THE STORY OFA PIECE OF COAL

WHAT IT IS, WHENCE IT COMES, AND WHITHER IT GOES

BYEDWARD A. MARTIN, F. G. S. 

1896

PREFACE. 

The knowledge of the marvels which a piece of coal possesses withinitself, and which in obedience to processes of man's invention it isalways willing to exhibit to an observant enquirer, is not so widespread, perhaps, as it should be, and the aim of this little book, this record ofone page of geological history, has been to bring together the principalfacts and wonders connected with it into the focus of a few pages, where, side by side, would be found the record of its vegetable and mineralhistory, its discovery and early use, its bearings on the greatfog-problem, its useful illuminating gas and oils, the question of thepossible exhaustion of British supplies, and other important andinteresting bearings of coal or its products. 

In the whole realm of natural history, in the widest sense of the term, there is nothing which could be cited which has so benefited, sointerested, I might almost say, so excited mankind, as have the wonderfuldiscoveries of the various products distilled from gas-tar, itself adistillate of coal. 

Coal touches the interests of the botanist, the geologist, and thephysicist; the chemist, the sanitarian, and the merchant. 

In the little work now before the reader I have endeavoured to recount, without going into unnecessary detail, the wonderful story of a piece ofcoal. 

E. A. M. 

THORNTON HEATH, _February_, 1896. 

CONTENTS. 

 I. THE ORIGIN OF COAL AND THE PLANTS OF WHICH IT IS COMPOSED

 II. A GENERAL VIEW OF THE COAL-BEARING STRATA

 III. VARIOUS FORMS OF COAL AND CARBON

 IV. THE COAL-MINE AND ITS DANGERS

 V. EARLY HISTORY--ITS USE AND ITS ABUSE

 VI. HOW GAS IS MADE--ILLUMINATING OILS AND BYE-PRODUCTS

 VII. THE COAL SUPPLIES OF THE WORLD

VIII. THE COAL-TAR COLOURS

CHART SHEWING THE PRODUCTS OF COAL

GENERAL INDEX

LIST OF ILLUSTRATIONS. 

FIG. 1. _Stigmaria_ " 2. _Annularia radiata_ " 3. _Rhacopteris inaequilatera_ " 4. Frond of _Pecopteris_ " 5. _Pecopteris Serlii_ " 6. _Sphenopteris affinis_ " 7. _Catamites Suckowii_ " 8. _Calamocladus grandis_ " 9. _Asterophyllites foliosa_ " 10. _Spenophyllum cuneifolium_ " 11. Cast of _Lepidodendron_ " 12. _Lepidodendron longifolium_ " 13. _Lepidodendron aculeatum_ " 14. _Lepidostrobus_ " 15. _Lycopodites_ " 16. _Stigmaria ficoides_ " 17. Section of _Stigmaria_ " 18. Sigillarian trunks in sandstone " 19. _Productus_ " 20. _Encrinite_ " 21. Encrinital limestone " 22. Various _encrinites_ " 23. _Cyathophyllum_ " 24. _Archegosaurus minor_ " 25. _Psammodus porosus_ " 26. _Orthoceras_ " 27. _Fenestella retepora_ " 28. _Goniatites_ " 29. _Aviculopecten papyraceus_ " 30. Fragment of _Lepidodendron_ " 31. Engine-house at head of a Coal-Pit " 32. Gas Jet and Davy Lamp " 33. Part of a Sigillarian trunk " 34. Inside a Gas-holder " 35. Filling Retorts by Machinery " 36. "Condensers" " 37. "Washers" " 38. "Purifiers"

CHAPTER I. 

THE ORIGIN OF COAL AND THE PLANTS OFWHICH IT IS COMPOSED. 

From the homely scuttle of coal at the side of the hearth to thegorgeously verdant vegetation of a forest of mammoth trees, might haveappeared a somewhat far cry in the eyes of those who lived some fiftyyears ago. But there are few now who do not know what was the origin ofthe coal which they use so freely, and which in obedience to their demandhas been brought up more than a thousand feet from the bowels of theearth; and, although familiarity has in a sense bred contempt for thatwhich a few shillings will always purchase, in all probability a straythought does occasionally cross one's mind, giving birth to feelings of amore or less thankful nature that such a store of heat and light was longago laid up in this earth of ours for our use, when as yet man was notdestined to put in an appearance for many, many ages to come. We canscarcely imagine the industrial condition of our country in the absenceof so fortunate a supply of coal; and the many good things which areobtained from it, and the uses to which, as we shall see, it can be put, do indeed demand recognition. 

Were our present forests uprooted and overthrown, to be covered bysedimentary deposits such as those which cover our coal-seams, the amountof coal which would be thereby formed for use in some future age, wouldamount to a thickness of perhaps two or three inches at most, and yet, inone coal-field alone, that of Westphalia, the 117 most important seams, if placed one above the other in immediate succession, would amount to noless than 294 feet of coal. From this it is possible to form a faint ideaof the enormous growths of vegetation required to form some of ourrepresentative coal beds. But the coal is not found in one continuousbed. These numerous seams of coal are interspersed between many thousandsof feet of sedimentary deposits, the whole of which form the"coal-measures. " Now, each of these seams represents the growth of aforest, and to explain the whole series it is necessary to suppose thatbetween each deposit the land became overwhelmed by the waters of the seaor lake, and after a long subaqueous period, was again raised into dryland, ready to become the birth-place of another forest, which wouldagain beget, under similarly repeated conditions, another seam of coal. Of the conditions necessary to bring these changes about we will speaklater on, but this instance is sufficient to show how inadequate thequantity of fuel would be, were we dependent entirely on our own existingforest growths. 

However, we will leave for the present the fascinating pursuit oftheorising as to the how and wherefore of these vast beds of coal, relegating the geological part of the study of the carboniferous systemto a future chapter, where will be found some more detailed account ofthe position of the coal-seams in the strata which contain them. Atpresent the actual details of the coal itself will demand our attention. 

Coal is the mineral which has resulted, after the lapse of thousands ofthousands of years, from the accumulations of vegetable material, causedby the steady yearly shedding of leaves, fronds and spores, from forestswhich existed in an early age; these accumulated where the trees grewthat bore them, and formed in the first place, perhaps, beds of peat; thebeds have since been subjected to an ever-increasing pressure ofaccumulating strata above them, compressing the sheddings of a wholeforest into a thickness in some cases of a few inches of coal, and havebeen acted upon by the internal heat of the earth, which has caused themto part, to a varying degree, with some of their component gases. If wereason from analogy, we are compelled to admit that the origin of coal isdue to the accumulation of vegetation, of which more scattered, but moredistinct, representative specimens occur in the shales and clays aboveand below the coal-seams. But we are also able to examine the textureitself of the various coals by submitting extremely thin slices to astrong light under the microscope, and are thus enabled to decide whetherthe particular coal we are examining is formed of conifers, horse-tails, club-mosses, or ferns, or whether it consists simply of the accumulatedsheddings of all, or perhaps, as in some instances, of innumerablespores. 

In this way the structure of coal can be accurately determined. Were weartificially to prepare a mass of vegetable substance, and covering it upentirely, subject it to great pressure, so that but little of thevolatile gases which would be formed could escape, we might in the courseof time produce something approaching coal, but whether we obtainedlignite, jet, common bituminous coal, or anthracite, would depend uponthe possibilities of escape for the gases contained in the mass. 

Everybody has doubtless noticed that, when a stagnant pool which containsa good deal of decaying vegetation is stirred, bubbles of gas rise to thesurface from the mud below. This gas is known as marsh-gas, or lightcarburetted hydrogen, and gives rise to the _ignis fatuus_ which hoversabout marshy land, and which is said to lure the weary traveller to hisdoom. The vegetable mud is here undergoing rapid decomposition, as thereis nothing to stay its progress, and no superposed load of strataconfining its resulting products within itself. The gases thereforeescape, and the breaking-up of the tissues of the vegetation goes onrapidly. 

The chemical changes which have taken place in the beds of vegetation ofthe carboniferous epoch, and which have transformed it into coal, areeven now but imperfectly understood. All we know is that, under certaincircumstances, one kind of coal is formed, whilst under other conditions, other kinds have resulted; whilst in some cases the processes haveresulted in the preparation of large quantities of mineral oils, such asnaphtha and petroleum. Oils are also artificially produced from theso-called waste-products of the gas-works, but in some parts of the worldthe process of their manufacture has gone on naturally, and a yearlyincreasing quantity is being utilised. In England oil has been pumped upfrom the carboniferous strata of Coalbrook Dale, whilst in Sussex it hasbeen found in smaller quantities, where, in all probability, it has hadits origin in the lignitic beds of the Wealden strata. Immense quantitiesare used for fuel by the Russian steamers on the Caspian Sea, the Bakupetroleum wells being a most valuable possession. In Sicily, Persia, and, far more important, in the United States, mineral oils are found in greatquantity. 

In all probability coniferous trees, similar to the living firs, pines, larches, &c. , gave rise for the most part to the mineral oils. The classof living _coniferae_ is well known for the various oils which itfurnishes naturally, and for others which its representatives yield onbeing subjected to distillation. The gradually increasing amount of heatwhich we meet the deeper we go beneath the surface, has been the cause ofa slow and continuous distillation, whilst the oil so distilled has foundits way to the surface in the shape of mineral-oil springs, or hasaccumulated in troughs in the strata, ready for use, to be drawn up whena well has been sunk into it. 

The plants which have gone to make up the coal are not at once apparentto the naked eye. We have to search among the shales and clays andsandstones which enclose the coal-seams, and in these we find petrifiedspecimens which enable us to build up in our mind pictures of thevegetable creation which formed the jungles and forests of theseimmensely remote ages, and which, densely packed together on the oldforest floor of those days, is now apparent to us as coal. 

[Illustration: Fig. 2. --_Annularia radiata. _ Carboniferous sandstone. ]

A very large proportion of the plants which have been found in thecoal-bearing strata consists of numerous species of ferns, the number ofactual species which have been preserved for us in our English coal, being double the number now existing in Europe. The greater part of thesedo not seem to have been very much larger than our own living ferns, and, indeed, many of them bear a close resemblance to some of our own livingspecies. The impressions they have left on the shales of thecoal-measures are most striking, and point to a time when the sandy claywhich imbedded them was borne by water in a very tranquil manner, to bedeposited where the ferns had grown, enveloping them gradually, andconsolidating them into their mass of future shale. In one species knownas the _neuropteris_, the nerves of the leaves are as clear and asapparent as in a newly-grown fern, the name being derived from two Greekwords meaning "nerve-fern. " It is interesting to consider the history ofsuch a leaf, throughout the ages that have elapsed since it was part of aliving fern. First it grew up as a new frond, then gradually unfoldeditself, and developed into the perfect fern. Then it became cut off bythe rising waters, and buried beneath an accumulation of sediment, andwhile momentous changes have gone on in connection with the surface ofthe earth, it has lain dormant in its hiding-place exactly as we see it, until now excavated, with its contemporaneous vegetation, to form fuelfor our winter fires. 

[Illustration: FIG. 3. --_Rhacopteris inaequilatera. _ Carboniferouslimestone. ]

Although many of the ferns greatly resembled existing species, yet therewere others in these ancient days utterly unlike anything indigenous toEngland now. There were undoubted tree-ferns, similar to those whichthrive now so luxuriously in the tropics, and which throw out theirgraceful crowns of ferns at the head of a naked stem, whilst on the barkare the marks at different levels of the points of attachment of formerleaves. These have left in their places cicatrices or scars, showing theplaces from which they formerly grew. Amongst the tree-ferns found are_megaphyton_, _paloeopteris_, and _caulopteris_, all of which have thesemarks upon them, thus proving that at one time even tree-ferns had ahabitat in England. 

[Illustration: Fig. 4. --Frond of _Pecopteris. _ Coal-shale. ]

One form of tree-fern is known by the name of _Psaronius_, and this waspeculiar in the possession of masses of aerial roots grouped round thestem. Some of the smaller species exhibit forms of leaves which areutterly unknown in the nomenclature of living ferns. Most have had namesassigned to them in accordance with certain characteristics which theypossess. This was the more possible since the fossilised impressions hadbeen retained in so distinct a manner. Here before us is a specimen in ashale of _pecopteris_, as it is called, (_pekos_, a comb). The leaf insome species is not altogether unlike the well-known living fern_osmunda_. The position of the pinnules on both sides of the centralstalk are seen in the fossil to be shaped something like a comb, or asaw, whilst up the centre of each pinnule the vein is as prominent andnoticeable as if the fern were but yesterday waving gracefully in theair, and but to-day imbedded in its shaly bed. 

[Illustration: FIG. 5. --_Pecopteris Serlii_. Coal-shale. ]

_Sphenopteris_, or "wedge-fern, " is the name applied to anothercoal-fern; _glossopteris_, or "tongue-leaf"; _cyclopteris_, or"round-leaf"; _odonlopteris, _ or "tooth-leaf, " and many others, showtheir chief characteristics in the names which they individually bear. _Alethopteris_ appears to have been the common brake of the coal-period, and in some respects resembles _pecopteris_. 

[Illustration: Fig. 6. --_Sphenopteris Affinis. _ Coal-shale. ]

In some species of ferns so exact are the representations which they haveimpressed on the shale which contains them, that not only are the veinsand nerves distinctly visible, but even the fructification still remainsin the shape of the marks left by the so-called seeds on the backs of theleaves. Something more than a passing look at the coal specimens in agood museum will well repay the time so spent. 

What are known as septarian nodules, or snake-stones, are, at certainplaces, common in the carboniferous strata. They are composed of layersof ironstone and sandstone which have segregated around some centralobject, such as a fern-leaf or a shell. When the leaf of a fern has beenfound to be the central object, it has been noticed that the leaf cansometimes be separated from the stone in the form of a carbonaceous film. 

Experiments were made many years ago by M. Goppert to illustrate theprocess of fossilisation of ferns. Having placed some living ferns in amass of clay and dried them, he exposed them to a red heat, and obtainedthereby striking resemblances to fossil plants. According to the degreeof heat to which they were subjected, the plants were found to be eitherbrown, a shining black, or entirely lost. In the last mentioned case, only the impression remained, but the carbonaceous matter had gone tostain the surrounding clay black, thus indicating that the dark colour ofthe coal-shales is due to the carbon derived from the plants which theyincluded. 

Another very prominent member of the vegetation of the coal period, wasthat order of plants known as the _Calamites_. The generic distinctionsbetween fossil and living ferns were so slight in many cases as to bealmost indistinguishable. This resemblance between the ancient and themodern is not found so apparent in other plants. The Calamites of thecoal-measures bore indeed a very striking resemblance, and were closelyrelated, to our modern horse-tails, as the _equiseta_ are popularlycalled; but in some respects they differed considerably. 

Most people are acquainted with the horse-tail (_equisetum fluviatile)_of our marshes and ditches. It is a somewhat graceful plant, and standserect with a jointed stem. The foliage is arranged in whorls around thejoints, and, unlike its fossil representatives, its joints are protectedby striated sheaths. The stem of the largest living species rarelyexceeds half-an-inch in diameter, whilst that of the calamite attained athickness of five inches. But the great point which is noticeable in thefossil calamites and _equisetites_ is that they grew to a far greaterheight than any similar plant now living, sometimes being as much aseight feet high. In the nature of their stems, too, they exhibited a morehighly organised arrangement than their living representatives, having, according to Dr Williamson, a "fistular pith, an exogenous woody stem, and a thick smooth bark. " The bark having almost al ways disappeared hasleft the fluted stem known to us as the calamite. The foliage consistedof whorls of long narrow leaves, which differed only from the fern_asterophyllites_ in the fact that they were single-nerved. Sir WilliamDawson assigns the calamites to four sub-types: _calamite_ proper, _calamopitus, calamodendron_, and _eucalamodendron_. 

[Image: FIG. 7. --Root of _Catamites Suckowii_. Coal-shale. ]

[Image: FIG 8. --_Calamocladus grandis_. Carboniferous sandstone. ]

Having used the word "exogenous, " it might be as well to pay a littleattention, in passing, to the nomenclature and broad classification ofthe various kinds of plants. We shall then doubtless find it far easierthoroughly to understand the position in the scale of organisation towhich the coal plants are referable. 

[Illustration: FIG. 9. --_Asterophyllites foliosa_. Coal-measures. ]

The plants which are lowest in organisation are known as _Cellular_. Theyare almost entirely composed of numerous cells built up one above theother, and possess none of the higher forms of tissue and organisationwhich are met with elsewhere. This division includes the lichens, sea-weeds, confervae (green aquatic scum), fungi (mushrooms, dry-rot), &c. 

The division of _Vascular_ plants includes the far larger proportion ofvegetation, both living and fossil, and these plants are built up ofvessels and tissues of various shapes and character. 

All plants are divided into (1) Cryptogams, or Flowerless, such asmosses, ferns, equisetums, and (2) Phanerogams, or Flowering. Floweringplants are again divided into those with naked seeds, as the conifers andcycads (gymnosperms), and those whose seeds are enclosed in vessels, orovaries (angiosperms). 

Angiosperms are again divided into the monocotyledons, as the palms, anddicotyledons, which include most European trees. 

Thus:--

-------------------------------------------------------------------| (M. A. Brongniart). | |(Lindley). ||CELLULAR | | || _Cryptogams_ (Flowerless) |Fungi, seaweeds, |Thallogens || | lichens | || | | ||VASCULAR | | || _Cryptogams_ (Flowerless) |Ferns, equisetums, |Acrogens || | mosses, lycopodiums| || _Phanerogams_ (Flowering) | | || Gymnosperms (having |Conifers and |Gymnogens || naked seeds) | cycads | || Two or more Cotyledons | | || Angiosperms (having | | || enclosed seeds) | | || Monocotyledons |Palms, lilies, |Endogens || | grasses | || Dicotyledons |Most European |Exogens || | trees and shrubs | |-------------------------------------------------------------------

Adolphe Brongniart termed the coal era the "Age of Acrogens, " because, aswe shall see, of the great predominance in those times of vascularcryptogamic plants, known in Dr Lindley's nomenclature as "Acrogens. "

[Illustration: FIG. 10. --_Spenophyllum cuneifolium. _ Coal-shale. ]

Two of these families have already been dealt with, viz. , the ferns(_felices_), and the equisetums, (_calamites_ and _equisetites_), and wenow have to pass on to another family. This is that which includes thefossil representatives of the Lycopodiums, or Club-mosses, and which goesto make up in some coals as much as two-thirds of the whole mass. Everyone is more or less familiar with some of the living Lycopodiums, those delicate little fern-like mosses which are to be found in many ahome. They are but lowly members of our British flora, and it may seemsomewhat astounding at first sight that their remote ancestors occupiedso important a position in the forests of the ancient period of which weare speaking. Some two hundred living species are known, most of thembeing confined to tropical climates. They are as a rule, low creepingplants, although some few stand erect. There is room for astonishmentwhen we consider the fact that the fossil representatives of the family, known as _Lepidodendra_, attained a height of no less than fifty feet, and, there is good ground for believing, in many cases, a far greatermagnitude. They consist of long straight stems, or trunks which branchconsiderably near the top. These stems are covered with scars or scales, which have been caused by the separation of the petioles or leaf-stalks, and this gives rise to the name which the genus bears. The scars arearranged in a spiral manner the whole of the way up the stem, and thestems often remain perfectly upright in the coal-mines, and reach intothe strata which have accumulated above the coal-seam. 

[Illustration: FIG. 11. --Cast of _lepidodendron_ in sandstone. ]

Count Sternberg remarked that we are unacquainted with any existingspecies of plant, which like the _Lepidodendron_, preserves at all ages, and throughout the whole extent of the trunk, the scars formed by theattachment of the petioles, or leaf-stalks, or the markings of the leavesthemselves. The yucca, dracaena, and palm, entirely shed their scaleswhen they are dried up, and there only remain circles, or rings, arrangedround the trunk in different directions. The flabelliform palms preservetheir scales at the inferior extremity of the trunk only, but lose themas they increase in age; and the stem is entirely bare, from the middleto the superior extremity. In the ancient _Lepidodendron_, on the otherhand, the more ancient the scale of the leaf-stalk, the more apparent itstill remains. Portions of stems have been discovered which containleaf-scars far larger than those referred to above, and we deduce fromthese fragments the fact that those individuals which have been foundwhole, are not by any means the largest of those which went to form solarge a proportion of the ancient coal-forests. The _lepidodendra_ borelinear one-nerved leaves, and the stems always branched dichotomously andpossessed a central pith. Specimens variously named _knorria, lepidophloios, halonia_, and _ulodendron_ are all referable to thisfamily. 

[Illustration: FIG. 12. --_Lepidodendron longifolium. _ Coal-shale. ]

[Illustration: FIG. 13. --_Lepidodendron aculeatum_ in sandstone. ]

In some strata, as for instance that of the Shropshire coalfield, quantities of elongated cylindrical bodies known as _lepidostrobi_ havebeen found, which, it was early conjectured, were the fruit of the giantclub-mosses about which we have just been speaking. Their appearance canbe called to mind by imagining the cylindrical fruit of the maize orIndian corn to be reduced to some three or four inches in length. Thesporangia or cases which contained the microscopic spores or seeds werearranged around a central axis in a somewhat similar manner to that inwhich maize is found. These bodies have since been found actuallysituated at the end of branches of _lepidodendron_, thus placing theirtrue nature beyond a doubt. The fossil seeds (spores) do not appear tohave exceeded in volume those of recent club-mosses, and this althoughthe actual trees themselves grew to a size very many times greater thanthe living species. This minuteness of the seed-germs goes to explain thereason why, as Sir Charles Lyell remarked, the same species of_lepidodendra_ are so widely distributed in the coal measures of Europeand America, their spores being capable of an easy transportation by thewind. 

[Illustration: FIG. 14. --_Lepidostrobus. _ Coal-shale. ]

One striking feature in connection with the fruit of the _lepidodendron_and other ancient representatives of the club-moss tribe, is that thebituminous coals in many, if not in most, instances, are made up almostentirely of their spores and spore-cases. Under a microscope, a piece ofsuch coal is seen to be thronged with the minute rounded bodies of thespores interlacing one another and forming almost the whole mass, whilstlarger than these, and often indeed enclosing them, are flattenedbag-like bodies which are none other than the compressed sporangia whichcontained the former. 

[Illustration: FIG. 15. --_Lycopodites_. Coal sandstone. ]

Now, the little Scottish or Alpine club-moss which is so familiar, produces its own little cones, each with its series of outside scales orleaves; these are attached to the bags or spore-cases, which are crowdedwith spores. Although in miniature, yet it produces its fruit in just thesame way, at the terminations of its little branches, and the spores, theactual germs of life, when examined microscopically, are scarcelydistinguishable from those which are contained in certain bituminouscoals. And, although ancient club-mosses have been found in a fossilisedcondition at least forty-nine feet high, the spores are no larger thanthose of our miniature club-mosses of the present day. 

The spores are more or less composed of pure bitumen, and the bituminousnature of the coal depends largely on the presence or absence of thesemicroscopic bodies in it. The spores of the living club-mosses contain somuch resinous matter that they are now largely used in the making offireworks, and upon the presence of this altered resinous matter in coaldepends its capability of providing a good blazing coal. 

At first sight it seems almost impossible that such a minute cause shouldresult in the formation of huge masses of coal, such an inconceivablenumber of spores being necessary to make even the smallest fragment ofcoal. But if we look at the cloud of spores that can be shaken from asingle spike of a club-moss, then imagine this to be repeated a thousandtimes from each branch of a fairly tall tree, and then finally picture awhole forest of such trees shedding in due season their copious showersof spores to earth, we shall perhaps be less amazed than we were at firstthought, at the stupendous result wrought out by so minute an object. 

Another well-known form of carboniferous vegetation is that known as the_Sigillaria_, and, connected with this form is one, which was longfamiliar under the name of _Stigmaria_, but which has since beensatisfactorily proved to have formed the branching root of thesigillaria. The older geologists were in the habit of placing theseplants among the tree-ferns, principally on account of the cicatriceswhich were left at the junctions of the leaf-stalks with the stem, afterthe former had fallen off. No foliage had, however, been met with whichwas actually attached to the plants, and hence, when it was discoveredthat some of them had long attenuated leaves not at all like thosepossessed by ferns, geologists were compelled to abandon thisclassification of them, and even now no satisfactory reference toexisting orders of them has been made, owing to their anomalousstructure. The stems are fluted from base to stem, although this is notso apparent near the base, whilst the raised prominences which now formthe cicatrices, are arranged at regular distances within the verticalgrooves. 

