; some are our friends and are ashelpful to us as are cultivated plants and domesticated animals. Amongthe most important of the microörganisms are bacteria, which includeamong their number both friend and foe. In the household, bacteria area fruitful source of trouble, but some of them are distinctly friends. The delicate flavor of butter and the sharp but pleasing taste ofcheese are produced by bacteria. On the other hand, bacteria are thecause of many of the most dangerous diseases, such as typhoid fever, tuberculosis, influenza, and la grippe. 

By careful observation and experimentation it has been shownconclusively that sunlight rapidly kills bacteria, and that it is onlyin dampness and darkness that bacteria thrive and multiply. Althoughsunlight is essential to the growth of most plants and animals, itretards and prevents the growth of bacteria. Dirt and dust exposed tothe sunlight lose their living bacteria, while in damp cellars anddark corners the bacteria thrive, increasing steadily in number. Forthis reason our houses should be kept light and airy; blinds should beraised, even if carpets do fade; it is better that carpets andfurniture should fade than that disease-producing bacteria should finda permanent abode within our dwellings. Kitchens and pantries inparticular should be thoroughly lighted. Bedclothes, rugs, andclothing should be exposed to the sunlight as frequently as possible;there is no better safeguard against bacterial disease than light. Ina sick room sunlight is especially valuable, because it not only killsbacteria, but keeps the air dry, and new bacteria cannot get a startin a dry atmosphere. 

CHAPTER XIII

COLOR

126. The Rainbow. One of the most beautiful and well-known phenomenain nature is the rainbow, and from time immemorial it has beenconsidered Jehovah's signal to mankind that the storm is over and thatthe sunshine will remain. Practically everyone knows that a rainbowcan be seen only when the sun's rays shine upon a mist of tiny dropsof water. It is these tiny drops which by their refraction and theirscattering of light produce the rainbow in the heavens. 

The exquisite tints of the rainbow can be seen if we look at an objectthrough a prism or chandelier crystal, and a very simple experimentenables us to produce on the wall of a room the exact colors of therainbow in all their beauty. 

[Illustration: FIG. 86. --White light is a mixture of lights of rainbowcolors. ]

127. How to produce Rainbow Colors. _The Spectrum. _ If a beam ofsunlight is admitted into a dark room through a narrow opening in theshade, and is allowed to fall upon a prism, as shown in Figure 86, abeautiful band of colors will appear on the opposite wall of the room. The ray of light which entered the room as ordinary sunlight has notonly been refracted and bent from its straight path, but it has beenspread out into a band of colors similar to those of the rainbow. 

Whenever light passes through a prism or lens, it is dispersed orseparated into all the colors which it contains, and a band of colorsproduced in this way is called a spectrum. If we examine such aspectrum we find the following colors in order, each colorimperceptibly fading into the next: violet, indigo, blue, green, yellow, orange, red. 

128. Sunlight or White Light. White light or sunlight can bedispersed or separated into the primary colors or rainbow hues, asshown in the preceding Section. What seems even more wonderful is thatthese spectral colors can be recombined so as to make white light. 

If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placedbehind _A_ in a reversed position, it will undo the dispersion of _A_, bending upward the seven different beams in such a way that theyemerge together and produce a white spot on the screen. Thus we see, from two simple experiments, that all the colors of the rainbow may beobtained from white light, and that these colors may be in turnrecombined to produce white light. 

[Illustration: FIG. 87. --Rainbow colors recombined to form whitelight. ]

White light is not a simple light, but is composed of all the colorswhich appear in the rainbow. 

129. Color. If a piece of red glass is held in the path of thecolored beam of light formed as in Section 127, all the colors on thewall will disappear except the red, and instead of a beautifulspectrum of all colors there will be seen the red color alone. The redglass does not allow the passage through it of any light except redlight; all other colors are absorbed by the red glass and do not reachthe eye. Only the red ray passes through the red glass, reaches theeye, and produces a sensation of color. 

If a piece of blue glass is substituted for the red glass, the blueband remains on the wall, while all the other colors disappear. Ifboth blue and red pieces of glass are held in the path of the beam, sothat the light must pass through first one and then the other, theentire spectrum disappears and no color remains. The blue glassabsorbs the various rays with the exception of the blue ones, and thered glass will not allow these blue rays to pass through it; hence nolight is allowed passage to the eye. 

An emerald looks green because it freely transmits green, but absorbsthe other colors of which ordinary daylight is composed. A diamondappears white because it allows the passage through it of all thevarious rays; this is likewise true of water and window panes. 

Stained-glass windows owe their charm and beauty to the presence inthe glass of various dyes and pigments which absorb in differentamounts some colors from white light and transmit others. Thesepigments or dyes are added to the glass while it is in the moltenstate, and the beauty of a stained-glass window depends largely uponthe richness and the delicacy of the pigments used. 

130. Reflected Light. _Opaque Objects. _ In Section 106 we learnedthat most objects are visible to us because of the light diffuselyreflected from them. A white object, such as a sheet of paper, awhitewashed fence, or a table cloth, absorbs little of the light whichfalls upon it, but reflects nearly all, thus producing the sensationof white. A red carpet absorbs the light rays incident upon it exceptthe red rays, and these it reflects to the eye. 

Any substance or object which reflects none of the rays which fallupon it, but absorbs all, appears black; no rays reach the eye, andthere is an absence of any color sensation. Coal and tar and soot aregood illustrations of objects which absorb all the light which fallsupon them. 

131. How and Why Colors Change. _Matching Colors. _ Most women preferto shop in the morning and early afternoon when the sunlightilluminates shops and factories, and when gas and electricity do notthrow their spell over colors. Practically all people know thatribbons and ties, trimmings and dresses, frequently look different atnight from what they do in the daytime. It is not safe to match colorsby artificial light; cloth which looks red by night may be almostpurple by day. Indeed, the color of an object depends upon the colorof the light which falls upon it. Strange sights are seen on theFourth of July when variously colored fireworks are blazing. The childwith a white blouse appears first red, then blue, then green, according as his powders burn red, blue, or green. The face of thechild changes from its normal healthy hue to a brilliant red and thento ghastly shades. 

Suppose, for example, that a white hat is held at the red end of thespectrum or in any red light. The characteristics of white objects istheir ability to reflect _all_ the various rays that fall upon them. Here, however, the only light which falls upon the white hat is redlight, hence the only light which the hat has to reflect is red lightand the hat consequently appears red. Similarly, if a white hat isplaced in a blue light, it will reflect all the light which falls uponit, namely, blue light, and will appear blue. If a red hat is held ina red light, it is seen in its proper color. If a red hat is held in ablue light, it appears black; it cannot reflect any of the blue lightbecause that is all absorbed and there is no red light to reflect. 

A child wearing a green frock on Independence Day seems at night to bewearing a black frock, if standing near powders burning with red, blue, or violet light. 

132. Pure, Simple Colors--Things as they Seem. To the eye whitelight appears a simple, single color. It reveals its compound natureto us only when passed through a prism, when it shows itself to becompounded of an infinite number of colors which Sir Isaac Newtongrouped in seven divisions: violet, indigo, blue, green, yellow, orange, and red. 

We naturally ask ourselves whether these colors which compose whitelight are themselves in turn compound? To answer that question, let usvery carefully insert a second prism in the path of the rays whichissue from the first prism, carefully barring out the remaining sixkinds of rays. If the red light is compound, it will be broken up intoits constituent parts and will form a typical spectrum of its own, just as white light did after its passage through a prism. But the redrays pass through the second prism, are refracted, and bent from thiscourse, and no new colors appear, no new spectrum is formed. Evidentlya ray of spectrum red is a simple color, not a compound color. 

If a similar experiment is made with the remaining spectrum rays, theresult is always the same: the individual spectrum colors remainsimple, pure colors. _The individual spectrum colors are groups ofsimple, pure colors. _

[Illustration: FIG. 88. --Violet and green give blue. Green, blue, andred give white. ]

133. Colors not as they Seem--Compound Colors. If one half of acardboard disk (Fig. 88) is painted green, and the other half violet, and the disk is slipped upon a toy top, and spun rapidly, the rotatingdisk will appear blue; if red and green are used in the same wayinstead of green and violet, the rotating disk will appear yellow. Acombination of red and yellow will give orange. The colors formed inthis way do not appear to the eye different from the spectrum colors, but they are actually very different. The spectrum colors, as we sawin the preceding Section, are pure, simple colors, while the colorsformed from the rotating disk are in reality compounded of severaltotally different rays, although in appearance the resulting colorsare pure and simple. 

If it were not that colors can be compounded, we should be limited inhue and shade to the seven spectral colors; the wealth and beauty ofcolor in nature, art, and commerce would be unknown; the flowers withtheir thousands of hues would have a poverty of color undreamed of;art would lose its magenta, its lilac, its olive, its lavender, andwould have to work its wonders with the spectral colors alone. Bycompounding various colors in different proportions, new colors can beformed to give freshness and variety. If one third of the rotatingdisk is painted blue, and the remainder white, the result is lavender;if fifteen parts of white, four parts of red, and one part of blue arearranged on the disk, the result is lilac. Olive is obtained from acombination of two parts green, one part red, and one part black; andthe soft rich shades of brown are all due to different mixtures ofblack, red, orange, or yellow. 

134. The Essential Colors. Strange and unexpected facts await us atevery turn in science! If the rotating cardboard disk (Fig. 88) ispainted one third red, one third green, and one third blue, theresulting color is white. While the mixture of the spectral colorsproduces white, it is not necessary to have all of the spectral colorsin order to obtain white; because a mixture of the following colorsalone, red, green, and blue, will give white. Moreover, by the mixtureof these three colors in proper proportions, any color of thespectrum, such as yellow or indigo or orange, may be obtained. Thethree spectral colors, red, green, and blue, are called primary oressential hues, because all known tints of color may be produced bythe careful blending of blue, green, and red in the properproportions; for example, purple is obtained by the blending of redand blue, and orange by the blending of red and yellow. 

135. Color Blindness. The nerve fibers of the eye which carry thesensation of color to the brain are particularly sensitive to theprimary colors--red, green, blue. Indeed, all color sensations areproduced by the stimulation of three sets of nerves which aresensitive to the primary colors. If one sees purple, it is because theoptic nerves sensitive to red and blue (purple equals red plus blue)have carried their separate messages to the brain, and the blending ofthe two distinct messages in the brain has given the sensation ofpurple. If a red rose is seen, it is because the optic nervessensitive to red have been stimulated and have carried the message tothe brain. 

A snowy field stimulates equally all three sets of optic nerves--thered, the green, and the blue. Lavender, which is one part blue andthree parts white, would stimulate all three sets of nerves, but witha maximum of stimulation for the blue. Equal stimulation of the threesets would give the impression of white. 

A color-blind person has some defect in one or more of the three setsof nerves which carry the color message to the brain. Suppose thenerve fibers responsible for carrying the red are totally defective. If such a person views a yellow flower, he will see it as a greenflower. Yellow contains both red and green, and hence both the red andgreen nerve fibers should be stimulated, but the red nerve fibers aredefective and do not respond, the green nerve fibers alone beingstimulated, and the brain therefore interprets green. 

A well-known author gives an amusing incident of a dinner party, atwhich the host offered stewed tomato for apple sauce. What colornerves were defective in the case of the host?

In some employments color blindness in an employee would be fatal tomany lives. Engineers and pilots govern the direction and speed oftrains and boats largely by the colored signals which flash out in thenight's darkness or move in the day's bright light, and any mistake inthe reading of color signals would imperil the lives of travelers. Forthis reason a rigid test in color is given to all persons seeking suchemployment, and the ability to match ribbons and yarns of all ordinaryhues is an unvarying requirement for efficiency. 

CHAPTER XIV

HEAT AND LIGHT AS COMPANIONS

 "The night has a thousand eyes, And the day but one; Yet the light of the bright world dies With the dying sun. "

136. Most bodies which glow and give out light are hot; the stovewhich glows with a warm red is hot and fiery; smoldering wood is blackand lifeless; glowing coals are far hotter than black ones. Thestained-glass window softens and mellows the bright light of the sun, but it also shuts out some of the warmth of the sun's rays; the shadyside of the street spares our eyes the intense glare of the sun, butmay chill us by the absence of heat. Our illumination, whether it beoil lamp or gas jet or electric light, carries with it heat; indeed, so much heat that we refrain from making a light on a warm summer'snight because of the heat which it unavoidably furnishes. 

137. Red a Warm Color. We have seen that heat and light usually gohand in hand. In summer we lower the shades and close the blinds inorder to keep the house cool, because the exclusion of light means theexclusion of some heat; in winter we open the blinds and raise theshades in order that the sun may stream into the room and flood itwith light and warmth. The heat of the sun and the light of the sunseem boon companions. 

We can show that when light passes through a prism and is refracted, forming a spectrum, as in Section 127, it is accompanied by heat. Ifwe hold a sensitive thermometer in the violet end of the spectrum sothat the violet rays fall upon the bulb, the reading of the mercurywill be practically the same as when the thermometer is held in anydark part of the room; if, however, the thermometer is slowly movedtoward the red end of the spectrum, a change occurs and the mercuryrises slowly but steadily, showing that heat rays are present at thered end of the spectrum. This agrees with the popular notion, formedindependently of science, which calls the reds the warm colors. Everyone of us associates red with warmth; in the summer red is rarelyworn, it looks hot; but in winter red is one of the most pleasingcolors because of the sense of warmth and cheer it brings. 

_All light rays are accompanied by a small amount of heat, but the redrays carry the most. _

What seems perhaps the most unexpected thing, is that the temperature, as indicated by a sensitive thermometer, continues to rise if thethermometer is moved just beyond the red light of the spectrum. Thereactually seems to be more heat beyond the red than in the red, but ifthe thermometer is moved too far away, the temperature again falls. Later we shall see what this means. 

138. The Energy of the Sun. It is difficult to tell how much of theenergy of the sun is light and how much is heat, but it is easy todetermine the combined effect of heat and light. 

[Illustration: FIG. 89. --The energy of the sun can be measured in heatunits. ]

Suppose we allow the sun's rays to fall perpendicularly upon a metalcylinder coated with lampblack and filled with a known quantity ofwater (Fig. 89); at the expiration of a few hours the temperature ofthe water will be considerably higher. Lampblack is a good absorber ofheat, and it is used as a coating in order that all the light rayswhich fall upon the cylinder may be absorbed and none lost byreflection. 

Light and heat rays fall upon the lampblack, pass through thecylinder, and heat the water. We know that the red light rays have thelargest share toward heating the water, because if the cylinder issurrounded by blue glass which absorbs the red rays and prevents theirpassage into the water, the temperature of the water begins to fall. That the other light rays have a small share would have been clearfrom the preceding Section. 

All the energy of the sunshine which falls upon the cylinder, both asheat and as light, is absorbed in the form of heat, and the totalamount of this energy can be calculated from the increase in thetemperature of the water. The energy which heated the water would havepassed onward to the surface of the earth if its path had not beenobstructed by the cylinder of water; and we can be sure that theenergy which entered the water and changed its temperature wouldordinarily have heated an equal area of the earth's surface; and fromthis, we can calculate the energy falling upon the entire surface ofthe earth during any one day. 

Computations based upon this experiment show that the earth receivesdaily from the sun the equivalent of 341, 000, 000, 000 horse power--anamount inconceivable to the human mind. 

Professor Young gives a striking picture of what this energy of thesun could do. A solid column of ice 93, 000, 000 miles long and 2-1/4miles in diameter could be melted in a single second if the sun couldconcentrate its entire power on the ice. 

While the amount of energy received daily from the sun by the earth isactually enormous, it is small in comparison with the whole amountgiven out by the sun to the numerous heavenly bodies which make up theuniverse. In fact, of the entire outflow of heat and light, the earthreceives only one part in two thousand million, and this is a verysmall portion indeed. 

139. How Light and Heat Travel from the Sun to Us. Astronomers tellus that the sun--the chief source of heat and light--is 93, 000, 000miles away from us; that is, so far distant that the fastest expresstrain would require about 176 years to reach the sun. How do heat andlight travel through this vast abyss of space?

[Illustration: FIG. 90. --Waves formed by a pebble. ]

A quiet pool and a pebble will help to make it clear to us. If wethrow a pebble into a quiet pool (Fig. 90), waves or ripples form andspread out in all directions, gradually dying out as they become moreand more distant from the pebble. It is a strange fact that while wesee the ripple moving farther and farther away, the particles of waterare themselves not moving outward and away, but are merely bobbing upand down, or are vibrating. If you wish to be sure of this, throw thepebble near a spot where a chip lies quiet on the smooth pond. Afterthe waves form, the chip rides up and down with the water, but doesnot move outward; if the water itself were moving outward, it wouldcarry the chip with it, but the water has no forward motion, and hencethe chip assumes the only motion possessed by the water, that is, anup-and-down motion. Perhaps a more simple illustration is theappearance of a wheat field or a lawn on a windy day; the wind sweepsover the grass, producing in the grass a wave like the water waves ofthe ocean, but the blades of grass do not move from their accustomedplace in the ground, held fast as they are by their roots. 

If a pebble is thrown into a quiet pool, it creates ripples or waveswhich spread outward in all directions, but which soon die out, leaving the pool again placid and undisturbed. If now we could quicklywithdraw the pebble from the pool, the water would again be disturbedand waves would form. If the pebble were attached to a string so thatit could be dropped into the water and withdrawn at regular intervals, the waves would never have a chance to disappear, because there wouldalways be a regularly timed definite disturbance of the water. Learnedmen tell us that all hot bodies and all luminous bodies are composedof tiny particles, called molecules, which move unceasingly back andforth with great speed. In Section 95 we saw that the molecules of allsubstances move unceasingly; their speed, however, is not so great, nor are their motions so regularly timed as are those of theheat-giving and the light-giving particles. As the particles of thehot and luminous bodies vibrate with great speed and force theyviolently disturb the medium around them, and produce a series ofwaves similar to those produced in the water by the pebble. If, however, a pebble is thrown into the water very gently, thedisturbance is slight, sometimes too slight to throw the water intowaves; in the same way objects whose molecules are in a state ofgentle motion do not produce light. 

The particles of heat-giving and light-giving bodies are in a state ofrapid vibration, and thereby disturb the surrounding medium, whichtransmits or conveys the disturbance to the earth or to other objectsby a train of waves. When these waves reach their destination, thesensation of light or heat is produced. 

We see the water waves, but we can never see with the eye the heat andlight waves which roll in to us from that far-distant source, the sun. We can be sure of them only through their effect on our bodies, and bythe visible work they do. 

140. How Heat and Light Differ. If heat and light are alike due tothe regular, rapid motion of the particles of a body, and aresimilarly conveyed by waves, how is it, then, that heat and light areapparently so different?

Light and heat differ as much as the short, choppy waves of the oceanand the slow, long swell of the ocean, but not more so. The sailorhandles his boat in one way in a choppy sea and in a different way ina rolling sea, for he knows that these two kinds of waves actdissimilarly. The long, slow swell of the ocean would correspond withthe longer, slower waves which travel out from the sun, and which onreaching us are interpreted as heat. The shorter, more frequent wavesof the ocean would typify the short, rapid waves which leave the sun, and which on reaching us are interpreted as light. 

CHAPTER XV

ARTIFICIAL LIGHTING

141. We seldom consider what life would be without our wonderfulmethods of illumination which turn night into day, and prolong thehours of work and pleasure. Yet it was not until the nineteenthcentury that the marvelous change was made from the short-lived candleto the more enduring oil lamp. Before the coming of the lamp, even inlarge cities like Paris, the only artificial light to guide thebelated traveler at night was the candle required to be kept burningin an occasional window. 

With the invention of the kerosene lamp came more efficient lightingof home and street, and with the advent of gas and electricity came alight so effective that the hours of business, manufacture, andpleasure could be extended far beyond the setting of the sun. 

The production of light by candle, oil, and gas will be considered inthe following paragraphs, while illumination by electricity will bereserved for a later Chapter. 

142. The Candle. Candles were originally made by dipping a wick intomelting tallow, withdrawing it, allowing the adhered tallow to harden, and repeating the dipping until a satisfactory thickness was obtained. The more modern method consists in pouring a fatty preparation into amold, at the center of which a wick has been placed. 

The wick, when lighted, burns for a brief interval with a faint, uncertain light; almost immediately, however, the intensity of thelight increases and the illumination remains good as long as thecandle lasts. The heat of the burning tallow melts more of the tallownear it, and this liquid fat is quickly sucked up into the burningwick. The heat of the flame is sufficient to change most of thisliquid into a gas, that is, to vaporize the liquid, and furthermore toset fire to the gas thus formed. These heated gases burn with a brightyellow flame. 

143. The Oil Lamp. The simple candle of our ancestors was nowreplaced by the oil lamp, which gave a brighter, steadier, and morepermanent illumination. The principle of the lamp is similar to thatof the candle, except that the wick is saturated with kerosene or oilrather than with fat. The heat from the burning wick is sufficient tochange the oil into a gas and then to set fire to the gas. By placinga chimney over the burning wick, a constant and uniform draught of airis maintained around the blazing gases, and hence a steady, unflickering light is obtained. Gases and carbon particles are setfree by the burning wick. In order that the gases may burn and thesolid particle glow, a plentiful supply of oxygen is necessary. If thequantity of air is insufficient, the carbon particles remain unburnedand form soot. A lamp "smokes" when the air which reaches the wick isinsufficient to burn the rapidly formed carbon particles; thisexplains the danger of turning a lamp wick too high and producing morecarbon particles than can be oxidized by the air admitted through thelamp chimney. 

One great disadvantage of oil lamps and oil stoves is that they cannotbe carried safely from place to place. It is almost impossible tocarry a lamp without spilling the oil. The flame soon spreads from thewick to the overflowing oil and in consequence the lamp blazes and anexplosion may result. Candles, on the other hand, are safe fromexplosion; the dripping grease is unpleasant but not dangerous. 

The illumination from a shaded oil lamp is soft and agreeable, but thetrimming of the wicks, the refilling of bowls, and the cleaning ofchimneys require time and labor. For this reason, the introduction ofgas met with widespread success. The illumination from an ordinary gasjet is stronger than that from an ordinary lamp, and the strongerillumination added to the greater convenience has made gas a verypopular source of light. 

144. Gas Burners and Gas Mantles. For a long time, the only gasflame used was that in which the luminosity resulted in heatingparticles of carbon to incandescence. Recently, however, that has beenwidely replaced by use of a Bunsen flame upon an incandescent mantle, such as the Welsbach. The principle of the incandescent mantle is verysimple. When certain substances, such as thorium and cerium, areheated, they do not melt or vaporize, but glow with an intense brightlight. Welsbach made use of this fact to secure a burner in which theillumination depends upon the glowing of an incandescent, solidmantle, rather than upon the blazing of a burning gas. He made acylindrical mantle of thin fabric, and then soaked it in a solution ofthorium and cerium until it became saturated with the chemical. Themantle thus impregnated with thorium and cerium is placed on the gasjet, but before the gas is turned on, a lighted match is held to themantle in order to burn away the thin fabric. After the fabric hasbeen burned away, there remains a coarse gauze mantle of the desiredchemicals. If now the gas cock is opened, the escaping gas is ignited, the heat of the flame will raise the mantle to incandescence and willproduce a brilliant light. A very small amount of burning gas issufficient to raise the mantle to incandescence, and hence, by the useof a mantle, intense light is secured at little cost. The mantle savesus gas, because the cock is usually "turned on full" whether we use aplain burner or a mantle burner. But, nevertheless, gas is saved, because when the mantle is adjusted to the gas jet, the pressure ofthe gas is lessened by a mechanical device and hence less gas escapesand burns. By actual experiment, it has been found that an ordinaryburner consumes about five times as much gas per candle power as thebest incandescent burner, and hence is about five times as expensive. One objection to the mantles is their tendency to break. But if themantles are carefully adjusted on the burner and are not roughlyjarred in use, they last many months; and since the best quality costonly twenty-five cents, the expense of renewing the mantles is slight. 

145. Gas for Cooking. If a cold object is held in the bright flameof an ordinary gas jet, it becomes covered with soot, or particles ofunburned carbon. Although the flame is surrounded by air, the centralportion of it does not receive sufficient oxygen to burn up thenumerous carbon particles constantly thrown off by the burning gas, and hence many carbon particles remain in the flame as glowing, incandescent masses. That some unburned carbon is present in a flameis shown by the fact that whenever a cold object is held in the flame, it becomes "smoked" or covered with soot. If enough air were suppliedto the flame to burn up the carbon as fast as it was set free, therewould be no deposition of soot on objects held over the flame or init, because the carbon would be transformed into gaseous matter. 

Unburned carbon would be objectionable in cooking stoves whereutensils are constantly in contact with the flame, and for this reasoncooking stoves are provided with an arrangement by means of whichadditional air is supplied to the burning gas in quantities adequateto insure complete combustion of the rapidly formed carbon particles. An opening is made in the tube through which gas passes to the burner, and as the gas moves past this opening, it carries with it a draft ofair. These openings are visible on all gas stoves, and should be keptclean and free of clogging, in order to insure complete combustion. Solong as the supply of air is sufficient, the flame burns with a dullblue color, but when the supply falls below that needed for completeburning of the carbon, the blue color disappears, and a yellow flametakes its place, and with the yellow flame the deposition of soot isinevitable. 

146. By-products of Coal Gas. Many important products besidesilluminating gas are obtained from the distillation of soft coal. Ammonia is made from the liquids which collect in the condensers;anilin, the source of exquisite dyes, is made from the thick, tarrydistillate, and coke is the residue left in the clay retorts. The coaltar yields not only anilin, but also carbolic acid and naphthalene, both of which are commercially valuable, the former as a widely useddisinfectant, and the latter as a popular moth preventive. 

From a ton of good gas-producing coal can be obtained about 10, 000cubic feet of illuminating gas, and as by-products 6 pounds ofammonia, 12 gallons of coal tar, and 1300 pounds of coke. 

147. Natural Gas. Animal and vegetable matter buried in the depth ofthe earth sometimes undergoes natural distillation, and as a resultgas is formed. The gas produced in this way is called natural gas. Itis a cheap source of illumination, but is found in relatively fewlocalities and only in limited quantity. 

148. Acetylene. In 1892 it was discovered that lime and coal fusedtogether in the intense heat of the electric furnace formed acrystalline, metallic-looking substance called calcium carbide. As aresult of that discovery, this substance was soon made on a largescale and sold at a moderate price. The cheapness of calcium carbidehas made it possible for the isolated farmhouse to discard oil lampsand to have a private gas system. When the hard, gray crystals ofcalcium carbide are put in water, they give off acetylene, a colorlessgas which burns with a brilliant white flame. If bits of calciumcarbide are dropped into a test tube containing water, bubbles of gaswill be seen to form and escape into the air, and the escaping gas maybe ignited by a burning match held near the mouth of the test tube. When chemical action between the water and carbide has ceased, and gasbubbles have stopped forming, slaked lime is all that is left of thedark gray crystals which were put into the water. 

When calcium carbide is used as a source of illumination, the crystalsare mechanically dropped into a tank containing water, and the gasgenerated is automatically collected in a small sliding tank, whenceit passes through pipes to the various rooms. The slaked lime, formedwhile the gas was generated, collects at the bottom of the tanks andis removed from time to time. 

The cost of an acetylene generator is about $50 for a small house, andthe cost of maintenance is not more than that of lamps. The generatordoes not require filling oftener than once a week, and the labor isless than that required for oil lamps. In a house in which there weretwenty burners, the tanks were filled with water and carbide but oncea fortnight. Acetylene is seldom used in large cities, but it is verywidely used in small communities and is particularly convenient inmore or less remote summer residences. 

Electric Lights. The most recent and the most convenient lighting isthat obtained by electricity. A fine, hairlike filament within a glassbulb is raised to incandescence by the heat of an electric current. This form of illumination will be considered in connection withelectricity. 

CHAPTER XVI

MAN'S WAY OF HELPING HIMSELF

149. Labor-saving Devices. To primitive man belonged more especiallythe arduous tasks of the out-of-door life: the clearing of pathsthrough the wilderness; the hauling of material; the breaking up ofthe hard soil of barren fields into soft loam ready to receive theseed; the harvesting of the ripe grain, etc. 

[Illustration: FIG. 91. --Prying a stone out of the ground. ]

The more intelligent races among men soon learned to help themselvesin these tasks. For example, our ancestors in the field soon learnedto pry stones out of the ground (Fig. 91) rather than to undertake thealmost impossible task of lifting them out of the earth in which theywere embedded; to swing fallen trees away from a path by means of ropeattached to one end rather than to attempt to remove themsingle-handed; to pitch hay rather than to lift it; to clear a fieldwith a rake rather than with the hands; to carry heavy loads inwheelbarrows (Fig. 92) rather than on the shoulders; to roll barrelsup a plank (Fig. 93) and to raise weights by ropes. In every case, whether in the lifting of stones, or the felling of trees, or thetransportation of heavy weights, or the digging of the ground, manused his brain in the invention of mechanical devices which wouldrelieve muscular strain and lighten physical labor. 

If all mankind had depended upon physical strength only, the worldto-day would be in the condition prevalent in parts of Africa, Asia, and South America, where the natives loosen the soil with their handsor with crude implements (Fig. 94), and transport huge weights ontheir shoulders and heads. 

[Illustration: FIG. 92. --The wheelbarrow lightens labor. ]

Any mechanical device (Figs. 95 and 96), whereby man's work can bemore conveniently done, is called a machine; the machine itself neverdoes any work--it merely enables man to use his own efforts to betteradvantage. 

[Illustration: FIG. 93. --Rolling barrels up a plank. ]

150. When do we Work? Whenever, as a result of effort or force, anobject is moved, work is done. If you lift a knapsack from the floorto the table, you do work because you use force and move the knapsackthrough a distance equal to the height of the table. If the knapsackwere twice as heavy, you would exert twice as much force to raise itto the same height, and hence you would do double the work. If youraised the knapsack twice the distance, --say to your shouldersinstead of to the level of the table, --you would do twice the work, because while you would exert the same force you would continue itthrough double the distance. 

[Illustration: FIG. 94. --Crude method of farming. ]

Lifting heavy weights through great distances is not the only way inwhich work is done. Painting, chopping wood, hammering, plowing, washing, scrubbing, sewing, are all forms of work. In painting, themoving brush spreads paint over a surface; in chopping wood, thedescending ax cleaves the wood asunder; in scrubbing, the wet moprubbed over the floor carries dirt away; in every conceivable form ofwork, force and motion occur. 

A man does work when he walks, a woman does work when she rocks in achair--although here the work is less than in walking. On a windy daythe work done in walking is greater than normal. The wind resists ourprogress, and we must exert more force in order to cover the samedistance. Walking through a plowed or rough field is much more tiringthan to walk on a smooth road, because, while the distance covered maybe the same, the effort put forth is greater, and hence more work isdone. Always the greater the resistance encountered, the greater theforce required, and hence the greater the work done. 

The work done by a boy who raises a 5-pound knapsack to his shoulderwould be 5x4, or 20, providing his shoulders were 4 feet from theground. 

The amount of work done depends upon the force used and the distancecovered (sometimes called displacement), and hence we can say that

 Work = force multiplied by distance, or _W = f × d_. 

151. Machines. A glance into our machine shops, our factories, andeven our homes shows how widespread is the use of complex machinery. But all machines, however complicated in appearance, are in realitybut modifications and combinations of one or more of four simplemachines devised long ago by our remote ancestors. These simpledevices are known to-day, as (1) the lever, represented by a crowbar, a pitchfork; (2) the inclined plane, represented by the plank uponwhich barrels are rolled into a wagon; (3) the pulley, represented byalmost any contrivance for the raising of furniture to upper stories;(4) the wheel and axle, represented by cogwheels and coffee grinders. 

[Illustration: FIG. 95. --Primitive method of grinding corn. ]

Suppose a 600-pound bowlder which is embedded in the ground is neededfor the tower of a building. The problem of the builder is to get theheavy bowlder out of the ground, to load it on a wagon fortransportation, and finally to raise it to the tower. Obviously, hecannot do this alone; the greatest amount of force of which he iscapable would not suffice to accomplish any one of these tasks. Howthen does he help himself and perform the impossible? Simply, by theuse of some of the machine types mentioned above, illustrations ofwhich are known in a general way to every schoolboy. The very knifewith which a stick is whittled is a machine. 

[Illustration: FIG. 96. --Separating rice grains by flailing. ]

[Illustration: FIG. 97. --The principle of the lever. ]

152. The Lever. Balance a foot rule, containing a hole at its middlepoint _F_, as shown in Figure 97. If now a weight of 1 pound issuspended from the bar at some point, say 12, the balance isdisturbed, and the bar swings about the point _F_ as a center. Thebalance can be regained by suspending an equivalent weight at theopposite end of the bar, or by applying a 2-pound weight at a point 3inches to the left of _F_. In the latter case a force of 1 poundactually balances a force of 2 pounds, but the 1-pound weight is twiceas far from the point of suspension as is the 2-pound weight. Thesmall weight makes up in distance what it lacks in magnitude. 

Such an arrangement of a rod or bar is called a lever. In any form oflever there are only three things to be considered: the point wherethe weight rests, the point where the force acts, and the point calledthe fulcrum about which the rod rotates. 

The distance from the force to the fulcrum is called the force arm. The distance from the weight to the fulcrum is called the weight arm;and it is a law of levers, as well as of all other machines, that theforce multiplied by the length of the force arm must equal the weightmultiplied by the length of the weight arm. 

 Force × force arm = weight × weight arm. 

A force of 1 pound at a distance of 6, or with a force arm 6, willbalance a weight of 2 pounds with a weight arm 3; that is, 1 × 6 = 2 × 3. 

Similarly a force of 10 pounds may be made to sustain a weight of 100pounds, providing the force arm is 10 times longer than the weightarm; and a force arm of 800 pounds, at a distance of 10 feet from thefulcrum, may be made to sustain a weight of 8000 pounds, providing theweight is 1 foot from the fulcrum. 

153. Applications of the Lever. By means of a lever, a 600-poundbowlder can be easily pried out of the ground. Let the lever, anystrong metal bar, be supported on a stone which serves as fulcrum;then if a man exerts his force at the end of the rod somewhat as inFigure 91 (p. 154), the force arm will be the distance from the stoneor fulcrum to the end of the bar, and the weight arm will be thedistance from the fulcrum to the bowlder itself. The man pushes downwith a force of 100 pounds, but with that amount succeeds in prying upthe 600-pound bowlder. If, however, you look carefully, you will seethat the force arm is 6 times as long as the weight arm, so that thesmaller force is compensated for by the greater distance through whichit acts. 

At first sight it seems as though the man's work were done for him bythe machine. But this is not so. The man must lower his end of thelever 3 feet in order to raise the bowlder 6 inches out of the ground. He does not at any time exert a large force, but he accomplishes hispurpose by exerting a small force continuously through acorrespondingly greater distance. He finds it easier to exert a forceof 100 pounds continuously until his end has moved 3 feet rather thanto exert a force of 600 pounds on the bowlder and move it 6 inches. 

By the time the stone has been raised the man has done as much work asthough the stone had been raised directly, but his inability to putforth sufficient muscular force to raise the bowlder directly wouldhave rendered impossible a result which was easily accomplished whenthrough the medium of the lever he could extend his small forcethrough greater distance. 

154. The Wheelbarrow as a Lever. The principle of the lever isalways the same; but the relative position of the important points mayvary. For example, the fulcrum is sometimes at one end, the force atthe opposite end, and the weight to be lifted between them. 

[Illustration: FIG. 98. --A slightly different form of lever. ]

Suspend a stick with a hole at its center as in Figure 98, and hang a4-pound weight at a distance of 1 foot from the fulcrum, supportingthe load by means of a spring balance 2 feet from the fulcrum. Thepointer on the spring balance shows that the force required to balancethe 4-pound load is but 2 pounds. 

The force is 2 feet from the fulcrum, and the weight (4) is 1 footfrom the fulcrum, so that

 Force × distance = Weight × distance, or 2 × 2 = 4 × 1. 

Move the 4-pound weight so that it is very near the fulcrum, say but 6inches from it; then the spring balance registers a force only onefourth as great as the weight which it suspends. In other words aforce of 1 at a distance of 24 inches (2 feet) is equivalent to aforce of 4 at a distance of 6 inches. 

[Illustration: FIG. 99. --The wheelbarrow lightened labor. ]

One of the most useful levers of this type is the wheelbarrow (Fig. 99). The fulcrum is at the wheel, the force is at the handles, theweight is on the wheelbarrow. If the load is halfway from the fulcrumto the man's hands, the man will have to lift with a force equal toone half the load. If the load is one fourth as far from the fulcrumas the man's hands, he will need to lift with a force only one fourthas great as that of the load. 

[Illustration: FIG. 100. --A modified wheelbarrow. ]

This shows that in loading a wheelbarrow, it is important to arrangethe load as near to the wheel as possible. 

[Illustration: FIG. 101. --The nutcracker is a lever. ]

The nutcracker (Fig. 101) is an illustration of a double lever of thewheelbarrow kind; the nearer the nut is to the fulcrum, the easier thecracking. 

[Illustration: FIG. 102. --The hand exerts a small force over a longdistance and draws out a nail. ]

Hammers (Fig. 102), tack-lifters, scissors, forceps, are importantlevers, and if you will notice how many different levers (fig. 103)are used by all classes of men, you will understand how valuable amachine this simple device is. 

