The Story of a Piece of Coal Part 3

Again, older than the true carboniferous age, are the Silurian anthracites of Co. Cavan, and certain Norwegian coals, whilst in New South Wales we are confronted with an a.s.semblage of coal-bearing strata which extend apparently from the Devonian into Mesozoic times.

Still, the age we have considered more closely has an unrivalled right to the t.i.tle, coal appearing there not merely as an occasional bed, but as a marked 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 yet sufficient varieties to show probabilities of the existence of many with which we are unfamiliar. The highest forms yet found, show an advance as compared with those from earlier formations, and exhibit amphibian characteristics intermediate between the two great cla.s.ses of fishes and reptiles. 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, Saarbruck, Bavaria, Pennsylvania, and elsewhere. The _Archegosaurus,_ which we have figured, and the _Anthracosaurus,_ are forms which appear to have existed in great numbers in the swamps and lakes of the age. The fish of the period belong almost entirely to the ancient orders of the ganoids and placoids. Of the ganoids, the great _megalichthys Hibberti_ ranges throughout the whole of the system.

Wonderful acc.u.mulations of fish remains are found at the base of the system, in the bone-bed of the Bristol coal-field, as well as in a similar bed at Armagh. Many fishes were armed with powerful conical teeth, but the majority, like the existing Port Jackson shark, were possessed of ma.s.sive palates, suited in some cases for crushing, and in others for cutting.

[Ill.u.s.tration: FIG. 24.--_Archegosaurus minor_. Coal-measures.]

[Ill.u.s.tration: FIG. 25.--_Psammodus porosus_. Crushing palate of a fish.]

[Ill.u.s.tration: FIG. 26.--_Orthoceras_. Mountain limestone.]

In the mountain limestone we see, of course, the predominance of marine types, encrinital remains forming the greater proportion of the ma.s.s.

There are occasional plant remains which bear evidence of having drifted for some distance from the sh.o.r.e. But next to the _encrinites_, the corals are the most important and persistent. Corals of most beautiful forms and capable of giving polished marble-like sections, are in abundance. _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 six feet long, and _goniat.i.tes_, 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 the coal-shales, but not in the limestones. _Aviculopecten_ is there abundant also. In the mountain limestone the last of the trilobites (_Phillipsia_) is found.

[Ill.u.s.tration: FIG. 27.--_Fenestella retipora_. Mountain limestone.]

[Ill.u.s.tration: FIG. 28.--_Goniat.i.tes_. 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 hollow tree, although the fossil is an extremely rare one. The same may be said of the only two species of land-snail which have been found connected with the coal forests, viz., _pupa vetusta_ and _zonites priscus_, both discovered in the cliffs of Nova Scotia. These are sufficient to demonstrate that the fauna of the period had already reached a high stage of development. In the estuaries of the day, ma.s.ses of a species of freshwater mussel (_anthracosia_) were in existence, and these have left their remains in the shape of extensive beds of sh.e.l.ls. They are familiar to the miner as _mussel-binds_, and are as noticeable a feature of this long ago period, as are the aggregations of mussels on every coast at the present day.

[Ill.u.s.tration: 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 the substance called "carbon" is, and its nature and properties generally, since this it is which forms such a large percentage of all kinds of coal, and which indeed forms the actual basis of it. In the shape of c.o.ke, of course, we have a fairly pure form of carbon, and this being produced, as we shall see presently, by the driving off of the volatile or vaporous const.i.tuents of coal, we are able to perceive by the residue how great a proportion of coal consists of carbon. In fact, the two have almost an identical meaning in the popular mind, and the fact that the great ma.s.ses of strata, in which are contained our princ.i.p.al and most valuable seams of coal, are termed "carboniferous," from the Latin _carbo_, coal, and _fero_, I bear, tends to perpetuate the existence of the idea.

There is always a certain, though slight, quant.i.ty of carbon in the air, and this remains fairly constant in the open country. Small though it may be in proportion to the quant.i.ty of pure air in which it is found, it is yet sufficient to provide the carbon which is necessary to the growth of vegetable life. Just as some of the animals known popularly as the _zoophytes_, which are attached during life to rocks beneath the sea, are fed by means of currents of water which bring their food to them, so the leaves, which inhale carbon-food during the day through their under-surfaces, are provided with it by means of the currents of air which are always circulating around them; and while the fuel is being taken in beneath, the heat and light are being received from above, and the sun supplies the motive power to digestion.

It is a.s.sumed that it is, within the knowledge of all that, for the origin of the various seams and beds of coaly combinations which exist in the earth's crust, we must look to the vegetable world. If, however, we could go so far back in the world's history as the period when our incandescent orb had only just severed connection with a gradually-diminishing sun, we should probably find the carbon there, but locked up in the bonds of chemical affinities with other elements, and existing therewith in a gaseous condition. But, as the solidifying process went on, and as the vegetable world afterwards made its appearance, the carbon became, so to speak, wrenched from its combinations, and being absorbed by trees and plants, finally became deposited amongst the ruins of a former vegetable world, and is now presented to us in the form of coal.

We are able to trace the gradual changes through which the pasty ma.s.s of decaying vegetation pa.s.sed, in consequence of the fact that we have this material locked up in various stages of carbonisation, in the strata beneath our feet. These we propose to deal with individually, in as unscientific and untechnical a manner as possible.