When they have remained standing for some length of time, and the stratahave been allowed quietly to accumulate around the trunks, they haveescaped compression. They were evidently, to a great extent, hollow likea reed, so that in those trees which still remain vertical, the interiorhas become filled up by a coat of sandstone, whilst the bark has becometransformed into an envelope of an inch, or half an inch of coal. Butmany are found lying in the strata in a horizontal plane. These have beencast down and covered up by an ever-increasing load of strata, so thatthe weight has, in the course of time, compressed the tree into simplythe thickness of the double bark, that is, of the two opposite sides ofthe envelope which covered it when living. 

_Sigillarae_ grew to a very great height without branching, somespecimens having measured from 60 to 70 feet long. In accordance withtheir outside markings, certain types are known as _syringodendron_, _favularia_, and _clathraria_. _Diploxylon_ is a term applied to aninterior stem referable to this family. 

[Illustration: FIG. 16. --_Stigmaria ficoides_. Coal-shale. ]

But the most interesting point about the _sigillariae_ is the root. Thiswas for a long time regarded as an entirely distinct individual, and theolder geologists explained it in their writings as a species of succulentaquatic plant, giving it the name of _stigmaria_. They realized the factthat it was almost universally found in those beds which occurimmediately beneath the coal seams, but for a long time it did not strikethem that it might possibly be the root of a tree. In an old edition ofLyell's "Elements of Geology, " utterly unlike existing editions inquality, quantity, or comprehensiveness, after describing it as anextinct species of water-plant, the author hazarded the conjecture thatit might ultimately be found to have a connection with some otherwell-known plant or tree. It was noticed that above the coal, in theroof, stigmariae were absent, and that the stems of trees which occurredthere, had become flattened by the weight of the overlying strata. Thestigmariae on the other hand, abounded in the _underclay_, as it iscalled, and were not in any way compressed but retained what appeared tobe their natural shape and position. Hence to explain their appearance, it was thought that they were water-plants, ramifying the mud in everydirection, and finally becoming overwhelmed and covered by the muditself. On botanical grounds, Brongniart and Lyell conjectured that theyformed the roots of other trees, and this became the more apparent as itcame to be acknowledged that the underclays were really ancient soils. All doubt was, however, finally dispelled by the discovery by Mr Binney, of a sigillaria and a stigmaria in actual connection with each other, inthe Lancashire coal-field. 

Stigmariae have since been found in the Cape Breton coal-field, attachedto Lepidodendra, about which we have already spoken, and a similardiscovery has since been made in the British coal-fields. This, therefore, would seem to shew the affinity of the sigillaria to thelepidodendron, and through it to the living lycopods, orclub-mosses. 

Some few species of stigmarian roots had been discovered, and variousspecific names had been given to them before their actual nature was madeout. What for some time were thought to be long cylindrical leaves, havenow been found to be simply rootlets, and in specimens where these havebeen removed, the surface of the stigmaria has been noticed to be coveredwith large numbers of protuberant tubercles, which have formed the basesof the rootlets. There appears to have also been some special kind ofarrangement in their growth, since, unlike the roots of most livingplants, the tubercles to which these rootlets were attached, werearranged spirally around the main root. Each of these tubercles waspitted in the centre, and into these the almost pointed ends of therootlets fitted, as by a ball and socket joint. 

[Illustration: FIG. 17--_Section of stigmaria_. ]

"A single trunk of _sigillaria_ in an erect forest presents an epitome ofa coal-seam. Its roots represent the _stigmaria_ underclay; its bark thecompact coal; its woody axis, the mineral charcoal; its fallen leaves andfruits, with remains of herbaceous plants growing in its shade, mixedwith a little earthy matter, the layers of coarse coal. The condition ofthe durable outer bark of erect trees, concurs with the chemical theoryof coal, in showing the especial suitableness of this kind of tissue forthe production of the purer compact coals. "--(Dawson, "Structures inCoal. ")

There is yet one other family of plants which must be mentioned, andwhich forms a very important portion of the constituent _flora_ of thecoal period. This is the great family of the _coniferae_, which althoughdiffering in many respects from the highly organised dicotyledons of thepresent day, yet resembled them in some respects, especially in theformation of an annual ring of woody growth. 

The conifers are those trees which, as the name would imply, bear theirfruit in the form of cones, such as the fir, larch, cedar, and others. The order is one which is familiar to all, not only on account of thecones they bear, and their sheddings, which in the autumn strew theground with a soft carpet of long needle-like leaves, but also because ofthe gum-like secretion of resin which is contained in their tissues. Onlya few species have been found in the coal-beds, and these, on examinationunder the microscope, have been discovered to be closely related to thearaucarian division of pines, rather than to any of our common firs. Theliving species of this tree is a native of Norfolk Island, in thePacific, and here it attains a height of 200 feet, with a girth of 30feet. From the peculiar arrangement of the ducts in the elongatedcellular tissue of the tree, as seen under the microscope, the fossilconifers, which exhibit this structure, have been placed in the samedivision. 

The familiar fossil known to geologists as _Sternbergia_ has now beenshown to be the cast of the central pith of these conifers, amongst whichmay be mentioned _cordaites, araucarites_, and _dadoxylon. _. The centralcores had become replaced with inorganic matter after the pith had shrunkand left the space empty. This shrinkage of the pith is a process whichtakes place in many plants even when living, and instances will at onceoccur, in which the stems of various species of shrubs when broken openexhibit the remains of the shrunken pith, in the shape of thin discsacross the interval cavity. 

We might reasonably expect that where we find the remains of fossilconiferous trees, we should also meet with the cones or fruit which theybear. And such is the case. In some coal-districts fossil fruits, named_cardiocarpum_ and _trigonocarpum_, have been found in great quantities, and these have now been decided by botanists to be the fruits of certainconifers, allied, not to those which bear hard cones, but to those whichbear solitary fleshy fruits. Sir Charles Lyell referred them to a Chinesegenus of the yew tribe called _salisburia_. Dawson states that they arevery similar to both _taxus_ and _salisburia. _. They are abundant in somecoal-measures, and are contained, not only in the coal itself, but alsoin the sandstones and shales. The under-clays appear to be devoid ofthem, and this is, of course, exactly what might have been expected, since the seeds would remain upon the soil until covered up by vegetablematter, but would never form part of the clay soil itself. 

In connection with the varieties which have been distinguished in thefamilies of the conifers, calamites, and sigillariae, Sir William Dawsonmakes the following observations: "I believe that there was aconsiderably wide range of organisation in _cordaitinae_ as well as in_calamites_ and _sigillariae_, and that it will eventually be found thatthere were three lines of connection between the higher cryptogams(flowerless) and the phaenogams (flowering), one leading from thelycopodes by the _sigillariae_, another leading by the _cordaites_, andthe third leading from the _equisetums_ by the _calamites_. Still furtherback the characters, afterwards separated in the club-mosses, mare's-tails, and ferns, were united in the _rhizocarps_, or, as someprefer to call them, the heterosporous _filicinae_. "

In concluding this chapter dealing with the various kinds of plants whichhave been discovered as contributing to the formation ofcoal-measures, it would be as well to say a word or two concerning theclimate which must have been necessary to permit of the growth of such anabundance of vegetation. It is at once admitted by all botanists that amoist, humid, and warm atmosphere was necessary to account for theexistence of such an abundance of ferns. The gorgeous wavingtree-ferns which were doubtless an important feature of the landscape, would have required a moist heat such as does not now exist in thiscountry, although not necessarily a tropical heat. The magnificent giantlycopodiums cast into the shade all our living members of that class, thelargest of which perhaps are those that flourish in New Zealand. In NewZealand, too, are found many species of ferns, both those which arearborescent and those which are of more humble stature. Add to these thenumerous conifers which are there found, and we shall find that a forestin that country may represent to a certain extent the appearancepresented by a forest of carboniferous vegetation. The ferns, lycopods, and pines, however, which appear there, it is but fair to add, are mixedwith other types allied to more recent forms of vegetation. 

There are many reasons for believing that the amount of carbonic acid gasthen existing in the atmosphere was larger than the quantity which we nowfind, and Professor Tyndall has shown that the effect of this would be toprevent radiation of heat from the earth. The resulting forms ofvegetation would be such as would be comparable with those which are nowreared in the green-house or conservatory in these latitudes. The gaswould, in fact, act as a glass roof, extending over the whole world. 

CHAPTER II. 

A GENERAL VIEW OF THE COAL-BEARING STRATA. 

In considering the source whence coal is derived, we must be careful toremember that coal itself is but a minor portion of the whole formationin which it occurs. The presence of coal has indeed given the name to theformation, the word "carboniferous" meaning "coal-bearing, " but in takinga comprehensive view of the position which it occupies in the bowels ofthe earth, it will be necessary to take into consideration the strata inwhich it is found, and the conditions, so far as are known, under whichthese were deposited. 

Geologically speaking, the Carboniferous formation occurs near the closeof that group of systems which have been classed as "palaeozoic, " youngerin point of age than the well known Devonian and Old Red Sandstonestrata, but older by far than the Oolites, the Wealden, or the Cretaceousstrata. 

In South Wales the coal-bearing strata have been estimated at between11, 000 and 12, 000 feet, yet amongst this enormous thickness of strata, the whole of the various coal-seams, if taken together, probably does notamount to more than 120 feet. This great disproportion between the totalthickness and the thickness of coal itself shows itself in everycoal-field that has been worked, and when a single seam of coal isdiscovered attaining a thickness of 9 or 10 feet, it is so unusual athing in Great Britain as to cause it to be known as the "nine" or"ten-foot seam, " as the case may be. Although abroad many seams are foundwhich are of greater thicknesses, yet similarly the other portions of theformation are proportionately greater. 

It is not possible therefore to realise completely the significance ofthe coal-beds themselves unless there is also a knowledge of theremaining constituents of the whole formation. The strata found in thevarious coal-fields differ considerably amongst themselves in character. There are, however, certain well-defined characteristics which findrepresentation in most of the principal coal-fields, whether British orEuropean. Professor Hull classifies these carboniferous beds asfollows:--

 UPPER CARBONIFEROUS. _Upper coal-measures. _ Reddish and purple sandstones, red and grey clays and shales, thin bands of coal, ironstone and limestone, with _spirorbis_ and fish. 

 _Middle coal-measures. _ Yellow and gray sandstones, blue and black clays and shales, bands of coal and ironstone, fossil plants, bivalves and fish, occasional marine bands. 

 MIDDLE CARBONIFEROUS. _Gannister beds_ or _Lower coal-measures. _ _Millstone grit. _ Flagstone series in Ireland. _Yoredale beds. _ Upper shale series of Ireland. 

 LOWER CARBONIFEROUS. _Mountain limestone_. _Limestone shale_. 

Each of the three principal divisions has its representative in Scotland, Belgium, and Ireland, but, unfortunately for the last-named country, thewhole of the upper coal-measures are there absent. It is from thesemeasures that almost all our commercial coals are obtained. 

This list of beds might be further curtailed for all practical purposesof the geologist, and the three great divisions of the system would thusstand:--

 Upper Carboniferous, or Coal-measures proper. 

 Millstone grit. 

 Lower Carboniferous, or Mountain limestone. 

In short, the formation consists of masses of sandstone, shale, limestoneand coal, these also enclosing clays and ironstones, and, in thelimestone, marbles and veins of the ores of lead, zinc, and antimony, andoccasionally silver. 

[Illustration: FIG. 18. --Sigillarian trunks in current-bedded sandstone. St Etienne. ]

As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these weredeposited in the waters of the carboniferous ages, and this we can bestdo by considering the laws under which strata of a similar nature are nowbeing deposited as sedimentary beds. 

A great proportion consists of sandstone. Now sandstone is the result ofsand which has been deposited in large quantities, having becomeindurated or hardened by various processes brought to bear upon it. It isnecessary, therefore, first to ascertain whence came the sand, andwhether there are any peculiarities in its method of deposition whichwill explain its stratification. It will be noticed at once that it bearsa considerable amount of evidence of what is called "current-bedding, "that is to say, that the strata, instead of being regularly deposited, exhibit series of wedge-shaped masses, which are constantly thinning out. 

Sand and quartz are of the same chemical composition, and in allprobability the sand of which every sandstone in existence is composed, appeared on this earth in its first solid form in the shape of quartz. Now quartz is a comparatively heavy mineral, so also, therefore, willsand be. It is also very hard, and in these two respects it differsentirely from another product of sedimentary deposition, namely, mud orclay, with which we shall have presently to deal when coming to theshales. Since quartz is a hard mineral it necessarily follows that itwill suffer, without being greatly affected, a far greater amount ofwearing and knocking about when being transported by the agency ofcurrents and rivers, than will a softer substance, such as clay. An equalamount of this wearing action upon clay will reduce it to a fineimpalpable silt. The grains of sand, however, will still remain of anappreciable average size, and where both sand and clay are beingtransported to the sea in one and the same stream, the clay will betransported to long distances, whilst the sand, being heavier, bulk forbulk, and also consisting of grains larger in size than grains of clay, will be rapidly deposited, and form beds of sand. Of course, if thecurrent be a violent one, the sand is transported, not by being held insuspension, but rather by being pushed along the bed of the river; suchan action will then tend to cause the sand to become powdered into stillfiner sand. 

When a river enters the sea it soon loses its individuality; it becomesmerged in the body of the ocean, where it loses its current, and wheretherefore it has no power to keep in suspension the sediment which it hadbrought down from the higher lands. When this is the case, the sand bornein suspension is the first to be deposited, and this accumulates in banksnear the entrance of the river into the sea. We will suppose, forillustration, that a small river has become charged with a supply ofsand. As it gradually approaches the sea, and the current loses itsforce, the sand is the more sluggishly carried along, until finally itfalls to the bottom, and forms a layer of sand there. This layerincreases in thickness until it causes the depth of water above it tobecome comparatively shallow. On the shallowing process taking place, thecurrent will still have a certain, though slighter, hold on the sand insuspension, and will transport it yet a little further seaward, when itwill be thrown down, at the edge of the bank or layer already formed, thus tending to extend the bank, and to shallow a wider space ofriver-bed. 

As a result of this action, strata would be formed, shewingstratification diagonally as well as horizontally, represented in sectionas a number of banks which had seemingly been thrown down one above theother, ending in thin wedge-shaped terminations where the particularsupply of sediment to which each owed its formation had failed. 

The masses of sandstone which are found in the carboniferous formation, exhibit in a large degree these wedge-shaped strata, and we havetherefore a clue at once, both as to their propinquity to sea and land, and also as to the manner in which they were formed. 

[Illustration: FIG. 19. --_Productus_. Coal-measures. ]

There is one thing more, too, about them. Just as, in the case we wereconsidering, we could observe that the wedge-shaped strata always pointedaway from the source of the material which formed them, so we cansimilarly judge that in the carboniferous strata the same deduction holdsgood, that the diagonally-pointing strata were formed in the same way, and that their thinning out was simply owing to temporary failure ofsediment, made good, however, by a further deposition of strata when thenext supply was borne down. 

It is scarcely likely, however, that sand in a pure state was alwayscarried down by the currents to the sea. Sometimes there would be somesilt mixed with it. Just as in many parts large masses of almost puresandstone have been formed, so in other places shales, or, as they arepopularly known by miners, "bind, " have been formed. Shales are formedfrom the clays which have been carried down by the rivers in the shape ofsilt, but which have since become hardened, and now split up easily intothin parallel layers. The reader has no doubt often handled a piece ofhard clay when fresh from the quarry, and has remembered how that, whenhe has been breaking it up, in order, perhaps, to excavate apartially-hidden fossil, it has readily split up in thin flakes or layersof shaly substance. This exhibits, on a small scale, the chiefpeculiarity of the coal shales. 

The formation of shales will now demand our attention. When a river iscarrying down with it a quantity of mud or clay, it is transported as afine, dusty silt, and when present in quantities, gives the muddy tint tothe water which is so noticeable. We can very well see how that silt willbe carried down in greater quantities than sand, since nearly all riversin some part of their course will travel through a clayey district, andfinely-divided clay, being of a very light nature, will be carriedforward whenever a river passes over such a district. And a very slightcurrent being sufficient to carry it in a state of suspension, it followsthat it will have little opportunity of falling to the bottom, until, bysome means or other, the current, which is the means of its conveyance, becomes stopped or hindered considerably in its flow. 

When the river enters a large body of water, such as the ocean or a lake, in losing its individuality, it loses also the velocity of its current, and the silt tends to sink down to the bottom. But being less heavy thanthe sand, about which we have previously spoken, it does not sink all atonce, but partly with the impetus it has gained, and partly on account ofthe very slight velocity which the current still retains, even afterhaving entered the sea, it will be carried out some distance, and willthe more gradually sink to the bottom. The deeper the water in which itfalls the greater the possibility of its drifting farther still, since insinking, it would fall, not vertically, but rather as the drops of rainin a shower when being driven before a gale of wind. Thus we shouldnotice that clays and shales would exhibit a regularity and uniformity ofdeposition over a wide area. Currents and tides in the sea or lake wouldtend still further to retard deposition, whilst any stoppages in thesupply of silt which took place would give the former layer time toconsolidate and harden, and this would assist in giving it that beddedstructure which is so noticeable in the shales, and which causes it tosplit up into fine laminae. This uniformity of structure in the shalesover wide areas is a well ascertained characteristic of the coal-shales, and we may therefore regard the method of their deposition as given herewith a degree of certainty. 

There is a class of deposit found among the coal-beds, which is known asthe "underclay, " and this is the most regular of all as to the positionin which it is found. The underclays are found beneath every bed of coal. "Warrant, " "spavin, " and "gannister" are local names which are sometimesapplied to it, the last being a term used when the clay contains such alarge proportion of silicious matter as to become almost like a hardflinty rock. Sometimes, however, it is a soft clay, at others it is mixedwith sand, but whatever the composition of the underclays may be, theyalways agree in being unstratified. They also agree in this respect thatthe peculiar fossils known as _stigmariae_ abound in them, and in somecases to such an extent that the clay is one thickly-matted mass of thefilamentous rootlets of these fossils. We have seen how these graduallycame to be recognised as the roots of trees which grew in this age, andwhose remains have subsequently become metamorphosed into coal, and it isbut one step farther to come to the conclusion that these underclays arethe ancient soils in which the plants grew. 

No sketch of the various beds which go to form the coal-measures would becomplete which did not take into account the enormous beds of mountainlimestone which form the basis of the whole system, and which in thinnerbands are intercalated amongst the upper portion of the system, or thetrue coal-measures. 

Now, limestones are not formed in the same way in which we have seen thatsandstones and shales are formed. The last two mentioned owe their originto their deposition as sediment in seas, estuaries or lakes, but themasses of limestone which are found in the various geological formationsowe their origin to causes other than that of sedimentary deposition. 

In carboniferous times there lived numberless creatures which we knownowadays as _encrinites_. These, when growing, were fixed to the bed ofthe ocean, and extended upward in the shape of pliant stems composed oflimestone joints or plates; the stem of each encrinite then expanded atthe top in the shape of a gorgeous and graceful starfish, possessed ofnumberless and lengthy arms. These encrinites grew in such profusion thatafter death, when the plates of which their stems consisted, becameloosened and scattered over the bed of the sea, they accumulated andformed solid beds of limestone. Besides the encrinites, there were ofcourse other creatures which were able to create the hard parts of theirstructures by withdrawing lime from the sea, such as _foraminifera_, shell-fish, and especially corals, so that all these assisted after deathin the accumulation of beds of limestone where they had grown and lived. 

[Illustration: FIG. 20. --Encrinite. ]

[Illustration: FIG. 21. --Encrinital limestone. ]

There is one peculiarity in connection with the habitats of theencrinites and corals which goes some distance in supplying us with auseful clue as to the conditions under which this portion of thecarboniferous formation was formed. These creatures find it a difficultmatter, as a rule, to live and secrete their calcareous skeleton in anywater but that which is clear, and free from muddy or sandy sediment. They are therefore not found, generally speaking, where the otherdeposits which we have considered, are forming, and, as these are alwaysfound near the coasts, it follows that the habitats of the creaturesreferred to must be far out at sea where no muddy sediments, borne byrivers, can reach them. We can therefore safely come to the conclusionthat the large masses of encrinital limestone, which attain such anenormous thickness in some places, especially in Ireland, have beenformed far away from the land of the period; we can at the same time drawthe conclusion that if we find the encrinites broken and snapped asunder, and the limestone deposits becoming impure through being mingled with aproportion of clayey or sandy deposits, that we are approaching acoast-line where perhaps a river opened out, and where it destroyed thegrowth of encrinites, mixing with their dead remains the sedimentarydêbris of the land. 

[Illustration: FIG. 22. --Encrinites: various. Mountain limestone. ]

We have lightly glanced at the circumstances attending the deposition ofeach of the principal rocks which form the beds amongst which coal isfound, and have now to deal with the formation of the coal itself. Wehave already considered the various kinds of plants and trees which havebeen discovered as contributing their remains to the formation of coal, and have now to attempt an explanation of how it came to be formed in soregular a manner over so wide an area. 

Each of the British coal-fields is fairly extensive. The Yorkshire andDerbyshire coal-fields, together with the Lancashire coal-field, withwhich they were at one time in geological connection, give us an area ofnearly 1000 square miles, and other British coal-fields show at leastsome hundreds of square miles. And yet, spread over them, we find aseries of beds of coal which in many cases extend throughout the wholearea with apparent regularity. If we take it, as there seems every reasonto believe was the case, that almost all these coal-fields were not onlybeing formed at the same time, but were in most instances in continuationwith one another, this regularity of deposition of comparatively narrowbeds of coal, appears all the more remarkable. 

The question at once suggests itself, Which of two things is probable?Are we to believe that all this vegetable matter was brought down by somemighty river and deposited in its delta, or that the coal-plants grewjust where we now find the coal?

Formerly it was supposed that coal was formed out of dead leaves andtrees, the refuse of the vegetation of the land, which had been carrieddown by rivers into the sea and deposited at their mouths, in the sameway that sand and mud, as we have seen, are swept down and deposited. Ifthis were so, the extent of the deposits would require a river with anenormous embouchure, and we should be scarcely warranted in believingthat such peaceful conditions would there prevail as to allow of thelayers of coal to be laid down with so little disturbance and with suchregularity over these wide areas. But the great objection to this theoryis, that not only do the remains still retain their perfection ofstructure, but they are comparatively _pure, --i. E. , _ unmixed withsedimentary depositions of clay or sand. Now, rivers would not bring downthe dead vegetation alone; their usual burden of sediment would also bedeposited at their mouths, and thus dead plants, sand, and clay would bemixed up together in one black shaly or sandy mass, a mixture which wouldbe useless for purposes of combustion. The only theory which explainedall the recognised phenomena of the coal-measures was that the plantsforming the coal actually grew where the coal was formed, and where, indeed, we now find it. When the plants and trees died, their remainsfell to the ground of the forest, and these soon turned to a black, pasty, vegetable mass, the layer thus formed being regularly increasedyear by year by the continual accumulation of fresh carbonaceous matter. By this means a bed would be formed with regularity over a wide area; thecoal would be almost free from an admixture of sandy or clayey sediment, and probably the rate of formation would be no more rapid in one part ofthe forest than another. Thus there would be everywhere uniformity ofthickness. The warm and humid atmosphere, which it is probable thenexisted, would not only have tended towards the production of an abnormalvegetation, but would have assisted in the decaying and disintegratingprocesses which went on amongst the shed leaves and trees. 

When at last it was announced as a patent fact that every bed of coalpossessed its underclay, and that trees had been discovered actuallystanding upon their own roots in the clay, there was no room at all fordoubt that the correct theory had been hit upon--viz. , that coal is nowfound just where the trees composing it had grown in the past. 

But we have more than one coal-seam to account for. We have to explainthe existence of several layers of coal which have been formed over oneanother on the same spot at successive periods, divided by other periodswhen shale and sandstones only have been formed. 

A careful estimate of the Lancashire coal-field has been made byProfessor Hull for the Geological Survey. Of the 7000 feet ofcarboniferous strata here found, spread out over an area of 217 squaremiles, there are on the average eighteen seams of coal. 

This is only an instance of what is to be found elsewhere. Eighteencoal-seams! what does this mean? It means that, during carboniferoustimes, on no less than eighteen occasions, separate and distinct forestshave grown on this self-same spot, and that between each of theseoccasions changes have taken place which have brought it beneath thewaters of the ocean, where the sandstones and shales have been formedwhich divide the coal-seams from each other. We are met here by awonderful demonstration of the instability of the surface of the earth, and we have to do our best to show how the changes of level have beenbrought about, which have allowed of this game of geological see-saw totake place between sea and land. Changes of level! Many a hard geologicalnut has only been overcome by the application of the principle of changesof level in the surface of the earth, and in this we shall find a sureexplanation of the phenomena of the coal-measures. 