155. The Inclined Plane. A man wishes to load the 600-pound bowlderon a wagon, and proceeds to do it by means of a plank, as in Figure93. Such an arrangement is called an inclined plane. 

The advantage of an inclined plane can be seen by the followingexperiment. Select a smooth board 4 feet long and prop it so that theend _A_ (Fig. 104) is 1 foot above the level of the table; the lengthof the incline is then 4 times as great as its height. Fasten a metalroller to a spring balance and observe its weight. Then pull theroller uniformly upward along the plank and notice what the pull is onthe balance, being careful always to hold the balance parallel to theincline. 

When the roller is raised along the incline, the balance registers apull only one fourth as great as the actual weight of the roller. Thatis, when the roller weighs 12, a force of 3 suffices to raise it tothe height _A_ along the incline; but the smaller force must beapplied throughout the entire length of the incline. In many cases, itis preferable to exert a force of 30 pounds, for example, over thedistance _CA_ than a force of 120 pounds over the shorter distance_BA_. 

[Illustration: FIG. 103. --Primitive man tried to lighten his task bybalancing his burden. ]

Prop the board so that the end _A_ is 2 feet above the table level;that is, arrange the inclined plane in such a way that its length istwice as great as its height. In that case the steady pull on thebalance will be one half the weight of the roller; or a force of 6pounds will suffice to raise the 12-pound roller. 

[Illustration: FIG. 104. --Less force is required to raise the rolleralong the incline than to raise it to _A_ directly. ]

The steeper the incline, the more force necessary to raise a weight;whereas if the incline is small, the necessary lifting force isgreatly reduced. On an inclined plane whose length is ten times itsheight, the lifting force is reduced to one tenth the weight of theload. The advantage of an incline depends upon the relative length andheight, or is equal to the ratio of the length to the height. 

156. Application. By the use of an inclined plank a strong man canload the 600-pound bowlder on a wagon. Suppose the floor of the wagonis 2 feet above the ground, then if a 6-foot plank is used, 200 poundsof force will suffice to raise the bowlder; but the man will have topush with this force against the bowlder while it moves over theentire length of the plank. 

Since work is equal to force multiplied by distance, the man has donework represented by 200 × 6, or 1200. This is exactly the amount ofwork which would have been necessary to raise the bowlder directly. Aman of even enormous strength could not lift such a weight (600 lb. )even an inch directly, but a strong man can furnish the smaller force(200) over a distance of 6 feet; hence, while the machine does notlessen the total amount of work required of a man, it creates a newdistribution of work and makes possible, and even easy, results whichotherwise would be impossible by human agency. 

157. Railroads and Highways. The problem of the incline is animportant one to engineers who have under their direction theconstruction of our highways and the laying of our railroad tracks. Itrequires tremendous force to pull a load up grade, and most of us arefamiliar with the struggling horse and the puffing locomotive. Forthis reason engineers, wherever possible, level down the steep places, and reduce the strain as far as possible. 

[Illustration: FIG. 105. --A well-graded railroad bed. ]

The slope of the road is called its grade, and the grade itself issimply the number of feet the hill rises per mile. A road a mile long(5280 feet) has a grade of 132 if the crest of the hill is 132 feetabove the level at which the road started. 

[Illustration: FIG. 106. --A long, gradual ascent is better than ashorter, steeper one. ]

In such an incline, the ratio of length to height is 5280 ÷ 132, or40; and hence in order to pull a train of cars to the summit, theengine would need to exert a continuous pull equal to one fortieth ofthe combined weight of the train. 

If, on the other hand, the ascent had been gradual, so that the gradewas 66 feet per mile, a pull from the engine of one eightieth of thecombined weight would have sufficed to land the train of cars at thecrest of the grade. 

Because of these facts, engineers spend great sums in grading downrailroad beds, making them as nearly level as possible. In mountainousregions, the topography of the land prevents the elimination of allsteep grades, but nevertheless the attempt is always made to followthe easiest grades. 

158. The Wedge. If an inclined plane is pushed underneath or withinan object, it serves as a wedge. Usually a wedge consists of twoinclined planes (Fig. 107). 

[Illustration: FIG. 107. --By means of a wedge, the stump is split. ]

A chisel and an ax are illustrations of wedges. Perhaps the mostuniversal form of a wedge is our common pin. Can you explain how thisis a wedge?

159. The Screw. Another valuable and indispensable form of theinclined plane is the screw. This consists of a metal rod around whichpasses a ridge, and Figure 108 shows clearly that a screw is simply arod around which (in effect) an inclined plane has been wrapped. 

[Illustration: FIG. 108--A screw as a simple machine. ]

The ridge encircling the screw is called the thread, and the distancebetween two successive threads is called the pitch. It is easy to seethat the closer the threads and the smaller the pitch, the greater theadvantage of the screw, and hence the less force needed in overcomingresistance. A corkscrew is a familiar illustration of the screw. 

160. Pulleys. The pulley, another of the machines, is merely agrooved wheel around which a cord passes. It is sometimes moreconvenient to move a load in one direction rather than in another, andthe pulley in its simplest form enables us to do this. In order toraise a flag to the top of a mast, it is not necessary to climb themast, and so pull up the flag; the same result is accomplished muchmore easily by attaching the flag to a movable string, somewhat as inFigure 109, and pulling from below. As the string is pulled down, theflag rises and ultimately reaches the desired position. 

If we employ a stationary pulley, as in Figure 109, we do not changethe force, because the force required to balance the load is as largeas the load itself. The only advantage is that a force in onedirection may be used to produce motion in another direction. Such apulley is known as a fixed pulley. 

[Illustration: FIG. 109. --By means of a pulley, a force in onedirection produces motion in the opposite direction. ]

161. Movable Pulleys. By the use of a movable pulley, we are able tosupport a weight by a force equal to only one half the load. In Figure109, the downward pull of the weight and the downward pull of the handare equal; in Figure 110, the spring balance supports only one halfthe entire load, the remaining half being borne by the hook to whichthe string is attached. The weight is divided equally between the twoparts of the string which passes around the pulley, so that eachstrand bears only one half of the burden. 

We have seen in our study of the lever and the inclined plane that anincrease in force is always accompanied by a decrease in distance, andin the case of the pulley we naturally look for a similar result. Ifyou raise the balance (Fig. 110) 12 feet, you will find that theweight rises only 6 feet; if you raise the balance 24 inches, you willfind that the weight rises 12 inches. You must exercise a force of100 pounds over 12 feet of space in order to raise a weight of 200pounds a distance of 6 feet. When we raise 100 pounds through 12 feetor 200 pounds through 6 feet the total work done is the same; but thepulley enables those who cannot furnish a force of 200 pounds for thespace of 6 feet to accomplish the task by furnishing 100 pounds forthe space of 12 feet. 

[Illustration: FIG. 110. --A movable pulley lightens labor. ]

162. Combination of Pulleys. A combination of pulleys called blockand tackle is used where very heavy loads are to be moved. In Figure111 the upper block of pulleys is fixed, the lower block is movable, and one continuous rope passes around the various pulleys. The load issupported by 6 strands, and each strand bears one sixth of the load. If the hand pulls with a force of 1 pound at _P_, it can raise a loadof 6 pounds at _W_, but the hand will have to pull downward 6 feet at_P_ in order to raise the load at _W_ 1 foot. If 8 pulleys were used, a force equivalent to one eighth of the load would suffice to move_W_, but this force would have to be exerted over a distance 8 timesas great as that through which _W_ was raised. 

[Illustration: FIG. 111. --An effective arrangement of pulleys known asblock and tackle. ]

163. Practical Application. In our childhood many of us saw withwonder the appearance and disappearance of flags flying at the topsof high masts, but observation soon taught us that the flags wereraised by pulleys. In tenements, where there is no yard for the familywashing, clothes often appear flapping in mid-air. This seems mostmarvelous until we learn that the lines are pulled back and forth bypulleys at the window and at a distant support. By means of pulleys, awnings are raised and lowered, and the use of pulleys by furnituremovers, etc. , is familiar to every wide-awake observer on the streets. 

164. Wheel and Axle. The wheel and axle consists of a large wheeland a small axle so fastened that they rotate together. 

[Illustration: FIG. 112. --The wheel and axle. ]

When the large wheel makes one revolution, _P_ falls a distance equalto the circumference of the wheel. While _P_ moves downward, _W_likewise moves, but its motion is upward, and the distance it moves issmall, being equal only to the circumference of the small axle. But asmall force at _P_ will sustain a larger force at _W_; if thecircumference of the large wheel is 40 inches, and that of the smallwheel 10 inches, a load of 100 at _W_ can be sustained by a force of25 at _P_. The increase in force of the wheel and axle depends uponthe relative size of the two parts, that is, upon the circumference ofwheel as compared with circumference of axle, and since the ratiobetween circumference and radius is constant, the ratio of the wheeland axle combination is the ratio of the long radius to the shortradius. 

For example, in a wheel and axle of radii 20 and 4, respectively, agiven weight at _P_ would balance 5 times as great a load at _W_. 

165. Application. _Windlass, Cogwheels. _ In the old-fashionedwindlass used in farming districts, the large wheel is replaced by ahandle which, when turned, describes a circle. Such an arrangement isequivalent to wheel and axle (Fig. 112); the capstan used on shipboardfor raising the anchor has the same principle. The kitchen coffeegrinder and the meat chopper are other familiar illustrations. 

Cogwheels are modifications of the wheel and axle. Teeth cut in _A_fit into similar teeth cut in _B_, and hence rotation of _A_ causesrotation of _B_. Several revolutions of the smaller wheel, however, are necessary in order to turn the larger wheel through one completerevolution; if the radius of _A_ is one half that of _B_, tworevolutions of _A_ will correspond to one of _B_; if the radius of _A_is one third that of _B_, three revolutions of _A_ will correspond toone of _B_. 

[Illustration: FIG. 113. --Cogwheels. ]

Experiment demonstrates that a weight _W_ attached to a cogwheel ofradius 3 can be raised by a force _P_, equal to one third of _W_applied to a cogwheel of radius 1. There is thus a great increase inforce. But the speed with which _W_ is raised is only one third thespeed with which the small wheel rotates, or increase in power hasbeen at the decrease of speed. 

This is a very common method for raising heavy weights by small force. 

Cogwheels can be made to give speed at the decrease of force. A heavyweight _W_ attached to _B_ will in its slow fall cause rapid rotationof _A_, and hence rapid rise of _P_. It is true that _P_, the loadraised, will be less than _W_, the force exerted, but if speed is ouraim, this machine serves our purpose admirably. 

An extremely important form of wheel and axle is that in which the twowheels are connected by belts as in Figure 114. Rotation of _W_induces rotation of _w_, and a small force at _W_ is able to overcomea large force at _w_. An advantage of the belt connection is thatpower at one place can be transmitted over a considerable distance andutilized in another place. 

[Illustration: FIG. 114. --By means of a belt, motion can betransferred from place to place. ]

166. Compound Machines. Out of the few simple machines mentioned inthe preceding Sections has developed the complex machinery of to-day. By a combination of screw and lever, for example, we obtain theadvantage due to each device, and some compound machines have beenmade which combine all the various kinds of simple machines, and inthis way multiply their mechanical advantage many fold. 

A relatively simple complex machine called the crane (Fig. 116) maybeseen almost any day on the street, or wherever heavy weights are beinglifted. It is clear that a force applied to turn wheel 1 causes aslower rotation of wheel 3, and a still slower rotation of wheel 4, but as 4 rotates it winds up a chain and slowly raises _Q_. A verycomplex machine is that seen in Figure 117. 

[Illustration: FIG. 115. --A simple derrick for raising weights. ]

[Illustration: FIG. 116. --A traveling crane. ]

167. Measurement of Work. In Section 150, we learned that the amountof work done depends upon the force exerted, and the distancecovered, or that _W_ = force × distance. A man who raises 5 pounds aheight of 5 feet does far more work than a man who raises 5 ounces aheight of 5 inches, but the product of force by distance is 25 in eachcase. There is difficulty because we have not selected an arbitraryunit of work. The unit of work chosen and in use in practical affairsis the foot pound, and is defined as the work done when a force of 1pound acts through a distance of 1 foot. A man who moves 8 poundsthrough 6 feet does 48 foot pounds of work, while a man who moves 8ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth ofa foot pound of work. 

[Illustration: FIG. 117. --A farm engine putting in a crop. ]

168. The Power or the Speed with which Work is Done. A man can loada wagon more quickly than a growing boy. The work done by the one isequal to the work done by the other, but the man is more powerful, because the time required for a given task is very important. Anengine which hoists a 50-pound weight in 1 second is much morepowerful than a man who requires 50 seconds for the same task; hencein estimating the value of a working agent, whether animal ormechanical, we must consider not only the work done, but the speedwith which it is done. 

The rate at which a machine is able to accomplish a unit of work iscalled _power_, and the unit of power customarily used is the horsepower. Any power which can do 550 foot pounds of work per second issaid to be one horse power (H. P. ). This unit was chosen by James Watt, the inventor of a steam engine, when he was in need of a unit withwhich to compare the new source of power, the engine, with his oldsource of power, the horse. Although called a horse power it isgreater than the power of an average horse. 

An ordinary man can do one sixth of a horse power. The averagelocomotive of a railroad has more than 500 H. P. , while the engines ofan ocean liner may have as high as 70, 000 H. P. 

169. Waste Work and Efficient Work. In our study of machines weomitted a factor which in practical cases cannot be ignored, namely, friction. No surface can be made perfectly smooth, and when a barrelrolls over an incline, or a rope passes over a pulley, or a cogwheelturns its neighbor, there is rubbing and slipping and sliding. Motionis thus hindered, and the effective value of the acting force islessened. In order to secure the desired result it is necessary toapply a force in excess of that calculated. This extra force, whichmust be supplied if friction is to be counteracted, is in realitywaste work. 

If the force required by a machine is 150 pounds, while thatcalculated as necessary is 100 pounds, the loss due to friction is 50pounds, and the machine, instead of being thoroughly efficient, isonly two thirds efficient. 

Machinists make every effort to eliminate from a machine the waste dueto friction, leveling and grinding to the most perfect smoothness andadjustment every part of the machine. When the machine is in use, friction may be further reduced by the use of lubricating oil. Friction can never be totally eliminated, however, and machines ofeven the finest construction lose by friction some of theirefficiency, while poorly constructed ones lose by friction as much asone half of their efficiency. 

170. Man's Strength not Sufficient for Machines. A machine, an inertmass of metal and wood, cannot of itself do any work, but can onlydistribute the energy which is brought to it. Fortunately it is notnecessary that this energy should be contributed by man alone, becausethe store of energy possessed by him is very small in comparison withthe energy required to run locomotives, automobiles, sawmills, etc. Perhaps the greatest value of machines lies in the fact that theyenable man to perform work by the use of energy other than his own. 

[Illustration: FIG. 118. --Man's strength is not sufficient for heavywork. ]

Figure 118 shows one way in which a horse's energy can be utilized inlifting heavy loads. Even the fleeting wind has been harnessed by man, and, as in the windmill, made to work for him (Fig. 119). One seesdotted over the country windmills large and small, and in Holland, thecountry of windmills, the landowner who does not possess a windmill ispoor indeed. 

For generations running water from rivers, streams, and falls hasserved man by carrying his logs downstream, by turning the wheels ofhis mill, etc. ; and in our own day running water is used as anindirect source of electric lights for street and house, the energy ofthe falling water serving to rotate the armature of a dynamo (Section310). 

[Illustration: FIG. 119. --The windmill pumps water into the troughswhere cattle drink. ]

A more constant source of energy is that available from the burning offuel, such as coal and oil. The former is the source of energy inlocomotives, the latter in most automobiles. 

In the following Chapter will be given an account of water, wind, andfuel as machine feeders. 

CHAPTER XVII

THE POWER BEHIND THE ENGINE

171. Small boys soon learn the power of running water; swimming orrowing downstream is easy, while swimming or rowing against thecurrent is difficult, and the swifter the water, the easier the oneand the more difficult the other; the river assists or opposes us aswe go with it or against it. The water of a quiet pool or of a gentlestream cannot do work, but water which is plunging over a precipice ordam, or is flowing down steep slopes, may be made to saw wood, grindour corn, light our streets, run our electric cars, etc. A waterfall, or a rapid stream, is a great asset to any community, and for thisreason should be carefully guarded. Water power is as great a sourceof wealth as a coal bed or a gold mine. 

The most tremendous waterfall in our country is Niagara Falls, whichevery minute hurls millions of gallons of water down a 163-footprecipice. The energy possessed by such an enormous quantity of waterflowing at such a tremendous speed is almost beyond everydaycomprehension, and would suffice to run the engines of many cities farand near. Numerous attempts to buy from the United States the right toutilize some of this apparently wasted energy have been made byvarious commercial companies. It is fortunate that these negotiationshave been largely fruitless, because much deviation of the water forcommercial uses and the installation of machinery in the vicinity ofthe famous falls would greatly detract from the beauty of thisworld-known scene, and would rob our country of a natural beautyunequaled elsewhere. 

[Illustration: FIG. 120. --A mountain stream turns the wheels of themill. ]

172. Water Wheels. In Figure 120 the water of a small but rapidmountain stream is made to rotate a large wheel, which in turncommunicates its motion through belts to a distant sawmill or grinder. In more level regions huge dams are built which hold back the waterand keep it at a higher level than the wheel; from the dam the wateris conveyed in pipes (flumes) to the paddle wheel which it turns. Cogwheels or belts connect the paddle wheel with the factorymachinery, so that motion of the paddle wheel insures the running ofthe machinery. 

[Illustration: FIG. 121. --The Pelton water wheel. ]

One of the most efficient forms of water wheels is that shown inFigure 121, and called the Pelton wheel. Water issues in a narrow jetsimilar to that of the ordinary garden hose and strikes with greatforce against the lower part of the wheel, thereby causing rotation ofthe wheel. Belts transfer this motion to the machinery of factory ormill. 

173. Turbines. The most efficient form of water motor is theturbine, a strong metal wheel shaped somewhat like a pin wheel, inclosed in a heavy metal case. 

[Illustration: FIG. 122--A turbine at Niagara Falls. ]

Water is conveyed from a reservoir or dam through a pipe (penstock) tothe turbine case, in which is placed the heavy metal turbine wheel(Fig. 122). The force of the water causes rotation of the turbine andof the shaft which is rigidly fastened to it. The water which flowsinto the turbine case causes rotation of the wheel, escapes from thecase through openings, and flows into the tail water. 

The power which a turbine can furnish depends upon the quantity ofwater and the height of the fall, and also upon the turbine wheelitself. One of the largest turbines known has a horse power of about20, 000; that is, it is equivalent, approximately, to 20, 000 horses. 

174. How much is a Stream Worth? The work which a stream can performmay be easily calculated. Suppose, for example, that 50, 000 pounds ofwater fall over a 22-foot dam every second; the power of such a streamwould be 1, 100, 000 foot pounds per second or 2000 H. P. Naturally, apart of this power would be lost to use by friction within themachinery and by leakage, so that the power of a turbine run by a 2000H. P. Stream would be less than that value. 

Of course, the horse power to be obtained from a stream determines thesize of the paddle wheel or turbine which can be run by it. It wouldbe possible to construct a turbine so large that the stream would notsuffice to turn the wheel; for this reason, the power of a stream iscarefully determined before machine construction is begun, and thesize of the machinery depends upon the estimates of the water powerfurnished by expert engineers. 

A rough estimate of the volume of a stream may be made by the methoddescribed below:--

Suppose we allow a stream of water to flow through a rectangulartrough; the speed with which the water flows through the trough can bedetermined by noting the time required for a chip to float the lengthof the trough; if the trough is 10 feet long and the time required is5 seconds, the water has a velocity of 2 feet per second. 

[Illustration: FIG. 123. --Estimating the quantity of water which flowsthrough the trough each second. ]

The quantity of water which flows through the trough each seconddepends upon the dimensions of the trough and the velocity of thewater. Suppose the trough is 5 feet wide and 3 feet high, or has across section of 15 square feet. If the velocity of the water were 1foot per second, then 15 cubic feet of water would pass any givenpoint each second, but since the velocity of the water is 2 feet persecond, 30 cubic feet will represent the amount of water which willflow by a given point in one second. 

175. Quantity of Water Furnished by a River. Drive stakes in theriver at various places and note the time required for a chip to floatfrom one stake to another. If we know the distance between the stakesand the time required for the chip to float from one stake to another, the velocity of the water can be readily determined. 

The width of the stream from bank to bank is easily measured, and thedepth is obtained in the ordinary way by sounding; it is necessary totake a number of soundings because the bed of the river is by no meanslevel, and soundings taken at only one level would not give anaccurate estimate. If the soundings show the following depths: 30, 25, 20, 32, 28, the average depth could be taken as 30 + 25 + 20 + 32 + 28÷ 5, or 27 feet. If, as a result of measuring, the river at a givenpoint in its course is found to be 27 feet deep and 60 feet wide, thearea of a cross section at that spot would be 1620 square feet, and ifthe velocity proved to be 6 feet per second, then the quantity ofwater passing in any one second would be 1620 × 6, or 9720 cubic feet. By experiment it has been found that 1 cu. Ft. Of water weighs about62. 5 lb. The weight of the water passing each second would thereforebe 62. 5 × 9720, or 607, 500 lb. If this quantity of water plunges overa 10-ft. Dam, it does 607, 500 × 10, or 6, 075, 000 foot pounds of workper second, or 11, 045 H. P. Such a stream would be very valuable forthe running of machinery. 

176. Windmills. Those of us who have spent our vacation days in thecountry know that there is no ready-made water supply there as in thecities, but that as a rule the farmhouses obtain their drinking waterfrom springs and wells. In poorer houses, water is laboriouslycarried in buckets from the spring or is lifted from the well by thewindlass. In more prosperous houses, pumps are installed; this is animprovement over the original methods, but the quantity of waterconsumed by the average family is so great as to make the task ofpumping an arduous one. 

The average amount of water used per day by one person is 25 gallons. This includes water for drinking, cooking, dish washing, bathing, laundry. For a family of five, therefore, the daily consumption wouldbe 125 gallons; if to this be added the water for a single horse, cow, and pig, the total amount needed will be approximately 150 gallons perday. A strong man can pump that amount from an ordinary well in aboutone hour, but if the well is deep, more time and strength arerequired. 

The invention of the windmill was a great boon to country folksbecause it eliminated from their always busy life one task in whichlabor and time were consumed. 

177. The Principle of the Windmill. The toy pin wheel is a windmillin miniature. The wind strikes the sails, and causes rotation; and thestronger the wind blows, the faster will the wheel rotate. Inwindmills, the sails are of wood or steel, instead of paper, but theprinciple is identical. 

[Illustration: FIG. 124. --The toy pin wheel is a miniature windmill. ]

As the wheel rotates, its motion is communicated to a mechanicaldevice which makes use of it to raise and lower a plunger, and henceas long as the wind turns the windmill, water is raised. The waterthus raised empties into a large tank, built either in the windmilltower or in the garret of the house, and from the tank the waterflows through pipes to the different parts of the house. On very windydays the wheel rotates rapidly, and the tank fills quickly; in orderto guard against an overflow from the tank a mechanical device isinstalled which stops rotation of the wheel when the tank is nearlyfull. The supply tank is usually large enough to hold a supply ofwater sufficient for several days, and hence a continuous calm of aday or two does not materially affect the house flow. When once built, a windmill practically takes care of itself, except for oiling, and isan efficient and cheap domestic possession. 

[Illustration: FIG. 125. --The windmill pumps water into the tank. ]

178. Steam as a Working Power. If a delicate vane is held at anopening from which steam issues, the pressure of the steam will causerotation of the vane (Fig. 126), and if the vane is connected with amachine, work can be obtained from the steam. 

When water is heated in an open vessel, the pressure of its steam istoo low to be of practical value, but if on the contrary water isheated in an almost closed vessel, its steam pressure is considerable. If steam at high pressure is directed by nozzles against the blades ofa wheel, rapid rotation of the wheel ensues just as it did in Figure121, although in this case steam pressure replaces water pressure. After the steam has spent itself in turning the turbine, it condensesinto water and makes its escape through openings in an inclosing case. In Figure 127 the protecting case is removed, in order that the formof the turbine and the positions of the nozzles may be visible. 

[Illustration: FIG. 126. --Steam as a source of power. ]

[Illustration: FIG. 127. --Steam turbine with many blades and 4nozzles. ]

A single large turbine wheel may have as many as 800, 000 sails orblades, and steam may pour out upon these from many nozzles. 

The steam turbine is very much more efficient than its forerunner, thesteam engine. The installation of turbines on ocean liners has beenaccompanied by great increase in speed, and by an almost correspondingdecrease in the cost of maintenance. 

179. Steam Engines. A very simple illustration of the working of asteam engine is given in Figure 128. Steam under pressure entersthrough the opening _F_, passes through _N_, and presses upon thepiston _M_. As a result _M_ moves downward, and thereby inducesrotation in the large wheel _L_. 

[Illustration: FIG. 128. --The principle of the steam engine. ]

As _M_ falls it drives the air in _D_ out through _O_ and _P_ (theopening _P_ is not visible in the diagram). 

As soon as this is accomplished, a mechanical device draws up the rod_E_, which in turn closes the opening _N_, and thus prevents the steamfrom passing into the part of _D_ above _M_. 

But when the rod _E_ is in such a position that _N_ is closed, _O_ onthe other hand is open, and steam rushes through it into _D_ andforces up the piston. This up-and-down motion of the piston causescontinuous rotation of the wheel _L_. If the fire is hot, steam isformed quickly, and the piston moves rapidly; if the fire is low, steam is formed slowly, and the piston moves less rapidly. 

The steam engine as seen on our railroad trains is very complex, andcannot be discussed here; in principle, however, it is identical withthat just described. Figure 129 shows a steam harvester at work on amodern farm. 

[Illustration: FIG. 129. --Steam harvester at work. ]

In both engine and turbine the real source of power is not the steambut the fuel, such as coal or oil, which converts the water intosteam. 

180. Gas Engines. Automobiles have been largely responsible for thegas engine. To carry coal for fuel and water for steam would beimpracticable for most motor cars. Electricity is used in some cars, but the batteries are heavy, expensive, and short-lived, and are notalways easily replaceable. For this reason gasoline is extensivelyused, and in the average automobile the source of power is the forcegenerated by exploding gases. 

It was discovered some years ago that if the vapor of gasoline ornaphtha was mixed with a definite quantity of air, and a light wasapplied to the mixture, an explosion would result. Modern science usesthe force of such exploding gases for the accomplishment of work, suchas running of automobiles and launches. 

In connection with the gasoline supply is a carburetor or sprayer, from which the cylinder _C_ (Fig. 130) receives a fine mist ofgasoline vapor and air. This mixture is ignited by an automatic, electric sparking device, and the explosion of the gases drives thepiston _P_ to the right. In the 4-cycle type of gas engines (Fig. 130)--the kind used in automobiles--the four strokes are as follows:1. The mixture of gasoline and air enters the cylinder as the pistonmoves to the right. 2. The valves being closed, the mixture iscompressed as the piston moves to the left. 3. The electric sparkignites the compressed mixture and drives the piston to the right. 4. The waste gas is expelled as the piston moves to the left. The exhaustvalve is then closed, the inlet valve opened, and another cycle offour strokes begins. 

[Illustration: FIG. 130. --The gas engine. ]

The use of gasoline in launches and automobiles is familiar to many. Not only are launches and automobiles making use of gas power, but thegasoline engine has made it possible to propel aëroplanes through theair. 

CHAPTER XVIII

PUMPS AND THEIR VALUE TO MAN

181. "As difficult as for water to run up a hill!" Is there any onewho has not heard this saying? And yet most of us accept as a matterof course the stream which gushes from our faucet, or give no thoughtto the ingenuity which devised a means of forcing water upward throughpipes. Despite the fact that water flows naturally down hill, and notup, we find it available in our homes and office buildings, in some ofwhich it ascends to the fiftieth floor; and we see great streams of itdirected upon the tops of burning buildings by firemen in the streetsbelow. 

In the country, where there are no great central pumping stations, water for the daily need must be raised from wells, and the supply ofeach household is dependent upon the labor and foresight of itsmembers. The water may be brought to the surface either by laboriouslyraising it, bucket by bucket, or by the less arduous method ofpumping. These are the only means possible; even the windmill does noteliminate the necessity for the pump, but merely replaces the energyused by man in working it. 

In some parts of our country we have oil beds or wells. But if thisunderground oil is to be of service to man, it must be brought to thesurface, and this is accomplished, as in the case of water, by the useof pumps. 

An old tin can or a sponge may serve to bale out water from a leakingrowboat, but such a crude device would be absurd if employed on ourhuge vessels of war and commerce. Here a rent in the ship's side wouldmean inevitable loss were it not possible to rid the ship of theinflowing water by the action of strong pumps. 

Another and very different use to which pumps are put is seen in thecompression of gases. Air is forced into the tires of bicycles andautomobiles until they become sufficiently inflated to insure comfortin riding. Some present-day systems of artificial refrigeration(Section 93) could not exist without the aid of compressed gases. 

Compressed air has played a very important role in mining, being sentinto poorly ventilated mines to improve the condition of the air, andto supply to the miners the oxygen necessary for respiration. Diversand men who work under water carry on their backs a tank of compressedair, and take from it, at will, the amount required. 

There are many forms of pumps, and they serve widely differentpurposes, being essential to the operation of many industrialundertakings. In the following Sections some of these forms will bestudied. 

[Illustration: FIG. 131. --Carrying water home from the spring. ]

182. The Air as Man's Servant. Long before man harnessed water forturbines, or steam for engines, he made the air serve his purpose, andby means of it raised water from hidden underground depths to thesurface of the earth; likewise, by means of it, he raised to hisdwelling on the hillside water from the stream in the valley below. Those who live in cities where running water is always present in thehome cannot realize the hardship of the days when this "ready-made"supply did not exist, but when man laboriously carried to hisdwelling, from distant spring and stream, the water necessary for thedaily need. 

What are the characteristics of the air which have enabled man toaccomplish these feats? They are well known to us and may be brieflystated as follows:--

(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure, weighs 1-1/4 ounces. 

(2) The air around us presses with a force of about 15 pounds uponevery square inch of surface that it touches. 

(3) Air is elastic; it can be compressed, as in the balloon or bicycletire, but it expands immediately when pressure is reduced. As itexpands and occupies more space, its pressure falls and it exerts lessforce against the matter with which it comes in contact. If, forexample, 1 cubic foot of air is allowed to expand and occupy 2 cubicfeet of space, the pressure which it exerts is reduced one half. Whenair is compressed, its pressure increases, and it exerts a greaterforce against the matter with which it comes in contact. If 2 cubicfeet of air are compressed to 1 cubic foot, the pressure of thecompressed air is doubled. (See Section 89. )

[Illustration: FIG. 132. --The atmosphere pressing downward on _a_pushes water after the rising piston _b_. ]

183. The Common Pump or Lifting Pump. Place a tube containing aclose-fitting piston in a vessel of water, as shown in Figure 132. Then raise the piston with the hand and notice that the water rises inthe piston tube. The rise of water in the piston tube is similar tothe raising of lemonade through a straw (Section 77). The atmospherepresses with a force of 15 pounds upon every square inch of water inthe large vessel, and forces some of it into the space left vacant bythe retreating piston. The common pump works in a similar manner. Itconsists of a piston or plunger which moves back and forth in anair-tight cylinder, and contains an outward opening valve throughwhich water and air can pass. From the bottom of the cylinder a tuberuns down into the well or reservoir, and water from the well hasaccess to the cylinder through another outward-moving valve. Inpractice the tube is known as the suction pipe, and its valve as thesuction valve. 

In order to understand the action of a pump, we will suppose that nowater is in the pump, and we will pump until a stream issues from thespout. The various stages are represented diagrammatically by Figure133. In (1) the entire pump is empty of water but full of air atatmospheric pressure, and both valves are closed. In (2) the plungeris being raised and is lifting the column of air that rests on it. Theair and water in the inlet pipe, being thus partially relieved ofdownward pressure, are pushed up by the atmospheric pressure on thesurface of the water in the well. When the piston moves downward as in(3), the valve in the pipe closes by its own weight, and the air inthe cylinder escapes through the valve in the plunger. In (4) thepiston is again rising, repeating the process of (2). In (5) theprocess of (3) is being repeated, but water instead of air is escapingthrough the valve in the plunger. In (6) the process of (2) is beingrepeated, but the water has reached the spout and is flowing out. 

[Illustration: FIG. 133. Diagram of the process of pumping. ]

After the pump is in condition (6), motion of the plunger is followedby a more or less regular discharge of water through the spout, andthe quantity of water which gushes forth depends upon the speed withwhich the piston is moved. A strong man giving quick strokes canproduce a large flow; a child, on the other hand, is able to produceonly a thin stream. Whoever pumps must exert sufficient force to liftthe water from the surface of the well to the spout exit. For thisreason the pump has received the name of _lifting pump_. 

[Illustration: FIG. 134. --Force pump. ]

184. The Force Pump. In the common pump, water cannot not be raisedhigher than the spout. In many cases it is desirable to force waterconsiderably above the pump itself, as, for instance, in the firehose; under such circumstances a type of pump is employed which hasreceived the name of _force pump_. This differs but little from theordinary lift pump, as a reference to Figure 134 will show. Here bothvalves are placed in the cylinder, and the piston is solid, but theprinciple is the same as in the lifting pump. 

An upward motion of the plunger allows water to enter the cylinder, and the downward motion of the plunger drives water through _E_. (Isthis true for the lift pump as well?) Since only the downward motionof the plunger forces water through _E_, the discharge is intermittentand is therefore not practical for commercial purposes. In order toconvert this intermittent discharge into a steady stream, an airchamber is installed near the discharge tube, as in Figure 135. Thewater forced into the air chamber by the downward-moving pistoncompresses the air and increases its pressure. The pressure of theconfined air reacts against the water and tends to drive it out of thechamber. Hence, even when the plunger is moving upward, water isforced through the pipe because of the pressure of the compressedair. In this way a continuous flow is secured. 

[Illustration: FIG 135. --The air chamber _A_ insures a continuous flowof water. ]

The height to which the water can be forced in the pipe depends uponthe size and construction of the pump and upon the force with whichthe plunger can be moved. The larger the stream desired and thegreater the height to be reached, the stronger the force needed andthe more powerful the construction necessary. 

The force pump gets its name from the fact that the moving pistondrives or forces the water through the discharge tube. 

185. Irrigation and Drainage. History shows that the lifting pumphas been used by man since the fourth century before Christ; for manypresent-day enterprises this ancient form of pump is inconvenient andimpracticable, and hence it has been replaced in many cases by moremodern types, such as rotary and centrifugal pumps (Fig. 136). Inthese forms, rapidly rotating wheels lift the water and drive itonward into a discharge pipe, from which it issues with great force. There is neither piston nor valve in these pumps, and the quantity ofwater raised and the force with which it is driven through the pipesdepends solely upon the size of the wheels and the speed with whichthey rotate. 

Irrigation, or the artificial watering of land, is of the greatestimportance in those parts of the world where the land is naturally toodry for farming. In the United States, approximately two fifths of theland area is so dry as to be worthless for agricultural purposesunless artificially watered. In the West, several large irrigatingsystems have been built by the federal government, and at presentabout ten million acres of land have been converted from worthlessfarms into fields rich in crops. Many irrigating systems usecentrifugal pumps to force water over long distances and to supply itin quantities sufficient for vast agricultural needs. In many regions, the success of a farm or ranch depends upon the irrigation furnishedin dry seasons, or upon man's ability to drive water from a region ofabundance to a remote region of scarcity. 

[Illustration: FIG. 136. --Centrifugal pump with part of the casing]cut away to show the wheel. 

[Illustration: FIG. 137. --Agriculture made possible by irrigation. ]

The draining of land is also a matter of considerable importance;swamps and marshes which were at one time considered useless have beendrained and then reclaimed and converted into good farming land. Thesurplus water is best removed by centrifugal pumps, since sand andsticks which would clog the valves of an ordinary pump are passedalong without difficulty by the rotating wheel. 

[Illustration: FIG. 138. --Rice for its growth needs periodicalflooding, and irrigation often supplies the necessary water. ]

186. Camping. --Its Pleasures and its Dangers. The allurement of avacation camp in the heart of the woods is so great as to make manycampers ignore the vital importance of securing a safe water supply. Ariver bank may be beautiful and teeming with diversions, but if theriver is used as a source of drinking water, the results will almostalways be fatal to some. The water can be boiled, it is true, but fewcampers are willing to forage for the additional wood needed for thisapparently unnecessary requirement; then, too, boiled water does notcool readily in summer, and hence is disagreeable for drinkingpurposes. 

The only safe course is to abandon the river as a source of drinkingwater, and if a spring cannot be found, to drive a well. In manyregions, especially in the neighborhood of streams, water can befound ten or fifteen feet below the surface. Water taken from such adepth has filtered through a bed of soil, and is fairly safe for anypurpose. Of course the deeper the well, the safer will be the water. With the use of such a pump as will be described, campers can, withoutgrave danger, throw dish water, etc. , on the ground somewhat remotefrom the camp; this may not injure their drinking water because theliquids will slowly seep through the ground, and as they filterdownward will lose their dangerous matter. All the water which reachesthe well pipes will have filtered through the soil bed and thereforewill probably be safe. 

But while the careless disposal of wastes may not spoil the drinkingwater (in the well to be described), other laws of health demand athoughtful disposal of wastes. The malarial mosquito and the typhoidfly flourish in unhygienic quarters, and the only way to guard againsttheir dangers is to allow them neither food nor breeding place. 