Great changes of the level of the land are undoubtedly taking place evennow on the earth's surface, and in assuming that similar changes tookplace in carboniferous times, we shall not be assuming the formerexistence of an agent with which we are now unfamiliar. And when weconsider the thicknesses of sandstone and shale which intervene beneaththe coal-seams, we can realise to a certain extent the vast lapses ofyears which must have taken place between the existence of each forest;so that although now an individual passing up a coal-mine shaft mayrapidly pass through the remains of one forest after another, the rise ofthe strata above each forest-bed then was tremendously slow, and theperiod between the growth of each forest must represent the passing awayof countless ages. Perhaps it would not be too much to say that thestrata between some of the coal-seams would represent a period not lessthan that between the formation of the few tertiary coals with which weare acquainted, and a time which is still to us in the far-away future. 

The actual seams of coal themselves will not yield much information, fromwhich it will be possible to judge of the contour of the landmasses atthis ancient period. Of one thing we are sure, namely, that at the timeeach seam was formed, the spot where it accumulated was dry land. If, therefore, the seams which appear one above the other coincide fairlywell as to their superficial extent, we can conclude that each time theland was raised above the sea and the forest again grew, the contour ofthe land was very similar. This conclusion will be very useful to goupon, since whatever decision may be come to as an explanation of onesuccessive land-period and sea-period on the same spot, will beapplicable to the eighteen or more periods necessary for the completionof some of the coal-fields. 

We will therefore look at one of the sandstone masses which occur betweenthe coal-seams, and learn what lessons these have to teach us. Inconsidering the formation of strata of sand in the seas around ourriver-mouths, it was seen that, owing to the greater weight of theparticles of the sand over those of clay, the former the more readilysank to the bottom, and formed banks not very far away from the land. Itwas seen, too, that each successive deposition of sand formed awedge-shaped layer, with the point of the wedge pointing away from thesource of origin of the sediment, and therefore of the current whichconveyed the sediment. Therefore, if in the coal-measure sandstones thelayers were found with their wedges all pointing in one direction, weshould be able to judge that the currents were all from one direction, and that, therefore, they were formed by a single river. But this is justwhat we do not find, for instead of it the direction of the wedge-shapedstrata varies in almost every layer, and the current-bedding has beenbrought about by currents travelling in every direction. Such diversecurrent-bedding could only result from the fact that the spot where thesand was laid down was subject to currents from every direction, and theinference is that it was well within the sphere of influence of numerousstreams and rivers, which flowed from every direction. The only conditionof things which would explain this is that the sandstone was originallyformed in a closed sea or large lake, into which numerous rivers flowingfrom every direction poured their contents. 

Now, in the sandstones, the remains of numerous plants have been found, but they do not present the perfect appearance that they do when found inthe shales; in fact they appear to have suffered a certain amount ofdamage through having drifted some distance. This, together with the factthat sandstones are not formed far out at sea, justify the safeconclusion that the land could not have been far off. Wherever thecurrent-bedding shows itself in this manner we may be sure we areexamining a spot from which the land in every direction could not havebeen at a very great distance, and also that, since the heavy materialsof which sandstone is composed could only be transported by beingimpelled along by currents at the bed of the sea, and that in deep watersuch currents could not exist, therefore we may safely decide that thesea into which the rivers fell was a comparatively shallow one. 

Although the present coal-fields of England are divided from one anotherby patches of other beds, it is probable that some of them were formerlyconnected with others, and a very wide sheet of coal on each occasion waslaid down. The question arises as to what was the extent of the inlandsea or lake, and did it include the area covered by the coal basins ofScotland and Ireland, of France and Belgium? And if these, why not thoseof America and other parts? The deposition of the coal, according to thetheory here advanced, may as well have been brought about in a series oflarge inland seas and lakes, as by one large comprehensive sea, andprobably the former is the more satisfactory explanation of the two. Butthe astonishing part of it is that the changes in the level of the landmust have been taking place simultaneously over these large areas, although, of course, while one quarter may have been depressed beneaththe sea, another may have been raised above it. 

In connection with the question of the contour of the land during theexistence of the large lakes or inland seas, Professor Hull has prepared, in his series of maps illustrative of the Palaeo-Geography of the BritishIslands, a map showing on incontestible grounds the existence during thecoal-ages of a great central barrier or ridge of high land stretchingacross from Anglesea, south of Flint, Staffordshire, and Shropshirecoal-fields, to the eastern coast of Norfolk. He regards the Britishcoal-measures as having been laid down in two, or at most three, areas ofdeposition--one south of this ridge, the remainder to the north of it. Inregard to the extent of the former deposits of coal in Ireland, there isevery probability that the sister island was just as favourably treatedin this respect as Great Britain. Most unfortunately, Ireland has sincesuffered extreme denudation, notably from the great convulsions of natureat the close of the very period of their deposition, as well as in morerecent times, resulting in the removal of nearly all the valuable uppercarboniferous beds, and leaving only the few unimportantcoal-beds to which reference has been made. 

[Illustration: FIG. 23. --_Cyathophyllum_. Coral in encrinital limestone. ]

We are unable to believe in the continuity of our coal-beds with those ofAmerica, for the great source of sediment in those times was a continentsituated on the site of the Atlantic Ocean, and it is owing to thisextensive continent that the forms of _flora_ found in the coal-beds ineach country bear so close a resemblance to one another, and also thatthe encrinital limestone which was formed in the purer depths of theocean on the east, became mixed with silt, and formed masses of shalyimpure limestone in the south-western parts of Ireland. 

It must be noted that, although we may attribute to upheaval from beneaththe fact that the bed of the sea became temporarily raised at each periodinto dry land, the deposits of sand or shale would at the same time betending to shallow the bed, and this alone would assist the process ofupheaval by bringing the land at least very near to the surface of thewater. 

Each upheaval, however, could have been but a temporary arrest of thegreat movement of crust subsidence which was going on throughout the coalperiod, so that, at its close, when the last coal forest grew upon thesurface of the land, there had disappeared, in the case of South Wales, athickness of 11, 000 feet of material. 

Of the many remarkable things in connection with coal-beds, not the leastis the state of purity in which coal is found. On the floor of eachforest there would be many a streamlet or even small river which wouldwend its way to meet the not very distant sea, and it is surprising atfirst that so little sediment found its way into the coal itself. Butthis was cleverly explained by Sir Charles Lyell, who noticed, on one ofhis visits to America, that the water of the Mississippi, around the rankgrowths of cypress which form the "cypress swamps" at the mouths of thatriver, was highly charged with sediment, but that, having passed throughthe close undergrowth of the swamps, it issued in almost a pure state, the sediment which it bore having been filtered out of it andprecipitated. This very satisfactorily explained how in some placescarbonaceous matter might be deposited in a perfectly pure state, whilstin others, where sandstone or shale was actually forming, it might beimpregnated by coaly matter in such a way as to cause it to be stainedblack. In times of flood sediment would be brought in, even where purecoal had been forming, and then we should have a thin "parting" ofsandstone or shale, which was formed when the flood was at its height. Ora slight sinking of the land might occur, in which case also theformation of coal would temporarily cease, and a parting of foreignmatter would be formed, which, on further upheaval taking place, wouldagain give way to another forest growth. Some of the thicker beds havebeen found presenting this aspect, such as the South Staffordshireten-yard coal, which in some parts splits up into a dozen or so smallerbeds, with partings of sediment between them. 

In the face of the stupendous movements which must have happened in orderto bring about the successive growth of forests one above another on thesame spot, the question at once arises as to how these movements of thesolid earth came about, and what was the cause which operated in such amanner. We can only judge that, in some way or other, heat, or thewithdrawal of heat, has been the prime motive power. We can perceive, from what is now going on in some parts of the earth, how great aninfluence it has had in shaping the land, for volcanoes owe theiractivity to the hidden heat in the earth's interior, and afford us anidea of the power of which heat is capable in the matter of building upand destroying continents. No less certain is it that heat is the primefactor in those more gradual vertical movements of the land to which wehave referred elsewhere, but in regard to the exact manner in which itacts we are very much in the dark. Everybody knows that, in the majorityof instances, material substances of all kinds expand under the influenceof heat, and contract when the source of heat is withdrawn. If we canimagine movements in the quantity of heat contained in the solid crust, the explanation is easy, for if a certain tract of land receive anaccession of heat beneath it, it is certain that the principal effectwill be an elevation of the land, consequent on the expansion of itsmaterials, with a subsequent depression when the heat beneath the tractin question becomes gradually lessened. Should the heat be retained for along period, the strata would be so uplifted as to form an anticlinal, orsaddle-back, and then, should subsequent denudation take place, moreancient strata would be brought to view. It was thus in the instance ofthe tract bounded by the North and South Downs, which were formerlyentirely covered by chalk, and in the instance of the uprising of thecarboniferous limestone between the coal-fields of Lancashire, Staffordshire, and Derbyshire. 

How the heat-waves act, and the laws, if any, which they obey in theirsubterranean movements, we are unable to judge. From the properties whichheat possesses we know that its presence or absence produces markeddifferences in the positions of the strata of the earth, and fromobservations made in connection with the closing of some volcanoes, andthe opening up of fresh earth-vents, we have gone a long way towardsestablishing the probability that there are even now slow and ponderousmovements taking place in the heat stored in the earth's crust, whoseeffects are appreciably communicated to the outside of the thin rind ofsolid earth upon which we live. 

Owing to the great igneous and volcanic activity at the close of thedeposition of the carboniferous system of strata, the coal-measuresexhibit what are known as _faults_ in abundance. The mountain limestone, where it outcrops at the surface, is observed to be much jointed, so muchso that the work of quarrying the limestone is greatly assisted by thejointed structure of the rock. Faults differ from joints in that, whilstthe strata in the latter are still in relative position on each side ofthe joint, they have in the former slipped out of place. In such a casethe continuation of a stratum on the opposite side of a fault will befound to be depressed, perhaps a thousand feet or more. It will be seenat once how that, in sinking a new shaft into a coal-seam, thepossibility of an unknown fault has to be brought into consideration, since the position of the seam may prove to have been depressed to suchan extent as to cause it to be beyond workable depth. Many seams, on theother hand, which would have remained altogether out of reach of miningoperations, have been brought within workable depth by a series of_step-faults_, this being a term applied to a series of parallel faults, in none of which the amount of down-throw is great. 

The amount of the down-throw, or the slipping-down of the beds, ismeasured, vertically, from the point of disappearance of a layer to animaginary continuation of the same layer from where it again appearsbeyond the fault. The plane of a fault is usually more or less inclined, the amount of the inclination being known as the _hade_ of the fault, andit is a remarkable characteristic of faults that, as a general rule, theyhade to the down-throw. This will be more clearly understood when it isexplained that, by its action, a seam of coal, which is subject tonumerous faults, can never be pierced more than once by one and the sameboring. In mountainous districts, however, there are occasions when thehade is to the up-throw, and this kind of fault is known as an _invertedfault_. 

Lines of faults extend sometimes for hundreds of miles. The great PennineFault of England is 130 miles long, and others extend for much greaterdistances. The surfaces on both sides of a fault are often smooth andhighly polished by the movement which has taken place in the strata. Theythen show the phenomenon known as _slicken-sides_. Many faults havebecome filled with crystalline minerals in the form of veins of ore, deposited by infiltrating waters percolating through the naturalfissures. 

In considering the formation and structure of the better-knowncoal-bearing beds of the carboniferous age, we must not lose sight of thefact that important beds of coal also occur in strata of much more recentdate. There are important coal-beds in India of Permian age. There arecoal-beds of Liassic age in South Hungary and in Texas, and of Jurassicage in Virginia, as well as at Brora in Sutherlandshire; there are coalsof Cretaceous age in Moravia, and valuable Miocene Tertiary coals inHungary and the Austrian Alps. 

Again, older than the true carboniferous age, are the Siluriananthracites of Co. Cavan, and certain Norwegian coals, whilst in NewSouth Wales we are confronted with an assemblage of coal-bearing stratawhich extend apparently from the Devonian into Mesozoic times. 

Still, the age we have considered more closely has an unrivalled right tothe title, coal appearing there not merely as an occasional bed, but as amarked characteristic of the formation. 

The types of animal life which are found in this formation are varied, and although naturally enough they do not excel in number, there are yetsufficient varieties to show probabilities of the existence of many withwhich we are unfamiliar. The highest forms yet found, show an advance ascompared with those from earlier formations, and exhibit amphibiancharacteristics intermediate between the two great classes of fishes andreptiles. Numerous specimens proper to the extinct order of_labyrinthodontia_ have been arranged into at least a score of genera, these having been drawn from the coal-measures of Newcastle, Edinburgh, Kilkenny, Saärbruck, Bavaria, Pennsylvania, and elsewhere. The_Archegosaurus, _ which we have figured, and the _Anthracosaurus, _ areforms which appear to have existed in great numbers in the swamps andlakes of the age. The fish of the period belong almost entirely to theancient orders of the ganoids and placoids. Of the ganoids, the great_megalichthys Hibberti_ ranges throughout the whole of the system. Wonderful accumulations of fish remains are found at the base of thesystem, in the bone-bed of the Bristol coal-field, as well as in asimilar bed at Armagh. Many fishes were armed with powerful conicalteeth, but the majority, like the existing Port Jackson shark, werepossessed of massive palates, suited in some cases for crushing, and inothers for cutting. 

[Illustration: FIG. 24. --_Archegosaurus minor_. Coal-measures. ]

[Illustration: FIG. 25. --_Psammodus porosus_. Crushing palate of a fish. ]

[Illustration: FIG. 26. --_Orthoceras_. Mountain limestone. ]

In the mountain limestone we see, of course, the predominance of marinetypes, encrinital remains forming the greater proportion of the mass. There are occasional plant remains which bear evidence of having driftedfor some distance from the shore. But next to the _encrinites_, thecorals are the most important and persistent. Corals of most beautifulforms and capable of giving polished marble-like sections, are inabundance. _Polyzoa_ are well represented, of which the lace-coral(_fenestella_) and screw-coral (_archimedopora_) are instances. _Cephalopoda_ are represented by the _orthoceras_, sometimes five or sixfeet long, and _goniatites_, the forerunner of the familiar _ammonite_. Many species of brachiopods and lammellibranchs are met with. _Lingula_, most persistent throughout all geological time, is abundant in thecoal-shales, but not in the limestones. _Aviculopecten_ is there abundantalso. In the mountain limestone the last of the trilobites (_Phillipsia_)is found. 

[Illustration: FIG. 27. --_Fenestella retipora_. Mountain limestone. ]

[Illustration: FIG. 28. --_Goniatites_. Mountain limestone. ]

We have evidence of the existence in the forests of a variety of_centipede_, specimens having been found in the erect stump of a hollowtree, although the fossil is an extremely rare one. The same may be saidof the only two species of land-snail which have been found connectedwith the coal forests, viz. , _pupa vetusta_ and _zonites priscus_, bothdiscovered in the cliffs of Nova Scotia. These are sufficient todemonstrate that the fauna of the period had already reached a high stageof development. In the estuaries of the day, masses of a species offreshwater mussel (_anthracosia_) were in existence, and these have lefttheir remains in the shape of extensive beds of shells. They are familiarto the miner as _mussel-binds_, and are as noticeable a feature of thislong ago period, as are the aggregations of mussels on every coast atthe present day. 

[Illustration: FIG. 29. --_Aviculopecten papyraceus_. Coal-shale. ]

CHAPTER III. 

VARIOUS FORMS OF COAL AND CARBON. 

In considering the various forms and combinations into which coal enters, it is necessary that we should obtain a clear conception of what thesubstance called "carbon" is, and its nature and properties generally, since this it is which forms such a large percentage of all kinds ofcoal, and which indeed forms the actual basis of it. In the shape ofcoke, of course, we have a fairly pure form of carbon, and this beingproduced, as we shall see presently, by the driving off of the volatileor vaporous constituents of coal, we are able to perceive by the residuehow great a proportion of coal consists of carbon. In fact, the two havealmost an identical meaning in the popular mind, and the fact that thegreat masses of strata, in which are contained our principal and mostvaluable seams of coal, are termed "carboniferous, " from the Latin_carbo_, coal, and _fero_, I bear, tends to perpetuate the existence ofthe idea. 

There is always a certain, though slight, quantity of carbon in the air, and this remains fairly constant in the open country. Small though it maybe in proportion to the quantity of pure air in which it is found, it isyet sufficient to provide the carbon which is necessary to the growth ofvegetable life. Just as some of the animals known popularly as the_zoophytes_, which are attached during life to rocks beneath the sea, arefed by means of currents of water which bring their food to them, so theleaves, which inhale carbon-food during the day through theirunder-surfaces, are provided with it by means of the currents of airwhich are always circulating around them; and while the fuel is beingtaken in beneath, the heat and light are being received from above, andthe sun supplies the motive power to digestion. 

It is assumed that it is, within the knowledge of all that, for theorigin of the various seams and beds of coaly combinations which exist inthe earth's crust, we must look to the vegetable world. If, however, wecould go so far back in the world's history as the period when ourincandescent orb had only just severed connection with agradually-diminishing sun, we should probably find the carbon there, butlocked up in the bonds of chemical affinities with other elements, andexisting therewith in a gaseous condition. But, as the solidifyingprocess went on, and as the vegetable world afterwards made itsappearance, the carbon became, so to speak, wrenched from itscombinations, and being absorbed by trees and plants, finally becamedeposited amongst the ruins of a former vegetable world, and is nowpresented to us in the form of coal. 

We are able to trace the gradual changes through which the pasty mass ofdecaying vegetation passed, in consequence of the fact that we have thismaterial locked up in various stages of carbonisation, in the stratabeneath our feet. These we propose to deal with individually, in asunscientific and untechnical a manner as possible. 

First of all, when a mass of vegetable matter commences to decay, it soonloses its colour. There is no more noticeable proof of this, than thatwhen vitality is withdrawn from the leaves of autumn, they at oncecommence to assume a rusty or an ashen colour. Let the leaves but fall tothe ground, and be exposed to the early frosts of October, the damp mistsand rains of November, and the rapid change of colour is at onceapparent. Trodden under foot, they soon assume a dirty blackish hue, andeven when removed they leave a carbonaceous trace of themselves behindthem, where they had rested. Another proof of the rapid acquisition oftheir coaly hue is noticeable in the spring of the year. When the treeshave burst forth and the buds are rapidly opening, the cases in which thebuds of such trees as the horse-chestnut have been enclosed will be foundcast off, and strewing the path beneath. Moistened by the rains and thedamp night-mists, and trodden under foot, these cases assume a jet blackhue, and are to all appearance like coal in the very first stages offormation. 

But of course coal is not made up wholly and only of leaves. The branchesof trees, twigs of all sizes, and sometimes whole trunks of trees arefound, the last often remaining in their upright position, and piercingthe strata which have been formed above them. At other times they liehorizontally on the bed of coal, having been thrown down previously tothe formation of the shale or sandstone, which now rests upon them. Theyare often petrified into solid sandstone themselves, whilst leaving arind of coal where formerly was the bark. Although the trunk of a treelooks so very different to the leaves which it bears upon its branches, it is only naturally to be supposed that, as they are both built up afterthe same manner from the juices of the earth and the nourishment in theatmosphere, they would have a similar chemical composition. One verypalpable proof of the carbonaceous character of tree-trunks suggestsitself. Take in your hand a few dead twigs or sticks from which theleaves have long since dropped; pull away the dead parts of the ivy whichhas been creeping over the summer-house; or clasp a gnarled old monsterof the forest in your arms, and you will quickly find your hand coveredwith a black smut, which is nothing but the result of the first stagewhich the living plant has made, in its progress towards its condition asdead coal. But an easy, though rough, chemical proof of the constituentsof wood, can be made by placing a few pieces of wood in a medium-sizedtest-tube, and holding it over a flame. In a short time a certainquantity of steam will be driven off, next the gaseous constituents ofwood, and finally nothing will be left but a few pieces of black brittlecharcoal. The process is of course the same in a fire-grate, only thathere more complete combustion of the wood takes place, owing to its beingintimately exposed to the action of the flames. If we adopt the sameexperiment with some pieces of coal, the action is similar, only that inthis case the quantity of gases given off is not so great, coalcontaining a greater proportion of carbon than wood, owing to the factthat, during its long burial in the bowels of the earth, it has beenacted upon in such a way as to lose a great part of its volatileconstituents. 

From processes, therefore, which are to be seen going on around us, it iseasily possible to satisfy ourselves that vegetation will in the long runundergo such changes as will result in the formation of coal. 

There are certain parts in most countries, and particularly in Ireland, where masses of vegetation have undergone a still further stage inmetamorphism, namely, in the well-known and famous peat-bogs. Ireland is_par excellence_ the land of bogs, some three millions of acres beingsaid to be covered by them, and they yield an almost inexhaustible supplyof peat. One of the peat-bogs near the Shannon is between two and threemiles in breadth and no less than fifty in length, whilst its depthvaries from 13 feet to as much as 47 feet. Peat-bogs have in no wayceased to be formed, for at their surfaces the peat-moss grows afreshevery year; and rushes, horse-tails, and reeds of all descriptions growand thrive each year upon the ruins of their ancestors. The formation ofsuch accumulations of decaying vegetation would only be possible wherethe physical conditions of the country allowed of an abundant rainfall, and depressions in the surface of the land to retain the moisture. Whereextensive deforesting operations have taken place, peat-bogs have oftenbeen formed, and many of those in existence in Europe undoubtedly owetheir formation to that destruction of forests which went on under thesway of the Romans. Natural drainage would soon be obstructed by fallentrees, and the formation of marsh-land would follow; then with the growthof marsh-plants and their successive annual decay, a peaty mass wouldcollect, which would quickly grow in thickness without let or hindrance. 

In considering the existence of inland peat-bogs, we must not lose sightof the fact that there are subterranean forest-beds on various parts ofour coasts, which also rest upon their own beds of peaty matter, and verypossibly, when in the future they are covered up by marine deposits, theywill have fairly started on their way towards becoming coal. 

Peat-bogs do not wholly consist of peat, and nothing else. The trunks ofsuch trees as the oak, yew, and fir, are often found mingled with theremains of mosses and reeds, and these often assume a decidedly coalyaspect. From the famous Bog of Allen in Ireland, pieces of oak, generallyknown as "bog-oak, " which have been buried for generations in peat, havebeen excavated. These are as black as any coal can well be, and aresufficiently hard to allow of their being used in the manufacture ofbrooches and other ornamental objects. Another use to which peat of somekinds has been put is in the manufacture of yarn, the result being amaterial which is said to resemble brown worsted. On digging a ditch todrain a part of a bog in Maine, U. S. , in which peat to a depth of twentyfeet had accumulated, a substance similar to cannel coal itself wasfound. As we shall see presently, cannel coal is one of the earlieststages of true coal, and the discovery proved that under certainconditions as to heat and pressure, which in this case happened to bepresent, the materials which form peat may also be metamorphosed intotrue coal. 

Darwin, in his well-known "Voyage in the _Beagle_" gives a peculiarlyinteresting description of the condition of the peat-beds in the ChonosArchipelago, off the Chilian coast, and of their mode of formation. "Inthese islands, " he says, "cryptogamic plants find a most congenialclimate, and within the forest the number of species and great abundanceof mosses, lichens, and small ferns, is quite extraordinary. In Tierradel Fuego every level piece of land is invariably covered by a thick bedof peat. In the Chonos Archipelago where the nature of the climate moreclosely approaches that of Tierra del Fuego, every patch of level groundis covered by two species of plants (_Astelia pumila_ and _Donatiamegellanica_), which by their joint decay compose a thick bed of elasticpeat. 

"In Tierra del Fuego, above the region of wood-land, the former of theseeminently sociable plants is the chief agent in the production of peat. Fresh leaves are always succeeding one to the other round the centraltap-root; the lower ones soon decay, and in tracing a root downwards inthe peat, the leaves, yet holding their places, can be observed passingthrough every stage of decomposition, till the whole becomes blended inone confused mass. The Astelia is assisted by a few other plants, --hereand there a small creeping Myrtus (_M. Nummularia_), with a woody stemlike our cranberry and with a sweet berry, --an Empetrum (_E. Rubrum_), like our heath, --a rush (_Juncus grandiflorus_), are nearly the only onesthat grow on the swampy surface. These plants, though possessing a veryclose general resemblance to the English species of the same genera, aredifferent. In the more level parts of the country the surface of the peatis broken up into little pools of water, which stand at differentheights, and appear as if artificially excavated. Small streams of water, flowing underground, complete the disorganisation of the vegetablematter, and consolidate the whole. 

"The climate of the southern part of America appears particularlyfavourable to the production of peat. In the Falkland Islands almostevery kind of plant, even the coarse grass which covers the whole surfaceof the land, becomes converted into this substance: scarcely anysituation checks its growth; some of the beds are as much as twelve feetthick, and the lower part becomes so solid when dry that it will hardlyburn. Although every plant lends its aid, yet in most parts the Asteliais the most efficient. 

"It is rather a singular circumstance, as being so very different fromwhat occurs in Europe, that I nowhere saw moss forming by its decay anyportion of the peat in South America. With respect to the northern limitat which the climate allows of that peculiar kind of slow decompositionwhich is necessary for its production, I believe that in Chiloe (lat. 41°to 42°), although there is much swampy ground, no well characterised peatoccurs; but in the Chonos Islands, three degrees farther southward, wehave seen that it is abundant. On the eastern coast in La Plata (lat. 35°) I was told by a Spanish resident, who had visited Ireland, that hehad often sought for this substance, but had never been able to find any. He showed me, as the nearest approach to it which he had discovered, ablack peaty soil, so penetrated with roots as to allow of an extremelyslow and imperfect combustion. "

The next stage in the making of coal is one in which the change hasproceeded a long way from the starting-point. _Lignite_ is the name whichhas been applied to a form of impure coal, which sometimes goes under thename of "brown coal. " It is not a true coal, and is a very long way fromthat final stage to which it must attain ere it takes rank with the mostvaluable of earth's products. From the very commencement, an action hasbeing going on which has caused the amount of the gaseous constituents tobecome less and less, and which has consequently caused the carbonremaining behind to occupy an increasingly large proportion of the wholemass. So, when we arrive at the lignite stage, we find that aconsiderable quantity of volatile matter has already been parted with, and that the carbon, which in ordinary living wood is about 50 per cent. Of the whole, has already increased to about 67 per cent. In mostlignites there is, as a rule, a comparatively large proportion ofsulphur, and in such cases it is rendered useless as a domestic fuel. Ithas been used as a fuel in various processes of manufacture, and thelignite of the well-known Bovey Tracey beds has been utilised in this wayat the neighbouring potteries. As compared with true coal, it isdistinguished by the abundance of smoke which it produces and the chokingsulphurous fumes which also accompany its combustion, but it is largelyused in Germany as a useful source of paraffin and illuminating oils. InSilesia, Saxony, and in the district about Bonn, large quantities oflignite are mined, and used as fuel. Large stores of lignite are known toexist in the Weald of the south-east of England, and although the miningoperations which were carried on at one time at Heathfield, Bexhill, andother places, were failures so far as the actual discovery of true coalwas concerned, yet there can be no doubt as to the future value of thelignite in these parts, when England's supplies of coal approachexhaustion, and attention is turned to other directions for the futuresource of her gas and paraffin oils. 

Beside the Bovey Tracey lignitic beds to which we have above referred, other tertiary clays are found to contain this early promise of coal. The_eocene_ beds of Brighton are an important instance of a tertiarylignite, the seam of _surturbrand_, as it is locally called, being asomewhat extensive deposit. 

We have now closely approached to true coal, and the next step which weshall take will be to consider the varieties in which the black mineralitself is found. The principal of these varieties are as follows, againsteach being placed the average proportion of pure carbon which itcontains:--

 Splint or Hard Coal, 83 per cent. ; Cannel, Candle or Parrott Coal, 84 per cent. ; Cherry or Soft Coal, 85 per cent. ; Common Bituminous, or Caking Coal, 88 per cent. ; Anthracite, Blind Coal, Culm, Glance, or Stone Coal, from South Wales, 93 per cent. 

As far as the gas-making properties of the first three are concerned, therelative proportions of carbon and volatile products are much the same. Everybody knows a piece of cannel coal when it is seen, how it appearsalmost to have been once in a molten condition, and how it breaks with aconchoidal fracture, as opposed to the cleavage of bituminous coal intothin layers; and, most apparent and most noticeable of all, how it doesnot soil the hands after the manner of ordinary coal. It is at times sodense and compact that it has been fashioned into ornaments, and iscapable of receiving a polish like jet. From the large percentage ofvolatile products which it contains, it is greatly used in gasworks. 

Caking coal and the varieties of coal which exist between it andanthracite, are familiar to every householder; the more it approaches thecomposition of the latter the more difficult it is to get it to burn, butwhen at last fairly alight it gives out great heat, and what is moreimportant, a less quantity of volatile constituents in the shape of gas, smoke, ammonia, ash and sulphurous acid. For this reason it has beenproposed to compel consumers to adopt anthracite as _the_ domestic coalby Act of Parliament. Certainly by this means the amount of impurities inthe air might be appreciably lessened, but as it would involve thereconstruction of some millions of fire-places, and an increase in pricein consequence of the general demand for it, it is not likely that agovernment would be so rash as to attempt to pass such a measure; even ifpassed, it would probably soon become as dead and obsolete and impotentas those many laws with which our ancestors attempted, first to arrest, and then to curb the growth in the use of coal of any sort. Anthracite isnot a "homely" coal. If we use it alone it will not give us that brightand cheerful blaze which English-speaking people like to obtain fromtheir fires. 

It is a significant fact, and one which proves that the various kinds ofcoal which are found are nothing but stages begotten by different degreesof disentanglement of the contained gases, that where, as in some parts, a mass of basalt has come into contact with ordinary bituminous coal, thecoal has assumed the character of anthracite, whilst the change has insome instances gone so far as to convert the anthracite into graphite. The basalt, which is one of the igneous rocks, has been erupted into thecoal-seam in a state of fusion, and the heat contained in it has beensufficient to cause the disentanglement of the gases, the extraction ofwhich from the coal brings about the condition of anthracite andgraphite. 

The mention of graphite brings us to the next stage. Graphite, plumbago, or, as it is more commonly called, black-lead, which, we may say inpassing, has nothing of lead about it at all, is best known in the shapeof that very useful and cosmopolitan article, the black-lead pencil. Thisis even purer carbon than anthracite, not more than 5 per cent. Of ashand other impurities being present. It is well-known by its grey metalliclustre; the chemist uses it mixed with fire-clay to make his crucibles;the engineer uses it, finely powdered, to lubricate his machinery; thehouse-keeper uses it to "black-lead" her stoves to prevent them fromrusting. An imperfect graphite is found inside some of the hottestretorts from which gas is distilled, and this is used as the negativeelement in zinc and carbon electricity-making cells, whilst its use asthe electrodes or carbons of the arc-lamp is becoming more and morewidely adopted, as installations of electric light become more general. 

One great source of true graphite for many years was the famous mine atBorrowdale, in Cumberland, but this is now almost exhausted. The vein laybetween strata of slate, and was from eight to nine feet thick. As muchas £100, 000 is said to have been realised from it in one year. Extensivesupplies of graphite are found in rocks of the Laurentian age in Canada. In this formation nothing which can undoubtedly be classed as organic hasyet been discovered. Life at this early period must have found its homein low and humble forms, and if the _eozoön_ of Dawson, which has beenthought to represent the earliest type of life, turns out after all notto be organic, but only a deceptive appearance assumed by certain of thestrata, we at least know that it must have been in similarly humble formsthat life, if it existed at all, did then exist. We can scarcely, therefore, expect that the vegetable world had made any great advance incomplexity of organism at this time, otherwise the supplies of graphiteor plumbago which are found in the formation, would be attributed todense forest growths, acted upon, after death, in a similar manner tothat which awaited the vegetation which, ages after, went to form beds ofcoal. At present we know of no source of carbon except through theintervention and the chemical action of plants. Like iron, carbon isseldom found on the earth except in combination. If there were no growthof vegetation at this far-away period to give rise to these deposits ofgraphite, we are compelled to ask ourselves whether, perchance, there didnot then exist conditions of which we are not now cognisant on the earth, and which allowed graphite to be formed without assistance from thevegetable kingdom. At present, however, science is in the dark as to anyother process of its formation, and we are left to assume that thevegetable growth of the time was enormous in quantity, although there isnothing to show the kind of vegetation, whether humble mosses or tallforest trees, which went to constitute the masses of graphite. Geologistswill agree that this is no small assumption to make, since, if true, itmay show that there was an abundance of vegetation at a time when animallife was hidden in one or more very obscure forms, one only of which hasso far been detected, and whose very identity is strongly doubted bynearly all competent judges. At the same time there _may_ have been anabundance of both animal and vegetable life at the time. We must notforget that it is a well-ascertained fact that in later ages, the minuteseed-spores of forest trees were in such abundance as to form importantseams of coal in the true carboniferous era, the trees which gave birthto them being now classed amongst the humble _cryptogams_, the ferns, andclub-mosses, &c. The graphite of Laurentian age may not improbably havebeen caused by deposits of minute portions of similar lowly specimens ofvegetable life, and if the _eozoön_ the "dawn-animalcule, " does representthe animal life of the time, life whose types were too minute to leaveundoubted traces of their existence, both animal life and vegetable lifemay be looked upon as existing side by side in extremely humble forms, neither as yet having taken an undoubted step forward in advance of theother in respect to complexity of organism. 

[Illustration: FIG 30. --_Lepidodendron_. Portion of Sandstone stem afterremoval of bark of a giant club-moss]

There is but one more form of carbon with which we have to deal inrunning through the series. We have seen that coal is not the _summumbonum_ of the series. Other transformations take place after the stage ofcoal is reached, which, by the continued disentanglement of gases, finally bring about the plumbago stage. 

What the action is which transforms plumbago or some other form of carboninto the condition of a diamond cannot be stated. Diamond is the purestform of carbon found in nature. It is a beautiful object, alike from theresults of its powers of refraction, as also from the form into which itscarbon has been crystallised. How Nature, in her wonderful laboratory, has precipitated the diamond, with its wonderful powers of spectrumanalysis, we cannot say with certainty. Certain chemists have, at a greatexpense, produced crystals which, in every respect, stand the tests oftrue diamonds; but the process of their production at a great expense hasin no way diminished the value of the natural product. 

The process by which artificial diamonds have been produced is sointeresting, and the subject may prove to be of so great importance, thata few remarks upon the process may not be unacceptable. 

The experiments of the great French chemist, Dumas, and others, satisfactorily proved the fact, which has ever since been consideredthoroughly established, that the diamond is nothing but carboncrystallised in nearly a pure state, and many chemists have since beenengaged in the hitherto futile endeavour to turn ordinary carbon into thetrue diamond. 

Despretz at one time considered that he had discovered the process, whichconsisted in his case of submitting a piece of charcoal to the action ofan electric battery, having in his mind the similar process ofelectrolysis, by which water is divided up into the two gases, hydrogenand oxygen. He obtained a microscopic deposit on the poles of thebattery, which he pronounced to be diamond dust, but which, a long timeafter, was proved to be nothing but graphite in a crystallised state. This was, however, certainly a step in the right direction. 

The honour of first accomplishing the task fell to Mr Hannay, of Glasgow, who succeeded in producing very small but comparatively soft diamonds, byheating lampblack under great pressure, in company with one or two otheringredients. The process was a costly one, and beyond being a greatscientific feat, the discovery led to little result. 

A young French chemist, M. Henri Moissau, has since come to the front, and the diamonds which he has produced have stood every test for the truediamond to which they could be subjected; above all, the density of theproduct is 3. 5, _i. E. _, that of the diamond, that of graphite reaching 2only. 

He recognised that in all diamonds which he had consumed--and he consumedsome £150 worth in order to assure himself of the fact--there were alwaystraces of iron in their composition. He saw that iron in fusion, likeother metals, always dissolves a certain quantity of carbon. Might it notbe that molten iron, cooling in the presence of carbon, deep in volcanicdepths where there was little scope for the iron to expand in assumingthe solid form, would exert such tremendous pressure upon the particlesof carbon which it absorbed, that these would assume the crystallinestate?

He packed a cylinder of soft iron with the carbon of sugar, and placedthe whole in a crucible filled with molten iron, which was raised to atemperature of 3000° by means of an electric furnace. The soft cylindermelted, and dissolved a large portion of the carbon. The crucible wasthrown into water, and a mass of solid iron was formed. It was allowedfurther to cool in the open air, but the expansion which the iron wouldhave undergone on cooling, was checked by the crucible which containedit. The result was a tremendous pressure, during which the carbon, whichwas still dissolved, was crystallised into minute diamonds. 

These showed themselves as minute points which were easily separable fromthe mass by the action of acids. Thus the wonderful transformation fromsugar to the diamond was accomplished. 

It should be mentioned that iron, silver, and water, alone possess thepeculiar property of expanding when passing from the liquid to the solidstate. 

The diamonds so obtained were of both kinds. The particles of whitediamond resembled in every respect the true brilliant. But there was alsoan appreciable quantity of the variety known as the "black diamond. "These diamonds seem to approximate more closely to carbon as we are mostfamiliar with it. They are not considered as of such value as thetransparent form, but they are still of considerable commercial value. The _carbonado_, as this kind is called, possesses so great a degree ofhardness that by means of it it is possible to bore through the hardestrocks. The diamond drill, used for boring purposes, is furnished aroundthe outer edge of the cylinder of the "boring bit, " as it is called, withperhaps a dozen black diamonds, together with another row of Braziliandiamonds on the inside. By the rotation of the boring tool the sharpedges of the diamonds cut their way through rocks of all degrees ofhardness, leaving a core of the rock cut through, in the centre of thecylindrical drill. It is found that the durability of the natural edge ofthe diamond is far greater than that of the edge caused by _artificial_cutting and trimming. The cutting of a pane of glass by means of a ringset with an artificially-cut diamond, cannot therefore be done withoutinjuring to a slight extent the edge of the stone. 

The diamond is the hardest of all known substances, leaving a scratch onany substance across which it may be drawn. Yet it is one whose form canbe changed, and whose hardness can be completely destroyed, by the simpleprocess of combustion. It can be deprived of its high lustre, and of itspower of breaking up by refraction the light of the sun into the varioustints of the solar spectrum, simply by heating it to a red heat, and thenplunging it into a jar of oxygen gas. It immediately expands, changesinto a coky mass, and burns away. The product left behind is a mixture ofcarbon and oxygen, in the proportions in which it is met with incarbonic-anhydride, or, carbonic acid gas deprived of its water. This isindeed a strange transformation, from the most valuable of all ourprecious stones to a compound which is the same in chemical constituentsas the poisonous gas which we and all animals exhale. But there is thisto be said. Probably in the far-away days when the diamond began to beformed, the tree or other vegetable product which was its far-removedancestor abstracted carbonic acid gas from the atmosphere, just as do ourplants in the present day. By this means it obtained the carbon wherewithto build up its tissues. Thus the combustion of the diamond intocarbonic-anhydride now is, after all, only a return to the same compoundout of which it was originally formed. How it was formed is a secret:probably the time occupied in the formation of the diamond may be countedby centuries, but the time of its re-transformation into a mass of cokymatter is but the work of seconds!

There is another form of carbon which was formerly of much greaterimportance than it is now, and which, although not a natural product, isyet deserving of some notice here. Charcoal is the substance referred to. 

In early days the word "coal, " or, as it was also spelt, "cole, " wasapplied to any substance which was used as fuel; hence we have areference in the Bible to a "fire of coals, " so translated when themeaning to be conveyed was probably not coal as we know it. Wood wasformerly known as coal, whilst charred wood received the name ofcharred-coal, which was soon corrupted into charcoal. Thecharcoal-burners of years gone by were a far more flourishing communitythan they are now. When the old baronial halls and country-seats dependedon them for the basis of their fuel, and the log was a more frequentoccupant of the fire-grate than now, these occupiers of midforest were apeople of some importance. 

We must not overlook the fact that there is another form of charcoal, namely, animal charcoal or bone-black. This can be obtained by heatingbones to redness in closed iron vessels. In the refining of raw sugar thediscoloration of the syrup is brought about by filtering it throughanimal-charcoal; by this means the syrup is rendered colourless. 

When properly prepared, charcoal exhibits very distinctly the rings ofannual growth which may have characterised the wood from which it wasformed. It is very light in consequence of its porous nature, and it iswonderfully indestructible. 

But its greatest, because it is its most useful property, is undoubtedlythe power which it has of absorbing great quantities of gas into itself. It is in fact what may be termed an all-round purifier. It is adeodoriser, a disinfectant, and a decoloriser. It is an absorbent of badodours, and partially removes the smell from tainted meat. It has beenused when offensive manures have been spread over soils, with the sameobject in view, and its use for the purification of water is well knownto all users of filters. Some idea of its power as a disinfectant may begained by the fact that one volume of wood-charcoal will absorb no lessthan 90 volumes of ammonia, 35 volumes of carbonic anhydride, and 65volumes of sulphurous anhydride. 

Other forms of carbon which are well-known are (1) coke, the residue leftwhen coal has been subjected to a great heat in a closed retort, but fromwhich all the bye-products of coal have been allowed to escape; (2) sootand lamp-black, the former of which is useful as a manure in consequenceof ammonia being present in it, whilst the latter is a specially preparedsoot, and is used in the manufacture of Indian ink and printers' ink. 

CHAPTER IV. 

THE COAL-MINE AND ITS DANGERS. 

It is somewhat strange to think that where once existed the solitudes ofan ancient carboniferous forest now is the site of a busy undergroundtown. For a town it really is. The various roads and passages which arecut through the solid coal as excavation of a coal-mine proceeds, represent to a stranger all the intricacies of a well-planned town. Noris the extent of these underground towns a thing to be despised. There isan old pit near Newcastle which contains not less than fifty miles ofpassages. Other pits there are whose main thoroughfares in a direct lineare not less than four or five miles in length, and this, it must beborne in mind, is the result of excavation wrought by human hands andhuman labour. 

So great an extent of passages necessarily requires some special means ofkeeping the air within it in a pure state, such as will render it fit forthe workers to breathe. The further one would go from the mainthoroughfare in such a mine, the less likely one would be to find air ofsufficient purity for the purpose. It is as a consequence necessary totake some special steps to provide an efficient system of ventilationthroughout the mine. This is effectually done by two shafts, calledrespectively the downcast and the upcast shaft. A shaft is in reality avery deep well, and may be circular, rectangular or oval in form. Inorder to keep out water which may be struck in passing through thevarious strata, it is protected by plank or wood tubbing, or the shaft isbricked over, or sometimes even cast-iron segments are sunk. In manyshafts which, owing to their great depth, pass through strata of everydegree of looseness or viscosity, all three methods are utilised in turn. In Westphalia, where coal is worked beneath strata of more recentgeological age, narrow shafts have been, in many cases, sunk by means ofboring apparatus, in preference to the usual process of excavation, andthe practice has since been adopted in South Wales. In England the usualform of the pit is circular, but elliptical and rectangular pits are alsoin use. On the Continent polygonal-shaped shafts are not uncommon, all ofthem, of whatever shape, being constructed with a view to resist thegreat pressure exerted by the rock around. 

[Illustration: FIG. 31. --Engine-House and Buildings at head of aCoal-Pit. ]

If there be one of these shafts at one end of the mine, and another at aremote distance from it, a movement of the air will at once begin, and arough kind of ventilation will ensue. This is, however, quiteinsufficient to provide the necessary quantity of air for inhalation bythe army of workers in the coal-mine, for the current thus set up doesnot even provide sufficient force to remove the effete air and impuritieswhich accumulate from hundreds of perspiring human bodies. 

It is therefore necessary to introduce some artificial means, by which astrong and regular current shall pass down one shaft, through the mine inall its workings, and out at the other shaft. This is accomplished invarious ways. It took many years before those interested in mines camethoroughly to understand how properly to secure ventilation, and inbygone days the system was so thoroughly bad that a tremendous amount ofsickness prevailed amongst the miners, owing to the poisonous effects ofbreathing the same air over and over again, charged, as it was, with moreor less of the gases given off by the coal itself. Now, those miners whodo so great a part in furnishing the means of warming our houses inwinter, have the best contrivances which can be devised to furnish themwith an ever-flowing current of fresh air. 

Amongst the various mechanical appliances which have been used to ensureventilation may be mentioned pumps, fans, and pneumatic screws. There is, as we have said, a certain, though slight, movement of the air in the twocolumns which constitute the upcast and the downcast shafts, but in orderthat a current may flow which shall be equal to the necessities of theminers, some means are necessary, by which this condition of almostequilibrium shall be considerably disturbed, and a current created whichshall sweep all foul gases before it. One plan was to force fresh airinto the downcast, which should in a sense push the foetid air away bythe upcast. Another was to exhaust the upcast, and so draw the gases inthe train of the exhausted air. In other cases the plan was adopted ofproviding a continual falling of water down the downcast shaft. 

These various plans have almost all given way to that which is the mostserviceable of all, namely, the plan of having an immense furnaceconstantly burning in a specially-constructed chamber at the bottom ofthe upcast. By this means the column of air above it becomes rarefiedunder the heat, and ascends, whilst the cooler air from the downcastrushes in and spreads itself in all directions whence the bad air hasalready been drawn. On the other hand, to so great a state of perfectionhave ventilating fans been brought, that one was recently erected whichwould be capable of changing the air of Westminster Hall thirty times inone hour. 

Having procured a current of sufficient power, it will be at onceunderstood that, if left to its own will, it would take the nearest pathwhich might lie between its entrance and its exit, and, in this way, ventilating the principal street only, would leave all the manyoff-shoots from it undisturbed. It is consequently manipulated by meansof barriers and tight-fitting doors, in such a way that the current isbound in turn to traverse every portion of the mine. A large number ofboys, known as trappers, are employed in opening the doors to all comers, and in carefully closing the doors immediately after they have passed, inorder that the current may not circulate through passages along which itis not intended that it should pass. 

The greatest dangers which await the miners are those which result, inthe form of terrible explosions, from the presence of inflammable gasesin the mines. The great walls of coal which bound the passages in minesare constantly exuding supplies of gas into the air. When a bank of coalis brought down by an artificial explosion, by dynamite, by limecartridges, or by some other agency, large quantities of gas aresometimes disengaged, and not only is this highly detrimental to thehealth of the miners, if not carried away by proper ventilation, but itconstitutes a constant danger which may at any time cause an explosionwhen a naked light is brought into contact with it. Fire-damp may besometimes heard issuing from fiery seams with a peculiar hissing sound. If the volume be great, the gas forms what is called a _blower_, and thisoften happens in the neighbourhood of a fault. When coal is brought downin any large volume, the blowers which commence may be exhausted in a fewmoments. Others, however, have been known to last for years, this beingthe case at Wallsend, where the blower gave off 120 feet of gas perminute. In such cases the gas is usually conveyed in pipes to a placewhere it can be burned in safety. 

In the early days of coal-mining the explosions caused by this gas soonreceived the serious attention of the scientific men of the age. In the_Philosophical Transactions of the Royal Society_ we find a record of agas explosion in 1677. The amusing part of such records was that theexplosions were ascribed by the miners to supernatural agencies. Littleattention seemed to have been paid to the fact, which has since sothoroughly been established, that the explosions were caused byaccumulations of gas, mixed in certain proportions with air. As aconsequence, tallow candles with an exposed flame were freely used, especially in Britain. These were placed in niches in the workings, wherethey would give to the pitman the greatest amount of light. Previous tothe introduction of the safety-lamp, workings were tested before the menentered them, by "trying the candle". Owing to the specific gravity offire-damp (. 555) being less than that of air, it always finds a lodgementat the roofs of the workings, so that, to test the condition of the air, it was necessary to steadily raise the candle to the roof at certainplaces in the passages, and watch carefully the action of the flame. Thepresence of fire-damp would be shown by the flame assuming a blue colour, and by its elongation; the presence of other gases could be detected byan experienced man by certain peculiarities in the tint of the flame. This testing with the open flame has almost entirely ceased since theintroduction of the perfected Davy lamp. 

The use of candles for illumination soon gave place in most of the largecollieries to the introduction of small oil-lamps. In the less fierymines on the Continent, oil-lamps of the well-known Etruscan pattern arestill in use, whilst small metal lamps, which can conveniently beattached to the cap of the worker, occasionally find favour in theshallower Scotch mines. These lamps are very useful in getting the coalfrom the thinner seams, where progress has to be made on the hands andfeet. At the close of the last century, as workings began to be carrieddeeper, and coal was obtained from places more and more infested withfire-damp, it soon came to be realised that the old methods ofillumination would have to be replaced by others of a safer nature. 

It is noteworthy that mere red heat is insufficient in itself to ignitefire-damp, actual contact with flame being necessary for this purpose. Bearing this in mind, Spedding, the discoverer of the fact, invented whatis known as the "steel-mill" for illuminating purposes. In this a toothedwheel was made to play upon a piece of steel, the sparks thus causedbeing sufficient to give a moderate amount of illumination. It was found, however, that this method was not always trustworthy, and lamps wereintroduced by Humboldt in 1796, and by Clanny in 1806. In these lamps theair which fed the flame was isolated from the air of the mine by havingto bubble through a liquid. Many miners were not, however, provided withthese lamps, and the risks attending naked lights went on as merrily asever. 