The burning of garbage, the discharge of waters into cesspools, or, intemporary camps, the discharge of wastes to distant points through theagency of a cheap sewage pipe will insure safety to campers, willlessen the trials of flies and mosquitoes, and will add but little tothe expense. 

187. A Cheap Well for Campers. A two-inch galvanized iron pipe witha strong, pointed end containing small perforations is driven into theground with a sledge hammer. After it has penetrated for a few feet, another length is added and the whole is driven down, and this isrepeated until water is reached. A cheap pump is then attached to theupper end of the drill pipe and serves to raise the water. During thedrilling, some soil particles get into the pipe through theperforations, and these cloud the water at first; but after the pipehas once been cleaned by the upward-moving water, the supply remainsclear. The flow from such a well is naturally small; first, becausewater is not abundant near the surface of the earth, and second, because cheap pumps are poorly constructed and cannot raise a largeamount. But the supply will usually be sufficient for the needs ofsimple camp life, and many a small farm uses this form of well, notonly for household purposes, but for watering the cattle in winter. 

If the cheapness of such pumps were known, their use would be moregeneral for temporary purposes. The cost of material need not exceed$5 for a 10-foot well, and the driving of the pipe could be made asmuch a part of the camping as the pitching of the tent itself. If thecamping site is abandoned at the close of the vacation, the pump canbe removed and kept over winter for use the following summer inanother place. In this way the actual cost of the water supply can bereduced to scarcely more than $3, the removable pump being a permanentpossession. In rocky or mountain regions the driven well is notpracticable, because the driving point is blunted and broken by therock and cannot pierce the rocky beds of land. 

[Illustration: FIG. 139--A driven well. ]

[Illustration: FIG. 140. --Diagram showing how supplying a city withgood water lessens sickness and death. The lines _b_ show the relativenumber of people who died of typhoid fever before the water wasfiltered; the lines _a_ show the numbers who died after the water wasfiltered. The figures are the number of typhoid deaths occurringyearly out of 100, 000 inhabitants. ]

188. Our Summer Vacation. It has been asserted by some city healthofficials that many cases of typhoid fever in cities can be traced tothe unsanitary conditions existing in summer resorts. The drinkingwater of most cities is now under strict supervision, while that ofisolated farms, of small seaside resorts, and of scattered mountainhotels is left to the care of individual proprietors, and in only toomany instances receives no attention whatever. The sewage disposal isoften inadequate and badly planned, and the water becomes dangerouslycontaminated. A strong, healthy person, with plenty of outdoorexercise and with hygienic habits, may be able to resist the diseasegerms present in the poor water supply; more often the summer guestscarry back with them to their winter homes the germs of disease, andthese gain the upper hand under the altered conditions of city andbusiness life. It is not too much to say that every man and womanshould know the source of his summer table water and the method ofsewage disposal. If the conditions are unsanitary, they cannot beremedied at once, but another resort can be found and personal dangercan be avoided. Public sentiment and the loss of trade will go far infurthering an effort toward better sanitation. 

In the driven well, water cannot reach the spout unless it has firstfiltered through the soil to the depth of the driven pipe; after sucha journey it is fairly safe, unless very large quantities of sewageare present; generally speaking, such a depth of soil is able tofilter satisfactorily the drainage of the limited number of peoplewhich a driven well suffices to supply. 

[Illustration: FIG. 141. --A deep well with the piston in the water. ]

Abundant water is rarely reached at less than 75 feet, and it wouldusually be impossible to drive a pipe to such a depth. When a largequantity of water is desired, strong machines drill into the groundand excavate an opening into which a wide pipe can be lowered. Irecently spent a summer in the Pocono Mountains and saw such a wellcompleted. The machine drilled to a depth of 250 feet before muchwater was reached and to over 300 feet before a flow was obtainedsufficient to satisfy the owner. The water thus obtained was to be thesole water supply of a hotel accommodating 150 persons; the proprietorcalculated that the requirements of his guests, for bath, toilet, laundry, kitchen, etc. , and the domestics employed to serve them, together with the livery at their disposal, demanded a flow of 10gallons per minute. The ground was full of rock and difficult topenetrate, and it required 6 weeks of constant work for two skilledmen to drill the opening, lower the suction pipe, and install thepump, the cost being approximately $700. 

[Illustration: FIG. 142. --Showing how drinking water can becontaminated from cesspool _(c)_ and wash water _(w)_. ]

The water from such a well is safe and pure except under theconditions represented in Figure 142. If sewage or slops be pouredupon the ground in the neighborhood of the well, the liquid will seepthrough the ground and some may make its way into the pump before ithas been purified by the earth. The impure liquid will thuscontaminate the otherwise pure water and will render it decidedlyharmful. For absolute safety the sewage discharge should be at least75 feet from the well, and in large hotels, where there is necessarilya large quantity of sewage, the distance should be much greater. Asthe sewage seeps through the ground it loses its impurities, but thequantity of earth required to purify it depends upon its abundance; asmall depth of soil cannot take care of an indefinite amount ofsewage. Hence, the greater the number of people in a hotel, or themore abundant the sewage, the greater should be the distance betweenwell and sewer. 

By far the best way to avoid contamination is to see to it that thesewage discharges into the ground _below_ the well; that is, to digthe well in such a location that the sewage drainage will be away fromthe well. 

In cities and towns and large summer communities, the sewage ofindividual buildings drains into common tanks erected at publicexpense; the contents of these are discharged in turn into harbors andstreams, or are otherwise disposed of at great expense, although theycontain valuable substances. It has been estimated that the drainageor sewage of England alone would be worth $ 80, 000, 000 a year if usedas fertilizer. 

A few cities, such as Columbus and Cleveland, Ohio, realize the needof utilizing this source of wealth, and by chemical means deodorizetheir sewage and change it into substances useful for agricultural andindustrial purposes. There is still a great deal to be learned on thissubject, and it is possible that chemically treated sewage may be madea source of income to a community rather than an expense. 

189. Pumps which Compress Air. The pumps considered in the precedingSections have their widest application in agricultural districts, where by means of them water is raised to the surface of the earth oris pumped into elevated tanks. From a commercial and industrialstandpoint a most important class of pump is that known as thecompression type; in these, air or any other gas is compressed ratherthan rarefied. 

Air brakes and self-opening and self-closing doors on cars areoperated by means of compression pumps. The laying of bridge and pierfoundations, in fact all work which must be done under water, ispossible only through the agency of compression pumps. Those who havevisited mines, and have gone into the heart of the undergroundlabyrinth, know how difficult it is for fresh air to make its way tothe miners. Compression pumps have eliminated this difficulty, andto-day fresh air is constantly pumped into the mines to supply thelaborers there. Agricultural methods also have been modified by thecompression pump. The spraying of trees (Fig. 143), formerly doneslowly and laboriously, is now a relatively simple matter. 

[Illustration: FIG. 143. --Spraying trees by means of a compressionpump. ]

190. The Bicycle Pump. The bicycle pump is the best known of allcompression pumps. Here, as in other pumps of its type, the valvesopen inward rather than outward. When the piston is lowered, compressed air is driven through the rubber tubing, pushes open aninward-opening valve in the tire, and thus enters the tire. When thepiston is raised, the lower valve closes, the upper valve is openedby atmospheric pressure, and air from outside enters the cylinder; thenext stroke of the piston drives a fresh supply of air into the tire, which thus in time becomes inflated. In most cheap bicycle pumps, thepiston valve is replaced by a soft piece of leather so attached to thepiston that it allows air to slip around it and into the cylinder, butprevents its escape from the cylinder (Fig. 144). 

[Illustration: FIG. 144. --The bicycle foot pump. ]

191. How a Man works under Water. Place one end of a piece of glasstube in a vessel of water and notice that the water rises in the tube(Fig. 145). Blow into the tube and see whether you can force the waterwholly or partially down the tube. If the tube is connected to a smallcompression pump, sufficient air can be sent into the tube to causethe water to sink and to keep the tube permanently clear of water. This is, in brief, the principle employed for work under water. Acompression pump forces air through a tube into the chamber in whichmen are to work (Fig. 146). The air thus furnished from above suppliesthe workmen with oxygen, and by its pressure prevents water fromentering the chamber. When the task has been completed, the chamber israised and later lowered to a new position. 

[Illustration: FIG. 145. --Water does not enter the tube as long as weblow into it. ]

Figure 147 shows men at work on a bridge foundation. Workmen, tools, and supplies are lowered in baskets through a central tube _BC_provided with an air chamber _L_, having air-tight gates at _A_ and_A'_. The gate _A_ is opened and workmen enter the air chamber. Thegate _A_ is then closed and the gate _A'_ is opened slowly to give themen time to get accustomed to the high pressure in _B_, and then themen are lowered to the bottom. Excavated earth is removed in a similarmanner. Air is supplied through a tube _DD_. Such an arrangement forwork under water is called a caisson. It is held in position by a massof concrete _EE_. 

[Illustration: FIG. 146--The principle of work under water. ]

[Illustration: FIG. 147--Showing how men can work under water. ]

In many cases men work in diving suits rather than in caissons; thesesuits are made of rubber except for the head piece, which is of metalprovided with transparent eyepieces. Air is supplied through aflexible tube by a compression pump. The diver sometimes carries onhis back a tank of compressed air, from which the air escapes througha tube to the space between the body and the suit. When the air hasbecome foul, the diver opens a valve in his suit and allows it to passinto the water, at the same time admitting a fresh supply from thetank. The valve opens outward from the body, and hence will allow ofthe exit of air but not of the entrance of water. When the diverceases work and desires to rise to the surface, he signals and isdrawn up by a rope attached to the suit. 

192. Combination of Pumps. In many cases the combined use of bothexhaust and compression pumps is necessary to secure the desiredresult; as, for example, in pneumatic dispatch tubes. These areemployed in the transportation of letters and small packages frombuilding to building or between parts of the same building. A pumpremoves air from the part of the tube ahead of the package, and thusreduces the resistance, while a compression pump forces air into thetube behind the package and thus drives it forward with great speed. 

CHAPTER XIX

THE WATER PROBLEM OF A LARGE CITY

193. It is by no means unusual for the residents of a large city ortown to receive through the newspapers a notification that the citywater supply is running low and that economy should be exercised inits use. The problem of supplying a large city with an abundance ofpure water is among the most difficult tasks which city officials haveto perform, and is one little understood and appreciated by theaverage citizen. 

Intense interest in personal and domestic affairs is natural, butevery citizen, rich or poor, should have an interest in civic affairsas well, and there is no better or more important place to begin thanwith the water supply. One of the most stirring questions in New Yorkto-day has to do with the construction of huge aqueducts designed toconvey to the residents of the city, water from the distant CatskillMountains. The growth of the population has been so phenomenally rapidthat the combined output of all available near-by sources does notsuffice to meet the increasing consumption. 

Where does your city obtain its water? Does it bring it to itsreservoirs in the most economic way possible, and is there anylegitimate excuse for the scarcity of water which many communitiesface in dry seasons?

194. Two Possibilities. Sometimes a city is fortunate enough to besituated near hills and mountains through which streams flow, and inthat case the water problem is simple. In such a case all that isnecessary is to run pipes, usually underground, from the elevatedlakes or streams to the individual houses, or to common reservoirsfrom which it is distributed to the various buildings. 

[Illustration: FIG. 148. --The elevated mountain lake serves as asource of water. ]

Figure 148 illustrates in a simple way the manner in which a mountainlake may serve to supply the inhabitants of a valley. The city ofDenver, for example, is surrounded by mountains abounding in streamsof pure, clear water; pipes convey the water from these heights to thecity, and thus a cheap and adequate flow is obtained. Such a system isknown as the gravity system. The nearer and steeper the elevation, thegreater the force with which the water flows through the valley pipes, and hence the stronger the discharge from the faucets. 

Relatively few cities and towns are so favorably situated as regardswater; more often the mountains are too distant, or the elevation istoo slight, to be of practical value. Cities situated in plains andremote from mountains are obliged to utilize the water of such streamsas flow through the land, forcing it to the necessary height by meansof pumps. Streams which flow through populated regions are apt to becontaminated, and hence water from them requires public filtration. Cities using such a water supply thus have the double expense ofpumping and filtration. 

195. The Pressure of Water. No practical business man would erect aturbine or paddle wheel without calculating in advance the value ofhis water power. The paddle wheel might be so heavy that the streamcould not turn it, or so frail in comparison with the water force thatthe stream would destroy it. In just as careful a manner, the size andthe strength of municipal reservoirs and pumps must be calculated. Thegreater the quantity of water to be held in the reservoir, the heavierare the walls required; the greater the elevation of the houses, thestronger must be the pumps and the engines which run them. 

In order to understand how these calculations are made, we must studythe physical characteristics of water just as we studied the physicalcharacteristics of air. 

When we measure water, we find that 1 cubic foot of it weighs about62. 5 pounds; this is equivalent to saying that water 1 foot deeppresses on the bottom of the containing vessel with a force of 62. 5pounds to the square foot. If the water is 2 feet deep, the loadsupported by the vessel is doubled, and the pressure on each squarefoot of the bottom of the vessel will be 125 pounds, and if the wateris 10 feet deep, the load borne by each square foot will be 625pounds. The deeper the water, the greater will be the weight sustainedby the confining vessel and the greater the pressure exerted by thewater. 

[Illustration: FIG. 149. --Water 1 foot deep exerts a pressure of 62. 5pounds a square foot. ]

Since the pressure borne by 1 square foot of surface is 62. 5 pounds, the pressure supported by 1 square inch of surface is 1/144 of 62. 5pounds, or . 43 pound, nearly 1/2 pound. Suppose a vessel held water tothe depth of 10 feet, then upon every square inch of the bottom ofthat vessel there would be a pressure of 4. 34 pounds. If a one-inchtap were inserted in the bottom of the vessel so that the water flowedout, it would gush forth with a force of 4. 34 pounds. If the waterwere 20 feet deep, the force of the outflowing water would be twice asstrong, because the pressure would be doubled. But the flow would notremain constant, because as the water leaves the outlet, less and lessof it remains in the vessel, and hence the pressure gradually sinksand the flow drops correspondingly. 

In seasons of prolonged drought, the streams which feed a cityreservoir are apt to contain less than the usual amount of water, hence the level of the water supply sinks, the pressure at the outletfalls, and the force of the outflowing water is lessened (Fig. 150). 

[Illustration: FIG. 150. --The pressure at an outlet decreases as thelevel of the water supply sinks. ]

196. Why the Water Supply is not uniform in All Parts of the City. In the preceding Section, we saw that the flow from a faucet dependsupon the height of the reserve water above the tap. Houses on a levelwith the main supply pipes (Figs. 148 and 151) have a strong flowbecause the water is under the pressure of a column _A_; housessituated on elevation _B_ have less flow, because the water is underthe pressure of a shorter column _B_; and houses at a considerableelevation _C_ have a less rapid flow corresponding to the diminisheddepth _(C)_. 

Not only does the flow vary with the elevation of the house, but itvaries with the location of the faucet within the house. Unless thereservoir is very high, or the pumps very powerful, the flow on theupper floors is noticeably less than that in the cellar, and in theupper stories of some high building the flow is scarcely more than afeeble trickle. 

[Illustration: FIG. 151. --Water pressure varies in different parts ofa water system. ]

When the respective flows at _A_, _B_, and _C_ (Fig. 151) are measured, they are found to be far lower than the pressures which columns ofwater of the heights _A_, _B_, and _C_ have been shown by actualdemonstration to exert. This is because water, in flowing from placeto place, expends force in overcoming the friction of the pipes andthe resistance of the air. The greater the distance traversed by thewater in its journey from reservoir to faucet, the greater the wasteforce and the less the final flow. 

In practice, large mains lead from the reservoir to the city, smallermains convey the water to the various sections of the city, andservice pipes lead to the individual house taps. During this longjourney, considerable force is expended against friction, and hencethe flow at a distance from the reservoir falls to but a fraction ofits original strength. For this reason, buildings situated near themain supply have a much stronger flow (Fig. 152) than those on thesame level but remote from the supply. Artificial reservoirs areusually constructed on the near outskirts of a town in order that thefrictional force lost in transmission may be reduced to a minimum. 

[Illustration: FIG. 152. --The more distant the fountain, the weakerthe flow. ]

In the case of a natural reservoir, such as an elevated lake orstream, the distance cannot be planned or controlled. New York, forexample, will secure an abundance of pure water from the CatskillMountains, but it will lose force in transmission. Los Angeles isundertaking one of the greatest municipal projects of the day. Hugeaqueducts are being built which will convey pure mountain water adistance of 250 miles, and in quantities sufficient to supply twomillion people. According to calculations, the force of the water willbe so great that pumps will not be needed. 

197. Why Water does not always flow from a Faucet. Most of us haveat times been annoyed by the inability to secure water on an upperstory, because of the drawing off of a supply on a lower floor. During the working hours of the day, immense quantities of water aredrawn off from innumerable faucets, and hence the quantity in thepipes decreases considerably unless the supply station is able todrive water through the vast network of pipes as fast as it is drawnoff. Buildings at a distance from the reservoir suffer under suchcircumstances, because while the diminished pressure is ordinarilypowerful enough to supply the lower floors, it is frequently too weakto force a continuous stream to high levels. At night, however, andout of working hours, few faucets are open, less water is drawn off atany one time, and the intricate pipes are constantly full of waterunder high pressure. At such times, a good flow is obtainable even onthe uppermost floors. 

In order to overcome the disadvantage of a decrease in flow during theday, standpipes (Fig. 153) are sometimes placed in various sections. These are practically small steel reservoirs full of water andconnecting with the city pipes. During "rush" hours, water passes fromthese into the communicating pipes and increases the available supply, while during the night, when the faucets are turned off, wateraccumulates in the standpipe against the next emergency (Figs. 151 and154). The service rendered by the standpipe is similar to that of theair cushion discussed in Section 184. 

[Illustration: FIG. 153. --A standpipe. ]

198. The Cost of Water. In the gravity system, where an elevatedlake or stream serves as a natural reservoir, the cost of the city'swaterworks is practically limited to the laying of pipes. But when thesource of the supply is more or less on a level with the surroundingland, the cost is great, because the supply for the entire city musteither be pumped into an artificial reservoir, from which it can bedistributed, or else must be driven directly through the mains (Fig. 154). 

[Illustration: FIG. 154. --Water must be got to the houses by means ofpumps. ]

A gallon of water weighs approximately 8. 3 pounds, and hence the workdone by a pump in raising a gallon of water to the top of an averagehouse, an elevation of 50 feet, is 8. 3 × 50, or 415 foot pounds. Asmall manufacturing town uses at least 1, 000, 000 gallons daily, andthe work done by a pump in raising that amount to an elevation of 50feet would be 8. 3 × 1, 000, 000 × 50, or 415, 000, 000 foot pounds. 

The total work done during the day by the pump, or the engine drivingthe pump, is 415, 000, 000 foot pounds, and hence the work done duringone hour would be 1/24 of 415, 000, 000, or 17, 291, 666 foot pounds; thework done in one minute would be 1/60 of 17, 291, 666, or 288, 194 footpounds, and the work done each second would be 1/60 of 288, 194, or4803 foot pounds. 

A 1-H. P. Engine does 550 foot pounds of work each second, andtherefore if the pump is to be operated by an engine, the strength ofthe latter would have to be 8. 7 H. P. An 8. 7-H. P. Pumping engineworking at full speed every second of the day and night would be ableto supply the town with the necessary amount of water. When, however, we consider the actual height to which the water is raised above thepumping station, and the extra pumping which must be done in order tobalance the frictional loss, it is easy to understand that in actualpractice a much more powerful engine would be needed. The larger thepiston and the faster it works, the greater is the quantity of waterraised at each stroke, and the stronger must be the engine whichoperates the pump. 

In many large cities there is no one single pumping station from whichsupplies run to all parts of the city, but several pumping stationsare scattered throughout the city, and each of them supplies arestricted territory. 

199. The Bursting of Dams and Reservoirs. The construction of a safereservoir is one of the most important problems of engineers. InOctober, 1911, a town in Pennsylvania was virtually wiped out ofexistence because of the bursting of a dam whose structure was ofinsufficient strength to resist the strain of the vast quantity ofwater held by it. A similar breakage was the cause of the fatalJohnstown flood in 1889, which destroyed no less than seven towns, andin which approximately 2000 persons are said to have lost their lives. 

Water presses not only on the bottom of a vessel, but upon the sidesas well; a bucket leaks whether the hole is in its side or its bottom, showing that water presses not only downward but outward. Usually aleak in a dam or reservoir occurs near the bottom. Weak spots at thetop are rare and easily repaired, but a leak near the bottom isusually fatal, and in the case of a large reservoir the outflowingwater carries death and destruction to everything in its path. 

If the leak is near the surface, as at _a_ (Fig. 155), the waterissues as a feeble stream, because the pressure against the sides atthat level is due solely to the relatively small height of waterabove _a_ (Section 195). If the leak is lower, as at _b_, the issuingstream is stronger and swifter, because at that level the outwardpressure is much greater than at _a_, the increase being due to thefact that the height of the water above _b_ is greater than that above_a_. If the leak is quite low, as at _c_, the issuing stream has astill greater speed and strength, and gushes forth with a forcedetermined by the height of the water above _c_. 

[Illustration: FIG. 155. --The flow from an opening depends upon theheight of water above the opening. ]

The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, andso great was the force and speed of the escaping stream that within anhour after the break had occurred, the water had traveled a distanceof 18 miles, and had destroyed property to the value of millions ofdollars. 

If a reservoir has a depth of 100 feet, the pressure exerted upon eachsquare foot of its floor is 62. 5 × 100, or 6250 pounds; the weighttherefore to be sustained by every square foot of the reservoir flooris somewhat more than 3 tons, and hence strong foundations areessential. The outward lateral pressure at a depth of 25 feet would beonly one fourth as great as that on the bottom--hence the strain onthe sides at that depth would be relatively slight, and a lesspowerful construction would suffice. But at a depth of 50 feet thepressure on the sides would be one half that of the floor pressure, or1-1/2 tons. At a depth of 75 feet, the pressure on the sides would bethree quarters that on the bottom, or 2-1/4 tons. As the bottom of thereservoir is approached, the pressure against the sides increases, andmore powerful construction becomes necessary. 

Small elevated tanks, like those of the windmill, frequently haveheavy iron bands around their lower portion as a protection againstthe extra strain. 

Before erecting a dam or reservoir, the maximum pressure to be exertedupon every square inch of surface should be accurately calculated, andthe structure should then be built in such a way that the varyingpressure of the water can be sustained. It is not sufficient that thebottom be strong; the sides likewise must support their strain, andhence must be increased in strength with depth. This strengthening ofthe walls is seen clearly in the reservoir shown in Figure 152. Thebursting of dams and reservoirs has occasioned the loss of so manylives, and the destruction of so much property, that some states areconsidering the advisability of federal inspection of all suchstructures. 

[Illustration: FIG. 156. --The lock gates must be strong in order towithstand the great pressure of the water against them. ]

200. The Relation of Forests to the Water Supply. When heavy rainsfall on a bare slope, or when snow melts on a barren hillside, a smallamount of the water sinks into the ground, but by far the greater partof it runs off quickly and swells brooks and streams, thus causingfloods and freshets. 

When, however, rain falls on a wooded slope, the action is reversed; asmall portion runs off, while the greater portion sinks into the softearth. This is due partly to the fact that the roots of trees by theirconstant growth keep the soil loose and open, and form channels, as itwere, along which the water can easily run. It is due also to thepresence on the ground of decaying leaves and twigs, or humus. Thedecaying vegetable matter which covers the forest floor acts more orless as a sponge, and quickly absorbs falling rain and melting snow. The water which thus passes into the humus and the soil beneath doesnot remain there, but slowly seeps downward, and finally after weeksand months emerges at a lower level as a stream. Brooks and springsformed in this way are constant feeders of rivers and lakes. 

In regions where the land has been deforested, the rivers run low inseason of prolonged drought, because the water which should haveslowly seeped through the soil, and then supplied the rivers for weeksand months, ran off from the barren slopes in a few days. 

Forests not only lessen the danger of floods, but they conserve ourwaterways, preventing a dangerous high-water mark in the season ofheavy rains and melting snows, and then preventing a shrinkage in dryseasons when the only feeders of the rivers are the undergroundsources. In the summer of 1911, prolonged drought in North Carolinalowered the rivers to such an extent that towns dependent upon themsuffered greatly. The city of Charlotte was reduced for a time to apractically empty reservoir; washing and bathing were eliminated, machinery dependent upon water-power and steam stood idle, and everyglass of water drunk was carefully reckoned. Thousands of gallons ofwater were brought in tanks from neighboring cities, and were emptiedinto the empty reservoir from whence it trickled slowly through thecity mains. The lack of water caused not only personal inconvenienceand business paralysis, but it occasioned real danger of diseasethrough unflushed sewers and insufficiently drained pipes. 

The conservation of the forest means the conservation of ourwaterways, whether these be used for transportation or as sources ofdrinking water. 

CHAPTER XX

MAN'S CONQUEST OF SUBSTANCES

201. Chemistry. Man's mechanical inventions have been equaled by hischemical researches and discoveries, and by the application he hasmade of his new knowledge. 

The plain cotton frock of our grandmothers had its death knell soundeda few years ago, when John Mercer showed that cotton fabrics soaked incaustic soda assumed under certain conditions a silky sheen, and whendyed took on beautiful and varied hues. The demonstration of thissimple fact laid the foundation for the manufacture of a vast varietyof attractive dress materials known as mercerized cotton. 

Possibly no industry has been more affected by chemical discovery thanthat of dyeing. Those of us who have seen the old masterpieces inpainting, or reproductions of them, know the softness, the mellowness, the richness of tints employed by the old masters. But if we look forthe brilliancy and variety of color seen in our own day, the searchwill be fruitless, because these were unknown until a half centuryago. Up to that time, dyes were few in number and were extractedsolely from plants, principally from the indigo and madder plants. Butabout the year 1856 it was discovered that dyes in much greatervariety and in purer form could be obtained from coal tar. Thischemical production of dyes has now largely supplanted the originalmethod, and the industry has grown so rapidly that a single firmproduced in one year from coal tar a quantity of indigo dye whichunder the natural process of plant extraction would have required aquarter million acres of indigo plant. 

The abundance and cheapness of newspapers, coarse wrapping papers, etc. , is due to the fact that man has learned to substitute wood forrags in the manufacture of paper. Investigation brought out the factthat wood contained the substance which made rags valuable for papermaking. Since the supply of rags was far less than the demand, theproblem of the extraction from wood of the paper-forming substance wasa vital one. From repeated trials, it was found that caustic soda whenheated with wood chips destroyed everything in the wood except thedesired substance, cellulose; this could be removed, bleached, dried, and pressed into paper. The substitution of wood for rags has madepossible the daily issue of newspapers, for the making of whichsufficient material would not otherwise have been available. When wereflect that a daily paper of wide circulation consumes ten acres ofwood lot per day, we see that all the rags in the world would beinadequate to meet this demand alone, to say nothing of periodicals, books, tissue paper, etc. 

Chemistry plays a part in every phase of life; in the arts, theindustries, the household, and in the body itself, where digestion, excretion, etc. , result from the action of the bodily fluids uponfood. The chemical substances of most interest to us are those whichaffect us personally rather than industrially; for example, soap, which cleanses our bodies, our clothing, our household possessions;washing soda, which lightens laundry work; lye, which clears out thedrain pipe clogged with grease; benzine, which removes stains fromclothing; turpentine, which rids us of paint spots left by carelessworkmen; and hydrogen peroxide, which disinfects wounds and sores. 

In order to understand the action of several of these substances wemust study the properties of two groups of chemicals--knownrespectively as acids and bases; the first of these may be representedby vinegar, sulphuric acid, and oxalic acid; and the second, byammonia, lye, and limewater. 

202. Acids. All of us know that vinegar and lemon juice have a sourtaste, and it is easy to show that most acids are characterized by asour taste. If a clean glass rod is dipped into very dilute acid, suchas acetic, sulphuric, or nitric acid, and then lightly touched to thetongue, it will taste sour. But the best test of an acid is by sightrather than by taste, because it has been found that an acid is ableto discolor a plant substance called litmus. If paper is soaked in alitmus solution until it acquires the characteristic blue hue of theplant substance, and is then dried thoroughly, it can be used todetect acids, because if it comes in contact with even the minutesttrace of acid, it loses its blue color and assumes a red tint. Hence, in order to detect the presence of acid in a substance, one has merelyto put some of the substance on blue litmus paper, and note whether ornot the latter changes color. This test shows that many of our commonfoods contain some acid; for example, fruit, buttermilk, sour bread, and vinegar. 

The damage which can be done by strong acids is well known; if a jarof sulphuric acid is overturned, and some of it falls on the skin, iteats its way into the flesh and leaves an ugly sore; if it falls oncarpet or coat, it eats its way into the material and leaves anunsightly hole. The evil results of an accident with acid can belessened if we know just what to do and do it quickly, but for this wemust have a knowledge of bases, the second group of chemicals. 

203. Bases. Substances belonging to this group usually have a bittertaste and a slimy, soapy feeling. For our present purposes, the mostimportant characteristic of a base is that it will neutralize an acidand in some measure hinder the damage effected by the former. If, assoon as an acid has been spilled on cloth, a base, such as ammonia, isapplied to the affected region, but little harm will be done. In yourlaboratory experiments you may be unfortunate enough to spill acid onyour body or clothing; if so, quickly apply ammonia. If you delay, theacid does its work, and there is no remedy. If soda (a base) touchesblack material, it discolors it and leaves an ugly brown spot; but theapplication of a little acid, such as vinegar or lemon juice, willoften restore the original color and counteract the bad effects of thebase. Limewater prescribed by physicians in cases of illness is awell-known base. This liquid neutralizes the too abundant acidspresent in a weak system and so quiets and tones the stomach. 

The interaction of acids and bases may be observed in another way. Ifblue litmus paper is put into an acid solution, its color changes tored; if now the red litmus paper is dipped into a base solution, caustic soda, for example, its original color is partially restored. What the acid does, the base undoes, either wholly or in part. Basesalways turn red litmus paper blue. 

Bases, like acids, are good or bad according to their use; if theycome in contact with cloth, they eat or discolor it, unlessneutralized by an acid. But this property of bases, harmful in oneway, is put to advantage in the home, where grease is removed fromdrainpipe and sink by the application of lye, a strong base. If thelye is too concentrated, it will not only eat the grease, but willcorrode the metal piping; it is easy, however, to dilute basesolutions to such a degree that they will not affect piping, but willremove grease. Dilute ammonia is used in almost every home and is anindispensable domestic servant; diluted sufficiently, it isinvaluable in the washing of delicate fabrics and in the removing ofstains, and in a more concentrated form it is helpful as a smellingsalt in cases of fainting. 

Some concentrated bases are so powerful in their action on grease, cloth, and metal that they have received the designation _caustic_, and are ordinarily known as caustic soda, caustic potash (lye), andcaustic lime. These more active bases are generally called alkalies indistinction from the less active ones. 

204. Neutral Substances. To any acid solution add gradually a smallquantity of a base, and test the mixture from time to time with bluelitmus paper; at first the paper will turn red quickly, but as moreand more of the base is added to the solution, it has less and lesseffect on the blue litmus paper, and finally a point is reached when afresh strip of blue paper will not be affected. Such a resultindicates infallibly the absence of any acid qualities in thesolution. If now red litmus paper is tested in the same solution, itscolor also will remain unchanged; such a result indicates infalliblythe absence of any basic quality. The solution has the characteristicproperty of neither acid nor base and is said to be neutral. 

If to the neutral solution an extra portion of base is added, so thatthere is an excess of base over acid, the neutralization isoverbalanced and the red paper turns blue. If to the neutral solutionan extra portion of acid is added, so that there is an excess of acidover base, the neutralization is overbalanced in the oppositedirection, and the solution acquires acid characteristics. 

Most acids and bases will eat and corrode and discolor, while neutralsubstances will not; it is for this reason that soap, a slightlyalkaline substance, is the safest cleansing agent for laundry, bath, and general work. Good soaps, being carefully made, are so nearlyneutral that they will not fade the color out of clothing; the cheapsoaps are less carefully prepared and are apt to have a strong excessof the base ingredient; such soaps are not safe for delicate work. 

205. Soap. If we gather together scrapings of lard, butter, bits oftallow from burned-out candles, scraps of waste fat, or any other sortof grease, and pour a strong solution of lye over the mass, a softsoapy substance is formed. In colonial times, every family made itsown supply of soap, utilizing, for that purpose, household scrapsoften regarded by the housekeeper of to-day as worthless. Grease andfat were boiled with water and hardwood ashes, which are rich in lye, and from the mixture came the soft soap used by our ancestors. Inpractice, the wood ashes were boiled in water, which was then strainedoff, and the resulting filtrate, or lye, was mixed with the fats forsoap making. 

Most fats contain a substance of an acid nature, and are decomposed bythe action of bases such as caustic soda and caustic potash. The acidcomponent of the grease partially neutralizes the base, and a newsubstance is formed, namely, soap. 

With the advance of civilization the labor of soap making passed fromthe home to the factory, very much as bread making has done in our ownday. Different varieties of soaps appeared, of which the hard soap wasthe most popular, owing to the ease with which it could betransported. Within the last few years liquid soaps have come intofavor, especially in schools, railroad stations, and other publicplaces, where a cake of soap would be handled by many persons. Bymeans of a simple device (Fig. 157), the soap escapes from areceptacle when needed. The mass of the soap does not come in contactwith the skin, and hence the spread of contagious skin diseases islessened. 

[Illustration: FIG. 157. --Liquid soap container. ]

Commercial soaps are made from a great variety of substances, such astallow, lard, castor oil, coconut oil, olive oil, etc. ; or in cheapersoaps, from rosin, cottonseed oil, and waste grease. The fats which goto waste in our garbage could be made a source of income, not only tothe housewife, but to the city. In Columbus, Ohio, garbage is used asa source of revenue; the grease from the garbage being sold for soapmaking, and the tankage (Section 188) for fertilizer. 

206. Why Soap Cleans. The natural oil of the skin catches andretains dust and dirt, and makes a greasy film over the body. Thiscannot be removed by water alone, but if soap is used and a generouslather is applied to the skin, the dirt is "cut" and passes from thebody into the water. Soap affects a grease film and water very much asthe white of an egg affects oil and water. These two liquids alone donot mix, the oil remaining separate on the surface of the water; butif a small quantity of white of egg is added, an emulsion is formed, the oil separating into minute droplets which spread through thewater. In the same way, soap acts on a grease film, separating it intominute droplets which leave the skin and spread through the water, carrying with them the dust and dirt particles. The warmer the water, the better will be the emulsion, and hence the more effective theremoval of dirt and grease. This explanation holds true for theremoval of grease from any surface, whether of the body, clothing, furniture, or dishes. 

207. Washing Powders. Sometimes soap refuses to form a lather andinstead cakes and floats as a scum on the top of the water; this isnot the fault of the soap but of the water. As water seeps throughthe soil or flows over the land, it absorbs and retains various soilconstituents which modify its character and, in some cases, render italmost useless for household purposes. Most of us are familiar withthe rain barrel of the country house, and know that the housewifeprefers rain water for laundry and general work. Rain water, coming asit does from the clouds, is free from the chemicals gathered by groundwater, and is hence practically pure. While foreign substances do notnecessarily injure water for drinking purposes (Section 69), they areoften of such a nature as to prevent soap from forming an emulsion, and hence from doing its work. Under such circumstances the water issaid to be hard, and soap used with it is wasted. Even if water isonly moderately hard, much soap is lost. The substances which makewater hard are calcium and magnesium salts. When soap is put intowater containing one or both of these, it combines with the salts toform sticky insoluble scum. It is therefore not free to form anemulsion and to remove grease. As a cleansing agent it is valueless. The average city supply contains so little hardness that it issatisfactory for toilet purposes; but in the laundry, where there isneed for the full effect of the soap, and where the slightest losswould aggregate a great deal in the course of time, something must bedone to counteract the hardness. The addition of soda, or sodiumcarbonate to the water will usually produce the desired effect. Washing soda combines with calcium and magnesium and prevents themfrom uniting with soap. The soap is thus free to form an emulsion, just as in ordinary water. Washing powders are sometimes used insteadof washing soda. Most washing powders contain, in addition to asoftening agent, some alkali, and hence a double good is obtained fromtheir use; they not only soften the water and allow the soap to forman emulsion, but they also, through their alkali content, cut thegrease and themselves act as cleansers. In some cities where the wateris very hard, as in Columbus, Ohio, it is softened and filtered atpublic expense, before it leaves the reservoirs. But even under thesecircumstances, a moderate use of washing powder is general in laundrywork. 

If washing powder is put on clothes dry, or is thrown into a crowdedtub, it will eat the clothes before it has a chance to dissolve in thewater. The only safe method is to dissolve the powder before theclothes are put into the tub. The trouble with our public laundries isthat many of them are careless about this very fact, and do not taketime to dissolve the powder before mixing it with the clothes. 

The strongest washing powder is soda, and this cheap form is as goodas any of the more expensive preparations sold under fancy names. Borax is a milder powder and is desirable for finer work. 