In order to avoid explosions in mines which were known to give off largequantities of gas, "fiery" pits as they are called, Sir Humphrey Davy in1815 invented his safety lamp, the principle of which can be stated in afew words. 

If a piece of fine wire gauze be held over a gas-jet before it is lit, and the gas be then turned on, it can be lit above the gauze, but theflame will not pass downwards towards the source of the gas; at least, not until the gauze has become over-heated. The metallic gauze so rapidlyconducts away the heat, that the temperature of the gas beneath the gauzeis unable to arrive at the point of ignition. In the safety-lamp thelittle oil-lamp is placed in a circular funnel of fine gauze, whichprevents the flame from passing through it to any explosive gas that maybe floating about outside, but at the same time allows the rays of lightto pass through readily. Sir Humphrey Davy, in introducing his lamp, cautioned the miners against exposing it to a rapid current of air, whichwould operate in such a way as to force the flame through the gauze, andalso against allowing the gauze to become red-hot. In order to minimise, as far as possible, the necessity of such caution the lamp has beenconsiderably modified since first invented, the speed of the ventilatingcurrents not now allowing of the use of the simple Davy lamp, but theprinciple is the same. 

During the progress of Sir Humphrey Davy's experiments, he found thatwhen fire-damp was diluted with 85 per cent. Of air, and any lessproportion, it simply ignited without explosion. With between 85 percent. And 89 per cent. Of air, fire-damp assumed its most explosive form, but afterwards decreased in explosiveness, until with 94-1/4 per cent. Ofair it again simply ignited without explosion. With between 11 and 12 percent. Of fire-damp the mixture was most dangerous. Pure fire-damp itself, therefore, is not dangerous, so that when a small quantity enters thegauze which surrounds the Davy lamp, it simply burns with itscharacteristic blue flame, but at the same time gives the miner duenotice of the danger which he was running. 

[Illustration: FIG. 32. --Gas Jet and Davy Lamp. ]

With the complicated improvements which have since been made in the Davylamp, a state of almost absolute safety can be guaranteed, but still fromtime to time explosions are reported. Of the cause of many we areabsolutely ignorant, but occasionally a light is thrown upon their originby a paragraph appearing in a daily paper. Two men are charged before themagistrates with being in the possession of keys used exclusively forunlocking their miners' safety-lamps. There is no defence. These men knowthat they carry their lives in their hands, yet will risk their own andthose of hundreds of others, in order that they may be able to lighttheir pipes by means of their safety-lamps. Sometimes in an unexpectedmoment there is a great dislodgement of coal, and a tremendous quantityof gas is set free, which may be sufficient to foul the passages for somedistance around. The introduction or exposure of a naked light for evenso much as a second is sufficient to cause explosion of the mass; doorsare blown down, props and tubbing are charred up, and the volume ofsmoke, rushing up by the nearest shaft and overthrowing the engine-houseand other structures at the mouth, conveys its own sad message to thoseat the surface, of the dreadful catastrophe that has happened below. Perhaps all that remains of some of the workers consists of charred andscorched bodies, scarcely recognisable as human beings. Others escapewith scorched arms or legs, and singed hair, to tell the terrible tale tothose who were more fortunately absent; to speak of their own sufferingswhen, after having escaped the worst effects of the explosion, theyencountered the asphyxiating rush of the after-damp or choke-damp, whichhad been caused by the combustion of the fire-damp. "Choke-damp" in verytruth it is, for it is principally composed of our old acquaintancecarbonic acid gas (carbon dioxide), which is well known as anon-supporter of combustion and as an asphyxiator of animal life. 

It seems a terrible thing that on occasions the workings and wallsthemselves of a coal-mine catch fire and burn incessantly. Yet such isthe case. Years ago this happened in the case of an old colliery nearDudley, at the surface of which, by means of the heat and steam thusafforded, early potatoes for the London market, we are told, were grown;and it was no unusual thing to see the smoke emerging from cracks andcrevices in the rocks in the vicinity of the town. 

From fire on the one hand, we pass, on the other, to the danger whichawaits miners from a sudden inrush of water. During the great coal strikeof 1893, certain mines became unworkable in consequence of the quantityof water which flooded the mines, and which, continually passing alongthe natural fractures in the earth's crust, is always ready to find astorage reservoir in the workings of a coal-mine. This is a difficultywhich is always experienced in the sinking of shafts, and the shuttingoff of water engages the best efforts of mining engineers. 

Added to these various dangers which exist in the coal-mine, we must notomit to notice those accidents that are continually being caused by thefalling-in of roofs or of walls, from the falling of insecure timber, orof what are known as "coal-pipes" or "bell-moulds. " Then, again, everyman that enters the mine trusts his life to the cage by which he descendsto his labour, and shaft accidents are not infrequent. 

The following table shows the number of deaths from colliery accidentsfor a period of ten years, compiled by a Government inspector, and fromthis it will be seen that those resulting from falling roofs numberconsiderably more than one-third of the whole. 

-------------------------------------------------------------------| Causes of Death. | No. Of | Proportion || | Deaths. | per cent. |-------------------------------------------------------------------| Deaths resulting from fire-damp | | || explosions | 2019 | 20. 36 || | | || Deaths resulting from falling | | || roofs and coals | 3953 | 39. 87 || | | || Deaths resulting from shaft | | || accidents | 1710 | 17. 24 || | | || Deaths resulting from miscellaneous | | || causes and above ground | 2234 | 22. 53 || |------------|------------|| | 9916 | 100. 00 |------------------------------------------------------------------|

Every reader of the daily papers is familiar with the harrowing accountswhich are there given of coal-mine explosions. 

This kind of accident is one, which is, above all, associated in thepublic mind with the dangers of the coal-pit. Yet the accidents arisingfrom this cause number but 20 per cent. Of those recorded, and grantedthere be proper inspection, and the use of naked lights be absolutelyabolished, this low percentage might still be considerably reduced. 

A terrific explosion occurred at Whitwick Colliery, Leicestershire, in1893, when two lads were killed, whilst a third was rescued after a verynarrow escape. The lads, it is stated, _were working with naked lights_, when a sudden fall of coal released a quantity of gas, and an immediateexplosion was the natural result. Accidents had been so rare at this pitthat it was regarded as particularly safe, and it was alleged that theuse of naked lights was not uncommon. 

This is an instance of that large number of accidents which areundoubtedly preventable. 

An interesting commentary on the careless manner in which miners risktheir lives was shown in the discoveries made after an explosion at acolliery near Wrexham in 1889. Near the scene of the explosion anunsecured safety lamp was found, and the general opinion at the time wasthat the disaster was caused by the inexcusable carelessness of one ofthe twenty victims. Besides this, when the clothing of the bodiesrecovered was searched, the contents, taken, it should be noted, with thepitmen into the mines, consisted of pipes, tobacco, matches, and evenkeys for unlocking the lamps. It is a strange reflection on the manner inwhich this mine had been examined previous to the men entering upon theirwork, that the under-looker, but half an hour previously, had reportedthe pit to be free from gas. 

Another instance of the same foolhardiness on the part of the miners iscontained in the report issued in regard to an explosion which occurredat Denny, in Stirlingshire, on April 26th, 1895. By this accidentthirteen men lost their lives, and upon the bodies of eight of the numberthe following articles were found; upon Patrick Carr, tin matchbox halffull of matches and a contrivance for opening lamps; John Comrie, splitnail for opening lamps; Peter Conway, seven matches and split key foropening lamps; Patrick Dunton, split nail for opening lamps; John Herron, clay pipe and piece of tobacco; Henry M'Govern, tin matchbox half full ofmatches; Robert Mitchell, clay pipe and piece of tobacco; John Nicol, wooden pipe, piece of tobacco, one match, and box half full of matches. The report stated that the immediate cause of the disaster was theignition of fire-damp by naked light, the conditions of temperature beingsuch as to exclude the possibility of spontaneous combustion. HenryM'Govern had previously been convicted of having a pipe in the mine. Withregard to the question of sufficient ventilation it continued:--"And weare therefore led, on a consideration of the whole evidence, to theconclusion that the accident cannot be attributed to the absence ofventilation, which the mine owners were bound under the Mines RegulationAct and the special rules to provide. " The report concluded as follows:--"On the whole matter we have to report that, in our opinion, theexplosion at Quarter Pit on April 26th, 1895, resulting in the loss ofthirteen lives, was caused by the ignition of an accumulation or anoutburst of gas coming in contact with a naked light, 'other than an opensafety-lamp, ' which had been unlawfully kindled by one of the miners whowere killed. In our opinion, the intensity of the explosion wasaggravated, and its area extended, by the ignition of coal-dust. "

We have mentioned that accidents have frequently occurred from thefalling of "coal-pipes, " or, as they are also called, "bell-moulds. " Wemust explain what is meant by this term. They are simply what appear tobe solid trunks of trees metamorphosed into coal. If we go into atropical forest we find that the woody fibre of dead trees almostinvariably decays faster than the bark. The result is that what mayappear to be a sound tree is nothing but an empty cylinder of bark. Thisappears to have been the case with many of the trees in coal-mines, wherethey are seen to pierce the strata, and around which the miners areexcavating the coal. As the coaly mass collected around the trunk whenthe coal was being formed, the interior was undergoing a process ofdecomposition, while the bark assumed the form of coal. The hollowinterior then became filled with the shale or sandstone which forms theroof of the coal, and its sole support when the coal is removed fromaround it, is the thin rind of carbonised bark. When this falls topieces, or loses its cohesion, the sandstone trunk falls of its ownweight, often causing the death of the man that works beneath it. SirCharles Lyell mentions that in a colliery near Newcastle, no less thanthirty _sigillaria_ trees were standing in their natural position in anarea of fifty yards square, the interior in each case being sandstone, which was surrounded by a bark of friable coal. 

[Illustration: Fig. 33--Part of a trunk of _Sigillaria_, showing the thinouter carbonised bark, with leaf-scars, and the seal-like impressionswhere the bark is removed. ]

The last great danger to which we have here to make reference, is theexplosive action of a quantity of coal-dust in a dry condition. It isonly now commencing to be fully recognised that this is really a mostdangerous explosive. As we have seen, large quantities of coal are formedalmost exclusively of _lepidodendron_ spores, and such coal is productiveof a great quantity of dust. Explosions which are always more or lessattributable to the effects of coal-dust are generally considered, in theofficial statistics, to have been caused by fire-damp. The Act regulatingmines in Great Britain is scarcely up to date in this respect. There is aregulation which provides for the watering of all dry and dusty placeswithin twenty yards from the spot where a shot is fired, but theenforcement of this regulation in each and every pit necessarily devolveson the managers, many of whom in the absence of an inspector leave therequirement a dead letter. Every improvement which results in the betterventilation of a coal-mine tends to leave the dust in a more dangerouscondition. The air, as it descends the shaft and permeates the workings, becomes more and more heated, and licks up every particle of moisture itcan touch. Thorough ventilation results in more greatly freeing a mine ofthe dangerous fire-damp, but the remedy brings about another disease, viz. , the drying-up of all moisture. The dust is thus left in adangerously inflammable condition, acting like a train of gunpowder, tobe started, it may be, by the slightest breath of an explosion. There isapparently little doubt that the presence of coal-dust in a dry state ina mine appreciably increases the liability of explosion in that mine. 

So far as Great Britain is concerned, a Royal Commission was appointed byLord Rosebery's Government to inquire into and investigate the factsreferring to coal-dust. Generally speaking, the conclusion arrived at wasthat fine coal-dust was inflammable under certain conditions. There wasconsiderable difference of opinion as to what these conditions were. Somewere of opinion that coal-dust and air alone were of an explosive nature, whilst others thought that alone they were not, but that the addition ofa small quantity of fire-damp rendered the mixture explosive. Animportant conclusion was come to, that, with the combustion of coal-dustalone, there was little or no concussion, and that the flame was not ofan explosive character. 

Coal-dust was, however, admittedly dangerous, especially if in a drycondition. The effects of an explosion of gas might be considerablyextended by its presence, and there seems every reason to believe that, with a suitable admixture of air and a very small proportion of gas, itforms a dangerous explosive. Legislation in the direction of the reportof the Commission is urgently needed. 

We have seen elsewhere what it is in the dust which makes it dangerous, how that, for the most part, it consists of the dust-like spores of the_lepidodendron_ tree, fine and impalpable as the spores on the backs ofsome of our living ferns, and the fact that this consists of a largeproportion of resin makes it the easily inflammable substance it is. Nothing but an incessant watering of the workings in such cases willrender the dust innocuous. The dust is extremely fine, and is easilycarried into every nook and crevice, and when, as at Bridgend in 1892, itexplodes, it is driven up and out of the shaft, enveloping everythingtemporarily in dust and darkness. 

In some of the pits in South Wales a system of fine sprays of water is inuse, by which the water is ejected from pin-holes pricked in a series ofpipes which are carried through the workings. A fine mist is thus causedwhere necessary, which is carried forward by the force of the ventilatingcurrent. 

A thorough system of inspection in coal-mines throughout the world isundoubtedly urgently called for, in order to ensure the proper carryingout of the various regulations framed for their safety. It is extremelyunfortunate that so many of the accidents which happen are preventable, if only men of knowledge and of scientific attainments filled theresponsible positions of the overlookers. 

CHAPTER V. 

EARLY HISTORY--ITS USE AND ITS ABUSE. 

The extensive use of coal throughout the civilised world for purposes ofheating and illumination, and for the carrying on of manufactures andindustries, may be regarded as a well-marked characteristic of the age inwhich we live. 

Coal must have been in centuries past a familiar object to manygenerations. People must have long been living in close proximity to itsoutcrops at the sides of the mountains and at the surface of the land, yet without being acquainted with its practical value, and it seemsstrange that so little use was made of it until about three centuriesago, and that its use did not spread earlier and more quickly throughoutcivilised countries. 

A mineral fuel is mentioned by Theophrastus about 300 B. C. , from which itis inferred that thus early it was dug from some of the more shallowdepths. The Britons before the time of the Roman invasion are creditedwith some slight knowledge of its industrial value. Prehistoricexcavations have been found in Monmouthshire, and at Stanley, inDerbyshire, and the flint axes there actually found imbedded in the layerof coal are reasonably held to indicate its excavation by neolithic orpalaeolithic (stone-age) workmen. 

The fact that coal cinders have been found on old Roman walls inconjunction with Roman tools and implements, goes to prove that its use, at least for heating purposes, was known in England prior to the Saxoninvasion, whilst some polygonal chambers in the six-foot seam near theriver Douglas, in Lancashire, are supposed also to be Roman. 

The Chinese were early acquainted with the existence of coal, and knew ofits industrial value to the extent of using it for the baking ofporcelain. 

The fact of its extensive existence in Great Britain, and the valuableuses to which it might be put, did not, however, meet with much noticeuntil the ninth century, when, owing to the decrease of theforest-area, and consequently of the supply of wood-charcoal therefrom, it began to attract attention as affording an excellent substitute forcharcoal. 

The coal-miner was, however, still a creation of the future, and even aspeat is collected in Ireland at the present day for fuel, without thelaborious process of mining for it, so those people living incoal-bearing districts found their needs satisfied by the quantity ofcoal, small as it was, which appeared ready to hand on the sides of thecarboniferous mountains. Till then, and for a long time afterwards, theprincipal source of fuel consisted of vast forests, amidst which thecharcoal-burners, or "colliers" as they were even then called, lived outtheir lonely existence in preparing charcoal and hewing wood, for thefires of the baronial halls and stately castles then swarming throughoutthe land. As the forests became used up, recourse was had more and moreto coal, and in 1239 the first charter dealing with and recognising theimportance of the supplies was granted to the freemen of Newcastle, according them permission to dig for coals in the Castle fields. Aboutthe same time a coal-pit at Preston, Haddingtonshire, was granted to themonks of Newbattle. 

Specimens of Newcastle coal were sent to London, but the city was loth toadopt its use, objecting to the innovation as one prejudicial to thehealth of its citizens. By the end of the 16th century, two ships onlywere found sufficient to satisfy the demand for stone-coal in London. This slow progress may, perhaps, have been partially owing to thedifficulties which were placed in the way of its universal use. Greatopposition was experienced by those who imported it into the metropolis, and the increasing amount which was used by brewers and others about theyear 1300, caused serious complaints to be made, the effect of which wasto induce Parliament to obtain a proclamation from the King prohibitingits use, and empowering the justices to inflict a fine on those whopersisted in burning it. The nuisance which coal has since proved itself, in the pollution of the atmosphere and in the denuding of wide tracts ofcountry of all vegetation, was even thus early recognised, and had theefforts which were then made to stamp out its use, proved successful, those who live now in the great cities might never have become acquaintedwith that species of black winter fog which at times hangs like a pallover them, and transforms the brightness of day into a darkness littleremoved from that of night. At the same time, we must bear in mind thatit is universally acknowledged that England owes her prosperity, and herpre-eminence in commerce, in great part, to her happy possession of wideand valuable coal-fields, and many authorities have not hesitated to say, that, in their opinion, the length of time during which England willcontinue to hold her prominent position as an industrial nation islimited by the time during which her coal will last. 

The attempt to prohibit the burning of coal was not, however, verysuccessful, for in the reign of Edward III. A license was again grantedto the freemen of Newcastle to dig for coals. Newcastle was thus thefirst town to become famous as the home of the coal-miner, and the famewhich it early acquired, it has held unceasingly ever since. 

Other attempts at prohibition of the article were made at various timessubsequently, amongst them being one which was made in Elizabeth's reign. It was supposed that the health of the country squires, who came to townto attend the session of Parliament, suffered considerably during theirsojourn in London, and, to remedy this serious state of affairs, the useof stone-coal during the time Parliament was sitting was once moreprohibited. 

Coal was, however, by this time beginning to be recognised as a mostvaluable and useful article of fuel, and had taken a position in theindustrial life of the country from which it was difficult to remove it. Rather than attempt to have arrested the growing use of coal, Parliamentwould have been better employed had it framed laws compelling themanufacturers and other large burners to consume their own smoke, andinstead of aiming at total prohibition, have encouraged an intelligentand more economical use of it. 

In spite of all prohibition its use rapidly spread, and it was soonapplied to the smelting of iron and to other purposes. Iron had beenlargely produced in the south of England from strata of the Wealdenformation, during the existence of the great forest which at one timeextended for miles throughout Surrey and Sussex. The discovery of coal, however, and the opening up of many mines in the north, gave an importantimpetus to the smelting of iron in those counties, and as the forests ofthe Weald became exhausted, the iron trade gradually declined. Furnaceafter furnace became extinguished, until in 1809 that at Ashburnham, which had lingered on for some years, was compelled to bow to theinevitable fate which had overtaken the rest of the iron blast-furnaces. 

In referring to this subject, Sir James Picton says:--"Ironstone ofexcellent quality is found in various parts of the county, and was veryearly made use of. Even before the advent of the Romans, the Forest ofDean in the west, and the Forest of Anderida, in Sussex, in the east, were the two principal sources from which the metal was derived, and allthrough the mediaeval ages the manufacture was continued. After thediscovery of the art of smelting and casting iron in the sixteenthcentury, the manufacture in Sussex received a great impulse from theabundance of wood for fuel, and from that time down to the middle of thelast century it continued to flourish. One of the largest furnaces was atLamberhurst, on the borders of Kent, where the noble balustradesurrounding St Paul's Cathedral was cast at a cost of about £11, 000. Itis stated by the historian Holinshed that the first cast-iron ordnancewas manufactured at Buxted. Two specialities in the iron trade belongedto Sussex, the manufacture of chimney-backs, and cast-iron plates forgrave-stones. At the time when wood constituted the fuel the backs offire-places were frequently ornamented with neat designs. Specimens, bothof the chimney-backs and of the monuments, are occasionally met with. These articles were exported from Rye. The iron manufacture, of course, met with considerable discouragement on the discovery of smelting withpit-coal, and the rapid progress of iron works in Staffordshire and theNorth, but it lingered on until the great forest was cut down and thefuel exhausted. "

In his interesting work, "Sylvia, " published in 1661, Evelyn, in speakingof the noxious vapours poured out into the air by the increasing numberof coal fires, writes, "This is that pernicious smoke which sullies allher glory, superinducing a sooty crust or furr upon all that it lights, spoiling movables, tarnishing the plate, gildings and furniture, andcorroding the very iron bars and hardest stones with those piercing andacrimonious spirits which accompany its sulphur, and executing more inone year than the pure air of the country could effect in some hundreds. "The evils here mentioned are those which have grown and have becomeintensified a hundred-fold during the two centuries and a half which havesince elapsed. When the many efforts which were made to limit its use inthe years prior to 1600 are remembered; at which time, we are informed, two ships only were engaged in bringing coal to London, it at onceappears how paltry are the efforts made now to moderate these samebaneful influences on our atmosphere, at a time when the annualconsumption of coal in the United Kingdom has reached the enormous totalof 190 millions of tons. The various smoke-abatement associations whichhave started into existence during the last few years are doing a little, although very little, towards directing popular attention to the subject;but there is an enormous task before them, that of awakening everyindividual to an appreciation of the personal interest which he has intheir success, and to realise how much might at once be done if each wereto do his share, minute though it might be, towards mitigating the evilsof the present mode of coal-consumption. Probably very few householdersever realise what important factories their chimneys constitute, inbringing about air pollution, and the more they do away with the use ofbituminous coal for fuel, the nearer we shall be to the time when yellowfog will be a thing of the past. 

A large proportion of smoke consists of particles of pure unconsumedcarbon, and this is accompanied in its passage up our chimneys bysulphurous acid, begotten by the sulphur which is contained in the coalto the amount of about eight pounds in every thousand; by sulphurettedhydrogen, by hydro-carbons, and by vapours of various kinds of oils, small quantities of ammonia, and other bodies not by any meanscontributing to a healthy condition of the atmosphere. A good deal of theheavier carbon is deposited along the walls of chimneys in the form ofsoot, together with a small percentage of sulphate of ammonia; this is asa consequence very generally used for manure. The remainder is poured outinto the atmosphere, there to undergo fresh changes, and to become afruitful cause of those thick black fogs with which town-dwellers are sofamiliar. Sulphuretted hydrogen (H_{2}S) is a gas well known to studentsof chemistry as a most powerful reagent, its most characteristic externalproperty being the extremely offensive odour which it possesses, andwhich bears a strong resemblance to that of rotten eggs or decomposingfish. It tarnishes silver work and picture frames very rapidly. Oncombustion it changes to sulphurous acid (SO_{2}), and this in turn hasthe power of taking up from the air another atom of oxygen, formingsulphuric acid (SO_{3} + water), or, as we more familiarly know it, oilof vitriol. 

Yet the smoke itself, including as it does all the many impurities whichexist in coal, is not only evil in itself, but is evil in its influences. Dr Siemens has said:--"It has been shown that the fine dust resultingfrom the imperfect combustion of coal was mainly instrumental in theformation of fog; each particle of solid matter attracting to itselfaqueous vapour. These globules of fog were rendered particularlytenacious and disagreeable by the presence of tar vapour, another resultof imperfect combustion of raw fuel, which might be turned to betteraccount at the dyeworks. The hurtful influence of smoke upon publichealth, the great personal discomfort to which it gave rise, and the vastexpense it indirectly caused through the destruction of our monuments, pictures, furniture, and apparel, were now being recognised. "

The most effectual remedy would result from a general recognition of thefact that wherever smoke was produced, fuel was being consumedwastefully, and that all our calorific effects, from the largest furnaceto the domestic fire, could be realised as completely, and moreeconomically, without allowing any of the fuel employed to reach theatmosphere unburnt. This most desirable result might be effected by theuse of gas for all heating purposes, with or without the additional useof coke or anthracite. The success of the so-called smoke-consumingstoves is greatly open to question, whilst some of them have beenreported upon by those appointed to inspect them as actually accentuatingthe incomplete combustion, the abolition of which they were invented tobring about. 

The smoke nuisance is one which cuts at the very basis of our businesslife. The cloud which, under certain atmospheric conditions, rests like apall over our great cities, will not even permit at times of a single rayof sunshine permeating it. No one knows whence it rises, nor at what hourto expect it. It is like a giant spectre which, having lain dormant sincethe carboniferous age, has been raised into life and being at the call ofrestless humanity; it is now punishing us for our prodigal use of thewealth it left us, by clasping us in its deadly arms, cutting off ourbrilliant sunshine, and necessitating the use in the daytime ofartificial light; inducing all kinds of bronchial and throat affections, corroding telegraph and telephone wires, and weathering away the masonryof public buildings. 