One of the most disagreeable consequences of the use of hard water forbathing is the unavoidable scum which forms on the sides of bathtuband washbowl. The removal of the caked grease is difficult, and ifsoap alone is used, the cleaning of the tub requires both patience andhard scrubbing. The labor can be greatly lessened by moistening thescrubbing cloth with turpentine and applying it to the greasy film, which immediately dissolves and thus can be easily removed. Thepresence of the scum can be largely avoided by adding a small amountof liquid ammonia to the bath water. But many persons object to this;hence it is well to have some other easy method of removing theobjectionable matter. 

208. To remove Stains from Cloth. While soap is, generally speaking, the best cleansing agent, there are occasions when other substancescan be used to better advantage. For example, grease spots on carpetand non-washable dress goods are best removed by the application ofgasoline or benzine. These substances dissolve the grease, but do notremove it from the clothing; for that purpose a woolen cloth should belaid under the stain in readiness to absorb the benzine and the greasedissolved in it. If the grease is not absorbed while in solution, itremains in the clothing and after the evaporation of the benzinereappears in full force. 

Cleaners frequently clean suits by laying a blotter over a grease spotand applying a hot iron; the grease, when melted by the heat, takesthe easiest way of spreading itself and passes from cloth to blotter. 

209. Salts. A neutral liquid formed as in Section 204, by the actionof hydrochloric acid and the alkali solution of caustic soda, has abrackish, salty taste, and is, in fact, a solution of salt. This canbe demonstrated by evaporating the neutral liquid to dryness andexamining the residue of solid matter, which proves to be common salt. 

When an acid is mixed with a base, the result is a substance more orless similar in its properties to common salt; for this reason allcompounds formed by the neutralization of an acid with a base arecalled salts. If, instead of hydrochloric acid (HCl), we use an acidsolution of potassium tartrate, and if instead of caustic soda we usebicarbonate of soda (baking soda), the result is a brackish liquid asbefore, but the salt in the liquid is not common salt, but Rochellesalt. Different combinations of acids and bases produce differentsalts. Of all the vast group of salts, the most abundant as well asthe most important is common salt, known technically as sodiumchloride because of its two constituents, sodium and chlorine. 

We are not dependent upon neutralization for the enormous quantitiesof salt used in the home and in commerce. It is from the active, restless seas of the present, and from the dead seas of theprehistoric past that our vast stores of salt come. The waters of theMediterranean and of our own Great Salt Lake are led into shallowbasins, where, after evaporation by the heat of the sun, they leave aresidue of salt. By far the largest quantity of salt, however, comesfrom the seas which no longer exist, but which in far remote agesdried up and left behind them their burden of salt. Deposits of saltformed in this way are found scattered throughout the world, and inour own country are found in greatest abundance in New York. Thelargest salt deposit known has a depth of one mile and exists inGermany. 

Salt is indispensable on our table and in our kitchen, but the amountof salt used in this way is far too small to account for a yearlyconsumption of 4, 000, 000 tons in the United States alone. Themanufacture of soap, glass, bleaching powders, baking powders, washingsoda, and other chemicals depends on salt, and it is for these thatthe salt beds are mined. 

210. Baking Soda. Salt is by all odds the most important sodiumcompound. Next to it come the so-called carbonates: first, sodiumcarbonate, which is already familiar to us as washing soda; andsecond, sodium bicarbonate, which is an ingredient of baking powders. These are both obtained from sodium chloride by relatively simplemeans; that is, by treating salt with the base, ammonia, and withcarbon dioxide. 

Washing soda has already been discussed. Since baking powders in someform are used in almost all homes for the raising of cake and pastrydough, it is essential that their helpful and harmful qualities beclearly understood. 

The raising of dough by means of baking soda--bicarbonate of soda--isa very simple process. When soda is heated, it gives off carbondioxide gas; you can easily prove this for yourself by burning alittle soda in a test tube, and testing the escaping gas in a testtube of limewater. When flour and water alone are kneaded and bakedin loaves, the result is a mass so compact and hard that human teethare almost powerless to crush and chew it. The problem is to separatethe mass of dough or, in other words, to cause it to rise and lighten. This can be done by mixing a little soda in the flour, because theheat of the oven causes the soda to give off bubbles of gas, and thesein expanding make the heavy mass slightly porous. Bread is neverlightened with soda because the amount of gas thus given off is toosmall to convert heavy compact bread dough into a spongy mass; butbiscuit and cake, being by nature less compact and heavy, aresufficiently lightened by the gas given off from soda. 

But there is one great objection to the use of soda alone as aleavening agent. After baking soda has lost its carbon dioxide gas, itis no longer baking soda, but is transformed into its relative, washing soda, which has a disagreeable taste and is by no meansdesirable for the stomach. 

Man's knowledge of chemicals and their effect on each other hasenabled him to overcome this difficulty and, at the same time, toretain the leavening effect of the baking soda. 

211. Baking Powders. If some cooking soda is put into lemon juice orvinegar, or any acid, bubbles of gas immediately form and escape fromthe liquid. After the effervescence has ceased, a taste of the liquidwill show you that the lemon juice has lost its acid nature, and hasacquired in exchange a salty taste. Baking soda, when treated with anacid, is transformed into carbon dioxide and a salt. The variousbaking powders on the market to-day consist of baking soda and someacid substance, which acts upon the soda, forces it to give up itsgas, and at the same time unites with the residue to form a harmlesssalt. 

Cream of tartar contains sufficient acid to act on baking soda, and isa convenient and safe ingredient for baking powder. When soda andcream of tartar are mixed dry, they do not react on each other, neither do they combine rapidly in _cold_ moist dough, but as soon asthe heat of the oven penetrates the doughy mass, the cream of tartarcombines with the soda and sets free the gas needed to raise thedough. The gas expands with the heat of the oven, raising the doughstill more. Meanwhile, the dough itself is influenced by the heat andis stiffened to such an extent that it retains its inflated shape andspongy nature. 

Many housewives look askance at ready-made baking powders and preferto bake with soda and sour milk, soda and buttermilk, or soda andcream of tartar. Sour milk and buttermilk are quite as good as creamof tartar, because the lactic acid which they contain combines withthe soda and liberates carbon dioxide, and forms a harmless residue inthe dough. 

The desire of manufacturers to produce cheap baking powders led to theuse of cheap acids and alkalies, regardless of the character of theresulting salt. Alum and soda were popular for some time; but carefulexamination proved that the particular salt produced by thiscombination was not readily absorbed by the stomach, and that itsretention there was injurious to health. For this reason, many stateshave prohibited the use of alum in baking powders. 

It is not only important to choose the ingredients carefully; it isalso necessary to calculate the respective quantities of each, otherwise there will be an excess of acid or alkali for the stomach totake care of. A standard powder contains twice as much cream of tartaras of bicarbonate of soda, and the thrifty housewife who wishes toeconomize, can make for herself, at small cost, as good a bakingpowder as any on the market, by mixing tartar and soda in the aboveproportions and adding a little corn starch to keep the mixture dry. 

The self-raising flour, so widely advertised by grocers, is flour inwhich these ingredients or their equivalent have been mixed by themanufacturer. 

212. Soda Mints. Bicarbonate of soda is practically the soleingredient of the soda mints popularly sold for indigestion. Thesecorrect a tendency to sour stomach because they counteract the surplusacid in the stomach, and form with it a safe neutral substance. 

Seidlitz powder is a simple remedy consisting of two powders, onecontaining bicarbonate of soda, and the other, some acid such as creamof tartar. When these substances are dissolved in water and mixed, effervescence occurs, carbon dioxide escapes, and a solution ofRochelle salt remains. 

212_a_. Source of Soda. An enormous quantity of sodium carbonate, orsoda, as it is usually called, is needed in the manufacture of glass, soap, bleaching powders, and other commercial products. Formerly, thesupply of soda was very limited because man was dependent upon naturaldeposits and upon ashes of sea plants for it. Common salt, sodiumchloride, is abundant, and in 1775 a prize was offered to any one whowould find a way to obtain soda from salt. As a result of this, sodawas soon manufactured from common salt. In the most recent methods ofmanufacture, salt, water, ammonia, and carbon dioxide are made toreact. Baking soda is formed from the reaction. The baking soda isthen heated and decomposed into washing soda or the soda of commerce. 

CHAPTER XXI

FERMENTATION

213. While baking powder is universally used for biscuits and cake, it is seldom, if ever, used for bread, because it does not furnishsufficient gas to lighten the tough heavy mass of bread dough. Then, too, most people prefer the taste of yeast-raised bread. There is areason for this widespread preference, but to understand it, we mustgo somewhat far afield, and must study not only the bread of to-day, but the bread of antiquity, and the wines as well. 

If grapes are crushed, they yield a liquid which tastes like thegrapes; but if the liquid is allowed to stand in a warm place, itloses its original character, and begins to ferment, becoming, in thecourse of a few weeks, a strongly intoxicating drink. This is true notonly of grape juice but also of the juice of all other sweet fruits;apple juice ferments to cider, currant juice to currant wine, etc. This phenomenon of fermentation is known to practically all races ofmen, and there is scarcely a savage tribe without some kind offermented drink; in the tropics the fermented juice of the palm treeserves for wine; in the desert regions, the fermented juice of thecentury plant; and in still other regions, the root of the gingerplant is pressed into service. 

The fermentation which occurs in bread making is similar to that whichis responsible for the transformation of plant juices intointoxicating drinks. The former process is not so old, however, sincethe use of alcoholic beverages dates back to the very dawn of history, and the authentic record of raised or leavened bread is but littlemore than 3000 years old. 

214. The Bread of Antiquity. The original method of bread making andthe method employed by savage tribes of to-day is to mix crushed grainand water until a paste is formed, and then to bake this over a campfire. The result is a hard compact substance known as unleavenedbread. A considerable improvement over this tasteless mass isself-raised bread. If dough is left standing in a warm place a numberof hours, it swells up with gas and becomes porous, and when baked, isless compact and hard than the savage bread. Exposure to air andwarmth brings about changes in dough as well as in fruit juices, andalters the character of the dough and the bread made from it. Breadmade in this way would not seem palatable to civilized man of thepresent day, accustomed, as he is, to delicious bread made light andporous by yeast; but to the ancients, the least softening andlightening was welcome, and self-fermented bread, therefore, supplanted the original unleavened bread. 

Soon it was discovered that a pinch of this fermented dough acted as astarter on a fresh batch of dough. Hence, a little of the fermenteddough was carefully saved from a batch, and when the next bread wasmade, the fermented dough, or leaven, was worked into the fresh doughand served to raise the mass more quickly and effectively than mereexposure to air and warmth could do in the same length of time. Thisuse of leaven for raising bread has been practiced for ages. 

Grape juice mixed with millet ferments quickly and strongly, and theRomans learned to use this mixture for bread raising, kneading a verysmall amount of it through the dough. 

215. The Cause of Fermentation. Although alcoholic fermentation, andthe fermentation which goes on in raising dough, were known andutilized for many years, the cause of the phenomenon was a sealed bookuntil the nineteenth century. About that time it was discovered, through the use of the microscope, that fermenting liquids contain anarmy of minute plant organisms which not only live there, but whichactually grow and multiply within the liquid. For growth andmultiplication, food is necessary, and this the tiny plants get inabundance from the fruit juices; they feed upon the sugary matter andas they feed, they ferment it, changing it into carbon dioxide andalcohol. The carbon dioxide, in the form of small bubbles, passes offfrom the fermenting mass, while the alcohol remains in the liquid, giving the stimulating effect desired by imbibers of alcoholic drinks. The unknown strange organisms were called yeast, and they were thestarting point of the yeast cakes and yeast brews manufactured to-dayon a large scale, not only for bread making but for the commercialproduction of beer, ale, porter, and other intoxicating drinks. 

The grains, rye, corn, rice, wheat, from which meal is made, containonly a small quantity of sugar, but, on the other hand, they contain alarge quantity of starch which is easily convertible into sugar. Uponthis the tiny yeast plants in the dough feed, and, as in the case ofthe wines, ferment the sugar, producing carbon dioxide and alcohol. The dough is thick and sticky and the gas bubbles expand it into aspongy mass. The tiny yeast plants multiply and continue to makealcohol and gas, and in consequence, the dough becomes lighter andlighter. When it has risen sufficiently, it is kneaded and placed inan oven; the heat of the oven soon kills the yeast plants and drivesthe alcohol out of the bread; at the same time it expands theimprisoned gas bubbles and causes them to lighten and swell the breadstill more. Meanwhile, the dough has become stiff enough to supportitself. The result of the fermentation is a light, spongy loaf. 

216. Where does Yeast come From? The microscopic plants which wecall yeast are widely distributed in the air, and float around thereuntil chance brings them in contact with a substance favorable totheir growth, such as fruit juices and moist warm batter. Under thefavorable conditions of abundant moisture, heat, and food, they growand multiply rapidly, and cause the phenomenon of fermentation. Wildyeast settles on the skin of grapes and apples, but since it does nothave access to the fruit juices within, it remains inactive very muchas a seed does before it is planted. But when the fruit is crushed, the yeast plants get into the juice, and feeding on it, grow andmultiply. The stray yeast plants which get into the sirup arerelatively few, and hence fermentation is slow; it requires severalweeks for currant wine to ferment, and several months for the juice ofgrapes to be converted into wine. 

Stray yeast finds a favorable soil for growth in the warmth andmoisture of a batter; but although the number of these stray plants isvery large, it is insufficient to cause rapid fermentation, and if wedepended upon wild yeast for bread raising, the result would not be toour liking. 

When our remote ancestors saved a pinch of dough as leaven for thenext baking, they were actually cultivating yeast, although they didnot know it. The reserved portion served as a favorable breeding placeto the yeast plants within it; they grew and reproduced amazingly, andbecame so numerous, that the small mass of old dough in which theywere gathered served to leaven the entire batch at the next baking. 

As soon as man learned that yeast plants caused fermentation inliquors and bread, he realized that it would be to his advantage tocultivate yeast and to add it to bread and to plant juices rather thanto depend upon accidental and slow fermentation from wild yeast. Shortly after the discovery of yeast in the nineteenth century, mancommenced his attempt to cultivate the tiny organisms. Theirmicroscopic size added greatly to his trouble, and it was only afteryears of careful and tedious investigation that he was able to perfectthe commercial yeast cakes and yeast brews universally used by bakersand brewers. The well-known compressed yeast cake is simply a mass oflive and vigorous yeast plants, embedded in a soft, soggy material, and ready to grow and multiply as soon as they are placed under properconditions of heat, moisture, and food. Seeds which remain on ourshelves do not germinate, but those which are planted in the soil do;so it is with the yeast plants. While in the cake they are as lifelessas the seed; when placed in dough, or fruit juice, or grain water, they grow and multiply and cause fermentation. 

CHAPTER XXII

BLEACHING

217. The beauty and the commercial value of uncolored fabrics dependupon the purity and perfection of their whiteness; a man's whitecollar and a woman's white waist must be pure white, without theslightest tinge of color. But all natural fabrics, whether they comefrom plants, like cotton and linen, or from animals, like wool andsilk, contain more or less coloring matter, which impairs thewhiteness. This coloring not only detracts from the appearance offabrics which are to be worn uncolored, but it seriously interfereswith the action of dyes, and at times plays the dyer strange tricks. 

Natural fibers, moreover, are difficult to spin and weave unless somesoftening material such as wax or resin is rubbed lightly over them. The matter added to facilitate spinning and weaving generally detractsfrom the appearance of the uncolored fabric, and also interferes withsuccessful dyeing. Thus it is easy to see that the natural coloringmatter and the added foreign matter must be entirely removed fromfabrics destined for commercial use. Exceptions to this general factare sometimes made, because unbleached material is cheaper and moredurable than the bleached product, and for some purposes is entirelysatisfactory; unbleached cheesecloth and sheeting are frequentlypurchased in place of the more expensive bleached material. Formerly, the only bleaching agent known was the sun's rays, and linen andcotton were put out to sun for a week; that is, the unbleachedfabrics were spread on the grass and exposed to the bleaching actionof sun and dew. 

[Illustration: FIG. 158. --Preparing chlorine from hydrochloric acidand manganese dioxide. ]

218. An Artificial Bleaching Agent. While the sun's rays areeffective as a bleaching agent, the process is slow; moreover, itwould be impossible to expose to the sun's rays the vast quantity offabrics used in the civilized world of to-day, and the huge andnumerous bolts of material which daily come from our looms andfactories must therefore be whitened by artificial means. Thesubstance almost universally used as a rapid artificial bleachingagent is chlorine, best known to us as a constituent of common salt. Chlorine is never free in nature, but is found in combination withother substances, as, for example, in combination with sodium in salt, or with hydrogen in hydrochloric acid. 

The best laboratory method of securing free chlorine is to heat in awater bath a mixture of hydrochloric acid and manganese dioxide, acompound containing one part of manganese and two parts of oxygen. Theheat causes the manganese dioxide to give up its oxygen, whichimmediately combines with the hydrogen of the hydrochloric acid andforms water. The manganese itself combines with part of the chlorineoriginally in the acid, but not with all. There is thus some freechlorine left over from the acid, and this passes off as a gas and canbe collected, as in Figure 158. Free chlorine is heavier than air, andhence when it leaves the exit tube it settles at the bottom of thejar, displacing the air, and finally filling the bottle. 

Chlorine is a very active substance and combines readily with mostsubstances, but especially with hydrogen; if chlorine comes in contactwith steam, it abstracts the hydrogen and unites with it to formhydrochloric acid, but it leaves the oxygen free and uncombined. Thistendency of chlorine to combine with hydrogen makes it valuable as ableaching agent. In order to test the efficiency of chlorine as ableaching agent, drop a wet piece of colored gingham or calico intothe bottle of chlorine, and notice the rapid disappearance of colorfrom the sample. If unbleached muslin is used, the moist strip losesits natural yellowish hue and becomes a clear, pure white. Theexplanation of the bleaching power of chlorine is that the chlorinecombines with the hydrogen of the water and sets oxygen free; theuncombined free oxygen oxidizes the coloring matter in the cloth anddestroys it. 

Chlorine has no effect on dry material, as may be seen if we put drygingham into the jar; in this case there is no water to furnishhydrogen for combination with the chlorine, and no oxygen to be setfree. 

219. Bleaching Powder. Chlorine gas has a very injurious effect onthe human body, and hence cannot be used directly as a bleachingagent. It attacks the mucous membrane of the nose and lungs, andproduces the effect of a severe cold or catarrh, and when inhaled, causes death. But certain compounds of chlorine are harmless, and canbe used instead of chlorine for destroying either natural orartificial dyes. One of these compounds, namely, chloride of lime, isthe almost universal bleaching agent of commerce. It comes in the formof powder, which can be dissolved in water to form the bleachingsolution in which the colored fabrics are immersed. But fabricsimmersed in a bleaching powder solution do not lose their color aswould naturally be expected. The reason for this is that the chlorinegas is not free to do its work, but is restricted by its combinationwith the other substances. By experiment it has been found that theaddition to the bleaching solution of an acid, such as vinegar orlemon juice or sulphuric acid, causes the liberation of the chlorine. The chlorine thus set free reacts with the water and liberates oxygen;this in turn destroys the coloring matter in the fibers, andtransforms the material into a bleached product. 

The acid used to liberate the chlorine from the bleaching powder, andthe chlorine also, rot materials with which they remain in contact forany length of time. For this reason, fabrics should be removed fromthe bleaching solution as soon as possible, and should then be rinsedin some solution, such as ammonia, which is capable of neutralizingthe harmful substances; finally the fabric should be thoroughly rinsedin water in order that all foreign matter may be removed. The reasonhome bleaching is so seldom satisfactory is that most amateurs fail torealize the necessity of immediate neutralization and rinsing, andallow the fabric to remain too long in the bleaching solution, andallow it to dry with traces of the bleaching substances present in thefibers. Material treated in this way is thoroughly bleached, but is atthe same time rotten and worthless. Chloride of lime is frequentlyused in laundry work; the clothes are whiter than when cleaned withsoap and simple washing powders, but they soon wear out unless theprecaution has been taken to add an "antichlor" or neutralizer to thebleaching solution. 

220. Commercial Bleaching. In commercial bleaching the material tobe bleached is first moistened with a very weak solution of sulphuricacid or hydrochloric acid, and is then immersed in the bleachingpowder solution. As the moist material is drawn through the bleachingsolution, the acid on the fabric acts upon the solution and releaseschlorine. The chlorine liberates oxygen from the water. The oxygen inturn attacks the coloring matter and destroys it. 

[Illustration: FIG. 159. --The material to be bleached is drawn throughan acid _a_, then through a bleaching solution _b_, and finallythrough a neutralizing solution _c_. ]

The bleached material is then immersed in a neutralizing bath and isfinally rinsed thoroughly in water. Strips of cotton or linen manymiles long are drawn by machinery into and out of the varioussolutions (Fig. 159), are then passed over pressing rollers, andemerge snow white, ready to be dyed or to be used as white fabric. 

221. Wool and Silk Bleaching. Animal fibers like silk, wool, andfeathers, and some vegetable fibers like straw, cannot be bleached bymeans of chlorine, because it attacks not only the coloring matter butthe fiber itself, and leaves it shrunken and inferior. Cotton andlinen fibers, apart from the small amount of coloring matter presentin them, contain nothing but carbon, oxygen, and hydrogen, whileanimal fibers contain in addition to these elements some compounds ofnitrogen. The presence of these nitrogen compounds influences theaction of the chlorine and produces unsatisfactory results. For animalfibers it is therefore necessary to discard chlorine as a bleachingagent, and to substitute a substance which will have a less disastrousaction upon the fibers. Such a substance is to be had in sulphurousacid. When sulphur burns, as in a match, it gives off disagreeablefumes, and if these are made to bubble into a vessel containing water, they dissolve and form with the water a substance known as sulphurousacid. That this solution has bleaching properties is shown by the factthat a colored cloth dipped into it loses its color, and unbleachedfabrics immersed in it are whitened. The harmless nature of sulphurousacid makes it very desirable as a bleaching agent, especially in thehome. 

Silk, lace, and wool when bleached with chlorine become hard andbrittle, but when whitened with sulphurous acid, they retain theirnatural characteristics. 

This mild form of a bleaching substance has been put to uses which arenow prohibited by the pure food laws. In some canneries common corn iswhitened with sulphurous acid, and is then sold under falserepresentations. Cherries are sometimes bleached and then colored withthe bright shades which under natural conditions indicate freshness. 

Bleaching with chlorine is permanent, the dyestuff being destroyed bythe chlorine; but bleaching with sulphurous acid is temporary, becausethe milder bleach does not actually destroy the dyestuff, but merelymodifies it, and in time the natural yellow color of straw, cotton, and linen reappears. The yellowing of straw hats during the summer isfamiliar to everyone; the straw is merely resuming its natural colorwhich had been modified by the sulphurous acid solution applied to thestraw when woven. 

222. Why the Color Returns. Some of the compounds formed by thesulphurous acid bleaching process are gradually decomposed bysunlight, and in consequence the original color is in time partiallyrestored. The portion of a hat protected by the band retains itsfresh appearance because the light has not had access to it. Silks andother fine fabrics bleached in this way fade with age, and assume anunnatural color. One reason for this is that the dye used to color thefabric requires a clear white background, and loses its characteristichues when its foundation is yellow instead of white. Then, too, dyestuffs are themselves more or less affected by light, and fadeslowly under a strong illumination. 

Materials which are not exposed directly to an intense and prolongedillumination retain their whiteness for a long time, and hence dressmaterials and hats which have been bleached with sulphurous acidshould be protected from the sun's glare when not in use. 

223. The Removal of Stains. Bleaching powder is very useful in theremoval of stains from white fabrics. Ink spots rubbed with lemonjuice and dipped in bleaching solution fade away and leave on thecloth no trace of discoloration. Sometimes these stains can be removedby soaking in milk, and where this is possible, it is the bettermethod. 

Bleaching solution, however, while valuable in the removal of somestains, is unable to remove paint stains, because paints owe theircolor to mineral matter, and on this chlorine is powerless to act. Paint stains are best removed by the application of gasoline followedby soap and water. 

CHAPTER XXIII

DYEING

224. Dyes. One of the most important and lucrative industrialprocesses of the world to-day is that of staining and dyeing. Whetherwe consider the innumerable shades of leather used in shoes andharnesses and upholstery; the multitude of colors in the paper whichcovers our walls and reflects light ranging from the somber to thegay, and from the delicate to the gorgeous; the artificial scenerywhich adorns the stage and by its imitation of trees and flowers andsky translates us to the Forest of Arden; or whether we consider theuncounted varieties of color in dress materials, in carpets, and inhangings, we are dealing with substances which owe their beauty todyes and dyestuffs. 

The coloring of textile fabrics, such as cotton, wool, and silk, faroutranks in amount and importance that of leather, paper, etc. , andhence the former only will be considered here; but the theories andfacts relative to textile dyeing are applicable in a general way toall other forms as well. 

225. Plants as a Source of Dyes. Among the most beautiful examplesof man's handiwork are the baskets and blankets of the North AmericanIndians, woven with a skill which cannot be equaled by manufacturers, and dyed in mellow colors with a few simple dyes extracted from localplants. The magnificent rugs and tapestries of Persia and Turkey, andthe silks of India and Japan, give evidence that a knowledge of dyesis widespread and ancient. Until recently, the vegetable world wasthe source of practically all coloring matter, the pulverized root ofthe madder plant yielding the reds, the leaves and stems of the indigoplant the blues, the heartwood of the tropical logwood tree the blacksand grays, and the fruit of certain palm and locust trees yielding thesoft browns. So great was the commercial demand for dyestuffs thatlarge areas of land were given over to the exclusive cultivation ofthe more important dye plants. Vegetable dyes are now, however, rarelyused because about the year 1856 it was discovered that dyes could beobtained from coal tar, the thick sticky liquid formed as a by-productin the manufacture of coal gas. These artificial coal-tar, or aniline, dyes have practically undisputed sway to-day, and the vast areas ofland formerly used for the cultivation of vegetable dyes are now freefor other purposes. 

226. Wool and Cotton Dyeing. If a piece of wool is soaked in asolution of a coal-tar dye, such as magenta, the fiber of the clothdraws some of the dye out of the solution and absorbs it, becoming inconsequence beautifully colored. The coloring matter becomes "part andparcel, " as it were, of the wool fiber, because repeated washing ofthe fabric fails to remove the newly acquired color; the magentacoloring matter unites chemically with the fiber of the wool, andforms with it a compound insoluble in water, and hence fast towashing. 

But if cotton is used instead of wool, the acquired color is veryfaint, and washes off readily. This is because cotton fibers possessno chemical substance capable of uniting with the coloring matter toform a compound insoluble in water. 

If magenta is replaced by other artificial dyes, --for example, scarlets, --the result is similar; in general, wool material absorbsdye readily, and uniting with it is permanently dyed. Cotton material, on the other hand, does not combine chemically with coloring matterand therefore is only faintly tinged with color, and loses this whenwashed. When silk and linen are tested, it is found that the formerbehaves in a general way as did wool, while the linen has moresimilarity to the cotton. That vegetable fibers, such as cotton andlinen, should act differently toward coloring matter from animalfibers, such as silk and wool, is not surprising when we consider thatthe chemical nature of the two groups is very different; vegetablefibers contain only oxygen, carbon, and hydrogen, while animal fibersalways contain nitrogen in addition, and in many cases sulphur aswell. 

227. The Selection of Dyes. When silk and wool, cotton and linen, are tested in various dye solutions, it is found that the former have, in general, a great affinity for coloring matter and acquire apermanent color, but that cotton and linen, on the other hand, havelittle affinity for dyestuffs. The color acquired by vegetable fibersis, therefore, usually faint. 

There are, of course, many exceptions to the general statement thatanimal fibers dye readily and vegetable fibers poorly, because certaindyes fail utterly with woolen and silk material and yet are fairlysatisfactory when applied to cotton and linen fabrics. Then, too, adye which will color silk may not have any effect on wool in spite ofthe fact that wool, like silk, is an animal fiber; and certaindyestuffs to which cotton responds most beautifully are absolutelywithout effect on linen. 

The nature of the material to be dyed determines the coloring matterto be used; in dyeing establishments a careful examination is made ofall textiles received for dyeing, and the particular dyestuffs arethen applied which long experience has shown to be best suited to thematerial in question. Where "mixed goods, " such as silk and wool, orcotton and wool, are concerned, the problem is a difficult one, andthe countless varieties of gorgeously colored mixed materials giveevidence of high perfection in the art of dyeing and weaving. 

Housewives who wish to do successful home dyeing should therefore notpurchase dyes indiscriminately, but should select the kind best suitedto the material, because the coloring principle which will remake asilk waist may utterly ruin a woolen skirt or a linen suit. Powdersdesigned for special purposes may be purchased from druggists. 

228. Indirect Dyeing. We have seen that it is practically impossibleto color cotton and linen in a simple manner with any degree ofpermanency, because of the lack of chemical action between vegetablefibers and coloring matter. But the varied uses to which dyed articlesare put make fastness of color absolutely necessary. A shirt, forexample, must not be discolored by perspiration, nor a waist faded bywashing, nor a carpet dulled by sweeping with a dampened broom. Inorder to insure permanency of dyes, an indirect method was originatedwhich consisted of adding to the fibers a chemical capable of actingupon the dye and forming with it a colored compound insoluble inwater, and hence "safe. " For example, cotton material dyed directly inlogwood solution has almost no value, but if it is soaked in asolution of oxalic acid and alum until it becomes saturated with thechemicals, and is then transferred to a logwood bath, the coloracquired is fast and beautiful. 

This method of indirect dyeing is known as the mordanting process; itconsists of saturating the fabric to be dyed with chemicals which willunite with the coloring matter to form compounds unaffected by water. The chemicals are called mordants. 

229. How Variety of Color is Secured. The color which is fixed onthe fabric as a result of chemical action between mordant and dye isfrequently very different from that of the dye itself. Logwood dyewhen used alone produces a reddish brown color of no value either forbeauty or permanence; but if the fabric to be dyed is first mordantedwith a solution of alum and oxalic acid and is then immersed in alogwood bath, it acquires a beautiful blue color. 

Moreover, since the color acquired depends upon the mordant as well asupon the dye, it is often possible to obtain a wide range of colors byvarying the mordant used, the dye remaining the same. For example, with alum and oxalic acid as a mordant and logwood as a dye, blue isobtained; but with a mordant of ferric sulphate and a dye of logwood, blacks and grays result. Fabrics immersed directly in alizarin acquirea reddish yellow tint; when, however, they are mordanted with certainaluminium compounds they acquire a brilliant Turkey red, whenmordanted with chromium compounds, a maroon, and when mordanted withiron compounds, the various shades of purple, lilac, and violetresult. 

230. Color Designs in Cloth. It is thought that the earliestattempts at making "fancy materials" consisted in painting designs ona fabric by means of a brush. In more recent times the design was cutin relief on hard wood, the relief being then daubed with coloringmatter and applied by hand to successive portions of the cloth. Themost modern method of design-making is that of machine or rollerprinting. In this, the relief blocks are replaced by engraved copperrolls which rotate continuously and in the course of their rotationautomatically receive coloring matter on the engraved portion. Thecloth is to be printed is then drawn uniformly over the rotating roll, receiving color from the engraved design; in this way, the colorpattern is automatically printed on the cloth with perfect regularity. In cases where the fabrics do not unite directly with the coloringmatter, the design is supplied with a mordant and the impression madeon the fabric is that of the mordant; when the fabric is latertransferred to a dye bath, the mordanted portions, represented by thedesign, unite with the coloring matter and thus form the desired colorpatterns. 

Unless the printing is well done, the coloring matter does notthoroughly penetrate the material, and only a faint blurred designappears on the back of the cloth; the gaudy designs of cheap calicoesand ginghams often do not show at all on the under side. Suchcarelessly made prints are not fast to washing or light, and soonfade. But in the better grades of material the printing is well done, and the color designs are fairly fast, and a little care in thelaundry suffices to eliminate any danger of fading. 

Color designs of the greatest durability are produced by the weavingtogether of colored yarns. When yarn is dyed, the coloring matterpenetrates to every part of the fiber, and hence the patterns formedby the weaving together of well-dyed yarns are very fast to light andwater. 

If the color designs to be woven in the cloth are intricate, complexmachinery is necessary and skillful handwork; hence, patterns formedby the weaving of colored yarns are expensive and less common thanprinted fabrics. 

CHAPTER XXIV

CHEMICALS AS DISINFECTANTS AND PRESERVATIVES

231. The prevention of disease epidemics is one of the most strikingachievements of modern science. Food, clothing, furniture, and otherobjects contaminated in any way by disease germs may be disinfected bychemicals or by heat, and widespread infection from persons sufferingwith a contagious disease may be prevented. 

[Illustration: FIG. 160. --Pasteurizing apparatus, an arrangement bywhich milk is conveniently heated to destroy disease germs. ]

When disease germs are within the body, the problem is far fromsimple, because chemicals which would effectively destroy the germswould be fatal to life itself. But when germs are outside the body, asin water or milk, or on clothing, dishes, or furniture, they can beeasily killed. One of the best methods of destroying germs is tosubject them to intense heat. Contaminated water is made safe byboiling for a few minutes, because the strong heat destroys thedisease-producing germs. Scalded or Pasteurized milk saves the livesof scores of babies, because the germs of summer complaint which lurkin poor milk are killed and rendered harmless in the process ofscalding. Dishes used by consumptives, and persons suffering fromcontagious diseases, can be made harmless by thorough washing in thicksuds of almost boiling water. 

The bedding and clothing of persons suffering with diphtheria, tuberculosis, and other germ diseases should always be boiled and hungto dry in the bright sunlight. Heat and sunshine are two of the bestdisinfectants. 

232. Chemicals. Objects, such as furniture, which cannot be boiled, are disinfected by the use of any one of several chemicals, such assulphur, carbolic acid, chloride of lime, corrosive sublimate, etc. 

One of the simplest methods of disinfecting consists in burningsulphur in a room whose doors, windows, and keyholes have been closed, so that the burning fumes cannot escape, but remain in the room longenough to destroy disease germs. This is probably the most commonmeans of fumigation. 

For general purposes, carbolic acid is one of the very bestdisinfectants, but must be used with caution, as it is a deadly poisonexcept when very dilute. 

Chloride of lime when exposed to the air and moisture slowly gives offchlorine, and can be used as a disinfectant because the gas thus setfree attacks germs and destroys them. For this reason chloride of limeis an excellent disinfectant of drainpipes. Certain bowel troubles, such as diarrhoea, are due to microbes, and if the waste matter of aperson suffering from this or similar diseases is allowed passagethrough the drainage system, much damage may be done. But a smallamount of chloride of lime in the closet bowl will insuredisinfection. 

233. Personal Disinfection. The hands may gather germs from anysubstances or objects with which they come in contact; hence the handsshould be washed with soap and water, and especially before eating. Physicians who perform operations wash not only their hands, but theirinstruments, sterilizing the latter by placing them in boiling waterfor several minutes. 

Cuts and wounds allow easy access to the body; a small cut has beenknown to cause death because of the bacteria which found their wayinto the open wound and produced disease. In order to destroy anygerms which may have entered into the cut from the instrument, it iswell to wash out the wound with some mild disinfectant, such as verydilute carbolic acid or hydrogen peroxide, and then to bind the woundwith a clean cloth, to prevent later entrance of germs. 

234. Chemicals as Food Preservatives. The spoiling of meats andsoups, and the souring of milk and preserves, are due to germs which, like those producing disease, can be destroyed by heat and bychemicals. 

Milk heated to the boiling point does not sour readily, and successfulcanning consists in cooking fruits and vegetables until all the germsare killed, and then sealing the cans so that germs from outsidecannot find entrance and undo the work of the canner. 

Some dealers and manufacturers have learned that certain chemicalswill act as food preservatives, and hence they have replaced the safemethod of careful canning by the quicker and simpler plan of addingchemicals to food. Catchup, sauces, and jellies are now frequentlypreserved in this way. But the chemicals which destroy bacteriafrequently injure the consumer as well. And so much harm has been doneby food preservatives that the pure food laws require that cans andbottles contain a labeled statement of the kind and quantity ofchemicals used. 

Even milk is not exempt, but is doctored to prevent souring, thepreservative most generally used by milk dealers being formaldehyde. The vast quantity of milk consumed by young and old, sick and well, makes the use of formaldehyde a serious menace to health, because noconstitution can endure the injury done by the constant use ofpreservatives. 

The most popular and widely used preservatives of meats are borax andboric acid. These chemicals not only arrest decay, but partiallyrestore to old and bad meat the appearance of freshness; in this wayunscrupulous dealers are able to sell to the public in one form orother meats which may have undergone partial decomposition; sausagefrequently contains partially decomposed meat, restored as it were bychemicals. 

In jams and catchups there is abundant opportunity for preservatives;badly or partially decayed fruits are sometimes disinfected and usedas the basis of foods sold by so-called good dealers. Benzoate ofsoda, and salicylic acid are the chemicals most widely employed forthis purpose, with coal-tar dyes to simulate the natural color of thefruit. 

Many of the cheap candies sold by street venders are not fit forconsumption, since they are not only made of bad material, but arefrequently in addition given a light dipping in varnish as aprotection against the decaying influences of the atmosphere. 

The only wise preservatives are those long known and employed by ourancestors; salt, vinegar, and spices are all food preservatives, butthey are at the same time substances which in small amounts are notinjurious to the body. Smoked herring and salted mackerel arechemically preserved foods, but they are none the less safe anddigestible. 