The immense value to us of the coal-deposits which lie buried in suchprofusion in the earth beneath us, can only be appreciated when weconsider the many uses to which coal has been put. We must remember, aswe watch the ever-extending railway ramifying the country in everydirection, that the first railway and the first locomotive ever built, were those which were brought into being in 1814 by George Stephenson, for the purpose of the carriage of coals from the Killingworth Colliery. To the importance of coal in our manufactures, therefore, we owe thesubsequent development of steam locomotive power as the means of theintroduction of passenger traffic, and by the use of coal we are enabledto travel from one end of the country to the other in a space of timeinconceivably small as compared with that occupied on the same journey inthe old coaching days. The increased rapidity with which our vesselscross the wide ocean we owe to the use of coal; our mines are carried togreater depths owing to the power our pumping-engines obtain from coal inclearing the mines of water and in ensuring ventilation; the enormousdevelopment of the iron trade only became possible with the increasedblast power obtained from the consumption of coal, and the very hulls andengines of our steamships are made of this iron; our railroads andengines are mostly of iron, and when we think of the extensive use ofiron utensils in every walk in life, we see how important becomes thepower we possess of obtaining the necessary fuel to feed the smeltingfurnaces. Evaporation by the sun was at one time the sole means ofobtaining salt from seawater; now coal is used to boil the salt pans andto purify the brine from the salt-mines in the triassic strata ofCheshire. The extent to which gas is used for illuminating purposesreminds us of another important product obtained from coal. Paraffin oiland petroleum we obtain from coal, whilst candles, oils, dyes, lubricants, and many other useful articles go to attest the importance ofthe underground stores of that mineral which has well and deservedly beentermed the "black diamond. "

CHAPTER VI. 

HOW GAS IS MADE--ILLUMINATING OILS AND BYE-PRODUCTS. 

Accustomed as we are at the present day to see street after street ofwell-lighted thoroughfares, brilliantly illuminated by gas-lampsmaintained by public authority, we can scarcely appreciate the fact thatthe use of gas is, comparatively speaking, of but recent growth, andthat, like the use of coal itself, it has not yet existed a century inpublic favour. Valuable as coal is in very many different ways, perhapsnext in value to its actual use as fuel, ranks the use of the immediateproduct of its distillation--viz. , gas; and although gas is in somerespects waning before the march of the electric light in our day, yet, even as gas at no time has altogether superseded old-fashioned oil, so weneed not anticipate a time when gas in turn will be likely to besuperseded by the electric light, there being many uses to which the onemay be put, to which the latter would be altogether inapplicable; for, inthe words of Dr Siemens, assuming the cost of electric light to bepractically the same as gas, the preference for one or other would ineach application be decided upon grounds of relative convenience, butgas-lighting would hold its own as the poor man's friend. Gas is aninstitution of the utmost value to the artisan; it requires hardly anyattention, is supplied upon regulated terms, and gives, with what shouldbe a cheerful light, a genial warmth, which often saves the lighting of afire. 

The revolution which gas has made in the appearance of the streets, whereformerly the only illumination was that provided by each householder, who, according to his means, hung out a more or less efficient lantern, and consequently a more or less smoky one, cannot fail also to havebrought about a revolution in the social aspects of the streets, andtherefore is worthy to be ranked as a social reforming agent; and someslight knowledge of the process of its manufacture, such as it is hereproposed to give, should be in the possession of every educatedindividual. Yet the subjects which must be dealt with in this chapter areso numerous and of such general interest, that we shall be unable toenter more than superficially into any one part of the whole, but shallstrive to give a clear and comprehensive view, which shall satisfy theinquirer who is not a specialist. 

The credit of the first attempt at utilising the gaseous product of coalfor illumination appears to be due to Murdock, an engineer at Redruth, who, in 1792, introduced it into his house and offices, and who, tenyears afterwards, as the result of numerous experiments which he madewith a view to its utilisation, made a public display at Birmingham onthe occasion of the Peace of Amiens, in 1802. 

More than a century before, however, the gas obtained from coal had beenexperimented upon by a Dr Clayton, who, about 1690, conceived the idea ofheating coal until its gaseous constituents were forced out of it. Hedescribed how he obtained steam first of all, then a black oil, andfinally a "spirit, " as our ancestors were wont to term the gas. This, tohis surprise, ignited on a light being applied to it, and he considerablyamused his friends with the wonders of this inflammatory spirit. For acentury afterwards it remained in its early condition, a chemical wonder, a thing to be amused with; but it required the true genius and energy ofMurdock to show the great things of which it was capable. 

London received its first instalment of gas in 1807, and during the nextfew years its use became more and more extended, houses and streetsrapidly receiving supplies in quick succession. It was not, however, tillabout the year 1820 that its use throughout the country became at allgeneral, St James' Park being gas-lit in the succeeding year. This is notyet eighty years ago, and amongst the many wonderful things which havesprung up during the present century, perhaps we may place in theforemost rank for actual utility, the gas extracted from coal, conveyedas it is through miles upon miles of underground pipes into the veryhomes of the people, and constituting now almost as much a necessity of acomfortable existence as water itself. 

The use of gas thus rapidly extended for illuminating purposes, and to avery great extent superseded the old-fashioned means of illumination. 

[Illustration: FIG. 34. --Inside a Gas-Holder. ]

The gas companies which sprang up were not slow to notice that, seeingthe gas was supplied by meter, it was to their pecuniary advantage "togive merely the prescribed illuminating power, and to discourage theinvention of economical burners, in order that the consumption mightreach a maximum. The application of gas for heating purposes had not beenencouraged, and was still made difficult in consequence of theobjectionable practice of reducing the pressure in the mains duringdaytime to the lowest possible point consistent with prevention ofatmospheric indraught. "

The introduction of an important rival into the field in the shape of theelectric light has now given a powerful impetus to the invention andintroduction of effective gas-lamps, and amongst inventors of recentyears no name is, perhaps, in this respect so well known as the name ofSugg. As long as gas retained almost the monopoly, there was no incentiveto the gas companies to produce an effective light cheaply; but now thatthe question of the relative cheapness of gas and electricity is beingactively discussed, the gas companies, true to the instinct ofself-preservation, seem determined to show what can be done when gas isconsumed in a scientific manner. 

In order to understand how best a burner should be constructed in orderthat the gas that is burnt should give the greatest possible amount ofillumination, let us consider for a moment the composition of the gasflame. It consists of three parts, (1) an interior dark space, in whichthe elements of the gas are in an unconsumed state; (2) an inner ringaround the former, whence the greatest amount of light is obtained, andin which are numerous particles of carbon at a white heat, each awaitinga supply of oxygen in order to bring about combustion; and (3) an outerring of blue flame in which complete combustion has taken place, and fromwhich the largest amount of heat is evolved. 

The second of these portions of the flame corresponds with the "reducing"flame of the blow-pipe, since this part, if turned upon an oxide, willreduce it, i. E. , abstract its oxygen from it. This part also correspondswith the jet of the Bunsen burner, when the holes are closed by whichotherwise air would mingle with the gas, or with the flame from agas-stove when the gas ignites beneath the proper igniting-jets, andwhich gives consequently a white or yellow flame. 

The third portion, on the other hand, corresponds with the "oxidising"flame of the blow-pipe, since it gives up oxygen to bodies that arethirsting for it. This also corresponds with the ordinary blue flame ofthe Bunsen burner, and with the blue flame of gas-stoves where heat, andnot light, is required, the blue flame in both cases being caused by theadmixture of air with the gas. 

Thus, in order that gas may give the best illumination, we must increasethe yellow or white space of carbon particles at a white heat, and aburner that will do this, and at the same time hold the balance so thatunconsumed particles of carbon shall not escape in the way of smoke, willgive the most successful illuminating results. With this end in view theaddition of albo-carbon to a bulb in the gas-pipe has proved verysuccessful, and the incandescent gas-jet is constructed on exactly thesame chemical principle. The invention of burners which brought aboutthis desirable end has doubtless not been without effect in acting as apowerful obstacle to the widespread introduction of the electric light. 

Without entering into details of the manufacture of gas, it will be aswell just to glance at the principal parts of the apparatus used. 

The gasometer, as it has erroneously been called, is a familiar object tomost people, not only to sight but unfortunately also to the organs ofsmell. It is in reality of course only the gas-holder, in which the finalproduct of distillation of the coal is stored, and from which the gasimmediately passes into the distributing mains. 

The first, and perhaps, most important portion of the apparatus used ingas-making is the series of _retorts_ into which the coal is placed, andfrom which, by the application of heat, the various volatile productsdistil over. These retorts are huge cast-iron vessels, encased in strongbrick-work, usually five in a group, and beneath which a large furnace iskept going until the process is complete. Each retort has an iron exitpipe affixed to it, through which the gases generated by the furnace arecarried off. The exit pipes all empty themselves into what is known asthe _hydraulic main_, a long horizontal cylinder, and in this the gasbegins to deposit a portion of its impurities. The immediate products ofdistillation are, after steam and air, gas, tar, ammoniacal liquor, sulphur in various forms, and coke, the last being left behind in theretort. In the hydraulic main some of the tar and ammoniacal liquoralready begin to be deposited. The gas passes on to the _condenser_, which consists of a number of U-shaped pipes. Here the impurities arestill further condensed out, and are collected in the _tar-pit_ whilstthe gas proceeds, still further lightened of its impurities. It may bementioned that the temperature of the gas in the condenser is reduced toabout 60° F. , but below this some of the most valuable of the illuminantsof coal-gas would commence to be deposited in liquid form, and care hasto be taken to prevent a greater lowering of temperature. A mechanicalcontrivance known as the _exhauster_ is next used, by which the gas is, amongst other things, helped forward in its onward movement through theapparatus. The gas then passes to the _washers_ or _scrubbers_, a seriesof tall towers, from which water is allowed to fall as a fine spray, andby means of which large quantities of ammonia, sulphuretted hydrogen, carbonic acid and oxide, and cyanogen compounds, are removed. In thescrubber the water used in keeping the coke, with which it is filled, damp, absorbs these compounds, and the union of the ammonia with certainof them takes place, resulting in the formation of carbonate of ammonia(smelling salts), sulphide and sulphocyanide of ammonia. 

[Illustration: FIG. 35. --Filling Retorts by Machinery. ]

[Illustration: FIG. 36. --CONDENSERS. ]

Hitherto the purification of the gas has been brought about by mechanicalmeans, but the gas now enters the "_purifier_, " in which it undergoes afurther cleansing, but this time by chemical means. 

[Illustration: FIG. 37. ]

The agent used is either lime or hydrated oxide of iron, and by theirmeans the gas is robbed of its carbonic acid and the greater part of itssulphur compounds. The process is then considered complete, and the gaspasses on into the water chamber over which the gas-holder is reared, andin which it rises through the water, forcing the huge cylinder upwardaccording to the pressure it exerts. 

The gas-holder is poised between a number of upright pillars by a seriesof chains and pulleys, which allow of its easy ascent or descentaccording as the supply is greater or less than that drawn from it by thegas mains. 

[Illustration: FIG. 38. ]

When we see the process which is necessary in order to obtain pure gas, we begin to appreciate to what an extent the atmosphere is fouled whenmany of the products of distillation, which, as far as the production ofgas is concerned, may be called impurities, are allowed to escape freewithout let or hindrance. In these days of strict sanitary inspection itseems strange that the air in the neighbourhood of gas-works is stillallowed to become contaminated by the escape of impure compounds from thevarious portions of the gas-making apparatus. Go where one may, thepresence of these compounds is at once apparent to the nostrils within anone too limited area around them, and yet their deleterious effects canbe almost reduced to a minimum by the use of proper purifying agents, andby a scientific oversight of the whole apparatus. It certainly behovesall sanitary authorities to look well after any gas-works situated withintheir districts. 

Now let us see what these first five products of distillation actuallyare. 

Firstly, house-gas. Everybody knows what house-gas is. It cannot, however, be stated to be any one gas in particular, since it is amechanical mixture of at least three different gases, and often containssmall quantities of others. 

A very large proportion consists of what is known as marsh-gas, or lightcarburetted hydrogen. This occurs occluded or locked up in the pores ofthe coal, and often oozes out into the galleries of coal-mines, where itis known as firedamp (German _dampf_, vapour). It is disengaged wherevervegetable matter has fallen and has become decayed. If it were thencecollected, together with an admixture of ten times its volume of air, aminiature coal-mine explosion could be produced by the introduction of amatch into the mixture. Alone, however, it burns with a feebly luminousflame, although to its presence our house-gas owes a great portion of itsheating power. Marsh-gas is the first of the series of hydro-carbonsknown chemically as the _paraffins_, and is an extremely light substance, being little more than half the weight of an equal bulk of air. It iscomposed of four atoms of hydrogen to one of carbon (CH_{4}). 

Marsh-gas, together with hydrogen and the monoxide of carbon, the last ofwhich burns with the dull blue flame often seen at the surface of fires, particularly coke and charcoal fires, form about 87 per cent. Of thewhole volume of house-gas, and are none of them anything but poorilluminants. 

The illuminating power of house-gas depends on the presence therein ofolefiant gas (_ethylene_), or, as it is sometimes termed, heavycarburetted hydrogen. This is the first of the series of hydro-carbonsknown as the _olefines_, and is composed of two atoms of carbon to everyfour atoms of hydrogen (C_{2}H_{4}). Others of the olefines are presentin minute quantities. These assist in increasing the illuminosity, whichis sometimes greatly enhanced, too, by the presence of a small quantityof benzene vapour. These illuminants, however, constitute but about 6 percent. Of the whole. 

Added to these, there are four other usual constituents which in no wayincrease the value of gas, but which rather detract from it. They areconsequently as far as possible removed as impurities in the process ofgas-making. These are nitrogen, carbonic acid gas, and the destructivesulphur compounds, sulphuretted hydrogen and carbon bisulphide vapour. Itis to the last two to which are to be attributed the injurious effectswhich the burning of gas has upon pictures, books, and also thetarnishing which metal fittings suffer where gas is burnt, since theygive rise to the formation of oil of vitriol (sulphuric acid), which isbeing incessantly poured into the air. Of course the amount so given offis little as compared with that which escapes from a coal fire, but, fortunately for the inmates of the room, in this case the greaterquantity goes up the chimney; this, however, is but a method ofpostponing the evil day, until the atmosphere becomes so laden withimpurities that what proceeds at first up the chimney will finally againmake its way back through the doors and windows. A recent official reporttells us that, in the town, of St Helen's alone, sufficient sulphurescapes annually into the atmosphere to finally produce 110, 580 tons ofsulphuric acid, and a computation has been made that every square mile ofland in London is deluged annually with 180 tons of the samevegetation-denuding acid. It is a matter for wonder that any green thingcontinues to exist in such places at all. 

The chief constituents of coal-gas are, therefore, briefly asfollows:--

/ (1) Hydrogen, | (2) Marsh-gas (carburetted hydrogen or fire-damp), | (3) Carbon monoxide, | (4) Olefiant gas (ethylene, or heavy carburetted hydrogen), with\ other olefines, / (5) Nitrogen, | (6) Carbonic acid gas, | (7) Sulphuretted hydrogen, \ (8) Carbon bisulphide (vapour), the last four being regarded as impurities, which are removed as far aspossible in the manufacture. 

In the process of distillation of the coal, we have seen that variousother important substances are brought into existence. The final residueof coke, which is impregnated with the sulphur which has not beenvolatilised in the form of sulphurous gases, we need scarcely more thanmention here. But the gas-tar and the ammoniacal liquor are two importantproducts which demand something more than our casual attention. At onetime regarded by gas engineers as unfortunately necessary nuisances inthe manufacture of gas, they have both become so valuable on account ofmaterials which can be obtained from them, that they enable gas itself tobe sold now at less than half its original price. The waste of formergenerations is being utilised in this, and an instance is recorded inwhich tar, which was known to have been lying useless at the bottom of acanal for years, has been purchased by a gas engineer for distillingpurposes. It has been estimated that about 590, 000 tons of coal-tar aredistilled annually. 

Tar in its primitive condition has been used, as every one is aware, forpainting or tarring a variety of objects, such as barges and palings, infact, as a kind of protection to the object covered from the ravages ofinsects or worms, or to prevent corrosion when applied to metal piers. But it is worthy of a better purpose, and is capable of yielding far moreuseful and interesting substances than even the most imaginativeindividual could have dreamed of fifty years ago. 

In the process of distillation, the tar, after standing in tanks for sometime, in order that any ammoniacal liquor which may be present may riseto the surface and be drawn off, is pumped into large stills, where amoderate amount of heat is applied to it. The result is that some of themore volatile products pass over and are collected in a receiver. Thesefirst products are known as _first light oils_, or _crude coal-naphtha_, and to this naphtha all the numerous natural naphthas which have beendiscovered in various portions of the world, and to which have beenapplied numerous local names, bear a very close resemblance. Such an one, for instance, was that small but famous spring at Biddings, inDerbyshire, from which the late Mr Young--Paraffin Young--obtained hiswell-known paraffin oil, which gave the initial impetus to what has sincedeveloped into a trade of immense proportions in every quarter of theglobe. 

After a time the crude coal-naphtha ceases to flow over, and the heat isincreased. The result is that a fresh series of products, known as_medium oils_, passes over, and these oils are again collected and keptseparate from the previous series. These in turn cease to flow, when, bya further increase of heat, what are known as the _heavy oils_ finallypass over, and when the last of these, _green grease_, as it is called, distils over, pitch alone is left in the still. Pitch is used to a largeextent in the preparation of artificial asphalte, and also of a fuelknown as "briquettes. "

The products thus obtained at the various stages of the process arethemselves subjected to further distillation, and by the exercise ofgreat care, requiring the most delicate and accurate treatment, a largevariety of oils is obtained, and these are retailed under many andvarious fanciful names. 

One of the most important and best known products of the fractionaldistillation of crude coal-naphtha is that known as _benzene_, orbenzole, (C_{6}H_{6}). This, in its unrefined condition, is a lightspirit which distils over at a point somewhat below the boiling point ofwater, but a delicate process of rectification is necessary to producethe pure spirit. Other products of the same light oils are toluene andxylene. 

Benzene of a certain quality is of course a very familiar and usefulhousehold supplement. It is sometimes known and sold as _benzene collas_, and is used for removing grease from clothing, cleaning kid gloves, &c. If pure it is in reality a most dangerous spirit, being very inflammable;it is also extremely volatile, so much so that, if an uncorked bottle beleft in a warm room where there is a fire or other light near, its vapourwill probably ignite. Should the vapour become mixed with air beforeignition, it becomes a most dangerous explosive, and it will thus be seenhow necessary it is to handle the article in household use in a mostcautious manner. Being highly volatile, a considerable degree of cold isexperienced if a drop be placed on the hand and allowed to evaporate. 

Benzene, which is only a compound of carbon and hydrogen, was firstdiscovered by Faraday in 1825; it is now obtained in large quantitiesfrom coal-tar, not so much for use as benzene; is for its conversion, inthe first place, by the action of nitric acid, into _nitro-benzole, _ aliquid having an odour like the oil of bitter almonds, and which is muchused by perfumers under the name of _essence de mirbane_; and, in thesecond place, for the production from this nitro-benzole of the far-famed_aniline_. After the distillation of benzene from the crude coal-naphthais completed, the chief impurities in the residue are charred anddeposited by the action of strong sulphuric acid. By further distillationa lighter oil is given off, often known as _artificial turpentine oil_, which is used as a solvent for varnishes and lackers. This is veryfamiliar to the costermonger fraternity as the oil which is burned in theflaring lamps which illuminate the New Cut or the Elephant and Castle onSaturday and other market nights. 

By distillation of the _heavy oils_, carbolic acid and commercial_anthracene_ are produced, and by a treatment of the residue, a white andcrystalline substance known as _naphthalin_ (C_{10}H_{8}) is finallyobtained. 

Thus, by the continued operation of the chemical process known asfractional distillation of the immediate products of coal-tar, thesevarious series of useful oils are prepared. 

The treatment is much the same which has resulted in the production ofparaffin oil, to which we have previously referred, and an account of theproduction of coal-oils would be very far from satisfactory, which madeno mention of the production of similar commodities by the directdistillation of shale. Oil-shales, or bituminous shales, exist in allparts of the world, and may be regarded as mineral matter largelyimpregnated by the products of decaying vegetation. They thereforegreatly resemble some coals, and really only differ therefrom in degree, in the quantity of vegetable matter which they contain. Into the subjectof the various native petroleums which have been found--for theserock-oils are better known as petroleums--in South America, in Burmah(Rangoon Oil), at Baku, and the shores of the Caspian, or in the UnitedStates of America, we need not enter, except to note that in allprobability the action of heat on underground bituminous strata ofenormous extent has been the cause of their production, just as on asmaller scale the action of artificial heat has forced the reluctantshale to give up its own burden of mineral oil. However, previous to1847, although native mineral oil had been for some years a recognisedarticle of commerce, the causes which gave rise to the oil-wells, and thesource, probably a deep-seated one, of the supply of oil, does not appearto have been well known, or at least was not enquired after. But in thatyear Mr Young, a chemist at Manchester, discovered that by distillingsome petroleum, which he obtained from a spring at Riddings inDerbyshire, he was able to procure a light oil, which he used for burningin lamps, whilst the heavier product which he also obtained proved a mostuseful lubricant for machinery. This naturally distilled oil was soonfound to be similar to that oil which was noticed dripping from the roofof a coal-mine. Judging that the coal, being under the influence of heat, was the cause of the production of the oil, Mr Young tested thisconclusion by distilling the coal itself. Success attended his endeavourthus to procure the oil, and indelibly Young stamped his name upon theroll of famous men, whose industrial inventions have done so much towardsthe accomplishment of the marvellous progress of the present century. From the distillation he obtained the well-known Young's Paraffin Oil, and the astonishing developments of the process which have taken placesince he obtained his patent in 1850, for the manufacture of oils andsolid paraffin, must have been a source of great satisfaction to himbefore his death, which occurred in 1883. 

Cannel coal, Boghead or Bathgate coal, and bituminous shales of variousqualities, have all been requisitioned for the production of oils, andfrom these various sources the crude naphthas, which bear a variety ofnames according to some peculiarity in their origin, or place ofoccurrence, are obtained. Boghead coal, also known as "Torebanehillmineral, " gives Boghead naphtha, while the crude naphtha obtained fromshales is often quoted as shale-oil. In chemical composition thesenaphthas are closely related to one another, and by fractionaldistillation of them similar series of products are obtained as those wehave already seen as obtained from the crude coal-naphtha of coal-tar. 

In the direct distillation of cannel-coal for the production of paraffin, it is necessary that the perpendicular tubes or retorts into which thecoal is placed be heated only to a certain temperature, which isconsiderably lower than that applied when the object is the production ofcoal-gas. By this means nearly all the volatile matters pass over in theform of condensible vapours, and the crude oils are at once formed, fromwhence are obtained at different temperatures various volatile ethers, benzene, and artificial turpentine oil or petroleum spirit. After these, the well-known safety-burning paraffin oil follows, but it is essentialthat the previous three volatile products be completely cleared first, since, mixed with air, they form highly dangerous explosives. To the factthat the operation is carried on in the manufactories with great care andaccuracy can only be attributed the comparative rareness of explosions ofthe oil used in households. 

After paraffin, the heavy lubricating oils are next given off, stillincreasing the temperature, and, the residue being in turn subjected to avery low temperature, the white solid substance known as paraffin, somuch used for making candles, is the result. By a different treatment ofthe same residue is produced that wonderful salve for tender skins, cuts, and burns, known popularly as _vaseline_. Probably no suchwidely-advertised remedial substance has so deserved its success as thisuniversally-used waste product of petroleum. 

We have noticed the fact that in order to procure safety-burning oils, itis absolutely necessary that the more volatile portions be completelydistilled over first. By Act of Parliament a test is applied to all oilswhich are intended for purposes of illumination, and the test usedconsists of what is known as the flashing-point. Many of the morevolatile ethers, which are highly inflammable, are given off even atordinary temperatures, and the application of a light to the oil willcause the volatile portion to "flash, " as it is called. A safety-burningoil, according to the Act, must not flash under 100° Fahrenheit opentest, and all those portions which flash at a less temperature must bevolatilised off before the residue can be deemed a safe oil. It seemsprobable that the flashing-point will sooner or later be raised. 

One instance may be cited to show how necessary it is that the nativemineral oils which have been discovered should have this effectual testapplied to them. 

When the oil-wells were first discovered in America, the oil was obtainedsimply by a process of boring, and the fountain of oil which was boredinto at times was so prolific, that it rushed out with a force whichcarried all obstacles before it, and defied all control. In one instancea column of oil shot into the air to a height of forty feet, and defiedall attempts to keep it under. In order to prevent further accident, alllights in the immediate neighbourhood were extinguished, the nearestremaining being at a distance of four hundred feet. But in this crudenaphtha there was, as usual, a quantity of volatile spirit which wasbeing given off even at the temperature of the surrounding atmosphere. This soon became ignited, and with an explosion the column of oil wassuddenly converted into a roaring column of fire. The owner of theproperty was thrown a distance of twenty feet by the explosion, and soonafterwards died from the burns which he had received from it. Such anaccident could not now, however, happen. The tapping, stopping, andregulating of gushing wells can now be more effectually dealt with, andin the process of refining; the most inflammable portions are separated, with a result that, as no oil is used in the country which flashes under100° F. Open test, and as our normal temperature is considerably lessthan this, there is little to be feared in the way of explosion if theAct be complied with. 

When the results of Mr Young's labours became publicly known, a number ofcompanies were started with the object of working on the lines laid downin his patent, and these not only in Great Britain but also in the UnitedStates, whither quantities of cannel coal were shipped from England andother parts to feed the retorts. In 1860, according to the statisticsfurnished, some seventy factories were established in the United Statesalone with the object of extracting oil from coal and other mineralsources, such as bituminous shale, etc. When Young's patent finallyexpired, a still greater impetus was given to its production, and themanufacture would probably have continued to develop were it not thatattention had, two years previously, been forcibly turned to thosediscoveries of great stores of natural oil in existence beneath acomparatively thin crust of earth, and which, when bored into, spoutedout to tremendous heights. 

The discovery of these oil-fountains checked for a time the developmentof the industry, but with the great production there has apparently beena greatly increased demand for it, and the British industry once againappears to thrive, until even bituminous shales have been brought underrequisition for their contribution to the national wealth. 

Were it not for the nuisance and difficulty experienced in the propercleaning and trimming of lamps, there seems no other reason why mineraloil should not in turn have superseded the use of gas, even as gas had, years before, superseded the expensive animal and vegetable oils whichhad formerly been in use. 

Although this great development in the use of mineral oils has takenplace only within the last thirty years, it must not be thought thattheir use is altogether of modern invention. That they were notaltogether unknown in the fifth century before Christ is a matter ofcertainty, and at the time when the Persian Empire was at the zenith ofits glory, the fires in the temples of the fire-worshippers wereundoubtedly kept fed by the natural petroleum which the districts aroundafforded. It is thought by some that the legend which speaks of the firewhich came down from heaven, and which lit the altars of theZoroastrians, may have had its origin in the discovery of a hithertounknown petroleum spring. More recently, the remarks of Marco Polo in hisaccount of his travels in A. D. 1260 and following years, are particularlyinteresting as showing that, even then, the use of mineral oil forvarious purposes was not altogether unknown. He says that on the north ofArmenia the Greater is "Zorzania, in the confines of which a fountain isfound, from which a liquor like oil flows, and though unprofitable forthe seasoning of meat, yet is very fit for the supplying of lamps, and toanoint other things; and this natural oil flows constantly, and that inplenty enough to lade camels. "

From this we can infer that the nature of the oil was entirely unknown, for it was a "liquor like oil, " and was also, strange to say, "unprofitable for the seasoning of meat"! In another place in Armenia, Marco Polo states that there was a fountain "whence rises oil in suchabundance that a hundred ships might be at once loaded with it. It is notgood for eating, but very fit for fuel, for anointing the camels inmaladies of the skin, and for other purposes; for which reason peoplecome from a great distance for it, and nothing else is burned in all thiscountry. "

The remedial effects of the oil, when used as an ointment, were thusearly recognised, and the far-famed vaseline of the present day may beregarded as the lineal descendent, so to speak, of the crude medicinalagent to which Marco Polo refers. 

The term asphalt has been applied to so many and various mixtures, thatone scarcely associates it with natural mineral pitch which is found insome parts of the world. From time immemorial this compact, bituminous, resinous mineral has been discovered in masses on the shores of the DeadSea, which has in consequence received the well-known title of LakeAsphaltites. Like the naphthas and petroleums which have been noticed, this has had its origin in the decomposition of vegetable matter, andappears to be thrown up in a liquid form by the volcanic energies which, are still believed to be active in the centre of the lake, and which maybe existent beneath a stratum, or bed, of oil-producing bitumen. 

In connection with the formation of this substance, the remarks of SirCharles Lyell, the great geologist, may well be quoted, as showing thetransformation of vegetable matter into petroleum, and afterwards intosolid-looking asphalt. At Trinidad is a lake of bitumen which is a mileand a half in circumference. "The Orinoco has for ages been rolling downgreat quantities of woody and vegetable bodies into the surrounding sea, where, by the influence of currents and eddies, they may be arrested, andaccumulated in particular places. The frequent occurrence of earthquakesand other indications of volcanic action in those parts, lend countenanceto the opinion that these vegetable substances may have undergone, by theagency of subterranean fire, those transformations or chemical changeswhich produce petroleum; and this may, by the same causes, be forced upto the surface, where, by exposure to the air, it becomes inspissated, and forms those different varieties of earth-pitch or asphaltum soabundant in the island. "

It is interesting to note also that it was obtained, at an ancientperiod, from the oil-fountains of Is, and that it was put to considerableuse in the embalming of the bodies of the Egyptians. It appears, too, tohave been employed in the construction of the walls of Babylon, and thusfrom very early times these wonderful products and results of decayedvegetation have been brought into use for the service of man. 

Aniline has been previously referred (p. 135) to as having been preparedfrom nitro-benzole, or _essence de mirbane_, and its preparation, bytreating this substance with iron-filings and acetic acid, was one of theearly triumphs of the chemists who undertook the search after the unknowncontained in gas-tar. It had previously been obtained from oils distilledfrom bones. The importance of the substance lies in the fact that, by theaction of various chemical reagents, a series of colouring matters ofvery great richness are formed, and these are the well-known _anilinedyes_. 

As early as 1836, it was discovered that aniline, when heated withchloride of lime, acquired a beautiful blue tint. This discovery led tono immediate practical result, and it was not until twenty-one yearsafter that a further discovery was made, which may indeed be said to haveachieved a world-wide reputation. It was found that, by adding bichromateof potash to a solution of aniline and sulphuric acid, a powder wasobtained from which the dye was afterwards extracted, which is known as_mauve_. Since that time dyes in all shades and colours have beenobtained from the same source. _Magenta_ was the next dye to make itsappearance, and in the fickle history of fashion, probably no colourshave had such extraordinary runs of popularity as those of mauve andmagenta. Every conceivable colour was obtained in due course from thesame source, and chemists began to suspect that, in the course of time, the colouring matter of dyer's madder, which was known as _alizarin_, would also be obtained therefrom. Hitherto this had been obtained fromthe root of the madder-plant, but by dint of careful and well-reasonedresearch, it was obtained by Dr Groebe, from a solid crystalline coal-tarproduct, known as _anthracene_, (C_{12}H_{14}). This artificial alizarinyields colours which are purer than those of natural madder, and beingderived from what was originally regarded as a waste product, its cost ofproduction is considerably cheaper. 

We have endeavoured thus far to deal with (1) gas, and (2) tar, the twoprincipal products in the distillation of coal. We have yet to say a fewwords concerning the useful ammoniacal liquor, and the final residue inthe retorts, _i. E. _, coke. 

The ammoniacal liquor which has been passing over during distillation ofthe coal, and which has been collecting in the hydraulic main and inother parts of the gas-making apparatus, is set aside to be treated to avariety of chemical reactions, in order to wrench from it its usefulconstituents. Amongst these, of course, _ammonia_ stands in the firstrank, the others being comparatively unimportant. In order to obtainthis, the liquor is first of all neutralised by being treated with aquantity of acid, which converts the principal constituent of the liquor, viz. , carbonate of ammonia (smelling salts), into either sulphate ofammonia, or chloride of ammonia, familiarly known as sal-ammoniac, according as sulphuric acid or hydrochloric acid is the acid used. Thuscarbonate of ammonia with sulphuric acid will give sulphate of ammonia, but carbonate of ammonia with hydrochloric acid will give sal-ammoniac(chloride of ammonia). By a further treatment of these with lime, or, asit is chemically known, oxide of calcium, ammonia is set free, whilstchloride of lime (the well-known disinfectant), or sulphate of lime(gypsum, or "plaster of Paris" ), is the result. 

Thus:

Sulphate of ammonia + lime = plaster of Paris + ammonia. 

or, Sal-ammoniac + lime = chloride of lime + ammonia. 

Ammonia itself is a most powerful gas, and acts rapidly upon the eyes. Ithas a stimulating effect upon the nerves. It is not a chemical element, being composed of three parts of hydrogen by weight to one of nitrogen, both of which elements alone are very harmless, and, the latter indeed, very necessary to human life. Ammonia is fatal to life, producing greatirritation of the lungs. 

It has also been called "hartshorn, " being obtained by destructivedistillation of horn and bone. The name "ammonia" is said to have beenderived from the fact that it was first obtained by the Arabs near thetemple of Jupiter Ammon, in Lybia, North Africa, from the excrement ofcamels, in the form of sal-ammoniac. There are always traces of it in theatmosphere, especially in the vicinity of large towns and manufactorieswhere large quantities of coal are burned. 

Coke, if properly prepared, should consist of pure carbon. Good coalshould yield as much as 80 per cent. Of coke, but owing to theunsatisfactory manner of its production, this proportion is seldomyielded, whilst the coke which is familiar to householders, being theresidue left in the retorts after gas-making, usually contains so large aproportion of sulphur as to make its combustion almost offensive. Nodoubt the result of its unsatisfactory preparation has been that it hasfailed to make its way into households as it should have done, but thereis also another objection to its use, namely, the fact that, owing to thequantity of oxygen required in its combustion, it gives rise to feelingsof suffocation where insufficient ventilation of the room is provided. 

Large quantities of coke are, however, consumed in the feeding of furnacefires, and in the heating of boilers of locomotives, as well as inmetallurgical operations; and in order to supply the demand, largequantities of coal are "coked, " a process by which the volatile productsare completely combusted, pure coke remaining behind. This process istherefore the direct opposite to that of "distillation, " by which thevolatile products are carefully collected and re-distilled. 

The sulphurous impurities which are always present in the coal, and whichare, to a certain extent, retained in coke made at the gas-works, themselves have a value, which in these utilitarian days is not longlikely to escape the attention of capitalists. In coal, bands of brightshining iron pyrites are constantly seen, even in the homely scuttle, andwhen coal is washed, as it is in some places, the removal of the pyritesincreases the value of the coal, whilst it has a value of its own. 

The conversion of the sulphur which escapes from our chimneys intosulphuretted hydrogen, and then into sulphuric acid, or oil of vitriol, has already been referred to, and we can only hope that in these dayswhen every available source of wealth is being looked up, and when therethreatens to remain nothing which shall in the future be known as"waste, " that the atmosphere will be spared being longer the receptaclefor the unowned and execrated brimstone of millions of fires andfurnaces. 

CHAPTER VII. 

THE COAL SUPPLIES OF THE WORLD. 

As compared with some of the American coal-fields, those of Britain arebut small, both in extent and thickness. They can be regarded as fallingnaturally into three principal areas. 

 The northern coal-field, including those of Fife, Stirling, and Ayr in Scotland; Cumberland, Newcastle, and Durham in England; Tyrone in Ireland. 

 The middle coal-field, all geologically in union, including those of Yorkshire, Derbyshire, Shropshire, Staffordshire, Flint, and Denbigh. 

 The southern coal-field, including South Wales, Forest of Dean, Bristol, Dover, with an offshoot at Leinster, &c. , and Millstreet, Cork. 

Thus it will be seen that while England and Scotland are, in comparisonwith their extent of surface, bountifully supplied with coal-areas, inthe sister island of Ireland coal-producing areas are almost absent. Theisolated beds in Cork and Tipperary, in Tyrone and Antrim, are but theremnants left of what were formerly beds of coal extending the wholebreadth and length of Ireland. Such beds as there remain undoubtedlybelong to the base of the coal-measures, and observations all go to showthat the surface suffered such extreme denudation subsequent to thegrowth of the coal-forests, that the wealth which once lay there, hasbeen swept away from the surface which formerly boasted of it. 

On the continent of Europe the coal-fields, though not occupying so largea proportion of the surface of the country as in England, are very farfrom being slight or to be disregarded. The extent of forest-lands stillremaining in Germany and Austria are sufficing for the immediate needs ofthe districts where some of the best seams occur. It is only where thereis a dearth of handy fuel, ready to be had, perhaps, by the simplefelling of a few trees, that man commences to dig into the earth for hisfuel. But although on the continent not yet occupying so prominent aposition in public estimation as do coal-fields in Great Britain, thoseof the former have one conspicuous characteristic, viz. , the greatthickness of some of the individual seams. 

In the coal-field of Midlothian the seams of coal vary from 2 feet to 5feet in thickness. One of them is known as the "great seam, " and in spiteof its name attains a thickness only of from 8 to 10 feet thick. Thereare altogether about thirty seams of coal. When, however, we pass to thecontinent, we find many instances, such as that of the coal-field ofCentral France, in which the seams attain vast thicknesses, many of themactually reaching 40 and 60 feet, and sometimes even 80 feet. One of theseams in the district of St. Etienne varies from 30 to 70 feet thick, whilst the fifteen to eighteen workable seams give a thickness of 112feet, although the total area of the field is not great. Again, in theremarkable basin of the Saône-et-Loire, although there are but ten bedsof coal, two of them run from 30 to 60 feet each, whilst at Creusot themain seam actually runs locally to a thickness varying between 40 and 130feet. 

The Belgian coal-field stretches in the form of a narrow strip from 7 to9 miles wide by about 100 miles long, and is divided into three principalbasins. In that stretching from Liége to Verviers there are eighty-threeseams of coal, none of which are less than 3 feet thick. In the basin ofthe Sambre, stretching from Namur to Charleroi, there are seventy-threeseams which are workable, whilst in that between Mons and Thulin thereare no less than one hundred and fifty-seven seams. The measures here areso folded in zigzag fashion, that in boring in the neighbourhood of Monsto a depth of 350 yards vertical, a single seam was passed through noless than six times. 

Germany, on the west side of the Rhine, is exceptionally fortunate in thepossession of the famous Pfalz-Saarbrücken coal-field, measuring about 60miles long by 20 miles wide, and covering about 175 square miles. Much ofthe coal which lies deep in these coal-measures will always remainunattainable, owing to the enormous thickness of the strata, but acareful computation made of the coal which can be worked, gives anestimate of no less than 2750 millions of tons. There is a grand total oftwo hundred and forty-four seams, although about half of them areunworkable. 

Beside other smaller coal-producing areas in Germany, the coal-fields ofSilesia in the southeastern corner of Prussia are a possession unrivalledboth on account of their extent and thickness. It is stated that thereexist 333 feet of coal, all the seams of which exceed 2-1/2 feet, andthat in the aggregate there is here, within a workable depth, thescarcely conceivable quantity of 50, 000 million tons of coal. 

The coal-field of Upper Silesia, occupying an area about 20 miles long by15 miles broad, is estimated to contain some 10, 000 feet of strata, with333 feet of good coal. This is about three times the thickness containedin the South Wales coal-field, in a similar thickness of coal-measures. There are single seams up to 60 feet thick, but much of the coal iscovered by more recent rocks of New Red and Cretaceous age. In LowerSilesia there are numerous seams 3-1/2 feet to 5 feet thick, but owing totheir liability to change in character even in the same seam, their valueis inferior to the coals of Upper Silesia. 

When British supplies are at length exhausted, we may anticipate thatsome of the earliest coals to be imported, should coal then be needed, will reach Britain from the upper waters of the Oder. 

The coal-field of Westphalia has lately come into prominence inconnection with the search which has been made for coal in Kent andSurrey, the strata which are mined at Dortmund being thought to becontinuous from the Bristol coal-field. Borings have been made throughthe chalk of the district north of the Westphalian coal-field, and thesehave shown the existence of further coal-measures. The coal-field extendsbetween Essen and Dortmund a distance of 30 miles east and west, andexhibits a series of about one hundred and thirty seams, with anaggregate of 300 feet of coal. 

It is estimated that this coal-field alone contains no less than 39, 200millions of tons of coal. 

Russia possesses supplies of coal whose influence has scarcely yet beenfelt, owing to the sparseness of the population and the abundance offorest. Carboniferous rocks abut against the flanks of the UralMountains, along the sides of which they extend for a length of about athousand miles, with inter-stratifications of coal. Their actual contentshave not yet been gauged, but there is every reason to believe that thosecoal-beds which have been seen are but samples of many others which will, when properly worked, satisfy the needs of a much larger population thanthe country now possesses. 

Like the lower coals of Scotland, the Russian coals are found in thecarboniferous limestone. This may also be said of the coal-fields in thegovernments of Tula and Kaluga, and of those important coal-bearingstrata near the river Donetz, stretching to the northern corner of theSea of Azov. In the last-named, the seams are spread over an area of11, 000 square miles, in which there are forty-four workable seamscontaining 114 feet of coal. The thickest of known Russian coals occur atLithwinsk, where three seams are worked, each measuring 30 feet to 40feet thick. 

An extension of the Upper Silesian coal-field appears in Russian Poland. This is of upper Carboniferous age, and contains an aggregate of 60 feetof coal. 

At Ostrau, in Upper Silesia (Austria), there is a remarkable coal-field. Of its 370 seams there are no less than 117 workable ones, and thesecontain 350 feet of coal. The coals here are very full of gas, which evenpercolates to the cellars of houses in the town. A bore hole which wassunk in 1852 to a depth of 150 feet, gave off a stream of gas, whichignited, and burnt for many years with a flame some feet long. 

The Zwickau coal-field in Saxony is one of the most important in Europe. It contains a remarkable seam of coal, known as Russokohle or soot-coal, running at times 25 feet thick. It was separated by Geinitz and othersinto four zones, according to their vegetable contents, viz. :--

1. Zone of Ferns. 

2. Zone of Annularia and Calamites. 

3. Zone of Sigillaria. 

4. Zone of Sagenaria (in Silesia), equivalent to the culm-measures of Devonshire. 

Coals belonging to other than true Carboniferous age are found in Europeat Steyerdorf on the Danube, where there are a few seams of good coal instrata of Liassic age, and in Hungary and Styria, where there aretertiary coals which approach closely to those of true Carboniferous agein composition and quality. 

In Spain there are a few small scattered basins. Coal is found overlyingthe carboniferous limestone of the Cantabrian chain, the seams being from5 feet to 8 feet thick. In the Satero valley, near Sotillo, is a singleseam measuring from 60 feet to 100 feet thick. Coal of Neocomian ageappears at Montalban. 

When we look outside the continent of Europe, we may well be astonishedat the bountiful manner in which nature has laid out beds of coal uponthese ancient surfaces of our globe. 

Professor Rogers estimated that, in the United States of America, thecoal-fields occupy an area of no less than 196, 850 square miles. 

Here, again, it is extremely probable that the coal-fields which remain, in spite of their gigantic existing areas, are but the remnants of onetremendous area of deposit, bounded only on the east by the Atlantic, andon the west by a line running from the great lakes to the frontiers ofMexico. The whole area has been subjected to forces which have producedfoldings and flexures in the Carboniferous strata after deposition. Theseundulations are greatest near the Alleghanies, and between thesemountains and the Atlantic, whilst the flexures gradually dying outwestward, cause the strata there to remain fairly horizontal. In thetroughs of the foldings thus formed the coal-measures rest, thoseportions which had been thrown up as anticlines having suffered loss bydenudation. Where the foldings are greatest there the coal has beennaturally most altered; bituminous and caking-coals are characteristic ofthe broad flat areas west of the mountains, whilst, where the contortionsare greatest, the coal becomes a pure anthracite. 

It must not be thought that in this huge area the coal is all uniformlygood. It varies greatly in quality, and in some districts it occurs insuch thin seams as to be worthless, except as fuel for consumption by theactual coal-getters. There are, too, areas of many square miles inextent, where there are now no coals at all, the formation having beendenuded right down to the palaeozoic back-bone of the country. 

Amongst the actual coal-fields, that of Pennsylvania standspre-eminent. The anthracite here is in inexhaustible quantity, its outputexceeding that of the ordinary bituminous coal. The great field of whichthis is a portion, extends in an unbroken length for 875 miles N. E. AndS. W. , and includes the basins of Ohio, Maryland, Virginia, Kentucky, andTennessee. The workable seams of anthracite about Pottsville measure inthe aggregate from 70 to 207 feet. Some of the lower seams individuallyattain an exceptional thickness, that at Lehigh Summit mine containing aseam, or rather a bed, of 30 feet of good coal. 

A remarkable seam of coal has given the town of Pittsburg its name. Thisis 8 feet thick at its outcrop near the town, and although its thicknessvaries considerably, Professor Rogers estimates that the sheet of coalmeasures superficially about 14, 000 square miles. What a forest theremust have existed to produce so widespread a bed! Even as it is, it hasat a former epoch suffered great denudation, if certain detached basinsshould be considered as indicating its former extent. 

The principal seam in the anthracite district of central Pennsylvania, which extends for about 650 miles along the left bank of the Susquehanna, is known as the "Mammoth" vein, and is 29-1/2 feet thick at Wilkesbarre, whilst at other places it attains to, and even exceeds, 60 feet. 

On the west of the chain of mountains the foldings become gentler, andthe coal assumes an almost horizontal position. In passing through Ohiowe find a saddle-back ridge or anticline of more ancient strata than thecoal, and in consequence of this, we have a physical boundary placed uponthe coal-fields on each side. 

Passing across this older ridge of denuded Silurian and other rocks, wereach the famous Illinois and Indiana coal-field, whosecoal-measures lie in a broad trough, bounded on the west by the uprisingof the carboniferous limestone of the upper Mississippi. This limestoneformation appears here for the first time, having been absent on theeastern side of the Ohio anticline. The area of the coal-field isestimated at 51, 000 square miles. 

In connection with the coal-fields of the United States, it isinteresting to notice that a wide area in Texas, estimated at 3000 squaremiles, produces a large amount of coal annually from strata of theLiassic age. Another important area of production in eastern Virginiacontains coal referable to the Jurassic age, and is similar in fossilcontents to the Jurassic of Whitby and Brora. The main seam in easternVirginia boasts a thickness of from 30 to 40 feet of good coal. 

Very serviceable lignites of Cretaceous age are found on the Pacificslope, to which age those of Vancouver's Island and Saskatchewan Riverare referable. 

Other coal-fields of less importance are found between Lakes Huron andErie, where the measures cover an area of 5000 square miles, and also inRhode Island. 

In British North America we find extensive deposits of valuablecoal-measures. Large developments occur in New Brunswick and Nova Scotia. At South Joggins there is a thickness of 14, 750 feet of strata, in whichare found seventy-six coal-seams of 45 feet in total thickness. At Pictonthere are six seams with a total of 80 feet of coal. In the lowercarboniferous group is found the peculiar asphaltic coal of the Albertmine in New Brunswick. Extensive deposits of lignite are met with both inthe Dominion and in the United States, whilst true coal-measures flankboth sides of the Rocky Mountains. Coal-seams are often encountered inthe Arctic archipelago. 

The principal areas of deposit in South America are in Brazil, Uruguay, and Peru. The largest is the Candiota coal-field, in Brazil, wheresections in the valley of the Candiota River show five good seams with atotal of 65 feet of coal. It is, however, worked but little, theprincipal workings being at San Jeronimo on the Jacahahay River. 

In Peru the true carboniferous coal-seams are found on the higher groundof the Andes, whilst coal of secondary age is found in considerablequantities on the rise towards the mountains. At Porton, east ofTruxillo, the same metamorphism which has changed the ridge of sandstoneto a hard quartzite has also changed the ordinary bituminous coal into ananthracite, which is here vertical in position. The coals of Peru usuallyrise to more than 10, 000 feet above the sea, and they are practicallyinaccessible. 

Cretaceous coals have been found at Lota in Chili, and at Sandy Point, Straits of Magellan. 

Turning to Asia, we find that coal has been worked from time to time atHeraclea in Asia Minor. Lignites are met with at Smyrna and Lebanon. 

The coal-fields of Hindoostan are small but numerous, being found in allparts of the peninsula. There is an important coal-field at Raniganj, near the Hooghly, 140 miles north of Calcutta. It has an area of 500square miles. In the Raniganj district there are occasional seams 20 feetto 80 feet in thickness, but the coals are of somewhat inferior quality. 

The best quality amongst Indian coals has come from a small coal-field ofabout 11 square miles in extent, situated at Kurhurbali on the EastIndian Railway. Other coal-fields are found at Jherria and on the SoneRiver, in Bengal, and at Mopani on the Nerbudda. Much is expected infuture from the large coal-field of the Wardha and Chanda districts, inthe Central Provinces, the coal of which may eventually prove to be ofPermian age. 