235. The Preservation of Wood and Metal. The decaying of wood andthe rusting of metal are due to the action of air and moisture. Whenwood and metal are surrounded with a covering which neither air normoisture can penetrate, decay and rust are prevented. Paint affordssuch a protective covering. The main constituent of paint is acompound of white lead or other metallic substance; this is mixed withlinseed oil or its equivalent in order that it may be spread over woodand metal in a thin, even coating. After the mixture has been applied, it hardens and forms a tough skin fairly impervious to weathering. Forthe sake of ornamentation, various colored pigments are added to thepaint and give variety of effect. 

Railroad ties and street paving blocks are ordinarily protected by oilrather than paint. Wood is soaked in creosote oil until it becomesthoroughly saturated with the oily substance. The pores of the woodare thus closed to the entrance of air and moisture, and decay isavoided. Wood treated in this way is very durable. Creosote ispoisonous to insects and many small animals, and thus acts as apreservation not only against the elements but against animal life aswell. 

CHAPTER XXV

DRUGS AND PATENT MEDICINES

236. Stimulants and Narcotics. Man has learned not only the actionof substances upon each other, such as bleaching solution uponcoloring matter, washing soda upon grease, acids upon bases, but alsothe effect which certain chemicals have upon the human body. 

Drugs and their varying effects upon the human system have been knownto mankind from remote ages; in the early days, familiar leaves, roots, and twigs were steeped in water to form medicines which servedfor the treatment of all ailments. In more recent times, however, these simple herb teas have been supplanted by complex drugs, and nowmedicines are compounded not only from innumerable plant products, butfrom animal and mineral matter as well. Quinine, rhubarb, and arnicaare examples of purely vegetable products; iron, mercury, and arsenicare equally well known as distinctly mineral products, while cod-liveroil is the most familiar illustration of an animal remedy. Ordinarilya combination of products best serves the ends of the physician. 

Substances which, like cod-liver oil, serve as food to a worn-outbody, or, like iron, tend to enrich the blood, or, like quinine, aidin bringing an abnormal system to a healthy condition, are valuableservants and cannot be entirely dispensed with so long as man issubject to disease. 

But substances which, like opium, laudanum, and alcohol, are notrequired by the body as food, or as a systematic, intelligent aid torecovery, but are taken solely for the stimulus aroused or for theinsensibility induced, are harmful to man, and cannot be indulged inby him without ultimate mental, moral, and physical loss. Substancesof the latter class are known as narcotics and stimulants. 

237. The Cost of Health. In the physical as in the financial world, nothing is to be had without a price. Vigor, endurance, and mentalalertness are bought by hygienic living; that is, by proper food, fresh air, exercise, cleanliness, and reasonable hours. Some peoplewish vigor, endurance, etc. , but are unwilling to live the life whichwill develop these qualities. Plenty of sleep, exercise, and simplefood all tend to lay the foundations of health. Many, however, are notwilling to take the care necessary for healthful living, because itwould force them to sacrifice some of the hours of pleasure. Sooner orlater, these pleasure-seekers begin to feel tired and worn, and someof them turn to drugs and narcotics for artificial strength. At firstthe drugs seem to restore the lost energy, and without harm; however, the cost soon proves to be one of the highest Nature ever demands. 

238. The Uncounted Cost. The first and most obvious effect of opium, for example, is to deaden pain and to arouse pleasure; but while thedrug is producing these soothing sensations, it interferes with bodilyfunctions. Secretion, digestion, absorption of food, and the removalof waste matters are hindered. Continued use of the drug leads toheadache, exhaustion, nervous depression, and heart weakness. There isthus a heavy toll reckoned against the user, and the creditor isrelentless in demanding payment. 

Moreover, the respite allowed by a narcotic is exceedingly brief, anda depression which is long and deep inevitably follows. In order toovercome this depression, recourse is usually had to a further dose, and as time goes on, the intervals of depression become more frequentand lasting, and the necessity to overcome them increases. Thuswithout intention one finds one's self bound to the drug, its fastvictim. The sanatoria of our country are crowded with people who aretrying to free themselves of a drug habit into which they have driftedunintentionally if not altogether unknowingly. What is true of opiumis equally applicable to other narcotics. 

239. The Right Use of Narcotics. In the hands of the physician, narcotics are a great blessing. In some cases, by relieving pain, theygive the system the rest necessary for overcoming the cause of thepain. Only those who know of the suffering endured in former times canfully appreciate the decrease in pain brought about by the proper useof narcotics. 

240. Patent Medicines, Cough Sirups. A reputable physician issolicitous regarding the permanent welfare of his patient andadministers carefully chosen and harmless drugs. Mere medicinevenders, however, ignore the good of mankind, and flood the marketwith cheap patent preparations which delude and injure those whopurchase, but bring millions of dollars to those who manufacture. 

Practically all of these patent, or proprietary, preparations containa large proportion of narcotics or stimulants, and hence the benefitwhich they seem to afford the user is by no means genuine; examinationshows that the relief brought by them is due either to a temporarydeadening of sensibilities by narcotics or to a fleeting stimulationby alcohol and kindred substances. 

Among the most common ailments of both young and old are coughs andcolds; hence many patent cough mixtures have been manufactured andplaced on the market for the consumption of a credulous public. Such"quick cures" almost invariably contain one or more narcotic drugs, and not only do not relieve the cold permanently, but occasionsubsequent disorders. Even lozenges and pastilles are not free fromfraud, but have a goodly proportion of narcotics, containing in somecases chloroform, morphine, and ether. 

The widespread use of patent cough medicines is due largely to thefact that many persons avoid consulting a physician about so trivialan ailment as an ordinary cold, or are reluctant to pay a medical feefor what seems a slight indisposition and hence attempt to doctorthemselves. 

Catarrh is a very prevalent disease in America, and consequentlynumerous catarrh remedies have been devised, most of which contain ina disguised form the pernicious drug, cocaine. Laws have been enactedwhich require on the labels a declaration of the contents of thepreparation, both as to the kind of drug used and the amount, and thechoice of accepting or refusing such mixtures is left to theindividual. But the great mass of people are ignorant of the harmfulnature of drugs in general, and hence do not even read theself-accusing label, or if they do glance at it, fail to comprehendthe dangerous nature of the drugs specified there. In order tosafeguard the uninformed purchaser and to restrict the manufacture ofharmful patent remedies, some states limit the sale of allpreparations containing narcotics and thus give free rein to neitherconsumer nor producer. 

241. Soothing Sirups; Soft Drinks. The development of a race islimited by the mental and physical growth of its children, and yetthousands of its children are annually stunted and weakened by drugs, because most colic cures, teething concoctions, and soothing syrupsare merely agreeably flavored drug mixtures. Those who have used suchpreparations freely, know that a child usually becomes fretful andirritable between doses, and can be quieted only by larger and morefrequent supplies. A habit formed in this way is difficult toovercome, and many a child when scarcely over its babyhood had acraving which in later years may lead to systematic drug taking. Andeven though the pernicious drug craving is not created, considerableharm is done to the child, because its body is left weak andnon-resistant to diseases of infancy and childhood. 

Many of our soft drinks contain narcotics. The use of the coca leafand the kola nut for such preparations has increased very greatlywithin the last few years, and doubtless legislation will soon beinstituted against the indiscriminate sale of soft drinks. 

242. Headache Powders. The stress and strain of modern life hasopened wide the door to a multitude of bodily ills, among which may bementioned headache. Work must be done and business attended to, andthe average sufferer does not take time from his vocation toinvestigate the cause of the headache, but unthinkingly grasps at anyremedy which will remove the immediate pain, and utterly disregardslater injury. The relief afforded by most headache mixtures is due tothe presence of antipyrin or acetanilid, and it has been shownconclusively that these drugs weaken heart action, diminishcirculation, reduce the number of red corpuscles in the blood, andbring on a condition of chronic anemia. Pallid cheeks and blue lipsare visible evidence of the too frequent use of headache powders. 

The labels required by law are often deceptive and convey no adequateidea of the amount of drug consumed; for example, 240 grains ofacetanilid to an ounce seems a small quantity of drug for a powder, but when one considers that there are only 480 grains in an ounce, itwill be seen that each powder is one half acetanilid. 

Powders taken in small quantities and at rare intervals are apparentlyharmless; but they never remove the cause of the trouble, and hencethe discomfort soon returns with renewed force. Ordinarily, hygienicliving will eliminate the source of the trouble, but if it does not, aphysician should be consulted and medicine should be procured from himwhich will restore the deranged system to its normal healthycondition. 

243. Other Deceptions. Nearly all patent medicines contain somealcohol, and in many, the quantity of alcohol is far in excess of thatfound in the strongest wines. Tonics and bitters advertised as a curefor spring fever and a worn-out system are scarcely more than cheapcocktails, as one writer has derisively called them, and the amount ofalcohol in some widely advertised patent remedies is alarmingly largeand almost equal to that of strong whisky. 

[Illustration: FIG. 161. --Diagram showing the amount of alcohol insome alcoholic drinks and in one much used patent medicine. ]

Some conscientious persons who would not touch beer, wine, whisky, orany other intoxicating drink consume patent remedies containing largequantities of alcohol and thus unintentionally expose themselves tomental and physical danger. In all cases of bodily disorder, the onlysafe course is to consult a physician who has devoted himself to thestudy of the body and the methods by which a disordered system may berestored to health. 

CHAPTER XXVI

NITROGEN AND ITS RELATION TO PLANTS

244. Nitrogen. A substance which plays an important part in animaland plant life is nitrogen. Soil and the fertilizers which enrich it, the plants which grow on it, and the animals which feed on these, allcontain nitrogen or nitrogenous compounds. The atmosphere, which weordinarily think of as a storehouse of oxygen, contains far morenitrogen than oxygen, since four fifths of its whole weight is made upof this element. 

Nitrogen is colorless, odorless, and tasteless. Air is composedchiefly of oxygen and nitrogen; if, therefore, the oxygen in a vesselfilled with air can be made to unite with some other substance or canbe removed, there will be a residue of nitrogen. This can be done byfloating on water a light dish containing phosphorus, then ignitingthe phosphorus, and placing an inverted jar over the burningsubstance. The phosphorus in burning unites with the oxygen of the airand hence the gas that remains in the jar is chiefly nitrogen. It hasthe characteristics mentioned above and, in addition, does not combinereadily with other substances. 

245. Plant Food. Food is the course of energy in every living thingand is essential to both animal and plant life. Plants get their foodfrom the lifeless matter which exists in the air and in the soil;while animals get their food from plants. It is true that man and manyother animals eat fleshy foods and depend upon them for partialsustenance, but the ultimate source of all animal food is plant life, since meat-producing animals live upon plant growth. 

Plants get their food from the air, the soil, and moisture. From theair, the leaves take carbon dioxide and water and transform them intostarchy food; from the soil, the roots take water rich in mineralmatters dissolved from the soil. From the substances thus gathered, the plant lives and builds up its structure. 

A food substance necessary to plant life and growth is nitrogen. Sincea vast store of nitrogen exists in the air, it would seem that plantsshould never lack for this food, but most plants are unable to makeuse of the boundless store of atmospheric nitrogen, because they donot possess the power of abstracting nitrogen from the air. For thisreason, they have to depend solely upon nitrogenous compounds whichare present in the soil and are soluble in water. The solublenitrogenous soil compounds are absorbed by roots and are utilized byplants for food. 

246. The Poverty of the Soil. Plant roots are constantly takingnitrogen and its compounds from the soil. If crops which grow from thesoil are removed year after year, the soil becomes poorer in nitrogen, and finally possesses too little of it to support vigorous and healthyplant life. The nitrogen of the soil can be restored if we add to it afertilizer containing nitrogen compounds which are soluble in water. Decayed vegetable matter contains large quantities of nitrogencompounds, and hence if decayed vegetation is placed upon soil or isplowed into soil, it acts as a fertilizer, returning to the soil whatwas taken from it. Since man and all other animals subsist uponplants, their bodies likewise contain nitrogenous substances, andhence manure and waste animal matter is valuable as a fertilizer orsoil restorer. 

247. Bacteria as Nitrogen Gatherers. Soil from which crops are removedyear after year usually becomes less fertile, but the soil from whichcrops of clover, peas, beans, or alfalfa have been removed is richer innitrogen rather than poorer. This is because the roots of these plantsoften have on them tiny swellings, or tubercles, in which millions ofcertain bacteria live and multiply. These bacteria have the remarkablepower of taking free nitrogen from the air in the soil and of combiningit with other substances to form compounds which plants can use. Thebacteria-made compounds dissolve in the soil water and are absorbed intothe plant by the roots. So much nitrogen-containing material is made bythe root bacteria of plants of the pea family that the soil in whichthey grow becomes somewhat richer in nitrogen, and if plants whichcannot make nitrogen are subsequently planted in such a soil, they findthere a store of nitrogen. A crop of peas, beans, or clover isequivalent to nitrogenous fertilizer and helps to make ready the soilfor other crops. 

[Illustration: FIG. 162. --Roots of soy bean having tubercle-bearingbacteria. ]

248. Artificial Fertilizers. Plants need other foods besidesnitrogen, and they exhaust the soil not only of nitrogen, but also ofphosphorus and potash, since large quantities of these are necessaryfor plant life. There are many other substances absorbed from the soilby the plant, namely, iron, sodium, calcium, magnesium, but these areused in smaller quantities and the supply in the soil does not readilybecome exhausted. 

Commercial fertilizers generally contain nitrogen, phosphorus, andpotash in amounts varying with the requirements of the soil. Wheatrequires a large amount of phosphorus and quickly exhausts the groundof that food stuff; a field which has supported a crop of wheat isparticularly poor in phosphorus, and a satisfactory fertilizer forthat land would necessarily contain a large percentage of phosphorus. The fertilizer to be used in a soil depends upon the character of thesoil and upon the crops previously grown on it. 

[Illustration: FIG. 163. --Water cultures of buckwheat: 1, with all thefood elements; 2, without potash; 3, without nitrates. ]

The quantity of fertilizer needed by the farmers of the world isenormous, and the problem of securing the necessary substances inquantities sufficient to satisfy the demand bids fair to be serious. But modern chemistry is at work on the problem, and already it ispossible to make some nitrogen compounds on a commercial scale. Whennitrogen gas is in contact with heated calcium carbide, a reactiontakes place which results in the formation of calcium nitride, acompound suitable for enriching the soil. There are other commercialmethods for obtaining nitrogen compounds which are suitable forabsorption by plant roots. 

Phosphorus is obtained from bone ash and from phosphate rock which iswidely distributed over the surface of the earth. Bone ash andthousands of tons of phosphate rock are treated with sulphuric acid toform a phosphorus compound which is soluble in soil water and which, when added to soil, will be usable by the plants growing there. 

The other important ingredient of most fertilizers is potash. Woodashes are rich in potash and are a valuable addition to the soil. Butthe amount of potash thus obtained is far too limited to supply theneeds of agriculture; and to-day the main sources of potash are thevast deposits of potassium salts found in Prussia. 

Although Germany now furnishes the American farmer with the bulk ofhis potash, she may not do so much longer. In 1911 an indirect potashtax was levied by Germany on her best customer, the United States, towhom 15 million dollars' worth of potash had been sold the precedingyear. This led Americans to inquire whether potash could not beobtained at home. 

Geologists say that long ages ago Germany was submerged, that thewaters slowly evaporated and that the various substances in the seawater were deposited in thick layers. The deposits thus left by theevaporation of the sea water gradually became hidden by sediment andsoil, and lost to sight. From such deposits, potash is obtained. Geologists tell us that our own Western States were once submerged, and that the waters evaporated and disappeared from our land very muchas they did from Germany. The Great Salt Lake of Utah is a relic of agreat body of water. If it be true that waters once covered ourWestern States, there may be buried deposits of potash there, andto-day the search for the hidden treasure is going on with the energyand enthusiasm characteristic of America. 

Another probable source of potash is seaweed. The sea is a vastreservoir of potash, and seaweed, especially the giant kelp, absorbslarge quantities of this potash. A ton of dried kelp (dried by sun andwind) contains about 500 pounds of pure potash. The kelps areabundant, covering thousands of square miles in the Pacific Ocean, from Mexico to the Arctic Ocean. 

CHAPTER XXVII

SOUND

249. The Senses. All the information which we possess of the worldaround us comes to us through the use of the senses of sight, hearing, taste, touch, and smell. Of the five senses, sight and hearing aregenerally considered the most valuable. In preceding Chapters westudied the important facts relative to light and the power of vision;it remains for us to study Sound as we studied Light, and to learnwhat we can of sound and the power to hear. 

250. How Sound is Produced. If one investigates the source of anysound, he will always find that it is due to motion of some kind. Asudden noise is traced to the fall of an object, or to an explosion, or to a collision; in fact, is due to the motion of matter. A pianogives out sound whenever a player strikes the keys and sets in motionthe various wires within the piano; speech and song are caused by themotion of chest, vocal cords, and lips. 

[Illustration: FIG. 164. --Sprays of water show that the fork is inmotion. ]

If a large dinner bell is rung, its motion or vibration may be felt ontouching it with the finger. If a tuning fork is made to give forthsound by striking it against the knee, or hitting it with a rubberhammer, and is then touched to the surface of water, small sprays ofwater will be thrown out, showing that the prongs of the fork are inrapid motion. (A rubber hammer is made by putting a piece of glasstubing through a rubber cork. )

If a light cork ball on the end of a thread is brought in contact witha sounding fork, the ball does not remain at rest, but vibrates backand forth, being driven by the moving prongs. 

[Illustration: FIG. 165. --The ball does not remain at rest]

These simple facts lead us to conclude that all sound is due to themotion of matter, and that a sounding body of any kind is in rapidmotion. 

251. Sound is carried by Matter. In most cases sound reaches the earthrough the air; but air is not the only medium through which sound iscarried. A loud noise will startle fish, and cause them to dart away, so we conclude that the sound must have reached them through thewater. An Indian puts his ear to the ground in order to detect distantfootsteps, because sounds too faint to be heard through the air arecomparatively clear when transmitted through the earth. A gentletapping at one end of a long table can be distinctly heard at theopposite end if the ear is pressed against the table; if the ear isremoved from the wood, the sound of tapping is much fainter, showingthat wood transmits sound more readily than air. We see therefore thatsound can be transmitted to the ear by solids, liquids, or gases. 

Matter of any kind can transmit sound to the ear. The followingexperiments will show that matter is necessary for transmission. Attach a small toy bell to a glass rod (Fig. 166) by means of a rubbertube and pass the rod through one of two openings in a rubber cork. Insert the cork in a strong flask containing a small quantity of waterand shake the bell, noting the sound produced. Then heat the flask, allowing the water to boil briskly, and after the boiling hascontinued for a few minutes remove the flame and instantly close upthe second opening by inserting a glass stopper. Now shake the flaskand note that the sound is very much fainter than at first. As theflask was warmed, air was rapidly expelled; so that when the flask wasshaken the second time, less air was present to transmit the sound. Ifthe glass stopper is removed and the air is allowed to reenter theflask, the loudness of the sound immediately increases. 

[Illustration: FIG. 166. --Sound is carried by the air. ]

Since the sound of the bell grows fainter as air is removed, we inferthat there would be no sound if all the air were removed from theflask; that is to say, sound cannot be transmitted through empty spaceor a vacuum. If sound is to reach our ears, it must be through theagency of matter, such as wood, water, or air, etc. 

252. How Sound is transmitted through Air. We saw in Section 250that sound can always be traced to the motion or vibration of matter. It is impossible to conceive of an object being set into sudden andcontinued motion without disturbing the air immediately surroundingit. A sounding body always disturbs and throws into vibration the airaround it, and the air particles which receive motion from a soundingbody transmit their motion to neighboring particles, these in turn tothe next adjacent particles, and so on until the motion has traveledto very great distances. The manner in which vibratory motion istransmitted by the atmosphere must be unusual in character, since nomotion of the air is apparent, and since in the stillness of nightwhen "not a breath of air" is stirring, the shriek of a railroadwhistle miles distant may be heard with perfect clearness. Moreover, the most delicate notes of a violin can be heard in the remotestcorners of a concert hall, when not the slightest motion of the aircan be seen or felt. 

In our study of the atmosphere we saw that air can be compressed andrarefied; in other words, we saw that air is very elastic. It can beshown experimentally that whenever an elastic body in motion comes incontact with a body at rest, the moving body transfers its motion tothe second body and then comes to rest itself. Let two billiard ballsbe suspended in the manner indicated in Figure 167. If one of theballs is drawn aside and is then allowed to fall against the other, the second ball is driven outward to practically the height from whichthe first ball fell and the first ball comes to rest. 

[Illustration: FIG. 167. --Elastic balls. ]

[Illustration: FIG. 168. --Suspended billiard balls. ]

If a number of balls are arranged in line as in Figure 168 or Figure169, and the end ball is raised and then allowed to fall, or if _A_ ispushed against _C_, the last ball _B_ will move outward alone, with aforce nearly equal to that originally possessed by _A_ and to adistance nearly equal to that through which _A_ moved. But there willbe no _visible_ motion of the intervening balls. The force of themoving ball _A_ is given to the second ball, and the second ball inturn gives the motion to the third, and so on throughout the entirenumber, until _B_ is reached. But _B_ has no ball to give its motionto, hence _B_ itself moves outward, and moves with a force nearlyequal to that originally imparted by _A_ and to a distance nearlyequal to that through which _A_ fell. Motion at _A_ is transmitted to_B_ without any perceptible motion of the balls lying between thesepoints. Similarly the particles of air set into motion by a soundingbody impart their motion to each other, the motion being transmittedonward without any perceptible motion of the air itself. When thismotion reaches the ear, it sets the drum of the ear into vibration, and these vibrations are in turn transmitted to the auditory nerves, which interpret the motion as sound. 

[Illustration: FIG. 169. --Elastic balls transmit motion. ]

[Illustration: FIG. 170. --When a ball meets more than one ball, itdivides its motion. ]

253. Why Sound dies away with Distance. Since the last ball _B_ isdriven outward with a force nearly equal to that possessed by _A_, itwould seem that the effect on the ear drum should be independent ofdistance and that a sound should be heard as distinctly when remote aswhen near. But we know from experience that this is not true, becausethe more distant the source of sound, the fainter the impression; andfinally, if the distance between the source of sound and the hearerbecomes too great, the sound disappears entirely and nothing is heard. The explanation of this well-known fact is found in a further study ofthe elastic balls (Fig. 170). If _A_ hits two balls instead of one, the energy possessed by _A_ is given in part to one ball, and in partto the other, so that neither obtains the full amount. These balls, having each received less than the original energy, have less totransmit; each of these balls in turn meets with others, and hence themotion becomes more and more distributed, and distant balls receiveless and less impetus. The energy finally given becomes too slight toaffect neighboring balls, and the system comes to rest. This is whatoccurs in the atmosphere; a moving air particle meets not one but manyadjacent air particles, and each of these receives a portion of theoriginal energy and transmits a portion. When the original disturbancebecomes scattered over a large number of air particles, the energygiven to any one air particle becomes correspondingly small, andfinally the energy becomes so small that further particles are notaffected; beyond this limit the sound cannot be heard. 

If an air particle transmitted motion only to those air particlesdirectly in line with it, we should not be able to detect sound unlessthe ear were in direct line with the source. The fact that an airparticle divides its motion among all particles which it touches, thatis, among those on the sides as well as those in front, makes itpossible to hear sound in all directions. A good speaker is heard notonly by those directly in front of him, but by those on the side, andeven behind him. 

254. Velocity of Sound. The transmission of motion from particle toparticle does not occur instantaneously, but requires time. If thedistance is short, so that few air particles are involved, the timerequired for transmission is very brief, and the sound is heard atpractically the instant it is made. Ordinarily we are not consciousthat it requires time for sound to travel from its source to our ears, because the distance involved is too short. At other times werecognize that there is a delay; for example, thunder reaches our earsafter the lightning which caused the thunder has completelydisappeared. If the storm is near, the interval of time between thelightning and the thunder is brief, because the sound does not havefar to travel; if the storm is distant, the interval is much longer, corresponding to the greater distance through which the sound travels. Sound does not move instantaneously, but requires time for itstransmission. The report of a distant cannon is heard after the flashand smoke are seen; the report of a near cannon is heard the instantthe flash is seen. 

The speed with which sounds travels through the air, or its velocity, was first measured by noting the interval (54. 6 seconds) which elapsedbetween the flash of a cannon and the sound of the report. Thedistance of the cannon from the observer was measured and found to be61, 045 feet, and by dividing this distance by the number of seconds, we find that the distance traveled by sound in one second isapproximately 1118 feet. 

High notes and low notes, soft notes and shrill notes, all travel atthe same rate. If bass notes traveled faster or slower than sopranonotes, or if the delicate tones of the violin traveled faster orslower than the tones of a drum, music would be practicallyimpossible, because at a distance from the source of sound the varioustones which should be in unison would be out of time--some arrivinglate, some early. 

255. Sound Waves. Practically everyone knows that a hammock hungwith long ropes swings or vibrates more slowly than one hung withshort ropes, and that a stone suspended by a long string swings moreslowly than one suspended by a short string. No two rocking chairsvibrate in the same way unless they are exactly alike in shape, size, and material. An object when disturbed vibrates in a manner peculiarto itself, the vibration being slow, as in the case of the long-ropedswing, or quick, as in the case of the short-roped swing. The timerequired for a single swing or vibration is called the _period_ of thebody, and everything that can vibrate has a characteristic period. Size and shape determine to a large degree the period of a body; forexample, a short, thick tuning fork vibrates more rapidly than a tallslender fork. 

[Illustration: FIG. 171. --The two hammocks swing differently. ]

Some tuning forks when struck vibrate so rapidly that the prongs moveback and forth more than 5000 times per second, while other tuningforks vibrate so slowly that the vibrations do not exceed 50 persecond. In either case the distance through which the prongs move isvery small and the period is very short, so that the eye can seldomdetect the movement itself. That the prongs are in motion, however, isseen by the action of a pith ball when brought in contact with theprongs (see Section 250). 

[Illustration: FIG. 172. --The pitch given out by a fork depends uponits shape. ]

The disturbance created by a vibrating body is called a wave. 

256. Waves. While the disturbance which travels out from a soundingbody is commonly called a wave, it is by no means like the type ofwave best known to us, namely, the water wave. 

If a closely coiled heavy wire is suspended as in Figure 173 and theweight is drawn down and then released, the coil will assume theappearance shown; there is clearly an overcrowding or condensation insome places, and a spreading out or rarefaction in other places. Thepulse of condensation and rarefaction which travels the length of thewire is called a wave, although it bears little or no resemblance tothe familiar water wave. Sound waves are similar to the waves formedin the stretched coil. 

[Illustration: FIG. 173. --Waves in a coiled wire. ]

Sound waves may be said to consist of a series of condensations andrarefactions, and the distance between two consecutive condensationsand rarefactions may be defined as the wave length. 

257. How One Sounding Body produces Sound in Another Body. InSection 255 we saw that any object when disturbed vibrates in a mannerpeculiar to itself, --its natural period, --a long-roped hammockvibrating slowly and a short-roped hammock vibrating rapidly. Fromobservation we learn that it requires but little force to cause a bodyto vibrate in its natural period. If a sounding body is near a bodywhich has the same period as itself, the pulses of air produced by thesounding body will, although very small, set the second body intomotion and cause it to make a faint sound. When a piano is beingplayed, we are often startled to find that a window pane or anornament responds to some note of the piano. If two tuning forks ofexactly identical periods (that is, of the same frequency) are placedon a table as in Figure 174, and one is struck so as to give forth aclear sound, the second fork will likewise vibrate, even though thetwo forks may be separated by several feet of air. We can readily seethat the second fork is in motion, although it has not been struck, because it will set in motion a pith ball suspended beside it; atfirst the pith ball does not move, then it moves slightly, and finallybounces rapidly back and forth. If the periods of the two forks arenot identical, but differ in the slightest degree, the second forkwill not respond to the first fork, no matter how long or how loud thesound of the first fork. If we suppose that the fork vibrates 256times each second, then 256 gentle pulses of air are produced eachsecond, and these, traveling outward through the air, reach the silentfork and tend to set it in motion. A single pulse of air could notmove the solid, heavy prongs, but the accumulated action of 256vibrations per second soon makes itself felt, and the second forkbegins to vibrate, at first gently, then gradually stronger, andfinally an audible tone is given forth. 

[Illustration: FIG. 174. --When the first fork vibrates, the secondresponds. ]

The cumulative power of feeble forces acting frequently at definiteintervals is seen in many ways in everyday life. A small boy caneasily swing a much larger boy, provided he gives the swing a gentlepush in the right direction every time it passes him. But he must becareful to push at the proper instant, since otherwise his effort doesnot count for much; if he pushes forward when the swing is movingbackward, he really hinders the motion; if he waits until the swinghas moved considerably forward, his push counts for little. He mustpush at the proper instant; that is, the way in which his hand movesin giving the push must correspond exactly with the way in which theswing would naturally vibrate. A very striking experiment can be madeby suspending from the ceiling a heavy weight and striking this weightgently at regular, properly timed intervals with a small cork hammer. Soon the pendulum, or weight, will be set swinging. 

[Illustration: FIG. 175. --The hollow wooden box reënforces the sound. ]

258. Borrowed Sound. Picture frames and ornaments sometimes buzz andgive forth faint murmurs when a piano or organ is played. The wavessent out by a sounding body fall upon all surrounding objects and bytheir repeated action tend to throw these bodies into vibration. Ifthe period of any one of the objects corresponds with the period ofthe sounding body, the gentle but frequent impulses affect the object, which responds by emitting a sound. If, however, the periods do notcorrespond, the action of the sound waves is not sufficiently powerfulto throw the object into vibration, and no sound is heard. Bodieswhich respond in this way are said to be sympathetic and the responseproduced is called _resonance_. Seashells when held to the ear seem tocontain the roar of the sea; this is because the air within the shellis set into sympathetic vibrations by some external tone. If theseashell were held to the ear in an absolutely quiet room, no soundwould be heard, because there would be no external forces to set intovibration the air within the shell. 

Tuning forks do not produce strong tones unless mounted on hollowwooden boxes (Fig. 175), whose size and shape are so adjusted thatresonance occurs and strengthens the sound. When a human being talksor sings, the air within the mouth cavity is thrown into sympatheticvibration and strengthens the otherwise feeble tone of the speaker. 

259. Echo. If one shouts in a forest, the sound is sometimes heard asecond time a second or two later. This is because sound is reflectedwhen it strikes a large obstructing surface. If the sound wavesresulting from the shout meet a cliff or a mountain, they arereflected back, and on reaching the ear produce a later sensation ofsound. 

By observation it has been found that the ear cannot distinguishsounds which are less than one tenth of a second apart; that is, iftwo sounds follow each other at an interval less than one tenth of asecond, the ear recognizes not two sounds, but one. This explains whya speaker can be heard better indoors than in the open air. In theaverage building, the walls are so close that the reflected waves havebut a short distance to travel, and hence reach the ear at practicallythe same time as those which come directly from the speaker. In theopen, there are no reflecting walls or surfaces, and the originalsound has no reënforcement from reflection. 

If the reflected waves reach the ear too late to blend with theoriginal sound, that is, come later than one tenth of a second afterthe first impression, an echo is heard. What we call the rolling ofthunder is really the reflection and re-reflection of the originalthunder from cloud and cliff. 

Some halls are so large that the reflected sounds cause a confusion ofechoes, but this difficulty can be lessened by hanging draperies, which break the reflection. 

260. Motion does not always produce Sound. While we know that allsound can be traced to motion, we know equally well that motion doesnot always produce sound. The hammock swinging in the breeze does notgive forth a sound; the flag floating in the air does not give forth asound unless blown violently by the wind; a card moved slowly throughthe air does not produce sound, but if the card is moved rapidly backand forth, a sound becomes audible. 

Motion, in order to produce sound, must be rapid; a ball attached to astring and moved slowly through the air produces no sound, but thesame ball, whirled rapidly, produces a distinct buzz, which becomesstronger and stronger the faster the ball is whirled. 

261. Noise and Music. When the rapid motions which produce sound areirregular, we hear noise; when the motions are regular and definite, we have a musical tone; the rattling of carriage wheels on stones, theroar of waves, the rustling of leaves are noise, not music. In allthese illustrations we have rapid but irregular motion; no two stonesstrike the wheel in exactly the same way, no two waves produce pulsesin the air of exactly the same character, no two leaves rustle inprecisely the same way. The disturbances which reach the ear fromcarriage, waves, and leaves are irregular both in time and strength, and irritate the ear, causing the sensation which we call noise. 

The tuning fork is musical. Here we have rapid, regular motion;vibrations follow each other at perfectly definite intervals, and theair disturbance produced by one vibration is exactly like thedisturbance produced by a later vibration. The sound waves which reachthe ear are regular in time and kind and strength, and we call thesensation music. 

To produce noise a body must vibrate in such a way as to give short, quick shocks to the air; to produce music a body must not only impartshort, quick shocks to the air, but must impart these shocks withunerring regularity and strength. A flickering light irritates theeye; a flickering sound or noise irritates the ear; both are painfulbecause of the sudden and abrupt changes in effect which they cause, the former on the eye, the latter on the ear. 

The only thing essential for the production of a musical sound is thatthe waves which reach the ear shall be rapid and regular; it isimmaterial how these waves are produced. If a toothed wheel is mountedand slowly rotated, and a stiff card is held against the teeth of thewheel, a distinct tap is heard every time the card strikes the wheel. But if the wheel is rotated rapidly, the ear ceases to hear thevarious taps and recognizes a deep continuous musical tone. Theblending of the individual taps, occurring at regular intervals, hasproduced a sustained musical tone. A similar result is obtained if acard is drawn slowly and then rapidly over the teeth of a comb. 

[Illustration: FIG. 176. --A rotating disk. ]

That musical tones are due to a succession of regularly timed impulsesis shown most clearly by means of a rotating disk on which are cut twosets of holes, the outer set equally spaced, and the inner setunequally spaced (Fig. 176). 

If, while the disk is rotating rapidly, a tube is held over theoutside row and air is blown through the tube, a sustained musicaltone will be heard. If, however, the tube is held, during the rotationof the disk, over the inner row of unequally spaced holes, the musicaltone disappears, and a series of noises take its place. In the firstcase, the separate puffs of air followed each other regularly andblended into one tone; in the second case, the separate puffs of airfollowed each other at uncertain and irregular intervals and theresult was noise. 

Sound possesses a musical quality only when the waves or pulses followeach other at absolutely regular intervals. 

262. The Effect of the Rapidity of Motion on the Musical ToneProduced. If the disk is rotated so slowly that less than about 16puffs are produced in one second, only separate puffs are heard, and amusical tone is lacking; if, on the other hand, the disk is rotated insuch a way that 16 puffs or more are produced in one second, theseparate puffs will blend together to produce a continuous musicalnote of very low pitch. If the speed of the disk is increased so thatthe puffs become more frequent, the pitch of the resulting note rises;and at very high speeds the notes produced become so shrill andpiercing as to be disagreeable to the ear. If the speed of the disk islessened, the pitch falls correspondingly; and if the speed againbecomes so low that less than 16 puffs are formed per second, thesustained sound disappears and a series of intermittent noises isproduced. 

263. The Pitch of a Note. By means of an apparatus called the siren, it is possible to calculate the number of vibrations producing anygiven musical note, such, for example, as middle C on the piano. Ifair is forced continuously against the disk as it rotates, a series ofpuffs will be heard (Fig. 177). 

If the disk turns fast enough, the puffs blend into a musical sound, whose pitch rises higher and higher as the disk moves faster andfaster, and produces more and more puffs each second. 

The instrument is so constructed that clockwork at the top registersthe number of revolutions made by the disk in one second. The numberof holes in the disk multiplied by the number of revolutions a secondgives the number of puffs of air produced in one second. If we wish tofind the number of vibrations which correspond to middle C on thepiano, we increase the speed of the disk until the note given forth bythe siren agrees with middle C as sounded on the piano, as nearly asthe ear can judge; we then calculate the number of puffs of air whichtook place each second at that particular speed of the disk. In thisway we find that middle C is due to about 256 vibrations per second;that is, a piano string must vibrate 256 times per second in order forthe resultant note to be of pitch middle C. In a similar manner wedetermine the following frequencies:--

 |do |re |mi |fa |sol |la |si |do | |C |D |E |F |G |A |B |C' | |256 |288 |320 |341 |384 |427 |480 |512 |

[Illustration: FIG. 177. --A siren. ]

The pitch of pianos, from the lowest bass note to the very highesttreble, varies from 27 to about 3500 vibrations per second. No humanvoice, however, has so great a range of tone; the highest sopranonotes of women correspond to but 1000 vibrations a second, and thedeepest bass of men falls but to 80 vibrations a second. 

While the human voice is limited in its production of sound, --rarelyfalling below 80 vibrations a second and rarely exceeding 1000vibrations a second, --the ear is by no means limited to that range inhearing. The chirrup of a sparrow, the shrill sound of a cricket, andthe piercing shrieks of a locomotive are due to far greaterfrequencies, the number of vibrations at times equaling 38, 000 persecond or more. 

264. The Musical Scale. When we talk, the pitch of the voice changesconstantly and adds variety and beauty to conversation; a speakerwhose tone, or pitch, remains too constant is monotonous and dull, nomatter how brilliant his thoughts may be. 

While the pitch of the voice changes constantly, the changes arenormally gradual and slight, and the different tones merge into eachother imperceptibly. In music, however, there is a well-definedinterval between even consecutive notes; for example, in the musicalscale, middle C (do) with 256 vibrations is followed by D (re) with288 vibrations, and the interval between these notes is sharp and wellmarked, even to an untrained ear. The interval between two notes isdefined as the ratio of the frequencies; hence, the interval between Cand D (do and re) is 288/256, or 9/8. Referring to Section 263, we seethat the interval between C and E is 320/256, or 5/4, and the intervalbetween C and C' is 512/256, or 2; the interval between any note andits octave is 2. 

The successive notes in one octave of the musical scale are related asfollows:--

 |Key of C |C |D |E |F |G |A |B |C' | |No. Of vibrations | | | | | | | | | |per sec. |256 |288 |320 |341 |384 |427 |480 |512 | |Interval |9/8 |5/4 |4/3 |3/2 |5/3 |15/8 |2 | |

The intervals of F and A are not strictly 4/3 and 5/3, but are nearlyso; if F made 341. 3 vibrations per second instead of 341; and if Amade 426. 6 instead of 427, then the intervals would be exactly 4/3 and5/3. Since the real difference is so slight, we can assume the simplerratios without appreciable error. 

Any eight notes whose frequencies are in the ratio of 9/8, 5/4, etc. , will when played in succession give the familiar musical scale; forexample, the deepest bass voice starts a musical scale whose noteshave the frequencies 80, 90, 100, 107, 120, 133, 150, 160, but theintervals here are identical with those of a higher scale; theinterval between C and D, 80 and 90, is 9/8, just as it was beforewhen the frequencies were much greater; that is, 256 and 288. Insinging "Home, Sweet Home, " for example, a bass voice may start with anote vibrating only 132 times a second; while a tenor may start at ahigher pitch, with a note vibrating 198 times per second, and asoprano would probably take a much higher range still, with an initialfrequency of 528 vibrations per second. But no matter where the voicesstart, the intervals are always identical. The air as sung by the bassvoice would be represented by _A_. The air as sung by the tenor voicewould be represented by _B_. The air as sung by the soprano voicewould be represented by _C_. 

[Illustration: FIG. 178. --A song as sung by three voices of differentpitch. ]

CHAPTER XXVIII

MUSICAL INSTRUMENTS

265. Musical instruments maybe divided into three groups accordingto the different ways in which their tones are produced:--

_First. _ The stringed instruments in which sound is produced by thevibration of stretched strings, as in the piano, violin, guitar, mandolin. 

_Second. _ The wind instruments in which sound is produced by thevibrations of definite columns of air, as in the organ, flute, cornet, trombone. 

_Third. _ The percussion instruments, in which sound is produced by themotion of stretched membranes, as in the drum, or by the motion ofmetal disks, as in the tambourines and cymbals. 

266. Stringed Instruments. If the lid of a piano is opened, numerouswires are seen within; some long, some short, some coarse, some fine. Beneath each wire is a small felt hammer connected with the keys insuch a way that when a key is pressed, a string is struck by a hammerand is thrown into vibration, thereby producing a tone. 

If we press the lowest key, that is, the key giving forth the lowestpitch, we see that the longest wire is struck and set into vibration;if we press the highest key, that is, the key giving the highestpitch, we see that the shortest wire is struck. In addition, it isseen that the short wires which produce the high tones are fine, while the long wires which produce the low tones are coarse. Theshorter and finer the wire, the higher the pitch of the tone produced. The longer and coarser the wire, the lower the pitch of the toneproduced. 

[Illustration: FIG. 179. --Piano wires seen from the back. ]

The constant striking of the hammers against the strings stretches andloosens them and alters their pitch; for this reason each string isfastened to a screw which can be turned so as to tighten the string orto loosen it if necessary. The tuning of the piano is the adjustmentof the strings so that each shall produce a tone of the right pitch. When the strings are tightened, the pitch rises; when the strings areloosened, the pitch falls. 

What has been said of the piano applies as well to the violin, guitar, and mandolin. In the latter instruments the strings are few in number, generally four, as against eighty-eight in the piano; the hammer ofthe piano is replaced in the violin by the bow, and in the guitar bythe fingers; varying pitches on any one string are obtained by slidinga finger of the left hand along the wire, and thus altering itslength. 

Frequent tuning is necessary, because the fine adjustments are easilydisturbed. The piano is the best protected of all the stringedinstruments, being inclosed by a heavy framework, even when in use. 

[Illustration: FIG. 180. --Front view of an open piano. ]

267. Strings and their Tones. Fasten a violin string to a woodenframe or box, as shown in Figure 181, stretching it by means of someconvenient weight; then lay a yardstick along the box in order thatthe lengths may be determined accurately. If the stretched string isplucked with the fingers or bowed with the violin bow, a clear musicalsound of definite pitch will be produced. Now divide the string intotwo equal parts by inserting the bridge midway between the two ends;and pluck either half as before. The note given forth is of adecidedly higher pitch, and if by means of the siren we compare thepitches in the two cases, we find that the note sounded by the halfwire is the octave of the note sounded by the entire wire; thefrequency has been doubled by halving the length. If now the bridge isplaced so that the string is divided into two unequal portions such as1:3 and 2:3, and the shorter portion is plucked, the pitch will bestill higher; the shorter the length plucked, the higher the pitchproduced. This movable bridge corresponds to the finger of theviolinist; the finger slides back and forth along the string, thuschanging the length of the bowed portion and producing variations inpitch. 

[Illustration: FIG. 181. --The length of a string influences thepitch. ]

[Illustration: FIG. 182. --Only one half of the string is bowed, butboth halves vibrate. ]

If there were but one string, only one pitch could be sounded at anyone time; the additional strings of the violin allow of thesimultaneous production of several tones. 

268. The Freedom of a String. Some stringed instruments give forthtones which are clear and sweet, but withal thin and lacking inrichness and fullness. The tones sounded by two different strings mayagree in pitch and loudness and yet produce quite different effects onthe ear, because in one case the tone may be much more pleasing thanin the other. The explanation of this is, that a string may vibrate ina number of different ways. 

Touch the middle of a wire with the finger or a pencil (Fig. 182), thus separating it into two portions and draw a violin bow across thecenter of either half. Only one half of the entire string is struck, but the motion of this half is imparted to the other half and throwsit into similar motion, and if a tiny A-shaped piece of paper or rideris placed upon the unbowed half, it is hurled off. 

[Illustration: FIG. 183. --The string vibrates in three portions. ]

If the wire is touched at a distance of one third its length and a bowis drawn across the middle of the smaller portion, the string willvibrate in three parts; we cannot always see these various motions indifferent parts of the string, but we know of their existence throughthe action of the riders. 

Similarly, touching the wire one fourth of its length from an endmakes it vibrate in four segments; touching it one fifth of its lengthmakes it vibrate in five segments. 

In the first case, the string vibrated as a whole string and also astwo strings of half the length; hence, three tones must have beengiven out, one tone due to the entire string and two tones due to thesegments. But we saw in Section 267 that halving the length of astring doubles the pitch of the resulting tone, and produces theoctave of the original tone; hence a string vibrating as in Figure 183gives forth three tones, one of which is the fundamental tone of thestring, and two of which are the octave of the fundamental tone. Hence, the vibrating string produces two sensations, that of thefundamental note and of its octave. 

[Illustration: FIG. 184. --When a string vibrates as a whole, it givesout the fundamental note. ]

When a string is plucked in the middle without being held, it vibratessimply as a whole (Fig. 184), and gives forth but one note; this iscalled the fundamental. If the string is made to vibrate in two parts, it gives forth two notes, the fundamental, and a note one octavehigher than the fundamental; this is called the first overtone. Whenthe string is made to move as in Figure 183, three distinct motionsare called forth, the motion of the entire string, the motion of theportion plucked, and the motion of the remaining unplucked portion ofthe string. Here, naturally, different tones arise, corresponding tothe different modes of vibration. The note produced by the vibrationof one third of the original string is called the second overtone. 

The above experiments show that a string is able to vibrate in anumber of different ways at the same time, and to emit simultaneouslya number of different tones; also that the resulting complex soundconsists of the fundamental and one or more overtones, and that thenumber of overtones present depends upon how and where the string isplucked. 

[Illustration: FIG. 185. --A string can vibrate in a number ofdifferent ways simultaneously, and can produce different notessimultaneously. ]

269. The Value of Overtones. The presence of overtones determinesthe quality of the sound produced. If the string vibrates as a wholemerely, the tone given out is simple, and seems dull andcharacterless. If, on the other hand, it vibrates in such a way thatovertones are present, the tone given forth is full and rich and thesensation is pleasing. A tuning fork cannot vibrate in more than oneway, and hence has no overtones, and its tone, while clear and sweet, is far less pleasing than the same note produced by a violin or piano. The untrained ear is not conscious of overtones and recognizes onlythe strong dominant fundamental. The overtones blend in with thefundamental and are so inconspicuously present that we do not realizetheir existence; it is only when they are absent that we become awareof the beauty which they add to the music. A song played on tuningforks instead of on strings would be lifeless and unsatisfying becauseof the absence of overtones. 

It is not necessary to hold finger or pencil at the points 1:3, 1:4, etc. , in order to cause the string to vibrate in various ways; if astring is merely plucked or bowed at those places, the result will bethe same. It is important to remember that no matter where a string ofdefinite length is bowed, the note most distinctly heard will be thefundamental; but the quality of the emitted tone will vary with thebowing. For example, if a string is bowed in the middle, the effectwill be far less pleasing than though it were bowed near the end. Inthe piano, the hammers are arranged so as to strike near one end ofthe string, at a distance of about 1:7 to 1:9; and hence a largenumber of overtones combine to reënforce and enrich the fundamentaltone. 

270. The Individuality of Instruments. It has been shown that apiano string when struck by a hammer, or a violin string when bowed, or a mandolin string when plucked, vibrates not only as a whole, butalso in segments, and as a result gives forth not a simple tone, as weare accustomed to think, but a very complex tone consisting of thefundamental and one or more overtones. If the string whose fundamentalnote is lower C (128 vibrations per second) is thrown into vibration, the tone produced may contain, in addition to the prominentfundamental, any one or more of the following overtones: C', G'', C'', E'', C''', etc. 

The number of overtones actually present depends upon a variety ofcircumstances: in the piano, it depends largely upon the location ofthe hammer; in the violin, upon the place and manner of bowing. Mechanical differences in construction account for prominent andnumerous overtones in some instruments and for feeble and fewovertones in others. The oboe, for example, is so constructed thatonly the high overtones are present, and hence the sound gives a"pungent" effect; the clarinet is so constructed that theeven-numbered overtones are killed, and the presence of onlyodd-numbered overtones gives individuality to the instrument. In thesetwo instruments we have vibrating air columns instead of vibratingstrings, but the laws which govern vibrating strings are applicable tovibrating columns of air, as we shall see later. It is really thepresence or absence of overtones which enables us to distinguish thenote of the piano from that of the violin, flute, or clarinet. Ifovertones could be eliminated, then middle C, or any other note on thepiano, would be indistinguishable from that same note sounded on anyother instrument. The fundamental note in every instrument is thesame, but the overtones vary with the instrument and lendindividuality to each. The presence of high overtones in the oboe andthe presence of odd-numbered overtones in the clarinet enable us todistinguish without fail the sounds given out by these instruments. 

The richness and individuality of an instrument are due, not only tothe overtones which accompany the fundamental, but also to the"forced" vibrations of the inclosing case, or of the sounding board. If a vibrating tuning fork is held in the hand, the sound will beinaudible except to those quite near; if, however, the base of thefork is held against the table, the sound is greatly intensified andbecomes plainly audible throughout the room. 

The vibrations of the fork are transmitted to the table top and throwit into vibrations similar to its own, and these additional vibrationsintensify the original sound. Any fork, no matter what its frequency, can force the surface of the table into vibration, and hence the soundof any fork will be intensified by contact with a table or box. 

This is equally true of strings; if stretched between two posts andbowed, the sound given out by a string is feeble, but if stretchedover a sounding board, as in the piano, or over a wooden shell, as inthe violin, the sound is intensified. Any note of the instrument willforce the sounding body to vibrate, thus reënforcing the volume ofsound, but some tones, or modes of vibration, do this more easily thanothers, and while the sounding board or shell always responds, itresponds in varying degree. Here again we have not only enrichment ofsound but also individuality of instruments. 

271. The Kinds of Stringed Instruments. Stringed instruments may begrouped in the following three classes:--

_a_. Instruments in which the strings are set into motion byhammers--piano. 

_b_. Instruments in which the strings are set into motion bybowing--violin, viola, violoncello, double bass. 

_c_. Instruments in which the strings are set into motion byplucking--harp, guitar, mandolin. 

[Illustration: FIG. 186. --1, violin; 2, viola; 3, violoncello; 4, double bass. ]

 _a_. The piano is too well known to need comment. In passing, it may be mentioned that in the construction of the modern concert piano approximately 40, 000 separate pieces of material are used. The large number of pieces is due, partly, to the fact that the single string corresponding to any one key is usually replaced by no less than three or four similar strings in order that greater volume of sound may be obtained. The hammer connected to a key strikes four or more strings instead of one, and hence produces a greater volume of tone. 

 _b_. The viola is larger than the violin, has heavier and thicker strings, and is pitched to a lower key; in all other respects the two are similar. The violoncello, because of the length and thickness of its strings, is pitched a whole octave lower than the violin; otherwise it is similar. The unusual length and thickness of the strings of the double bass make it produce very low notes, so that it is ordinarily looked upon as the "bass voice" of the orchestra. 

 _c_. The harp has always been considered one of the most pleasing and perfect of musical instruments. Here the skilled performer has absolutely free scope for his genius, because his fingers can pluck the strings at will and hence regulate the overtones, and his feet can regulate at will the tension, and hence the pitch of the strings. 

 Guitar and mandolin are agreeable instruments for amateurs, but are never used in orchestral music. 

[Illustration: FIG. 187. --A harp. ]

272. Wind Instruments. In the so-called wind instruments, sound isproduced by vibrating columns of air inclosed in tubes or pipes ofdifferent lengths. The air column is thrown into vibration eitherdirectly, by blowing across a narrow opening at one end of a pipe asin the case of the whistle, or indirectly, by exciting vibrations in athin strip of wood or metal, called a reed, which in turn communicatesits vibrations to the air column within. 

The shorter the air column, the higher the pitch. This agrees with thelaw of vibrating strings which gives high pitches for short lengths. 

[Illustration: FIG. 188. --Open organ pipes of different pitch. ]

The pitch of the sound emitted by a column of air vibrating within apipe varies according to the following laws:

1. The shorter the pipe, the higher the pitch. 

2. The pitch of a note emitted by an open pipe is one octave higherthan that of a closed pipe of equal length. 

3. Air columns vibrate in segments just as do strings, and the toneemitted by a pipe of given length is complex, consisting of thefundamental and one or more overtones. The greater the number ofovertones present, the richer the tone produced. 

273. How the Various Pitches are Produced. With a pipe of fixedlength, for example, the clarinet (Fig. 189, 1), different pitches areobtained by pressing keys which open holes in the tube and thusshorten or lengthen the vibrating air column and produce a rise orfall in pitch. Changes in pitch are also produced by variation in theplayer's breathing. By blowing hard or gently, the number ofvibrations of the reed is increased or decreased and hence the pitchis altered. 

[Illustration: FIG. 189--1, clarinet; 2, oboe; 3, flute. ]

In the oboe (Fig. 189, 2) the vibrating air column is set into motionby means of two thin pieces of wood or metal placed in the mouthpieceof the tube. Variations in pitch are produced as in the clarinet bymeans of stops and varied breathing. In the flute, the air is set intomotion by direct blowing from the mouth, as is done, for instance, when we blow into a bottle or key. 

The sound given out by organ pipes is due to air blown across a sharpedge at the opening of a narrow tube. The air forced across the sharpedge is thrown into vibration and communicates its vibration to theair within the organ pipe. For different pitches, pipes of differentlengths are used: for very low pitches long, closed pipes are used;for very high pitches short, open pipes are used. The mechanism of theorgan is such that pressing a key allows the air to rush into thecommunicating pipe and a sound is produced characteristic of thelength of the pipe. 

[Illustration: FIG. 190. --1, horn; 2, trumpet; 3, trombone. ]

[Illustration: FIG. 191. --1, kettledrum; 2, bass drum; 3, cymbals. ]

[Illustration: FIG. 192. --The seating arrangement of the Philadelphiaorchestra. ]

In the brass wind instruments such as horn, trombone, and trumpet, thelips of the player vibrate and excite the air within. Varying pitchesare obtained partly by the varying wind pressure of the musician; ifhe breathes fast, the pitch rises; if he breathes slowly, the pitchfalls. All of these instruments, however, except the trombone possesssome valves which, on being pressed, vary the length of the tube andalter the pitch accordingly. In the trombone, valves are replaced by asection which slides in and out and shortens or lengthens the tube. 

274. The Percussion Instruments. The percussion instruments, including kettledrums, bass drums, and cymbals, are the leastimportant of all the musical instruments; and are usually of servicemerely in adding to the excitement and general effect of an orchestra. 

In orchestral music the various instruments are grouped somewhat asshown in Figure 192. 

CHAPTER XXIX

SPEAKING AND HEARING

[Illustration: FIG. 193. --The vibration of the vocal cords producesthe sound of the human voice. ]

275. Speech. The human voice is the most perfect of musicalinstruments. Within the throat, two elastic bands are attached to thewindpipe at the place commonly called Adam's apple; these flexiblebands have received the name of vocal cords, since by their vibrationall speech is produced. In ordinary breathing, the cords are loose andare separated by a wide opening through which air enters and leavesthe lungs. When we wish to speak, muscular effort stretches the cords, draws them closer together, and reduces the opening between them to anarrow slit, as in the case of the organ pipe. If air from the lungsis sent through the narrow slit, the vocal cords or bands are throwninto rapid vibration and produce sound. The pitch of the sound dependsupon the tension of the stretched membranes, and since this can bealtered by muscular action, the voice can be modulated at will. Intimes of excitement, when the muscles of the body in general are in astate of great tension, the pitch is likely to be uncommonly high. 

Women's voices are higher than men's because the vocal cords areshorter and finer; even though muscular tension is relaxed and thecords are made looser, the pitch of a woman's voice does not fall solow as that of a man's voice since his cords are naturally muchlonger and coarser. The difference between a soprano and an alto voiceis merely one of length and tension of the vocal cords. 

Successful singing is possible only when the vocal cords are readilyflexible and when the singer can supply a steady, continuous blast ofair through the slit between the cords. The hoarseness whichfrequently accompanies cold in the head is due to the thickening ofthe mucous membrane and to the filling up of the slit with mucus, because when this happens, the vocal cords cannot vibrate properly. 

The sounds produced by the vocal cords are transformed into speech bythe help of the tongue and lips, which modify the shape of the mouthcavity. Some of the lower animals have a speaking apparatus similar toour own, but they cannot perfectly transform sound into speech. Thebirds use their vocal cords to beautiful advantage in singing, farsurpassing us in many ways, but the power of speech is lacking. 

276. The Ear. The pulses created in the air by a sounding body arereceived by the ear and the impulses which they impart to the auditorynerve pass to the brain and we become conscious of a sound. The ear iscapable of marvelous discrimination and accuracy. "In order to form anidea of the extent of this power imagine an auditor in a large musichall where a full band and chorus are performing. Here, there aresounds mingled together of all varieties of pitch, loudness, andquality; stringed instruments, wood instruments, brass instruments, and voices, of many different kinds. And in addition to these theremay be all sorts of accidental and irregular sounds and noises, suchas the trampling and shuffling of feet, the hum of voices, the rustleof dress, the creaking of doors, and many others. Now it must beremembered that the only means the ear has of becoming aware of thesesimultaneous sounds is by the condensations and rarefactions whichreach it; and yet when the sound wave meets the nerves, the nervessingle out each individual element, and convey to the mind of thehearer, not only the tones and notes of every instrument in theorchestra, but the character of every accidental noise; and almost asdistinctly as if each single tone or noise were heard alone. "--POLE. 

[Illustration: FIG. 194. --The ear. ]

277. The Structure of the Ear. The external portion of the ear actsas a funnel for catching sound waves and leading them into the canal, where they strike upon the ear drum, or tympanic membrane, and throwit into vibration. Unless the ear drum is very flexible there cannotbe perfect response to the sound waves which fall upon it; for thisreason, the glands of the canal secrete a wax which moistens themembrane and keeps it flexible. Lying directly back of the tympanicmembrane is a cavity filled with air which enters by the Eustachiantube; from the throat air enters the Eustachian tube, moves along it, and passes into the ear cavity. The dull crackling noise noticed inthe ear when one swallows is due to the entrance and exit of air inthe tube. Several small bones stretch across the upper portion of thecavity and make a bridge, so to speak, from the ear drum to the farwall of the cavity. It is by means of these three bones that thevibrations of the ear drum are transmitted to the inner wall of thecavity. Behind the first cavity is a second cavity so complex andirregular that it is called the labyrinth of the ear. This labyrinthis filled with a fluid in which are spread out the delicate sensitivefibers of the auditory nerves; and it is to these that the vibrationsmust be transmitted. 

Suppose a note of 800 vibrations per second is sung. Then 800 pulsesof air will reach the ear each second, and the ear drum, beingflexible, will respond and will vibrate at the same rate. Thevibration of the ear drum will be transmitted by the three bones andthe fluid to the fibers of the auditory nerves. The impulses impartedto the auditory nerve reach the brain and in some unknown way aretranslated into sound. 

278. Care of the Ear. Most catarrhal troubles are accompanied by anoversupply of mucus which frequently clogs up the Eustachian tube andproduces deafness. For the same reason, colds and sore throatsometimes induce temporary deafness. 

The wax of the ear is essential for flexibility of the ear drum; if anextra amount accumulates, it can be got rid of by bathing the ear inhot water, since the heat will melt the wax. The wax should never bepicked out with pin or sharp object except by a physician, lest injurybe done to the tympanic membrane. 

279. The Phonograph. The invention of the phonograph by Edison in1878 marked a new era in the popularity and dissemination of music. Upto that time, household music was limited to those who were richenough to possess a real musical instrument, and who in addition hadthe understanding and the skill to use the instrument. The inventionof the phonograph has brought music to thousands of homes possessedof neither wealth nor skill. That the music reproduced by a phonographis not always of the highest order does not, in the least, detractfrom the interest and wonder of the instrument. It can reproduce whatit is called upon to reproduce, and if human nature demands thecommonplace, the instrument will be made to satisfy the demand. On theother hand, speeches of famous men, national songs, magnificent operaselections, and other pleasing and instructive productions can bereproduced fairly accurately. In this way the phonograph, perhaps morethan any other recent invention, can carry to the "shut-ins" a livelyglimpse of the outside world and its doings. 

[Illustration: FIG. 195. --A vibrating tuning fork traces a curved lineon smoked glass. ]

The phonograph consists of a cylinder or disk of wax upon which thevibrations of a sensitive diaphragm are recorded by means of a finemetal point. The action of the pointer in reporting the vibrations ofa diaphragm is easily understood by reference to a tuning fork. Fastena stiff bristle to a tuning fork by means of wax, allowing the end ofthe point to rest lightly upon a piece of smoked glass. If the glassis drawn under the bristle a straight line will be scratched on theglass, but if the tuning fork is struck so that the prongs vibrateback and forth, then the straight line changes to a wavy line and thetype of wavy line depends upon the fork used. 

In the phonograph, a diaphragm replaces the tuning fork and a cylinder(or a disk) coated with wax replaces the glass plate. When the speakertalks or the singer sings, his voice strikes against a delicatediaphragm and throws it into vibration, and the metal point attachedto it traces on the wax of a moving cylinder a groove of varying shapeand appearance called the "record. " Every variation in the speaker'svoice is repeated in the vibrations of the metal disk and hence in theminute motion of the pointer and in the consequent record on thecylinder. The record thus made can be placed in any other phonographand if the metal pointer of this new phonograph is made to pass overthe tracing, the process is reversed and the speaker's voice isreproduced. The sound given out in the this way is faint and weak, butcan be strengthened by means of a trumpet attached to the phonograph. 

[Illustration: FIG. 196. --A phonograph. In this machine the cylinderis replaced by a revolving disk. ]

CHAPTER XXX

ELECTRICITY

280. Many animals possess the five senses, but only man possessesconstructive, creative power, and is able to build on the informationgained through the senses. It is the constructive, creative powerwhich raises man above the level of the beast and enables him todevise and fashion wonderful inventions. Among the most important ofhis inventions are those which relate to electricity; inventions suchas trolley car, elevator, automobile, electric light, the telephone, the telegraph. Bell, by his superior constructive ability, madepossible the practical use of the telephone, and Marconi that ofwireless telegraphy. To these inventions might be added many otherswhich have increased the efficiency and production of the businessworld and have decreased the labor and strain of domestic life. 

[Illustration: FIG. 197. --A simple electric cell. ]

281. Electricity as first Obtained by Man. Until modern times theonly electricity known to us was that of the lightning flash, whichman could neither hinder nor make. But in the year 1800, electricityin the form of a weak current was obtained by Volta of Italy in a verysimple way; and even now our various electric batteries and cells arebut a modification of that used by Volta and called a voltaic cell. Astrip of copper and a strip of zinc are placed in a glass containingdilute sulphuric acid, a solution composed of oxygen, hydrogen, sulphur, and water. As soon as the plates are immersed in the acidsolution, minute bubbles of gas rise from the zinc strip and it beginsto waste away slowly. The solution gradually dissolves the zinc and atthe same time gives up some of the hydrogen which it contains; but ithas little or no effect on the copper, since there is no visiblechange in the copper strip. 

If, now, the strips are connected by means of metal wires, the zincwastes away rapidly, numerous bubbles of hydrogen pass over to thecopper strip and collect on it, and a current of electricity flowsthrough the connecting wires. Evidently, the source of the current isthe chemical action between the zinc and the liquid. 

Mere inspection of the connecting wire will not enable us to detectthat a current is flowing, but there are various ways in which thecurrent makes itself evident. If the ends of the wires attached to thestrips are brought in contact with each other and then separated, afaint spark passes, and if the ends are placed on the tongue, a twingeis felt. 

282. Experiments which grew out of the Voltaic Cell. Since chemicalaction on the zinc is the source of the current, it would seemreasonable to expect a current if the cell consisted of two zincplates instead of one zinc plate and one copper plate. But when thecopper strip is replaced by a zinc strip so that the cell consists oftwo similar plates, no current flows between them. In this case, chemical action is expended in heat rather than in the production ofelectricity and the liquid becomes hot. But if carbon and zinc areused, a current is again produced, the zinc dissolving away as before, and bubbles collecting on the carbon plate. By experiment it has beenfound that many different metals may be employed in the constructionof an electric cell; for example, current may be obtained from a cellmade with a zinc plate and a platinum plate, or from a cell made witha lead plate and a copper plate. Then, too, some other chemical, suchas bichromate of potassium, or ammonium chloride, may be used insteadof dilute sulphuric acid. 

Almost any two different substances will, under proper conditions, give a current, but the strength of the current is in some cases soweak as to be worthless for practical use, such as telephoning, orringing a door bell. What is wanted is a strong, steady current, andour choice of material is limited to the substances which will givethis result. Zinc and lead can be used, but the current resulting isweak and feeble, and for general use zinc and carbon are the mostsatisfactory. 

283. Electrical Terms. The plates or strips used in making anelectric cell are called electrodes; the zinc is called the negativeelectrode (-), and the carbon the positive electrode (+); the currentis considered to flow through the wire from the + to the-electrode. Asa rule, each electrode has attached to it a binding post to whichwires can be quickly fastened. 

The power that causes the current is called the electromotive force, and the value of the electromotive force, generally written E. M. F. , ofa cell depends upon the materials used. 

When the cell consists of copper, zinc, and dilute sulphuric acid, theelectromotive force has a definite value which is always the same nomatter what the size or shape of the cell. But the E. M. F. Has adecidedly different value in a cell composed of iron, copper, andchromic acid. Each combination of material has its own specificelectromotive force. 

284. The Disadvantage of a Simple Cell. When the poles of a simplevoltaic cell are connected by a wire, the current thus producedslowly diminishes in strength and, after a short time, becomes feeble. Examination of the cell shows that the copper plate is covered withhydrogen bubbles. If, however, these bubbles are completely brushedaway by means of a rod or stick, the current strength increases, butas the bubbles again gather on the + electrode the current strengthdiminishes, and when the bubbles form a thick film on the copperplate, the current is too weak to be of any practical value. The filmof bubbles weakens the current because it practically substitutes ahydrogen plate for a copper plate, and we saw in Section 282 that achange in any one of the materials of which a cell is composed changesthe current. 

This weakening of the current can be reduced mechanically by brushingaway the bubbles as soon as they are formed; or chemically, bysurrounding the copper plate with a substance which will combine withthe free hydrogen and prevent it from passing onward to the copperplate. 

[Illustration: FIG 198. The gravity cell. ]

In practically all cells, the chemical method is used in preference tothe mechanical one. The numerous types of cells in daily use differchiefly in the devices employed for preventing the formation ofhydrogen bubbles, or for disposing of them when formed. One of thebest-known cells in which weakening of the current is prevented bychemical means is the so-called gravity cell. 

285. The Gravity Cell. A large, irregular copper electrode is placedin the bottom of a jar (Fig. 198), and completely covered with asaturated solution of copper sulphate. Then a large, irregular zincelectrode is suspended from the top of the jar, and is completelycovered with dilute sulphuric acid which does not mix with the coppersulphate, but floats on the top of it like oil on water. The hydrogenformed by the chemical action of the dilute sulphuric acid on the zincmoves toward the copper electrode, as in the simple voltaic cell. Itdoes not reach the electrode, however, because, when it comes incontact with the copper sulphate, it changes places with the copperthere, setting it free, but itself entering into the solution. Thecopper freed from the copper sulphate solution travels to the copperelectrode, and is deposited on it in a clean, bright layer. Instead ofa deposit of hydrogen there is a deposit of copper, and falling off incurrent is prevented. 

The gravity cell is cheap, easy to construct, and of constantstrength, and is in almost universal use in telegraphic work. Practically all small railroad stations and local telegraph officesuse these cells. 

[Illustration: FIG. 199. --A dry cell. ]

286. Dry Cells. The gravity cell, while cheap and effective, isinconvenient for general use, owing to the fact that it cannot beeasily transported, and the _dry cell_ has largely supplanted allothers, because of the ease with which it can be taken from place toplace. This cell consists of a zinc cup, within which is a carbon rod;the space between the cup and rod is packed with a moist pastecontaining certain chemicals. The moist paste takes the place of theliquids used in other cells. 

[Illustration: FIG. 200. --A battery of three cells. ]

287. A Battery of Cells. The electromotive force of one cell may notgive a current strong enough to ring a door bell or to operate atelephone. But by using a number of cells, called a battery, thecurrent may be increased to almost any desired strength. If threecells are arranged as in Figure 200, so that the copper of one cell isconnected with the zinc of another cell, the electromotive force ofthe battery will be three times as great as the E. M. F. Of a singlecell. If four cells are arranged in the same way, the E. M. F. Of thebattery is four times as great as the E. M. F. Of a single cell; whenfive cells are combined, the resulting E. M. F. Is five times as great. 

CHAPTER XXXI

SOME USES OF ELECTRICITY

288. Heat. Any one who handles electric wires knows that they aremore or less heated by the currents which flow through them. If threecells are arranged as in Figure 200 and the connecting wire is coarse, the heating of the wire is scarcely noticeable; but if a shorter wireof the same kind is used, the heat produced is slightly greater; andif the coarse wire is replaced by a short, fine wire, the heating ofthe wire becomes very marked. We are accustomed to say that a wireoffers resistance to the flow of a current; that is, whenever acurrent meets resistance, heat is produced in much the same way aswhen mechanical motion meets an obstacle and spends its energy infriction. The flow of electricity along a wire can be compared to theflow of water through pipes: a small pipe offers a greater resistanceto the flow of water than a large pipe; less water can be forcedthrough a small pipe than through a large pipe, but the friction ofthe water against the sides of the small pipe is much greater than inthe large one. 

So it is with the electric current. In fine wires the resistance tothe current is large and the energy of the battery is expended in heatrather than in current. If the heat thus produced is very great, serious consequences may arise; for example, the contact of a hot wirewith wall paper or dry beams may cause fire. Insurance companiesdemand that the wires used in wiring a building for electric lights beof a size suitable to the current to be carried, otherwise they willnot take the risk of insurance. The greater the current to be carried, the coarser is the wire required for safety. 

289. Electric Stoves. It is often desirable to utilize the electriccurrent for the production of heat. For example, trolley cars areheated by coils of wire under the seats. The coils offer so muchresistance to the passage of a strong current through them that theybecome heated and warm the cars. 

[Illustration: FIG. 201. --An electric iron on a metal stand. ]

Some modern houses are so built that electricity is received into themfrom the great plants where it is generated, and by merely turning aswitch or inserting a plug, electricity is constantly available. Inconsequence, many practical applications of electricity are possible, among which are flatiron and toaster. 

[Illustration: FIG. 202. --The fine wires are strongly heated by thecurrent which flows through them. ]

Within the flatiron (Fig. 201), is a mass of fine wire coiled as shownin Figure 202; as soon as the iron is connected with the house supplyof electricity, current flows through the fine wire which thus becomesstrongly heated and gives off heat to the iron. The iron, when onceheated, retains an even temperature as long as the current flows, andthe laundress is, in consequence, free from the disadvantages of aslowly cooling iron, and of frequent substitution of a warm iron for acold one. Electric irons are particularly valuable in summer, becausethey eliminate the necessity for a strong fire, and spare thehousewife intense heat. In addition, the user is not confined to thelaundry, but is free to seek the coolest part of the house, the onlyrequisite being an electrical connection. 

[Illustration: FIG. 203. --Bread can be toasted by electricity. ]

The toaster (Fig. 203) is another useful electrical device, since bymeans of it toast may be made on a dining table or at a bedside. Thesmall electrical stove, shown in Figure 204, is similar in principleto the flatiron, but in it the heating coil is arranged as shown inFigure 205. To the physician electric stoves are valuable, since hisinstruments can be sterilized in water heated by the stove; and thatwithout fuel or odor of gas. 

A convenient device is seen in the heating pad (Fig. 206), asubstitute for a hot water bag. Embedded in some soft thick substanceare the insulated wires in which heat is to be developed, and overthis is placed a covering of felt. 

[Illustration: FIG. 204. --An electric stove. ]

290. Electric Lights. The incandescent bulbs which illuminate ourbuildings consist of a fine, hairlike thread inclosed in a glass bulbfrom which the air has been removed. When an electric current is sentthrough the delicate filament, it meets a strong resistance. The heatdeveloped in overcoming the resistance is so great that it makes thefilament a glowing mass. The absence of air prevents the filament fromburning, and it merely glows and radiates the light. 

[Illustration: FIG. 205. --The heating element in the electric stove. ]

291. Blasting. Until recently, dynamiting was attended with seriousdanger, owing to the fact that the person who applied the torch to thefuse could not make a safe retreat before the explosion. Now a finewire is inserted in the fuse, and when everything is in readiness, the ends of the wire are attached to the poles of a distant batteryand the heat developed in the wire ignites the fuse. 

[Illustration: FIG. 206. --An electric pad serves the same purpose as ahot water bag. ]

292. Welding of Metals. Metals are fused and welded by the use ofthe electric current. The metal pieces which are to be welded arepressed together and a powerful current is passed through theirjunction. So great is the heat developed that the metals melt andfuse, and on cooling show perfect union. 

293. Chemical Effects. _The Plating of Gold, Silver, and OtherMetals. _ If strips of lead or rods of carbon are connected to theterminals of an electric cell, as in Figure 208, and are then dippedinto a solution of copper sulphate, the strip in connection with thenegative terminal of the cell soon becomes thinly plated with acoating of copper. If a solution of silver nitrate is used in place ofthe copper sulphate, the coating formed will be of silver instead ofcopper. So long as the current flows and there is any metal present inthe solution, the coating continues to form on the negative electrode, and becomes thicker and thicker. 

[Illustration: FIG. 207. --An incandescent electric bulb. ]

The process by which metal is taken out of solution, as silver out ofsilver nitrate and copper out of copper sulphate, and is in turndeposited as a coating on another substance, is called electroplating. An electric current can separate a liquid into some of its variousconstituents and to deposit one of the metal constituents on thenegative electrode. 

[Illustration: FIG. 208. --Carbon rods in a solution of coppersulphate. ]

Since copper is constantly taken out of the solution of coppersulphate for deposit upon the negative electrode, the amount of copperremaining in the solution steadily decreases, and finally there isnone of it left for deposit. In order to overcome this, the positiveelectrode should be made of the same metal as that which is to bedeposited. The positive metal electrode gradually dissolves andreplaces the metal lost from the solution by deposit andelectroplating can continue as long as any positive electrode remains. 

[Illustration: FIG. 209. --Plating spoons by electricity. ]

Practically all silver, gold, and nickel plating is done in this way;machine, bicycle, and motor attachments are not solid, but are ofcheaper material electrically plated with nickel. When spoons are tobe plated, they are hung in a bath of silver nitrate side by side witha thick slab of pure silver, as in Figure 209. The spoons areconnected with the negative terminal of the battery, while the slab ofpure silver is connected with the positive terminal of the samebattery. The length of time that the current flows determines thethickness of the plating. 