The coal-deposits of China are undoubtedly of tremendous extent, althoughfrom want of exploration it is difficult to form any satisfactoryestimate of them. Near Pekin there are beds of coal 95 feet thick, whichafford ample provision for the needs of the city. In the mountainousdistricts of western China the area over which carboniferous strata areexposed has been estimated at 100, 000 square miles. The coal-measuresextend westward to the Mongolian frontier, where coal-seams 30 feet thickare known to lie in horizontal plane for 200 miles. Most of the Chinesecoal-deposits are rendered of small value, either owing to themountainous nature of the valleys in which they outcrop, or to theirinaccessibility from the sea. Japan is not lacking in good supplies ofcoal. A colliery is worked by the government on the island of Takasima, near Nagasaki, for the supply of coals for the use of the navy. 

The British possession of Labuan, off the island of Borneo, is rich in acoal of tertiary age, remarkable for the quantity of fossil resin which, it contains. Coal is also found in Sumatra, and in the MalayanArchipelago. 

In Cape Colony and Natal the coal-bearing Karoo beds are probably of NewRed age. The coal is reported to be excellent in quantity. 

In Abyssinia lignites are frequently met with in the high lands of theinterior. 

Coal is very extensively developed throughout Australasia. In New SouthWales, coal-measures occur in large detached portions between 29° and 35°S. Latitude. The Newcastle district, at the mouth of the Hunter river, isthe chief seat of the coal trade, and the seams are here found up to 30feet thick. Coal-bearing strata are found at Bowen River, in Queensland, covering an area of 24, 000 square miles, whilst important mines ofCretaceous age are worked at Ipswich, near Brisbane. In New Zealandquantities of lignite, described as a hydrous coal, are found andutilised; also an anhydrous coal which may prove to be either ofCretaceous or Jurassic age. 

We have thus briefly sketched the supplies of coal, so far as they areknown, which are to be found in various countries. But England has oflate years been concerned as to the possible failure of her home suppliesin the not very distant future, and the effects which such failure wouldbe likely to produce on the commercial prosperity of the country. 

Great Britain has long been the centre of the universe in the supply ofthe world's coal, and as a matter of fact, has been for many yearsraising considerably more than one half of the total amount of coalraised throughout the whole world. There is, as we have seen, anabundance of coal elsewhere, which will, in the course of time, competewith her when properly worked, but Britain seems to have early taken thelead in the production of coal, and to have become the great universalcoal distributor. Those who have misgivings as to what will happen whenher coal is exhausted, receive little comfort from the fact that in NorthAmerica, in Prussia, in China and elsewhere, there are tremendoussupplies of coal as yet untouched, although a certain sense of relief isexperienced when that fact becomes generally known. 

If by the time of exhaustion of the home mines Britain is still dependentupon coal for fuel, which, in this age of electricity, scarcely seemsprobable, her trade and commerce will feel with tremendous effect theblow which her prestige will experience when the first vessel, laden withforeign coal, weighs anchor in a British harbour. In the great coallock-out of 1893, when, for the greater part of sixteen weeks scarcely aton of coal reached the surface in some of her principal coal-fields, itwas rumoured, falsely as it appeared, that a collier from America hadindeed reached those shores, and the importance which attached to thesupposed event was shown by the anxious references to it in the publicpress, where the truth or otherwise of the alarm was actively discussed. Should such a thing at any time actually come to pass, it will indeed bea retribution to those who have for years been squandering theirinheritance in many a wasteful manner of coal-consumption. 

Thirty years ago, when so much small coal was wasted and wantonlyconsumed in order to dispose of it in the easiest manner possible at thepitmouths, and when only the best and largest coal was deemed to be ofany value, louder and louder did scientific men speak in protest againstthis great and increasing prodigality. Wild estimates were set on footshowing how that, sooner or later, there would be in Britain no nativesupply of coal at all, and finally a Royal Commission was appointed in1866, to collect evidence and report upon the probable time during whichthe supplies of Great Britain would last. 

This Commission reported in 1871, and the outcome of it was that a periodof twelve hundred and seventy-three years was assigned as the periodduring which the coal would last, at the then-existing rate ofconsumption. The quantity of workable coal within a depth of 4000 feetwas estimated to be 90, 207 millions of tons, or, including that atgreater depths, 146, 480 millions of tons. Since that date, however, therehas been a steady annual increase in the amount of coal consumed, andsubsequent estimates go to show that the supplies cannot last for morethan 250 years, or, taking into consideration a possible decrease inconsumption, 350 years. Most of the coal-mines will, indeed, have beenworked out in less than a hundred years hence, and then, perhaps, thecompetition brought about by the demand for, and the scarcity of, coalfrom the remaining mines, will have resulted in the dreaded importationof coal from abroad. 

In referring to the outcome of the Royal Commission of 1866, although theCommissioners fixed so comparatively short a period as the probableduration of the coal supplies, it is but fair that it should be statedthat other estimates have been made which have materially differed fromtheir estimate. Whereas one estimate more than doubled that of the RoyalCommission, that of Sir William Armstrong in 1863 gave it as 212 years, and Professor Jevons, speaking in 1875 concerning Armstrong's estimate, observed that the annual increase in the amount used, which was allowedfor in the estimate, had so greatly itself increased, that the 212 yearsmust be considerably reduced. 

One can scarcely thoroughly appreciate the enormous quantity of coal thatis brought to the surface annually, and the only wonder is that there areany supplies left at all. The Great Pyramid which is said by Herodotus tohave been twenty years in building, and which took 100, 000 men to build, contains 3, 394, 307 cubic yards of stone. The coal raised in 1892 wouldmake a pyramid which would contain 181, 500, 000 cubic yards, at the lowestimate that one ton could be squeezed into one cubic yard. 

The increase in the quantity of coal which has been raised in succeedingyears can well be seen from the following facts. 

In 1820 there were raised in Great Britain about 20 millions of tons. By1855 this amount had increased to 64-1/2 millions. In 1865 this again hadincreased to 98 millions, whilst twenty years after, viz. , in 1885, thishad increased to no less than 159 millions, such were the giant strideswhich the increase in consumption made. 

In the return for 1892, this amount had farther increased to 181-1/2millions of tons, an advance in eight years of a quantity more than equalto the total raised in 1820, and in 1894 the total reached199-1/2 millions; this was produced by 795, 240 persons, employed in andabout the mines. 

CHAPTER VIII. 

THE COAL-TAR COLOURS. 

In a former chapter some slight reference has been made to thosebye-products of coal-tar which have proved so valuable in the productionof the aniline dyes. It is thought that the subject is of so interestinga nature as to deserve more notice than it was possible to bestow upon itin that place. With abstruse chemical formulae and complex chemicalequations it is proposed to have as little as possible to do, but eventhe most unscientific treatment of the subject must occasionallynecessitate a scientific method of elucidation. 

The dyeing industry has been radically changed during the last halfcentury by the introduction of what are known as the _artificial_ dyes, whilst the _natural_ colouring matters which had previously been the solebasis of the industry, and which had been obtained by very simplechemical methods from some of the constituents of the animal kingdom, orwhich were found in a natural state in the vegetable kingdom, have verylargely given place to those which have been obtained from coal-tar, aproduct of the mineralised vegetation of the carboniferous age. 

The development and discovery of the aniline colouring matters were not, of course, possible until after the extensive adoption ofhouse-gas for illuminating purposes, and even then it was many yearsbefore the waste products from the gas-works came to have an appreciablevalue of their own. This, however, came with the increased utilitarianismof the commerce of the present century, but although aniline was firstdiscovered in 1826 by Unverdorben, in the materials produced by the drydistillation of indigo (Portuguese, _anil_, indigo), it was not untilthirty years afterwards, namely, in 1856, that the discovery of themethod of manufacture of the first aniline dye, mauveine, was announced, the discovery being due to the persistent efforts of Perkin, to whom, together with other chemists working in the same field, is due the greatadvance which has been made in the chemical knowledge of the carbon, hydrogen, and oxygen compounds. Scientists appeared to work along twoplanes; there were those who discovered certain chemical compounds in theresulting products of reactions in the treatment of _existing_vegetation, and there were those who, studying the wonderful constituentsin coal-tar, the product of a _past_ age, immediately set to work to findtherein those compounds which their contemporaries had alreadydiscovered. Generally, too, with signal success. 

The discovery of benzene in 1825 by Faraday was followed in the course ofa few years by its discovery in coal-tar by Hofmann. Toluene, which wasdiscovered in 1837 by Pelletier, was recognised in the fractionaldistillation of crude naphtha by Mansfield in 1848. Although the methodof production of mauveine on a large scale was not accomplished until1856, yet it had been noticed in 1834, the actual year of its recognitionas a constituent of coal-tar, that, when brought into contact withchloride of lime, it gave brilliant colours, but it required aconsiderable cheapening of the process of aniline manufacture before thedyes commenced to enter into competition with the old natural dyes. 

The isolation of aniline from coal-tar is expensive, in consequence ofthe small quantities in which it is there found, but it was discovered byMitscherlich that by acting upon benzene, one of the early distillates ofcoal-tar, for the production of nitro-benzole, a compound was producedfrom which aniline could be obtained in large quantities. There were thustwo methods of obtaining aniline from tar, the experimental and thepractical. 

In producing nitrobenzole (nitrobenzene), chemically represented as(C_{6}H_{5}NO_{2}), the nitric acid used as the reagent with benzene, ismixed with a quantity of sulphuric acid, with the object of absorbingwater which is formed during the reaction, as this would tend to dilutethe efficiency of the nitric acid. The proportions are 100 parts ofpurified benzene, with a mixture of 115 parts of concentrated nitric acid(HNO_{3}) and 160 parts of concentrated sulphuric acid. The mixture isgradually introduced into the large cast-iron cylinder into which thebenzene has been poured. The outside of the cylinder is supplied with anarrangement by which fine jets of water can be made to play upon it inthe early stages of the reaction which follows, and at the end of fromeight to ten hours the contents are allowed to run off into a storagereservoir. Here they arrange themselves into two layers, the top of whichconsists of the nitrobenzene which has been produced, together with somebenzene which is still unacted upon. The mixture is then freed from thelatter by treatment with a current of steam. Nitrobenzene presents itselfas a yellowish oily liquid, with a peculiar taste as of bitter almonds. It was formerly in great demand by perfumers, but its poisonousproperties render it a dangerous substance to deal with. In practice agiven quantity of benzene will yield about 150 per cent of nitrobenzene. Stated chemically, the reaction is shown by the following equation:--

C_{6}H_{6} + HNO_{3} = C_{6}H_{5}NO_{2}, + H_{2}O(Benzene) (Nitric acid) (Nitrobenzene) (Water). 

The water which is thus formed in the process, by the freeing of one ofthe atoms of hydrogen in the benzene, is absorbed by the sulphuric acidpresent, although the latter takes no actual part in the reaction. 

From the nitrobenzene thus obtained, the aniline which is now used soextensively is prepared. The component atoms of a molecule of aniline areshown in the formula C_{6}H_{5}NH_{2}. It is also known as phenylamine oramido-benzole, or commercially as aniline oil. There are various methodsof reducing nitrobenzene for aniline, the object being to replace theoxygen of the former by an equivalent number of atoms of hydrogen. Theprocess generally used is that known as Béchamp's, with slightmodifications. Equal volumes of nitrobenzene and acetic acid, togetherwith a quantity of iron-filings rather in excess of the weight of thenitrobenzene, are placed in a capacious retort. A brisk effervescenceensues, and to moderate the increase of temperature which is caused bythe reaction, it is found necessary to cool the retort. Instead of aceticacid hydrochloric acid has been a good deal used, with, it is said, certain advantageous results. From 60 to 65 per cent. Of aniline on thequantity of nitrobenzene used, is yielded by Béchamp's process. 

Stated in a few words, the above is the process adopted on all hands forthe production of commercial aniline, or aniline oil. The details of thedistillation and rectification of the oil are, however, as varied as theycan well be, no two manufacturers adopting the same process. Many of theaniline dyes depend entirely for their superiority, on the quality of theoil used, and for this reason it is subject to one or more processes ofrectification. This is performed by distilling, the distillates at thevarious temperatures being separately collected. 

When pure, aniline is a colourless oily liquid, but on exposure rapidlyturns brown. It has strong refracting powers and an agreeable aromaticsmell. It is very poisonous when taken internally; its sulphate is, however, sometimes used medicinally. It is by the action upon aniline ofcertain oxidising agents, that the various colouring matters so wellknown as aniline dyes are obtained. 

Commercial aniline oil is not, as we have seen, the purest form ofrectified aniline. The aniline oils of commerce are very variable incharacter, the principal constituents being pure aniline, para- andmeta-toluidine, xylidines, and cumidines. They are best known to thecolour manufacturer in four qualities--

(_a_) Aniline oil for blue and black. 

(_b_) Aniline oil for magenta. 

(_c_) Aniline oil for safranine. 

(_d_) _Liquid toluidine. 

From the first of these, which is almost pure aniline, aniline black isderived, and a number of organic compounds which are further used for theproduction of dyes. The hydrochloride of aniline is important and isknown commercially as "aniline salt. "

The distillation and rectification of aniline oil is practised on asimilar principle to the fractional distillation which we have noticed asbeing used for the distillation of the naphthas. First, light anilineoils pass over, followed by others, and finally by the heavy oils, or"aniline-tailings. " It is a matter of great necessity to those engaged incolour manufacture to apply that quality oil which is best for theproduction of the colour required. This is not always an easy matter, andthere is great divergence of opinion and in practice on these points. 

The so-called aniline colours are not all derived from aniline, suchcolouring matters being in some cases derived from other coal-tarproducts, such as benzene and toluene, phenol, naphthalene, andanthracene, and it is remarkable that although the earlier dyes wereproduced from the lighter and more easily distilled products ofcoal-tar, yet now some of the heaviest and most stubborn of thedistillates are brought under requisition for colouring matters, thosewhich not many years ago were regarded as fit only to be used aslubricants or to be regarded as waste. 

It is scarcely necessary or advisable in a work of this kind to pursuethe many chemical reactions, which, from the various acids and bases, result ultimately in the many shades and gradations of colour which areto be seen in dress and other fabrics. Many of them, beautiful in theextreme, are the outcome of much careful and well-planned study, and toprint here the complicated chemical formulae which show the great changestaking place in compounds of complex molecules, or to mention even thenames of these many-syllabled compounds, would be to destroy the purposeof this little book. The Rosanilines, the Indulines, and Safranines; theOxazines, the Thionines: the Phenol and Azo dyes are all substances whichare of greater interest to the chemical students and to the colourmanufacturer than to the ordinary reader. Many of the names of the basesof various dyes are unknown outside the chemical dyeworks, although eachand all have complicated; reactions of their own. In the reds arerosanilines, toluidine xylidine, &c. ; in the blues--phenyl-rosanilines, diphenylamine, toluidine, aldehyde, &c. ; violets--rosaniline, mauve, phenyl, ethyl, methyl, &c. ; greens--iodine, aniline, leucaniline, chrysotoluidine, aldehyde, toluidine, methyl-anilinine, &c. ; yellows andorange--leucaniline, phenylamine, &c. ; browns--chrysotoluidine, &c. ;blacks--aniline, toluidine, &c. 

To take the rosanilines as an instance of the rest. 

Aniline red, magenta, azaleine, rubine, solferino, fuchsine, chryaline, roseine, erythrobenzine, and others, are colouring matters in this groupwhich are salts of rosaniline, and which are all recognised in commerce. 

The base rosaniline is known chemically by the formula C_{20}H_{l9}N_{3}, and is prepared by heating a mixture of magenta aniline, toluidine, andpseudotoluidine, with arsenic acid and other oxidising agents. It isimportant that water should be used in such quantities as to prevent thesolution of arsenic acid from depositing crystals on cooling. Unlesscarefully crystallised rosaniline will contain a slight proportion of thearseniate, and when articles of clothing are dyed with the salt, it islikely to produce an inflammatory condition of skin, when worn. Someyears ago there was a great outcry against hose and other articles dyedwith aniline dyes, owing to the bad effects which were produced, and thishas no doubt proved very prejudicial to aniline dyes as a whole. 

Again, the base known as mauve, or mauveine, has a composition shown bythe formula C_{27}H_{24}N_{4}. It is produced from the sulphate ofaniline by mixing it with a cold saturated solution of bichromate ofpotash, and allowing the mixture to stand for ten or twelve hours. Ablue-black precipitate is then formed, which, after undergoing a processof purification, is dissolved in alcohol and evaporated to dryness. Ametallic-looking powder is then obtained, which constitutes thisall-important base. Mauve forms with acids a series of well-defined saltsand is capable of expelling ammonia from its combinations. Mauve was thefirst aniline dye which was produced on a large scale, this beingaccomplished by Perkin in 1856. 

The substance known as carbolic acid is so useful a product of a piece ofcoal that a description of the method of its production must necessarilyhave a place here. It is one of the most powerful antiseptic agents withwhich we are acquainted, and has strong anaesthetic qualities. Someuseful dyes are also obtained from it. It is obtained in quantities fromcoal-tar, that portion of the distillate known as the light oils beingits immediate source. The tar oil is mixed with a solution of causticsoda, and the mixture is violently agitated. This results in the causticsoda dissolving out the carbolic acid, whilst the undissolved oilscollect upon the surface, allowing the alkaline solution to be drawn frombeneath. The soda in the solution is then neutralised by the addition ofa suitable quantity of sulphuric acid, and the salt so formed sinks whilethe carbolic acid rises to the surface. 

Purification of the product is afterwards carried out by a process offractional distillation. There are various other methods of preparingcarbolic acid. 

Carbolic acid is known chemically as C_{6}H_{5}(HO). When pure it appearsas colourless needle-like crystals, and is exceedingly poisonous. It hasbeen used with marked success in staying the course of disease, such ascholera and cattle plague. It is of a very volatile nature, and itsefficacy lies in its power of destroying germs as they float in theatmosphere. Modern science tells us that all diseases have their originin certain germs which are everywhere present and which seek only asuitable _nidus_ in which to propagate and flourish. Unlike meredeodorisers which simply remove noxious gases or odours; unlikedisinfectants which prevent the spread of infection, carbolic acidstrikes at the very root and origin of disease by oxidising and consumingthe germs which breed it. So powerful is it that one part in fivethousand parts of flour paste, blood, &c. , will for months preventfermentation and putrefaction, whilst a little of its vapour in theatmosphere will preserve meat, as well as prevent it from becomingfly-blown. Although it has, in certain impure states, a slightlydisagreeable odour, this is never such as to be in any way harmful, whilst on the other hand it is said to act as a tonic to those connectedwith its preparation and use. 

The new artificial colouring matters which are continually being broughtinto the market, testify to the fact that, even with the many beautifultints and hues which have been discovered, finality and perfection havenot yet been reached. A good deal of popular prejudice has arisen againstcertain aniline dyes on account of their inferiority to many of the olddye-stuffs in respect to their fastness, but in recent years themanufacture of many which were under this disadvantage of looseness ofdye, has entirely ceased, whilst others have been introduced which arequite as fast, and sometimes even faster than the natural dyes. 

It is convenient to express the constituents of coal-tar, and thedistillates of those constituents, in the form of a genealogical chart, and thus, by way of conclusion, summarise the results which we havenoticed. 

 COAL. | . ----------+-----------+----+-------------------+--------+----. | | | | | | Water House-gas Coal-tar Ammoniacal Coke | | liquor | . ---------+-------+---------+---------. | Sulphur | | | | | | (sulphurreted First Second Heavy Anthracene Pitch | hydrogen: light light oils (green | sulphurous oils oils (creosote oils) | acid: oil | (crude oils) | | of vitriol) . ----+----. Naphtha) | Anthracene | | | | | | |Ammoniacal Benzene | | Alizarin or | liquor toluene, | | dyer's madder | &c. | | | | | | | | Sulphuric acid=Carbonate of=Hydrochloric | | | ammonia acid | | | (smelling | | | salts) | | | | | Lime=Sulphate of Lime=Chloride of | | | ammonia | ammonia (sal | | | | ammoniac) | | | | | | . ----+----. . ----+----. | | | | | | | | Ammonia Sulphate Ammonia Chloride | | of lime of lime. | | (Plaster of Paris) | | | . --+-----+----------. | | | | | Crude Carbolic Naphthalin | Creosote acid | . --------------+---+--+-------+--------+-----------. | | | | | Benzene=Nitric Acid Toluene Nylene Artificial Burning | turpentine oils Nitrobenzene= } Iron filings oil (solvent (Essence de | } and acetic acid naphtha) mirbane) | | Aniline=Various reagents | Aniline dyes

INDEX. 

A. 

Accidents, causes of mining"Age of _Acrogens_"_Alethopteris_AlizarinAmerican coal-fieldsAmmoniacal liquorAnilineAniline dyesAniline oil, commercialAniline saltAniline "tailings"AnthraceneAnthraciteArtificial turpentine oilAsphaltAustralian coals_Aviculopecten_

B. 

Béchamp's processBenzeneBindBitumen in Trinidad"Blower" aBoghead coalBog-oakBoring diamondsBorrowdale graphite mineBovey Tracey ligniteBritish coal-fieldsBritish North-American coal-measuresBriquettes

C. 

_Calamites_, extinct horsetailsCarbolic acidCarboniferous formation, the_Cardiocarpum_, fossil fruitCarelessness of minersCauses of earth-movementsChanges of levelCharcoal as a disinfectantChemistry of a gas-flameChinese coalsClanny's safety-lampClayton's experiments with gasClay, regularity in deposition ofClub-mosses, great height of fossilCoal-dust, danger fromCoal formed in large lakes or closed seasCoal formation, geological position ofCoal formed by escape of gasesCoal-mine, theCoal not the result of drifted vegetationCoal-period, climate of"Coal-pipes"Coal-plants, classification ofCoal-seam, each, a forest growthCoals of non-carboniferous ageCoal, vegetable origin ofCoke"Cole""Condensers"Cones of _Lepidodendra_Conifers in coal-measuresCurrent-bedding in sandstone

D. 

Davy-lampDangers of benzeneDarwin on the Chonos ArchipelagoDiamonds, how made artificiallyDisintegration of vegetable substancesDisproportion in relative thickness of coal and coal-measures

E. 

Early use of coalEffects of an explosionEncrinital limestone_Equiseta_"Essence de mirbane"European coal-fieldsEvelyn on the use of coalExperiments illustrating fossilisation

F. 

Filling retorts by machineryFiredampFire, mines onFirst light oilsFirst record of an explosionFlashing-point of oilFlooding of pitsFog and smoke_Foraminifera_Fossil fernsFructification on fossil-fernsFurnace, ventilating

G. 

Gas, coalGasholder, theGas, house, constituents of_Glossopteris_Graphite"Green Grease"

H. 

Hannay, of GlasgowHeavy oilsHumboldt's safety-lampHydraulic Main

I. 

Impurities in house-gasIndian coalsInsertion of rootlets of _stigmaria_Insufficiency of modern forest growthsIreland denuded of coal-bedsIron, supplies of

L. 

_Lepidodendra__Lepidostrobi_LigniteLondon lit by gas

M. 

Mammoth treesMarco PoloMarsh gasMedium oilsMetamorphism of coal by igneous agencyMethods of ventilationMountain limestoneMurdock's use of gasMussel beds

N. 

Napthalin_Neuropteris_Newcastle, charters toNitro-benzole

O. 

Objections to use of coalOils from coal and ligniteOil-wells of AmericaOlefiant gas_Orthoceras_

P. 

ParaffinsPeat_Pecopteris_Pennsylvanian anthracitePersian fire-worshippersPitchPlumbago_Polyzoa_Prejudice against aniline dyesProhibitions of the use of coalProportions of explosive mixtures_Psaronius_"Purifiers"Pyrites in coal

Q. 

Quantity of coal raised in Great Britain

R. 

Reptiles of the coal-eraResemblance of American and British coal-_flora_RetortsRoman use of coalRosanilines, theRoyal Commission of 1866

S. 

Sandstone, how formedShales_Sigillaria_South American coalsSpores of _lepidodrendron_Spores, resinous matter inSpores, inflammability ofSteel-mill_Sternbergia__Stigmaria_Subsidence throughout coal-eraSurturbrand at BrightonSussex iron-works

T. 

TarTesting pits by the candleTexas coalToluene, discovery ofTorbanehill mineralTrappers

U. 

UnderclaysUses to which coal is put

V. 

VaselineVegetation of the coal ageVentilation of coal-pits

W. 

"Washers"Waste of fuelWealden ligniteWestphalian coal-field

Y. 

Young's Paraffin Oil

Z. 

Zoroastrians