294. How Pure Metal is obtained from Ore. When ore is mined, itcontains in addition to the desired metal many other substances. Inorder to separate out the desired metal, the ore is placed in somesuitable acid bath, and is connected with the positive terminal of abattery, thus taking the place of the silver slab in the last Section. When current flows, any pure metal which is present is dissolved outof the ore and is deposited on a convenient negative electrode, whilethe impurities remain in the ore or drop as sediment to the bottom ofthe vessel. Metals separated from the ore by electricity are calledelectrolytic metals and are the purest obtainable. 

295. Printing. The ability of the electric current to decompose aliquid and to deposit a metal constituent has practicallyrevolutionized the process of printing. Formerly, type was arrangedand retained in position until the required number of impressions hadbeen made, the type meanwhile being unavailable for other uses. Moreover, the printing of a second edition necessitated practically asgreat labor as did the first edition, the type being necessarily setafresh. Now, however, the type is set up and a mold of it is taken inwax. This mold is coated with graphite to make it a conductor and isthen suspended in a bath of copper sulphate, side by side with a slabof pure copper. Current is sent through the solution as described inSection 293, until a thin coating of copper has been deposited on themold. The mold is then taken from the bath, and the wax is replaced bysome metal which gives strength and support to the thin copper plate. From this copper plate, which is an exact reproduction of the originaltype, many thousand copies can be printed. The plate can be preservedand used from time to time for later editions, and the original typecan be put back into the cases and used again. 

CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS

296. An Electric Current acts like a Magnet. In order to understandthe action of the electric bell, we must consider a third effect whichan electric current can cause. Connect some cells as shown in Figure200 and close the circuit through a stout heavy copper wire, dipping aportion of the wire into fine iron filings. A thick cluster of filingswill adhere to the wire (Fig. 210), and will continue to cling to itso long as the current flows. If the current is broken, the filingsfall from the wire, and only so long as the current flows through thewire does the wire have power to attract iron filings. An electriccurrent makes a wire equivalent to a magnet, giving it the power toattract iron filings. 

[Illustration: FIG. 210. --A wire carrying current attracts ironfilings. ]

[Illustration: FIG. 211. --A loosely wound coil of wire. ]

Although such a straight current bearing wire attracts iron filings, its power of attraction is very small; but its magnetic strength canbe increased by coiling as in Figure 211. Such an arrangement of wireis known as a helix or solenoid, and is capable of lifting or pullinglarger and more numerous filings and even good-sized pieces of iron, such as tacks. Filings do not adhere to the sides of the helix, butthey cling in clusters to the ends of the coil. This shows that theends of the helix have magnetic power but not the sides. 

If a soft iron nail (Fig. 212) or its equivalent is slipped within thecoil, the lifting and attractive power of the coil is increased, andcomparatively heavy weights can be lifted. 

[Illustration: FIG. 212. --Coil and soft iron rod. ]

A coil of wire traversed by an electric current and containing a coreof soft iron has the power of attracting and moving heavy ironobjects; that is, it acts like a magnet. Such an arrangement is calledan electromagnet. As soon as the current ceases to flow, theelectromagnet loses its magnetic power and becomes merely iron andwire without magnetic attraction. 

If many cells are used, the strength of the electromagnet isincreased, and if the coil is wound closely, as in Figure 213, insteadof loosely, as in Figure 211, the magnetic strength is still furtherincreased. The strength of any electromagnet depends upon the numberof coils wound on the iron core and upon the strength of the currentwhich is sent through the coils. 

[Illustration: FIG. 213. --An electromagnet. ]

[Illustration: FIG. 214. --A horseshoe electromagnet is powerful enoughto support heavy weights. ]

To increase the strength of the electromagnet still further, theso-called horseshoe shape is used (Fig. 214). In such an arrangementthere is practically the strength of two separate electromagnets. 

297. The Electric Bell. The ringing of the electric bell is due tothe attractive power of an electromagnet. By the pushing of a button(Fig. 215) connection is made with a battery, and current flowsthrough the wire wound on the iron spools, and further to the screw_P_ which presses against the soft iron strip or armature _S_; andfrom _S_ the current flows back to the battery. As soon as thecurrent flows, the coils become magnetic and attract the soft ironarmature, drawing it forward and causing the clapper to strike thebell. In this position, _S_ no longer touches the screw _P_, and hencethere is no complete path for the electricity, and the current ceases. But the attractive, magnetic power of the coils stops as soon as thecurrent ceases; hence there is nothing to hold the armature down, andit flies back to its former position. In doing this, however, thearmature makes contact at _P_ through the spring, and the currentflows once more; as a result the coils again become magnets, thearmature is again drawn forward, and the clapper again strikes thebell. But immediately afterwards the armature springs backward andmakes contact at _P_ and the entire operation is repeated. So long aswe press the button this process continues producing what sounds likea continuous jingle; in reality the clapper strikes the bell everytime a current passes through the electromagnet. 

[Illustration: FIG. 215. --The electric bell. ]

298. The Push Button. The push button is an essential part of everyelectric bell, because without it the bell either would not ring atall, or would ring incessantly until the cell was exhausted. When thepush button is free, as in Figure 216, the cell terminals are notconnected in an unbroken path, and hence the current does not flow. When, however, the button is pressed, the current has a complete path, provided there is the proper connection at _S_. That is, the pressureon the push button permits current to flow to the bell. The flow ofthis current then depends solely upon the connection at _S_, which isalternately made and broken, and in this way produces sound. 

[Illustration: FIG. 216. --Push button. ]

The sign "Bell out of order" is usually due to the fact that thebattery is either temporarily or permanently exhausted. In warmweather the liquid in the cell may dry up and cause stoppage of thecurrent. If fresh liquid is poured into the vessel so that thechemical action of the acid on the zinc is renewed, the current againflows. Another explanation of an out-of-order bell is that the liquidmay have eaten up all the zinc; if this is the case, the insertion ofa fresh strip of zinc will remove the difficulty and the current willflow. If dry cells are used, there is no remedy except in the purchaseof new cells. 

299. How Electricity may be lost to Use. In the electric bell, wesaw that an air gap at the push button stopped the flow ofelectricity. If we cut the wire connecting the poles of a battery, thecurrent ceases because an air gap intervenes and electricity does notreadily pass through air. Many substances besides air stop the flow ofelectricity. If a strip of glass, rubber, mica, or paraffin isintroduced anywhere in a circuit, the current ceases. If a metal isinserted in the gap, the current again flows. Substances which, likean air gap, interfere with the flow of electricity are callednon-conductors, or, more commonly, insulators. Substances which, likethe earth, the human body, and all other moist objects, conductelectricity are conductors. If the telephone and electric light wiresin our houses were not insulated by a covering of thread, or cloth, orother non conducting material, the electricity would escape intosurrounding objects instead of flowing through the wire and producingsound and light. 

In our city streets, the overhead wires are supported on glass knobsor are closely wrapped, in order to prevent the escape of electricitythrough the poles to the ground. In order to have a steady, dependablecurrent, the wire carrying the current must be insulated. 

Lack of insulation means not only the loss of current for practicaluses, but also serious consequences in the event of the crossing ofcurrent-bearing wires. If two wires properly insulated touch eachother, the currents flow along their respective wires unaltered; if, however, two uninsulated wires touch, some of the electricity flowsfrom one to the other. Heat is developed as a result of thistransference, and the heat thus developed is sometimes so great thatfire occurs. For this reason, wires are heavily insulated and extraprotection is provided at points where numerous wires touch or cross. 

Conductors and insulators are necessary to the efficient and economicflow of a current, the insulator preventing the escape of electricityand lessening the danger of fire, and the conductor carrying thecurrent. 

300. The Telegraph. Telegraphy is the process of transmittingmessages from place to place by means of an electric current. Theprinciple underlying the action of the telegraph is the principle uponwhich the electric bell operates; namely, that a piece of soft ironbecomes a magnet while a current flows around it, but loses itsmagnetism as soon as the current ceases. 

In the electric bell, the electromagnet, clapper, push button, andbattery are relatively near, --usually all are located in the samebuilding; while in the telegraph the current may travel miles beforeit reaches the electromagnet and produces motion of the armature. 

[Illustration: FIG. 217. --Diagram of the electric telegraph. ]

The fundamental connections of the telegraph are shown in Figure 217. If the key _K_ is pressed down by an operator in Philadelphia, thecurrent from the battery (only one cell is shown for simplicity) flowsthrough the line to New York, passes through the electromagnet _M_, and thence back to Philadelphia. As long as the key _K_ is presseddown, the coil _M_ acts as a magnet and attracts and holds fast thearmature _A_; but as soon as _K_ is released, the current is broken, _M_ loses its magnetism, and the armature is pulled back by the spring_D_. By a mechanical device, tape is drawn uniformly under the lightmarker _P_ attached to the armature. If _K_ is closed for but a shorttime, the armature is drawn down for but a short interval, and themarker registers a dot on the tape. If _K_ is closed for a longertime, a short dash is made by the marker, and, in general, the lengthof time that _K_ is closed determines the length of the marks recordedon the tape. The telegraphic alphabet consists of dots and dashes andtheir various combinations, and hence an interpretation of the dot anddash symbols recorded on the tape is all that is necessary for thereceiving of a telegraphic message. 

The Morse telegraphic code, consisting of dots, dashes, and spaces, isgiven in Figure 218. 

[Illustration:

 |A . - |H . .. . |O . . |U . . - | |B -. .. |I . . |P . .. .. |V . .. - | |C . . . |J -. -. |Q . . -. |W . -- | |D -. . |K -. - |R . . . |X . -. . | |E . |L --- |S . .. |Y . . . . | |F . -. |M - - |T - |Z . .. . | |G --. |N -. | | |

FIG. 218. --The Morse telegraphic code. ]

The telegraph is now such a universal means of communication betweendistant points that one wonders how business was conducted before itsinvention in 1832 by S. F. B. Morse. 

[Illustration: FIG. 219. --The sounder. ]

301. Improvements. _The Sounder. _ Shortly after the invention oftelegraphy, operators learned that they could read the message by theclick of the marker against a metal rod which took the place of thetape. In practically all telegraph offices of the present day theold-fashioned tape is replaced by the sounder, shown in Figure 219. When current flows, a lever, _L_, is drawn down by the electromagnetand strikes against a solid metal piece with a click; when the currentis broken, the lever springs upward, strikes another metal piece andmakes a different click. It is clear that the working of the key whichstarts and stops the current in this line will be imitated by themotion and the resulting clicks of the sounder. By means of thesevarying clicks of the sounder, the operator interprets the message. 

[Illustration: FIG. 220. --Diagram of a modern telegraph system. ]

_The Relay. _ When a telegraph line is very long, the resistance of thewire is great, and the current which passes through the electromagnetis correspondingly weak, so feeble indeed that the armature must bemade very thin and light in order to be affected by the makes andbreaks in the current. The clicks of an armature light enough torespond to the weak current of a long wire are too faint to berecognized by the ear, and hence in such long circuits some devicemust be introduced whereby the effect is increased. This is usuallydone by installing at each station a local battery and a very delicateand sensitive electromagnet called the _relay_. Under these conditionsthe current of the main line is not sent through the sounder, butthrough the relay which opens and closes a local battery in connectionwith the strong sounder. For example, the relay is so arranged thatcurrent from the main line runs through it exactly as it runs through_M_ in Figure 217. When current is made, the relay attracts anarmature, which thereby closes a circuit in a local battery and thuscauses a click of the sounder. When the current in the main line isbroken, the relay loses its magnetic attraction, its armature springsback, connection is broken in the local circuit, and the sounderresponds by allowing its armature to spring back with a sharp sound. 

302. The Earth an Important Part of a Telegraphic System. We learnedin Section 299 that electricity could flow through many differentsubstances, one of which was the earth. In all ordinary telegraphlines, advantage is taken of this fact to utilize the earth as aconductor and to dispense with one wire. Originally two wires wereused, as in Figure 217; then it was found that a railroad track couldbe substituted for one wire, and later that the earth itself servedequally well for a return wire. The present arrangement is shown inFigure 220, where there is but one wire, the circuit being completedby the earth. No fact in electricity seems more marvelous than thatthe thousands of messages flashing along the wires overhead arelikewise traveling through the ground beneath. If it were not for thisuse of the earth as an unfailing conductor, the network of overheadwires in our city streets would be even more complex than it now is. 

303. Advances in Telegraphy. The mechanical improvements intelegraphy have been so rapid that at present a single operator caneasily send or receive forty words a minute. He can telegraph morequickly than the average person can write; and with a combination ofthe latest improvements the speed can be enormously increased. Recently, 1500 words were flashed from New York to Boston over asingle wire in one second. 

In actual practice messages are not ordinarily sent long distancesover a direct line, but are automatically transferred to new lines atdefinite points. For example, a message from New York to Chicago doesnot travel along an uninterrupted path, but is automaticallytransferred at some point, such as Lancaster, to a second line whichcarries it on to Pittsburgh, where it is again transferred to a thirdline which takes it farther on to its destination. 

CHAPTER XXXIII

MAGNETS AND CURRENTS

304. In the twelfth century, there was introduced into Europe fromChina a simple instrument which changed journeying on the sea fromuncertain wandering to a definite, safe voyage. This instrument wasthe compass (Fig. 221), and because of the property of the compassneedle (a magnet) to point unerringly north and south, sailors wereable to determine directions on the sea and to steer for the desiredpoint. 

[Illustration: FIG. 221. --The compass. ]

Since an electric current is practically equivalent to a magnet(Section 296), it becomes necessary to know the most important factsrelative to magnets, facts simple in themselves but of far-reachingvalue and consequences in electricity. Without a knowledge of themagnetic characteristics of currents, the construction of the motorwould have been impossible, and trolley cars, electric fans, motorboats, and other equally well-known electrical contrivances would beunknown. 

305. The Attractive Power of a Magnet. The magnet best known to usall is the compass needle, but for convenience we will use a magneticneedle in the shape of a bar larger and stronger than that employed inthe compass. If we lay such a magnet on a pile of iron filings, itwill be found on lifting the magnet that the filings cling to the endsin tufts, but leave it almost bare in the center (Fig. 222). Thepoints of attraction at the two ends are called the poles of themagnet. 

[Illustration: FIG. 222. --A magnet. ]

If a delicately made magnet is suspended as in Figure 223, and isallowed to swing freely, it will always assume a definite north andsouth position. The pole which points north when the needle issuspended is called the north pole and is marked _N_, while the polewhich points south when the needle is suspended is called the southpole and is marked _S_. 

A freely suspended magnet points nearly north and south. 

A magnet has two main points of attraction called respectively thenorth and south poles. 

[Illustration: FIG. 223. --The magnetic needle. ]

306. The Extent of Magnetic Attraction. If a thin sheet of paper orcardboard is laid over a strong, bar-shaped magnet and iron filingsare then gently strewn on the paper, the filings clearly indicate theposition of the magnet beneath, and if the cardboard is gently tapped, the filings arrange themselves as shown in Figure 224. If the paper isheld some distance above the magnet, the influence on the filings isless definite, and finally, if the paper is held very far away, thefilings do not respond at all, but lie on the cardboard as dropped. 

The magnetic power of a magnet, while not confined to the magnetitself, does not extend indefinitely into the surrounding region; theinfluence is strong near the magnet, but at a distance becomes so weakas to be inappreciable. The region around a magnet through which itsmagnetic force is felt is called the field of force, or simply themagnetic field, and the definite lines in which the filings arrangethemselves are called lines of force. 

[Illustration: FIG. 224. --Iron filings scattered over a magnet arrangethemselves in definite lines. ]

The magnetic power of a magnet is not limited to the magnet, butextends to a considerable distance in all directions. 

307. The Influence of Magnets upon Each Other. If while oursuspended magnetic needle is at rest in its characteristicnorth-and-south direction another magnet is brought near, thesuspended magnet is turned; that is, motion is produced (Fig. 225). Ifthe north pole of the free magnet is brought toward the south pole ofthe suspended magnet, the latter moves in such a way that the twopoles _N_ and _S_ are as close together as possible. If the north poleof the free magnet is brought toward the north pole of the suspendedmagnet, the latter moves in such a way that the two poles _N_ and _N_are as far apart as possible. In every case that can be tested, it isfound that a north pole repels a north pole, and a south pole repels asouth pole; but that a north and a south pole always attract eachother. 

[Illustration: FIG. 225. --A south pole attracts a north pole. ]

The main facts relative to magnets may be summed up as follows:--

_a_. A magnet points nearly north and south if it is allowed to swingfreely. 

_b_. A magnet contains two unlike poles, one of which persistentlypoints north, and the other of which as persistently points south, ifallowed to swing freely. 

_c_. Poles of the same name repel each other; poles of unlike nameattract each other. 

_d_. A magnet possesses the power of attracting certain substances, like iron, and this power of attraction is not limited to the magnetitself but extends into the region around the magnet. 

308. Magnetic Properties of an Electric Current. If acurrent-bearing wire is really equivalent in its magnetic powers to amagnet, it must possess all of the characteristics mentioned in thepreceding Section. We saw in Section 296 that a coiled wire throughwhich current was flowing would attract iron filings at the two endsof the helix. That a coil through which current flows possesses thecharacteristics _a_, _b_, _c_, and _d_ of a magnet is shown as follows:--

_a_, _b_. If a helix marked at one end with a red string is arranged sothat it is free to rotate and a strong current is sent through it, the helix will immediately turn and face about until it points northand south. If it is disturbed from this position, it will slowly swingback until it occupies its characteristic north and south position. The end to which the string is attached will persistently point eithernorth or south. If the current is sent through the coil in theopposite direction, the two poles exchange positions and the helixturns until the new north pole points north. 

[Illustration: FIG. 226. --A helix through which current flows alwayspoints north and south, if it is free to rotate. ]

_c_. If a coil conducting a current is held near a suspended magnet, one end of the helix will be found to attract the north pole of themagnet, while the opposite end will be found to repel the north poleof the magnet. In fact, the helix will be found to behave in everyway as a magnet, with a north pole at one end and a south pole at theother. If the current is sent through the helix in the oppositedirection, the north and south poles exchange places. 

[Illustration: FIG. 227. --A wire through which current flows issurrounded by a field of magnetic force. ]

If the number of turns in the helix is reduced until but a single loopremains, the result is the same; the single loop acts like a flatmagnet, one side of the loop always facing northward and onesouthward, and one face attracting the north pole of the suspendedmagnet and one repelling it. 

_d_. If a wire is passed through a card and a strong current is sentthrough the wire, iron filings will, when sprinkled upon the card, arrange themselves in definite directions (Fig. 227). A wire carryinga current is surrounded by a magnetic field of force. 

A magnetic needle held under a current-bearing wire turns on its pivotand finally comes to rest at an angle with the current. The fact thatthe needle is deflected by the wire shows that the magnetic power ofthe wire extends into the surrounding medium. 

The magnetic properties of current electricity were discovered byOersted of Denmark less than a hundred years ago; but since that timepractically all important electrical machinery has been based upon oneor more of the magnetic properties of electricity. The motors whichdrive our electric fans, our mills, and our trolley cars owe theirexistence entirely to the magnetic action of current electricity. 

[Illustration: FIG. 228. --The coil turns in such a way that its northpole is opposite the south pole of the magnet. ]

309. The Principle of the Motor. If a close coil of wire issuspended between the poles of a strong horseshoe magnet, it will notassume any characteristic position but will remain wherever placed. If, however, a current is sent through the wire, the coil faces aboutand assumes a definite position. This is because a coil, carrying acurrent, is equivalent to a magnet with a north and south face; and, in accordance with the magnetic laws, tends to move until its northface is opposite the south pole of the horseshoe magnet, and its southface opposite the north pole of the magnet. If, when the coil is atrest in this position, the current is reversed, so that the north poleof the coil becomes a south pole and the former south pole becomes anorth pole, the result is that like poles of coil and magnet face eachother. But since like poles repel each other, the coil will move, andwill rotate until its new north pole is opposite to the south pole ofthe magnet and its new south pole is opposite the north pole. Bysending a strong current through the coil, the helix is made to rotatethrough a half turn; by reversing the current when the coil is at thehalf turn, the helix is made to continue its rotation and to swingthrough a whole turn. If the current could be repeatedly reversed justas the helix completed its half turn, the motion could be prolonged;periodic current reversal would produce continuous rotation. This isthe principle of the motor. 

[Illustration: FIG. 229. --Principle of the motor. ]

It is easy to see that long-continued rotation would be impossible inthe arrangement of Figure 228, since the twisting of the suspendingwire would interfere with free motion. If the motor is to be used forcontinuous motion, some device must be employed by means of which thehelix is capable of continued rotation around its support. 

In practice, the rotating coil of a motor is arranged as shown inFigure 229. Wires from the coil terminate on metal disks and aresecurely soldered there. The coil and disks are supported by thestrong and well-insulated rod _R_, which rests upon braces, but whichnevertheless rotates freely with disks and coil. The current flows tothe coil through the thin metal strips called brushes, which restlightly upon the disks. 

When the current which enters at _B_ flows through the wire, the coilrotates, tending to set itself so that its north face is opposite thesouth face of the magnet. If, when the helix has just reached thisposition, the current is reversed--entering at _B'_ instead of_B_--the poles of the coil are exchanged; the rotation, therefore, does not cease, but continues for another half turn. Proper reversalsof the current are accompanied by continuous motion, and since thedisk and shaft rotate with the coil, there is continuous rotation. 

If a wheel is attached to the rotating shaft, weights can be lifted, and if a belt is attached to the wheel, the motion of the rotatinghelix can be transferred to machinery for practical use. 

The rotating coil is usually spoken of as the armature, and the largemagnet as the field magnet. 

310. Mechanical Reversal of the Current. _The Commutator_. It is notpossible by hand to reverse the current with sufficient rapidity andprecision to insure uninterrupted rotation; moreover, the physicalexertion of such frequent reversals is considerable. Hence, somemechanical device for periodically reversing the current is necessary, if the motor is to be of commercial value. 

[Illustration: FIG. 230. --The commutator. ]

The mechanical reversal of the current is accomplished by the use ofthe commutator, which is a metal ring split into halves, wellinsulated from each other and from the shaft. To each half of thisring is attached one of the ends of the armature wire. The brusheswhich carry the current are set on opposite sides of the ring and donot rotate. As armature, commutator, and shaft rotate, the brushesconnect first with one segment of the commutator and then with theother. Since the circuit is arranged so that the current always entersthe commutator through the brush _B_, the flow of the current into thecoil is always through the segment in contact with _B_; but thesegment in contact with _B_ changes at every half turn of the coil, and hence the direction of the current through the coil changesperiodically. As a result the coil rotates continuously, and producesmotion so long as current is supplied from without. 

311. The Practical Motor. A motor constructed in accordance withSection 309 would be of little value in practical everyday affairs;its armature rotates too slowly and with too little force. If a motoris to be of real service, its armature must rotate with sufficientstrength to impart motion to the wheels of trolley cars and mills, todrive electric fans, and to set into activity many other forms ofmachinery. 

The strength of a motor may be increased by replacing the singlycoiled armature by one closely wound on an iron core; in somearmatures there are thousands of turns of wire. The presence of softiron within the armature (Section 296) causes greater attractionbetween the armature and the outside magnet, and hence greater forceof motion. The magnetic strength of the field magnet influencesgreatly the speed of the armature; the stronger the field magnet thegreater the motion, so electricians make every effort to strengthentheir field magnets. The strongest known magnets are electromagnets, which, as we have seen, are merely coils of wire wound on an ironcore. For this reason, the field magnet is usually an electromagnet. 

When very powerful motors are necessary, the field magnet is soarranged that it has four or more poles instead of two; the armaturelikewise consists of several portions, and even the commutator may bevery complex. But no matter how complex these various parts may seemto be, the principle is always that stated in Section 309, and theparts are limited to field magnet, commutator, and armature. 

[Illustration: FIG. 231. --A modern power plant. ]

[Illustration: FIG. 232. --The electric street car. ]

The motor is of value because by means of it motion, or mechanicalenergy, is obtained from an electric current. Nearly all electricstreet cars (Fig. 232), are set in motion by powerful motors placedunder the cars. As the armature rotates, its motion is communicated bygears to the wheels, the necessary current reaching the motor throughthe overhead wires. Small motors may be used to great advantage in thehome, where they serve to turn the wheels of sewing machines, and tooperate washing machines. Vacuum cleaners are frequently run bymotors. 

CHAPTER XXXIV

HOW ELECTRICITY MAY BE MEASURED

312. Danger of an Oversupply of Current. If a small toy motor isconnected with one cell, it rotates slowly; if connected with twocells, it rotates more rapidly, and in general, the greater the numberof cells used, the stronger will be the action of the motor. But it ispossible to send too strong a current through our wire, therebyinterfering with all motion and destroying the motor. We have seen inSection 288 that the amount of current which can safely flow through awire depends upon the thickness of the wire. A strong current sentthrough a fine wire has its electrical energy transformed largely intoheat; and if the current is very strong, the heat developed may besufficient to burn off the insulation and melt the wire itself. Thisis true not only of motors, but of all electric machinery in whichthere are current-bearing wires. The current should not be greaterthan the wires can carry, otherwise too much heat will be developedand damage will be done to instruments and surroundings. 

The current sent through our electric stoves and irons should bestrong enough to heat the coils, but not strong enough to melt them. If the current sent through our electric light wires is too great forthe capacity of the wires, the heat developed will injure the wiresand may cause disastrous results. The overloading of wires isresponsible for many disastrous fires. 

The danger of overloading may be eliminated by inserting in thecircuit a fuse or other safety device. A fuse is made by combining anumber of metals in such a way that the resulting substance has a lowmelting point and a high electrical resistance. A fuse is inserted inthe circuit, and the instant the current increases beyond its normalamount the fuse melts, breaks the circuit, and thus protects theremaining part of the circuit from the danger of an overload. In thisway, a circuit designed to carry a certain current is protected fromthe danger of an accidental overload. The noise made by the burningout of a fuse in a trolley car frequently alarms passengers, but it isreally a sign that the system is in good working order and that thereis no danger of accident from too strong a current. 

313. How Current is Measured. The preceding Section has shownclearly the danger of too strong a current, and the necessity forlimiting the current to that which the wire can safely carry. Thereare times when it is desirable to know accurately the strength of acurrent, not only in order to guard against an overload, but also inorder to determine in advance the mechanical and chemical effectswhich will be produced by the current. For example, the strength ofthe current determines the thickness of the coating of silver whichforms in a given time on a spoon placed in an electrolytic bath; ifthe current is weak, a thin plating is made on the spoon; if thecurrent is strong, a thick plating is made. If, therefore, the exactvalue of the current is known, the exact amount of silver which willbe deposited on the spoon in a given time can be definitelycalculated. 

[Illustration: FIG. 233. --The principle of the galvanometer. ]

Current-measuring instruments, or galvanometers, depend for theiraction on the magnetic properties of current electricity. Theprinciple of practically all galvanometers is as follows:--

A closely wound coil of fine wire free to rotate is suspended as inFigure 233 between the poles of a strong magnet. When a current issent through the coil, the coil becomes a magnet and turns so that itsfaces will be towards the poles of the permanent magnet. But as thecoil turns, the suspending wire becomes twisted and hinders theturning. For this reason, the coil can turn only until the motioncaused by the current is balanced by the twist of the suspending wire. But the stronger the current through the coil, the stronger will bethe force tending to rotate the coil, and hence the less effectivewill be the hindrance of the twisting string. As a consequence, thecoil swings farther than before; that is, the greater the current, thefarther the swing. Usually a delicate pointer is attached to themovable coil and rotates freely with it, so that the swing of thepointer indicates the relative values of the current. If the source ofthe current is a gravity cell, the swing is only two thirds as greatas when a dry cell is used, indicating that the dry cell furnishesabout 1-1/2 times as much current as a gravity cell. 

314. Ammeters. A galvanometer does not measure the current, butmerely indicates the relative strength of different currents. But itis desirable at times to measure a current in units. Instruments formeasuring the strength of currents in units are called ammeters, andthe common form makes use of a galvanometer. 

A current is sent through a movable coil (the field magnet and coilare inclosed in the case) (Fig. 234), and the magnetic field thusdeveloped causes the coil to turn, and the pointer attached to it tomove over a scale graduated so that it reads current strengths. Thisscale is carefully graduated by the following method. 

If two silver rods (Fig. 208) are weighed and placed in a solution ofsilver nitrate, and current from a single cell is passed through theliquid for a definite time, we find, on weighing the two rods, thatone has gained in weight and the other has lost. If the current isallowed to flow twice as long, the amount of silver lost and gained bythe electrodes is doubled; and if twice the current is used, theresult is again doubled. 

As a result of numerous experiments, it was found that a definitecurrent of electricity will deposit a definite amount of silver in adefinite time, and that the amount of silver deposited on an electrodein one second might be used to measure the current of electricitywhich has flowed through the circuit in one second. 

A current is said to be one ampere strong if it will deposit silver onan electrode at the rate of 0. 001118 gram per second. 

[Illustration: FIG. 234. --An ammeter. ]

In marking the scale, an ammeter is placed in the circuit of anelectrolytic cell and the position of the pointer is marked on theblank card which lies beneath and which is to serve as a scale (Fig. 235). After the current has flowed for about an hour, the amount ofsilver which has been deposited is measured. Knowing the time duringwhich the current has run, and the amount of deposit, the strength ofthe current in amperes can be calculated. This number is writtenopposite the place at which the pointer stood during the experiment. 

The scale may be completed by marking the positions of the pointerwhen other currents of known strength flow through the ammeter. 

[Illustration: FIG. 235. --Marking the scale of an ammeter. ]

All electric plants, whether for heating, lighting, or for machinery, are provided with ammeters, such instruments being as important to anelectric plant as the steam gauge is to the boiler. 

315. Voltage and Voltmeters. Since electromotive force, or voltage, is the cause of current, it should be possible to compare differentelectromotive forces by comparing the currents which they produce in agiven circuit. But two voltages of equal value do not give equalcurrents unless the resistances met by the currents are equal. Forexample, the simple voltaic cell and the gravity cell haveapproximately equal voltages, but the current produced by the voltaiccell is stronger than that produced by the gravity cell. This isbecause the current meets more resistance within the gravity cellthan within the voltaic cell. Every cell, no matter what its nature, offers resistance to the flow of electricity through it and is said tohave internal resistance. If we are determining the voltages ofvarious cells by a comparison of the respective currents produced, theresult will be true only on condition that the resistances in thevarious circuits are equal. If a very large external resistance offine wire is placed in circuit with a gravity cell, the _total_resistance of the circuit (made up of the relatively small resistancein the cell and the larger resistance in the rest of the circuit) willdiffer but little from that of another circuit in which the gravitycell is replaced by a voltaic cell, or any other type of cell. 

With a high resistance in the outside circuit, the deflections of theammeter will be small, but such as they are, they will fairlyaccurately represent the electromotive forces which produce them. 

Voltmeters (Fig. 236), or instruments for measuring voltage, are likeammeters except that a wire of very high resistance is in circuit withthe movable coil. In external appearance they are not distinguishablefrom ammeters. 

[Illustration: FIG. 236. --A voltmeter. ]

The unit of electromotive force is called the _volt_. The voltage of adry cell is approximately 1. 5 volts, and the voltage of a voltaic celland of a gravity cell is approximately 1 volt. 

316. Current, Voltage, Resistance. We learned in Section 287 thatthe strength of a current increases when the electromotive forceincreases, and diminishes when the electromotive force diminishes. Later, in Section 288, we learned that the strength of the currentdecreases as the resistance in circuit increases. 

The strength of a steady current depends upon these two factors only, the electromotive force which causes it and the resistance which ithas to overcome. 

317. Resistance. Since resistance plays so important a rôle inelectricity, it becomes necessary to have a unit of resistance. Thepractical unit of resistance is called an ohm, and some idea of thevalue of an ohm can be obtained if we remember that a 300-foot lengthof common iron telegraph wire has a resistance of 1 ohm. Anapproximate ohm for rough work in the laboratory may be made bywinding 9 feet 5 inches of number 30 copper wire on a spool orarranging it in any other convenient form. 

In Section 299 we learned that substances differ very greatly in theresistance which they offer to electricity, and so it will notsurprise us to learn that while it takes 300 feet of iron telegraphwire to give 1 ohm of resistance, it takes but 39 feet of number 24copper wire, and but 2. 2 feet of number 24 German silver wire, to givethe same resistance. 

 NOTE. The number of a wire indicates its diameter; number 30, for example, being always of a definite fixed diameter, no matter what the material of the wire. 

If we wish to avoid loss of current by heating, we use a wire of lowresistance; while if we wish to transform electricity into heat, as inthe electric stove, we choose wire of high resistance, as Germansilver wire. 

CHAPTER XXXV

HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE

318. The Dynamo. We have learned that cells furnish current as aresult of chemical action, and that the substance usually consumedwithin the cell is zinc. Just as coal within the furnace furnishesheat, so zinc within the cell furnishes electricity. But zinc is amuch more expensive fuel than coal or oil or gas, and to run a largemotor by electricity produced in this way would be very much moreexpensive than to run the motor by water or steam. For weak andinfrequent currents such as are used in the electric bell, only smallquantities of zinc are needed, and the expense is small. But for theproduction of such powerful currents as are needed to drive trolleycars, elevators, and huge machinery, enormous quantities of zinc wouldbe necessary and the cost would be prohibitive. It is safe to say thatelectricity would never have been used on a large scale if some lessexpensive and more convenient source than zinc had not been found. 

319. A New Source of Electricity. It came to most of us as asurprise that an electric current has magnetic properties andtransforms a coil into a veritable magnet. Perhaps it will notsurprise us now to learn that a magnet in motion has electricproperties and is, in fact, able to produce a current within a wire. This can be proved as follows:--

[Illustration: FIG. 237. --The motion of a magnet within a coil of wireproduces a current of electricity. ]

Attach a closely wound coil to a sensitive galvanometer (Fig. 237);naturally there is no deflection of the galvanometer needle, becausethere is no current in the wire. Now thrust a magnet into the coil. Immediately there is a deflection of the needle, which indicates thata current is flowing through the circuit. If the magnet is allowed toremain at rest within the coil, the needle returns to its zeroposition, showing that the current has ceased. Now let the magnet bewithdrawn from the coil; the needle is deflected as before, but thedeflection is in the opposite direction, showing that a currentexists, but that it flows in the opposite direction. We learn, therefore, that a current may be induced in a coil by moving a magnetback and forth within the coil, but that a magnet at rest within thecoil has no such influence. 

An electric current transforms a coil into a magnet. A magnet inmotion induces electricity within a coil; that is, causes a current toflow through the coil. 

A magnet possesses lines of force, and as the magnet moves toward thecoil it carries lines of force with it, and the coil is cut, so tospeak, by these lines of force. As the magnet recedes from the coil, it carries lines of force away with it, this time reducing the numberof the lines which cut the coil. 

[Illustration: FIG. 238. --As long as the coil rotates between thepoles of the magnet, current flows. ]

320. A Test of the Preceding Statement. We will test the statementthat a magnet has electric properties by another experiment. Betweenthe poles of a strong magnet suspend a movable coil which is connectedwith a sensitive galvanometer (Fig. 237). Starting with the coil inthe position of Figure 228, when many lines of force pass through it, let the coil be rotated quickly until it reaches the positionindicated in Figure 238, when no lines of force pass through it. During the motion of the coil, a strong deflection of the galvanometeris observed; but the deflection ceases as soon as the coil ceases torotate. If, now, starting with the position of Figure 238, the coil isrotated forward to its starting point, a deflection occurs in theopposite direction, showing that a current is present, but that itflows in the opposite direction. So long as the coil is in motion, itis cut by a varying number of lines of force, and current is inducedin the coil. 

_The above arrangement is a dynamo in miniature_. By rotation of acoil (armature) within a magnetic field, that is, between the poles ofa magnet, current is obtained. 

In the _motor_, current produces motion. In the _dynamo_, motionproduces current. 

321. The Dynamo. As has been said, the arrangement of the precedingSection is a dynamo in miniature. Every dynamo, no matter how complexits structure and appearance, consists of a coil of wire which canrotate continuously between the poles of a strong magnet. Themechanical devices to insure easy rotation are similar in all respectsto those previously described for the motor. 

[Illustration: FIG. 239. --A modern electrical machine. ]

The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction. Such alternating currents are unsatisfactory for many purposes, and tobe of service are in many cases transformed into direct currents; thatis, current which flows steadily in one direction. This isaccomplished by the use of a commutator. In the construction of themotor, continuous _motion_ in one direction is obtained by the use ofa commutator (Section 310); in the construction of a dynamo, continuous _current_ in one direction is obtained by the use of asimilar device. 

322. Powerful Dynamos. The power and efficiency of a dynamo areincreased by employing the devices previously mentioned in connectionwith the motor. Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up ofmany coils wound on soft iron. The speed with which the armature isrotated influences the strength of the induced current, and hence thearmature is run at high speed. 

[Illustration: FIG. 240. --Thomas Edison, one of the foremostelectrical inventors of the present day. ]

A small dynamo, such as is used for lighting fifty incandescent lamps, has a horse power of about 33. 5, and large dynamos are frequently aspowerful as 7500 horse power. 

323. The Telephone. When a magnet is at rest within a closed coil ofwire, as in Section 319, current does not flow through the wire. Butif a piece of iron is brought near the magnet, current is induced andflows through the wire; if the iron is withdrawn, current is againinduced in the wire but flows in the opposite direction. As ironapproaches and recedes from the magnet, current is induced in the wiresurrounding the magnet. This is in brief the principle of thetelephone. When one talks into a receiver, _L_, the voice throws intovibration a sensitive iron plate standing before an electromagnet. Theback and forth motion of the iron plate induces current in theelectromagnet _c_. The current thus induced makes itself evident atthe opposite end of the line _M_, where by its magnetic attraction, itthrows a second iron plate into vibrations. The vibrations of thesecond plate are similar to those produced in the first plate by thevoice. The vibrations of the far plate thus reproduce the soundsuttered at the opposite end. 

[Illustration: FIG. 241. --Diagram of a simple telephone circuit. ]

324. Cost of Electric Power. The water power of a stream dependsupon the quantity of water and the force with which it flows. Theelectric power of a current depends upon the quantity of electricityand the force under which it flows. The unit of electric power iscalled the watt; it is the power furnished by a current of one amperewith a voltage of one volt. 

One watt represents a very small amount of electric power, and forpractical purposes a unit 1000 times as large is used, namely, thekilowatt. By experiment it has been found that one kilowatt isequivalent to about 1-1/3 horse power. Electric current is charged forby the watt hour. A current of one ampere, having a voltage of onevolt, will furnish in the course of one hour one watt hour of energy. Energy for electric lighting is sold at the rate of about ten centsper kilowatt hour. For other purposes it is less expensive. The meterscommonly used measure the amperes, volts, and time automatically, andregister the electric power supplied in watt hours. 

 INDEX

 Absorption, of heat by lampblack, 143-144. Of gases by charcoal, 57. Of light waves, 135-138. 

 Accommodation of the eye, 123. 

 Acetanilid, 259. 

 Acetylene, as illuminant, 152-153. Manufacture of, 152-153. Properties of, 220. 

 Acid, boric, 253. Carbolic, 152, 251, 252. Hydrochloric, 55, 80, 227, 238, 241. Lactic, 230. Oxalic, 247, 248. Salicylic, 253. Sulphuric, 55, 80, 240, 241, 307. Sulphurous, 242. 

 Acids, action on litmus, 220. 

 Adenoids, 51. 

 Adulterants, detection of, 16. 

 Air, characteristics of, 81-83, 86, 189. Compressibility of, 91. Expansion of, 10-11. Humidity, 38, 39. Pumps, 201-205. Transmits sound, 269. Weight of, 86. _See_ Atmosphere. 

 Alcohol, 234. In patent medicines, 260. 

 Alizarin, 248. 

 Alkali, 222. 

 Alternating current, 351. 

 Alum, 247. In baking powder, 230. 

 Ammeter, 341, 343. 

 Ammonia, 152. A base, 221-222. In bath, 226. In manufacture of ice, 98. Neutralizing chlorine, 240. 

 Ampere, 342. 

 Anemia, 259. 

 Angle, of incidence, 110. Of reflection, 110. Of refraction, 114. 

 Aniline, 152, 245. 

 Animal charcoal, 58. 

 Animal transportation, 132. 

 Antichlor, 240. 

 Antipyrin, 259. 

 Armature, 319, 320. Dynamo, 350. Motor, 335. 

 Artificial lighting, 148-153. 

 Atmosphere, 81. Carbon dioxide in, 54-55. Height of, 81. Nitrogen and oxygen in, 262. Pressure of, 82-86. Water vapor in, 36-38. Weight, 86. _See_ Air. 

 Atmospheric pressure, 82-86. 

 Atomizer, 92. 

 Atoms, 102. 

 Automobiles, gas engines, 185. 

 Axis of a lens, 119. 

 Bacteria, 133. As nitrogen makers, 263. Destroyed by sunlight, etc. , 133, 250, 251. Diseases caused by, 133. In butter and cheese, 133. 

 Baking powder, 229-230. 

 Baking soda, 227-229. 

 Barograph, 87. 

 Barometer, aneroid, 84-85. Mercury, 84. Use in weather predictions, 86-87. 

 Bases, action on litmus, 221-222. Properties, 220-222. 

 Battery, electric, 311. 

 Beans, as food, 66. Roots take in nitrogen, 263. 

 Bell, electric, 319-321. 

 Benzine, 150. As a cleaning agent, 227. 

 Benzoate of soda, 253. 

 Bicarbonate of soda, in fire extinguisher, 55, 56. In Rochelle salt, 227. In soda mints, 231. In seidlitz powder, 231. 

 Bicycle pumps, 202. 

 Blasting, by electricity, 314. 

 Bleaching, 237-243. By chlorine, 238-240. 

 Bleaching powder, 239-240. 

 Body, human, 63-64. A conductor of electricity, 292. 

 Boiling, 31. Amount of heat absorbed, 31-32. Of milk, 32. Of water, 77. Point, 15. 

 Bomb calorimeter, 61. 

 Borax, as meat preservative, 253. As washing powder, 226. 

 Boric acid, as meat preservative, 253. 

 Boyle's law, 95-96. 

 Bread, 232-233. Unleavened, 233. 

 Bread making, 232-235. 

 Breathing, hygienic habits of, 50. By mouth, 50-51. 

 Burns, treatment of, 52-53. 

 Butter, adulteration test, 16. Bacteria in, 133. 

 Buttermilk, 230. 

 Caisson, 203-204. 

 Calcium carbide, 152-153. In making nitrogenous fertilizer, 264. 

 Calico printing, 249. 

 Calorie, 27-28, 61-62. 

 Calorimeter, 61. 

 Camera, 128-129. Films, 129. Lens, 129. Plates, 129. 

 Camping, water supply, 195-197. 

 Candle, 148-149. As standard for light-measure, 104-105. 

 Candle-power, 105-107. 

 Carbide, calcium, 152-153, 264. 

 Carbohydrates, 64-65, 149. 

 Carbolic acid, 152. As disinfectant, 251. 

 Carbon, 56, 66. In voltaic cells, 308. 

 Carbon dioxide, 53. As fire extinguisher, 55-56. Commercial use, 55-56. In baking soda, 228. In fermentation, 234. In health, 54. In plants, 55. Preparation of, 55. Source of, 53. Test for, 228. 

 Catarrh, 259. 

 Caustic lime, 222. . 

 Caustic potash, 222. 

 Caustic soda, 218, 222. To make a salt, 227. 

 Caves and caverns, 71. 

 Cell, dry, 310. Gravity, 309-310. Voltaic, 306-308, 310. 

 Cells of human body, 63, 64, 66. 

 Centigrade thermometer, 15. 

 Central heating plant, 19. 

 Chalk, in making carbon dioxide, 55. 

 Charcoal as a filter, 57. Commercially, 57. Preparation, 57-58. 

 Chemical action, and electricity, 307, 315-317. And light, 126, 127. 

 Chemistry, in daily life, 218, 219. 

 Chills, 38. 

 Chloride of lime, in bleaching, 240. Disinfectant, 251. 

 Chlorine, and hydrogen, 239. Effect upon human body, 239. In bleaching, 238-240. Influence of light upon, 126. Presence in salt, 227. 

 Circuit, electric, 321. Local, in telegraph, 325-326. 

 City water supply, 206-212. 

 Clarinet, 297. 

 Cleaning of material, 226, 243. 

 Climate, influenced by presence of water, 29, 40. 

 Clover, nitrogen producers, 263. 

 Coal, 30. 

 Coal gas, 150, 151. By-products, 152. 

 Coal oil, 149, 150. 

 Coal tar dyes, 152, 218, 245. 

 Cogwheels, 170. 

 Coil, current-bearing, 320. Magnetic field about, 331-333. 

 Coke, 152. 

 Cold storage, 97. 

 Color, 134-141. And heat, 142, 143. Influenced by light, 137. Of opaque bodies, 136, 137. Of transparent bodies, 135, 136. 

 Color blindness, 140, 141. Designs in cloth, 248, 249. 

 Colors, compound, 138, 139. Essential, 139-140. Primary, 135. Simple, 138. Spectrum, 134-135. Variety in dyeing, 247, 248. 

 Combustion, heat of, 45. Spontaneous, 52. 

 Commutator, 335. 

 Compass, 328. 

 Compound colors, 138, 139. 

 Compound machine, 171. 

 Compound substances, 103. 

 Compression of air, 91, 92. Cause of heat, 96. 

 Compression pumps, 201, 205. 

 Concave lens, 118. 

 Condensation, 33. Heat set free, 40. 

 Conduction of heat, 25. 

 Conductivity metals, 321. 

 Conductors, electric, 321, 322. 

 Conservation, of energy, 58, 59. Of matter, 58, 59. 

 Convection, 24, 25. 

 Convex lens, 118. 

 Cooling, by evaporation, 35-36. By expansion, 97. 

 Copper, in electric cell, 307. 

 Core, iron, 319. 

 Corn, bleached with sulphurous acid, 242. 

 Cotton, mercerized, 218. Bleaching, 241. Dyeing, 245-247. 

 Cough sirup, 258. 

 Crane, compound machine, 172. 

 Cream of tartar, 229. 

 Creosote oil, 254. 

 Crude petroleum, 149, 150. 

 Current, electric, 306, 312. Alternating, 349. Induced, 346-347. Measurement of, 340. Resistance, 312, 343, 345. Strength, 339, 340, 344. 

 Dams, 214-216. 

 Decay, 49. 

 Decomposition of soil by water, 70-74. 

 Degrees Fahrenheit and Centigrade, 15. 

 Density, 11. 

 Designs in cloth, printed, 248, 249. Woven, 249. 

 Developer in photography, 128. 

 Dew, 36, 37. 

 Dew point, 38. 

 Diarrhea, 251. 

 Diet, 62, 66. Economy on table, 66-69. 

 Discord, reason for, 271. 

 Disease, and surface water, 76. Relation of light to, 131-132. 

 Disease disinfectants, 250, 251, 252. 

 Distillation, 34-35. In commerce, 35. Of petroleum, 149-150. Of soft coal, 150. Of water, 34, 35, 77. 

 Diving suits, 204. 

 Door bells, 319-321. 

 Drainage, of land, 194, 195. Sewage, 196, 198, 199, 201. 

 Drilled well, 199. 

 Drinking water, 75-77. In camping, 195-196. And rural supplies, 198, 201. 

 Driven well, 196-197. 

 Drought, 217. 

 Drugs, 255, 260. 

 Dry cell, 312. 

 Dyeing, 244-249. Color designs, 248. 

 Dyeing, direct, 245. Home, 247. Indirect, 247. Variety of color, 247. 

 Dyes, 218, 244, 245. 

 Dynamo, 346. Alternating current, 349. Source of energy, 346-347. 

 Ear, in man, 301-303. Care of, 303. 

 Earth, conductor of electricity, 326. 

 Echo, 277. 

 Economy in buying food, 66-69. 

 Effort, muscular, 155, 160. 

 Electric, battery, 311. Bell, 319-321. Bread toasters, 314. Conductors and non-conductors, 321-322. Cost of, energy, 352. Current, 306, 312. Flatiron, 313. Heating pad, 314. Lights, 314. Street cars, 337. 

 Electricity, heat, 312-315, 339. As a magnet, 319, 331-333. Practical uses of, 312-317. 

 Electrodes, of cell, 308. 

 Electrolytic metals, 317. 

 Electromagnets, 319. 

 Electromotive force, 308. Unit of, 344. 

 Electroplating, 315. 

 Electrotyping, 317. 

 Elements, 102-103. 

 Emulsion, 224. 

 Energy, conservation of, 58, 59. Transformations of, 58, 59. 

 Engine, steam, 183-185. Gas, 185-186. Horse power, 173. 

 Erosion, 73-74. 

 Essential colors, 139-140. 

 Evaporation, 35-39. Cooling effect, 35-36. Effect of temperature on, 35, 36. Effect of air on, 38. Freezing by, 98. Heat absorbed, 36. Of perspiration, 38. 

 Expansion, of air, 10, 11. Cooling effect of, 97. Disadvantage and advantage of, 11-13. Of liquids, 9-11. Of solids, 10, 11. Of water, 9, 10, 11, 12. Eye, 122-125. Headache, 124, 125. How focused, 122, 123. Nearsighted and farsighted, 123. Strain, 125. 

 Fahrenheit thermometer, 15. 

 Fats, 65. In soap making, 223. 

 Fermentation, 232-236. By yeast, 234-236. 

 Ferric compounds, 248. 

 Fertilizers, 262-265. Nitrogen, 262. Phosphorus, 263, 264. Potash, 263-265. 

 Field magnet, 336. 

 Filings, iron, 329. 

 Film, photographic, 129. 

 Filter, charcoal, 57. 

 Filtering water, 77. 

 Fire, 9. And oxygen, 45, 47. And tinder box, 47. Making of, 51. Primitive production of, 47. Produced by friction, 47. Spontaneous combustion, 52. Sores and burns, 52-53. Extinguisher, 55, 56. 

 Fireless cooker, 25, 26. 

 Fireplaces, 17, 18. 

 Fixing, in photography, 128. 

 Flame, hydrogen, 80. 

 Flood, Johnstown, 214, 215. Relation to forests, 217. 

 Flour, self-raising, 231. 

 Flume, 177. 

 Flute, 297. 

 Focal length, 118. 

 Focus, of lens, 118. 

 Fog, 37. 

 Food, 60-69. Carbohydrates, 64, 65. Economy in buying, 66-69. Fats, 65. Fuel value of, 60-62. Need of, 63, 64. Preservatives, 252. Proteids, 66. Value, 67. Waste, 60. Water in, 75. 

 Foot pound, 172. 

 Force and motion, 156, 157. And work, 156, 157. Magnetic lines of, 329-331, 334. Muscular, 155, 160. 

 Force pumps, 192, 193. 

 Forests and water supply, 216-217. 

 Forging of iron, 40, 41. 

 Formaldehyde, 253. 

 Freezing, effect of salt, 44. Effect on ground and rocks, 42. Expansion of water on, 41. Ice cream freezer, 44. 

 Frequency in music, 273, 275. 

 Fresh air, 22-24, 49. Amount consumed by gas burner, 22. And health, 49, 50. In underground work, 202. In work under water, 203-205. 

 Friction, 173, 174. Losses by, 174, 210. Source of heat and fire, 47. 

 Frost, 36, 37. 

 Fruit, canned, bleached with sulphurous acid, 242. Colored with coal tar dyes, 253. 

 Fuel value of foods, 60-62. Table of fuel values, 67. 

 Fulcrum, 159, 160. 

 Fumigation, 251. 

 Fundamental tone, 290, 291, 292. 

 Furnace, hot air, 19. 

 Fuse, 340. 

 Fusion, heat of, 40. 

 Galvanometer, 341. 

 Gas, acetylene, 152, 153. And unburned carbon, 151. Coal, 151, 152. Effect of heat on volume, 96, 97. Effect of pressure on volume, 95-96. Engine, 185-186. For cooking, 151, 152. Illuminating, 92, 93, 150, 151. Liquefaction, 97, 98. Meter, 93, 94. Natural, 152. 

 Gasolene, 149, 150. As cleaning agent, 227, 243. In gas engine, 185, 186. 

 Gauge, pressure, 92-94. 

 Gelatin, plate and film, 129. 

 Glass, kinds of, 119. Molding of, 40. Non-conductor, 321. 

 Grape juice, fermented with millet, 233. 

 Gravity cell, 309, 310. 

 Grease, and lye, 221. And soap making, 223. 

 Gulf Stream, 24. 

 Hard water, and soap, 225. 

 Harp, 295. 

 Headache, 124, 125. Powders, 259. 

 Health, effect of diet, 62, 64. 

 Heat, 9. Absorbed in boiling, 31-32. And disease germs, 250. And food, 252. And friction, 47. And light, 142, 147. And oxidation, 45, 48, 49. And wave motion, 145-147. Conduction, 25. Convection, 24, 25. From burning hydrogen, 80. From electricity, 312-315, 339. Needed to melt substances, 39. Of fusion, 40. Of vaporization, 32. Produced by compression, 96. Relation of water to weather, 29, 40. Set free by freezing water, 40. Sources of, 29-30. Specific, 28-29. Temperature, 27. Unit of, 27, 28. 

 Heating effect of electric current, 312-315. 

 Heating of buildings: central heating plant, 19. Fireplaces, 17-18. 

 Heating, furnaces, 19. Hot water, 19-22. 

 Helix, 318. 

 Horse power, 173, 351. 

 Hot water heating, 19-22. 

 Hues, primary, 135. 

 Humidity, 38. Proper percentage for health and comfort, 38, 39. 

 Humus, 216, 217. 

 Hydrocarbons, 149. 

 Hydrochloric acid, composition, 227. In bleaching, 241. To make a salt, 227. To make carbon dioxide, 55. To make chlorine, 238. To make hydrogen, 80. 

 Hydrogen, 65, 66. And chlorine, 239. And water, 79. Chemical conduct, 126-127. Flame, 80. In voltaic cell, 307. Peroxide, 53, 252. Preparation, 80. To liquefy, 97. 

 Ice, lighter than water, 42. Manufacture of, 98, 99. 

 Ice cream freezers, 44. 

 Illuminating gas, manufacture of, 150, 151. Measurement of quantity consumed, 93, 94. Test of pressure, 92, 93. 

 Illumination, intensity of, 105, 106. 

 Image, in mirror, 108, 111. 

 Incandescent lighting, 107, 314. 

 Incidence, angle of, 110. 

 Inclined plane, 162-166. Screw, 166. Wedge, 166. 

 Indigo, 218. 

 Induced current, 346-347. 

 Ink spots, removal of, 243. 

 Insoluble substances, 71. 

 Insulators, electric, 324. 

 Intensity, of light, 105-107. Of sound, 270-271. 

 Interval, in musical scale, 283. 

 Iron, forging, 41. Filings, 329. Galvanizing, 49. Oxidation of, 48. 

 Irrigation, 193-194. 

 Isobaric lines, 88, 91. 

 Isothermal lines, 89, 91. 

 Johnstown flood, 214, 215. 

 Kerosene, 149, 150. 

 Kilowatt, 351. 

 Lactic acid, 230. 

 Leaves, 132, 262. 

 Lens, 117-121. Concave, 118. Converging, 118. Crystalline, of eye, 122. Focal length, 118. Material, 119. Refractive power, 119. 

 Lever, 158-162. Examples, 160-162. Fulcrum, 159, 160. 

 Life, and carbon dioxide, 54. And nitrogen, 261. And oxygen, 49, 54. 

 Lifting pumps, 189-192. 

 Light, absorption, 135-138. And heat, 142-147. A wave motion, 145-147. Bent rays, 113, 114. Chemical action, 126-127. Disease, 131-132. Essential to life, 131, 132. Fading illumination, 105, 106. Influence on color, 134. Reflection of, 109-112. Refraction of, 113-125. Travels in a straight line, 108. White, composed of colors, 134. 

 Lighting, artificial, 148-153. 

 Lime, chloride of, 240, 251. 

 Limewater, 220. And carbon dioxide, 228. 

 Linen, bleaching, 241. Dyeing, 245-247. 

 Lines, of force, 329-331, 334. Isobaric, 88, 91. Isothermal, 89, 91. 

 Liquefaction of gases, 97, 98. 

 Liquid air, 98. 

 Liquid soap, 223, 224. 

 Litmus, action of acids, 220. Action of bases, 221, 222. Action of neutral substance, 222. 

 Logwood dyes, 245, 247, 248. 

 Los Angeles aqueduct, 211. 

 Lye, 221, 222. 

 Machines, compound, 171. Inclined plane, 162-166. Lever, 158-162. Pulley, 166-169. Wheel and axle, 169-171. 

 Madder, for dyes, 245. 

 Magnet, 328. Electro-, 319. Field of, 329-331. Lines of force about, 329-331. Poles of, 330-332. Properties of electricity, 318. 

 Magnetic, needle, 328. Poles, 329-331. 

 Magnifying power, of a lens, 115. Of a microscope, 115. Of a telescope, 115. 

 Mammoth Cave of Kentucky, 71. 

 Manganese dioxide, 46. Chlorine made from, 238. Oxygen made from, 46. 

 Marble, for carbon dioxide, 55. 

 Matches, 47. Safety, 47-48. 

 Matching colors, 137. 

 Matter, conservation of, 58, 59. 

 Meat, 66. Preservation of, 253. 

 Mechanical devices, 154, 155. 

 Melting, 39, 40. 

 Melting point, 40. 

 Melting substances without a definite melting point, 40. 

 Mercerized cotton, 218. 

 Mercury, barometer, 84. Thermometer, 14-17. 

 Metals, electroplating, 317. Preservation by paint, 253-254. Veins deposited by precipitation, 72, 73. Welding, 315. 

 Meter, gas, 93, 94. 

 Microörganisms, 132, 133. 

 Microscope, 115. 

 Milk, boiling point, 32. Pasteurized, 250. 

 Minerals, in foods, 62, 63. In water, 70, 71. 

 Mirrors, 108-112. Distance of image behind mirror, 111. Distance of object in front of mirror, 111. Image a duplicate of object. 111. 

 Molding of glass, 40. 

 Molecule, 100-103. 

 Mordants, 247, 248, 249. 

 Morphine, 257. 

 Morse, telegraphic code, 324. 

 Motion, in sound, 266, 278, 280. In work, 156. 

 Motor, electric, 336. Principle of, 333. Street car, 337. 

 Mouth breathing, 50. Cause of, 51. 

 Movable pulley, 167, 168. 

 Music, 278. 

 Musical instruments, percussion, 299. Stringed, 284-295. Wind, 295, 299. 

 Musical scale, 282. 

 Naphtha in gas engines, 185. 

 Naphthalene, 152. 

 Narcotics, 255. 

 Natural gas, 152. 

 Needle, magnetic, 328. 

 Negative, electrode, 308. Photographic, 130. 

 Neutral substance, 222. And litmus, 222. 

 Neutralization, 222. 

 Niagara Falls, 176. 

 Nitrogen, 66. And bacteria, 263. And plant life, 261. In atmosphere, 261. In fertilizer, 262-265. In food, 66. Preparation of, 261. Properties of, 261. 

 Noise in music, 280. 

 Non-conductors, of electricity, 321-322. Of heat, 25. 

 Nutcracker, as a lever, 162. 

 Oboe, 297. 

 Octave, 284. 

 Odors, 101. 

 Ohm, unit of resistance, 345. 

 Oil, gasoline, 149, 150. Kerosene, 149, 150. Lubricating, 174. Olive, 16. 

 Orchestra grouping, 299. 

 Ore, 72. 

 Organ pipes, 297. 

 Overtones, 290-293. 

 Oxalic acid, 247, 248. 

 Oxidation, 45-59. And decay, 49. Heat the result of, 49-52. In human body, 49, 53. Of iron, 48. 

 Oxygen, 66. And bleaching, 239. And combustion, 45. And food, 66. And plants, 55. And the human body, 50. And water, 79, 80. In the atmosphere, 45. Preparation of, 46. 

 Paint, as wood and metal preservatives, 253, 254. Removal of stains, 243. 

 Paper making, 219. 

 Paraffin, 150, 321. 

 Pasteurized milk, 250. 

 Patent medicines, 257-260. 

 Peas, sources of nitrogen, 263. 

 Pelton wheel, 177. 

 Percussion instruments, 299. 

 Period of a body, 273. 

 Peroxide of hydrogen, 53, 252

 Petrolatum, 150. 

 Petroleum, 149, 150. 

 Phonograph, 303-305. 

 Phosphorus, in fertilizer, 263, 264. In making nitrogen, 261. In matches, 47, 48. Poisoning by, 47. 

 Photography, 127-131. 

 Photometer, 107. 

 Pianos, 284-292. 

 Pin wheel, 181. 

 Pitch of sound, 280, 281. Cause of, 282. In wind instruments, 296-299. 

 Plane, inclined, 162-166. 

 Plants, and atmosphere, 55. And light, 131-132. And nitrogen, 261. 

 Plate developing, photographic, 128. 

 Pneumatic dispatch tube, 205. 

 Poles, magnetic, 330-332. Of cell, 308. 

 Positive electrode, 308. 

 Potash, in fertilizer, 263-265. 

 Potassium chlorate and oxygen, 46. Permanganate, 100. Tartrate and Rochelle salt, 227. 

 Power, candle, 105-107. Electric, 351. Horse, 173, 351. Sources of, 174, 175, 185. Transmission by belts, 171. Water, 176-180. 

 Precipitation, 72, 73. 

 Preservatives, food, 252. Wood and metal, 253-254. 

 Pressure, atmospheric, 82-86. Calculation of atmospheric, 83, 84. Calculation of gas, 92, 93. Calculation of water, 94. Gauge, 92-94. Of illuminating gas, 93. Relation of pressure of gas to volume, 95, 96. Water pressure, 208-211, 214-216. Within the body, 86. 

 Primary colors, 135. 

 Print, photographic, 131. 

 Printing, color designs in cloth, 248, 249. Electrotype, 317. 

 Prisms, 135. Refraction through, 117. 

 Proteids, 66. 

 Pulleys, 166-169. Applications of, 169. 

 Pump, 187-205. Air, 201-205. Force, 192, 193. Lifting, 189-192. 

 Pupil of the eye, 122. 

 Pure food laws, bleaching, 242. Preservatives, 252. 

 Purification of water, 77, 196. 

 Push button, 321. 

 Radiator, 19-21. 

 Railroads, grading of, 165-166. 

 Rain, 36, 37. 

 Rainbow, 134. 

 Rain water, 225. 

 Reflection, angle of, 110. Of light, 109-112. Of sound, 278, 279. 

 Refraction, angle of, 114. By atmosphere, 114. Of light, 113. Uses of, 115-116. 

 Relay, telegraph, 325. 

 Reservoir, 214. Artificial, 211. Construction of, 214-216. Natural, 211. 

 Resistance, electrical, 312. Internal, of cell, 343. Unit of, 345. 

 Resonance, 276. 

 River, volume and value of, 180. 

 Roads, application of inclined plane to, 165-166. 

 Rochelle salt, 227, 231. 

 Rocks, effect of freezing water on, 42-43. Water as a solvent, 71. 

 Rosin, obtained by distillation, 35. 

 Safety matches, 47-48. 

 Salicylic acid, 253. 

 Salt, 227-228. 

 Salts, 227. General properties, 227. In ocean, 227. Smelling, 222. 

 Saturation of air, 37. 

 Scale, musical, 282. 

 Screw, and inclined plane, 166. 

 Seaweed, 265. 

 Seidlitz powder, 231. 

 Self-raising flour, 231. 

 Sewage, disposition of, 198-199. Of camps, 196. Source of revenue, 201. 

 Sewer gas, 57. 

 Silk, bleaching, 241. Dyeing, 245-247. 

 Silver chloride, 127-131. 

 Simple colors, 138. 

 Simple substances, 103. 

 Siren, 280. 

 Smelling salts, 222. 

 Snow, 36-37. 

 Soap, 222-224. And hard water, 225. Liquid, 223-224. Preparation, 223. 

 Soda, baking, 227, 228-229. Benzoate, 253. Caustic, 218, 222, 223, 227. Washing, 225, 226, 229. 

 Soda mints, 231. 

 Sodium, bicarbonate, 56, 227, 228, 230-231. Carbonate, 228. Chloride, 228. 

 Soil, deposited by streams, 73. 

 Solenoid, 318. 

 Solution, 70. 

 Soothing sirup, 258. 

 Sound, and motion, 266, 278. Musical, 278. Nature of, 266. Reflection, 277. Speed of, 271-272. Transmission of, 267-271. Velocity of, 271-272. Waves, 272-274. 

 Sounder, telegraph, 324. 

 Sounding board, 277. 

 Sour milk in cooking, 230. 

 Specific heat, 28-29. 

 Spectrum, 134-135. 

 Speed, of sound, 271, 272. 

 Spontaneous combustion, 52. 

 Stains, removal of, 226, 243. 

 Standpipes, 212. 

 Starch, 65. 

 Steam, and work, 183-184. Engine, 183-185. Heat of vaporization, 32. Heating by, 33. Turbine, 183-184. 

 Steel, forging and annealing, 16. 

 Stoves, 18-19. 

 Streams, carriers of mud, 73. Volume of, 179-180. 

 Street cars, electric, 337. 

 Stringed instruments, 284-295. 

 Strings, vibrating, 286-290. 

 Sugar, 16, 65. Fermented by yeast, 234. 

 Sulphur, 66. As disinfectant, 251. In making sulphurous acid, 242. 

 Sulphuric acid, in bleaching, 240, 241. In fire extinguisher, 55. In making of hydrogen, 80. In voltaic cell, 307. 

 Sulphurous acid, in bleaching, 242. Preparation, 242. 

 Sun, energy derived from, 143-144. Source of heat, 29-30. 

 Sunlight, 135. And bacteria, 133. And chemical action, 126-127. 

 Sympathetic vibrations, 274-277. 

 Tallow, 105, 148. 

 Tartar, cream of, 229. 

 Telegraph, 322. Long distance, 327. Relay, 325. Sounder, 324. 

 Telephone, 350-351. 

 Temperature, 13-14. As measurement of heat present, 27. In detecting adulterants, 17. In forging steel, 16. In making sirups, 16. Measurement of, 14-15. 

 Thermometer, 14-17. Centigrade, 15. Fahrenheit, 15. 

 Tinder box, 47. 

 Transmission, of light, 145-147. Of sound, 267-271. 

 Tuning fork, 266, 273, 278, 290. 

 Turbine, steam, 183. Water, 178. 

 Turpentine, and grease, 226. By distillation, 35. 

 Unleavened bread, 233. 

 Vacuum, sound in, 268. 

 Vapor, in atmosphere, 36-38. 

 Vaporization, heat of, 32. 

 Varnish, on candies, 253. 

 Vegetable matter, and coal, 30. And gas, 30. And oil, 30. 

 Veins, formation in rock, 72-73. 

 Velocity, of sound, 271-272. 

 Ventilation, 21-24, 54. Need of, 38. 

 Vibration, of strings, 286-290. Sympathetic, 274-277. 

 Viola, 295. 

 Violin, 295. 

 Violoncello, 295. 

 Vocal cords, 300. 

 Voice, 300. 

 Volt, 344. 

 Voltage, 345. 

 Voltaic cell, 306-308, 310. 

 Voltmeter, 344. 

 Volume, of a stream, 179-180. Relation of pressure of a gas, 95-96. 

 Washing powders, 224-226. Soda, 229. 

 Water, action in nature, 70-74. Amount used daily per person, 181. And hydrogen, 79. And oxygen, 79, 80. As solvent, 70-71. Boiling, 77. Boiling point, 15. Composition, 79-80. Condensation, 33. Dams and reservoirs, 214-216. Density, 11. Distilled, 34, 77. Drinking, 75-77, 195-201. Electrolysis, 79-80. Evaporation, 33-34. Expansion, 9-10, 41-42. Filtration, 77. Freezing, 40-41. Hard, 225. Heat of fusion, 40. Impurities, 76-77. In atmosphere, 36-38. In food, 75. In human body, 75. In vegetables, 75. Influence on climate, 29, 40. Irrigation, 193-194. Minerals in, 70-71. Ocean, 265. Power, 176-180. Precipitates, 72, 73. Pressure, 208-211, 214-216. Purification, 77. Rain, 225. Running, value of, 178-180. Source of, 78. Steam, 32. Waves, 145-147. Weight, 208-209, 215. Wells, 195-201. Wheels, 176-180. Work under, 203-205. 

 Water supply, and forests, 216-217. Cost, 212-214. Of city, 206-212, 217. 

 Watt, 351. 

 Waves, heat, 145-147. Light, 145-147. Sound, 268, 272-274. Water, 145-147. 

 Weather, bureau, 87-91. Forecasts, 38-39, 86-88. Relation of water to, 29, 40. 

 Weather maps, 89-91. 

 Wedge, and inclined plane, 166. 

 Weight, of air, 86. Of water, 208-209, 215. 

 Welding, by electricity, 315. 

 Wells, 195-201. Drilled, 199. Driven, 196-197. 

 Wheel and axle, 169-171. Cogwheels, 170. Windlass, 169. 

 Wheelbarrow as lever, 160-161. 

 White light, nature of, 135. 

 Wind instruments, 297-301. 

 Windlass, 169. 

 Windmill, 174-175, 180-182. 

 Winds, 24. 

 Wine, 232, 234. 

 Wood, as source of charcoal, 58. Ashes in soap making, 223. In paper making, 219. Preservation, 253-254. 

 Wool, bleaching, 241. Dyeing, 245-247. 

 Work, 156-186. And steam, 183-184. And water, 176-180. Conservation, 174-175. Formula, 157. Machines, 157-175. Unit of, 172-173. Waste, 173. 

 Woven designs in cloth, 249. 

 Yeast, 234-236. Wild, 235-236. 

 Zinc, in galvanizing iron, 49. In making hydrogen, 80. In voltaic cell, 307-308. 

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MATHEMATICAL GEOGRAPHY, $1. 00

By WILLIS E. JOHNSON, Ph. D. , Vice-President and Professor ofGeography and Social Sciences, Northern Normal and Industrial School, Aberdeen, South Dakota. 

This work explains with great clearness and thoroughness that portionof the subject which not only is most difficult to understand, butalso underlies and gives meaning to all geographical knowledge. A vastnumber of facts which are much inquired about, but little known, aretaken up and explained. Simple formulas are given so that a studentunacquainted with geometry or trigonometry may calculate the heightsand distances of objects, the latitude and longitude of a place, theamount any body is lightened by the centrifugal force due to rotation, the deviation of a plumb-line from a true vertical, etc. 

AMERICAN BOOK COMPANY

ELEMENTS OF GEOLOGY

By ELIOT BLACKWELDER, Associate Professor of Geology, University ofWisconsin, and HARLAN H. BARROWS, Associate Professor of GeneralGeology and Geography, University of Chicago. 

$1. 40

An introductory course in geology, complete enough for collegeclasses, yet simple enough for high school pupils. The text isexplanatory, seldom merely descriptive, and the student gains aknowledge not only of the salient facts in the history of the earth, but also of the methods by which those facts have been determined. Thestyle is simple and direct. Few technical terms are used. The book isexceedingly teachable. 

The volume is divided into two parts, physical geology and historicalgeology. It differs more or less from its predecessors in the emphasison different topics and in the arrangement of its material. Factors ofminor importance in the development of the earth, such as earthquakes, volcanoes, and geysers, are treated much more briefly than iscustomary. This has given space for the extended discussion of mattersof greater significance. For the first time an adequate discussion ofthe leading modern conceptions concerning the origin and earlydevelopment of the earth is presented in an elementary textbook. 

The illustrations and maps, which are unusually numerous, reallyillustrate the text and are referred to definitely in the discussion. They are admirably adapted to serve as the basis for classroomdiscussion and quizzes, and as such constitute one of the mostimportant features of the book. The questions at the end of thechapters are distinctive in that the answers are in general not to befound in the text. They may, however, be reasoned out by the student, provided he has read the text with understanding. 

AMERICAN BOOK COMPANY

ESSENTIALS OF BIOLOGY

By GEORGE WILLIAM HUNTER, A. M. , Head of Department of Biology, DeWitt Clinton High School, New York City. 

$1. 25

This new first-year course treats the subject of biology as a whole, and meets the requirements of the leading colleges and associations ofscience teachers. Instead of discussing plants, animals, and man asseparate forms of living organisms, it treats of fife in acomprehensive manner, and particularly in its relations to theprogress of humanity. Each main topic is introduced by a problem, which the pupil is to solve by actual laboratory work. The text thatfollows explains and illustrates the meaning of each problem. The workthroughout aims to have a human interest and a practical value, and toprovide the simplest and most easily comprehended method ofdemonstration. At the end of each chapter are lists of references toboth elementary and advanced books for collateral reading. 

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SHARPE'S LABORATORY MANUAL IN BIOLOGY

$0. 75

In this Manual the 56 important problems of Hunter's Essentials ofBiology are solved; that is, the principles of biology are developedfrom the laboratory standpoint. It is a teacher's detailed directionsput into print. It states the problems, and then tells what materialsand apparatus are necessary and how they are to be used, how to avoidmistakes, and how to get at the facts when they are found. Followingeach problem and its solution is a full list of references to otherbooks. 

AMERICAN BOOK COMPANY

ESSENTIALS OF PHYSICS

By GEORGE A. HOADLEY, C. E. , Sc. D. , Professor of Physics, SwarthmoreCollege. 

$1. 25

This is the author's popular and successful Elements of Physicsenriched and brought up to date. Despite the many changes andmodifications made in this new edition, it retains the qualities whichhave secured so great a success for the previous book. 

It tells only what everyone should know, and it does this in astraightforward, concise, and interesting manner. It takes intoconsideration the character of high school needs and conditions, and, throughout, lays particular emphasis upon the intimate relationbetween physics and everyday life. 

While the subject matter, as a whole, is unchanged, the order oftopics in many cases has been altered to adapt the development of thesubject to the habits of thought of high school pupils. Instead ofbeginning the treatment of a subject with the definition andproceeding to a discussion of the sub-topics, the author starts with adiscussion of well-known phenomena and leads up to the definition ofthe subject discussed. The text, wherever possible, has beensimplified, more than fifty topics having been amplified, expanded, orreworded. More familiar illustrations of the topics treated are given, and the demonstrations of many of the experiments are simplified bythe use of materials that are readily obtainable in the classroom. 

There have been added a number of new topics, mostly in connectionwith the recent advances in applied science. The number both ofquestions and problems has been greatly increased and the data inthese all relate to actual, practical, physical phenomena. More thanone-fifth of the illustrations in the book are new, many of thepictures of apparatus having been redrawn to show modern forms. 

AMERICAN BOOK COMPANY