First of all, when a ma.s.s of vegetable matter commences to decay, it soon loses its colour. There is no more noticeable proof of this, than that when vitality is withdrawn from the leaves of autumn, they at once commence to a.s.sume a rusty or an ashen colour. Let the leaves but fall to the ground, and be exposed to the early frosts of October, the damp mists and rains of November, and the rapid change of colour is at once apparent. Trodden under foot, they soon a.s.sume a dirty blackish hue, and even when removed they leave a carbonaceous trace of themselves behind them, where they had rested. Another proof of the rapid acquisition of their coaly hue is noticeable in the spring of the year. When the trees have burst forth and the buds are rapidly opening, the cases in which the buds of such trees as the horse-chestnut have been enclosed will be found cast off, and strewing the path beneath. Moistened by the rains and the damp night-mists, and trodden under foot, these cases a.s.sume a jet black hue, and are to all appearance like coal in the very first stages of formation.

But of course coal is not made up wholly and only of leaves. The branches of trees, twigs of all sizes, and sometimes whole trunks of trees are found, the last often remaining in their upright position, and piercing the strata which have been formed above them. At other times they lie horizontally on the bed of coal, having been thrown down previously to the formation of the shale or sandstone, which now rests upon them. They are often petrified into solid sandstone themselves, whilst leaving a rind of coal where formerly was the bark. Although the trunk of a tree looks 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 after the same manner from the juices of the earth and the nourishment in the atmosphere, they would have a similar chemical composition. One very palpable proof of the carbonaceous character of tree-trunks suggests itself. Take in your hand a few dead twigs or sticks from which the leaves have long since dropped; pull away the dead parts of the ivy which has been creeping over the summer-house; or clasp a gnarled old monster of the forest in your arms, and you will quickly find your hand covered with a black s.m.u.t, which is nothing but the result of the first stage which the living plant has made, in its progress towards its condition as dead coal. But an easy, though rough, chemical proof of the const.i.tuents of wood, can be made by placing a few pieces of wood in a medium-sized test-tube, and holding it over a flame. In a short time a certain quant.i.ty of steam will be driven off, next the gaseous const.i.tuents of wood, and finally nothing will be left but a few pieces of black brittle charcoal. The process is of course the same in a fire-grate, only that here more complete combustion of the wood takes place, owing to its being intimately exposed to the action of the flames. If we adopt the same experiment with some pieces of coal, the action is similar, only that in this case the quant.i.ty of gases given off is not so great, coal containing a greater proportion of carbon than wood, owing to the fact that, during its long burial in the bowels of the earth, it has been acted upon in such a way as to lose a great part of its volatile const.i.tuents.

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

There are certain parts in most countries, and particularly in Ireland, where ma.s.ses of vegetation have undergone a still further stage in metamorphism, namely, in the well-known and famous peat-bogs. Ireland is _par excellence_ the land of bogs, some three millions of acres being said to be covered by them, and they yield an almost inexhaustible supply of peat. One of the peat-bogs near the Shannon is between two and three miles in breadth and no less than fifty in length, whilst its depth varies from 13 feet to as much as 47 feet. Peat-bogs have in no way ceased to be formed, for at their surfaces the peat-moss grows afresh every year; and rushes, horse-tails, and reeds of all descriptions grow and thrive each year upon the ruins of their ancestors. The formation of such acc.u.mulations of decaying vegetation would only be possible where the physical conditions of the country allowed of an abundant rainfall, and depressions in the surface of the land to retain the moisture. Where extensive deforesting operations have taken place, peat-bogs have often been formed, and many of those in existence in Europe undoubtedly owe their formation to that destruction of forests which went on under the sway of the Romans. Natural drainage would soon be obstructed by fallen trees, and the formation of marsh-land would follow; then with the growth of marsh-plants and their successive annual decay, a peaty ma.s.s would collect, which would quickly grow in thickness without let or hindrance.

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

Peat-bogs do not wholly consist of peat, and nothing else. The trunks of such trees as the oak, yew, and fir, are often found mingled with the remains of mosses and reeds, and these often a.s.sume a decidedly coaly aspect. From the famous Bog of Allen in Ireland, pieces of oak, generally known as "bog-oak," which have been buried for generations in peat, have been excavated. These are as black as any coal can well be, and are sufficiently hard to allow of their being used in the manufacture of brooches and other ornamental objects. Another use to which peat of some kinds has been put is in the manufacture of yarn, the result being a material which is said to resemble brown worsted. On digging a ditch to drain a part of a bog in Maine, U.S., in which peat to a depth of twenty feet had acc.u.mulated, a substance similar to cannel coal itself was found. As we shall see presently, cannel coal is one of the earliest stages of true coal, and the discovery proved that under certain conditions as to heat and pressure, which in this case happened to be present, the materials which form peat may also be metamorphosed into true coal.

Darwin, in his well-known "Voyage in the _Beagle_" gives a peculiarly interesting description of the condition of the peat-beds in the Chonos Archipelago, off the Chilian coast, and of their mode of formation. "In these islands," he says, "cryptogamic plants find a most congenial climate, and within the forest the number of species and great abundance of mosses, lichens, and small ferns, is quite extraordinary. In Tierra del Fuego every level piece of land is invariably covered by a thick bed of peat. In the Chonos Archipelago where the nature of the climate more closely approaches that of Tierra del Fuego, every patch of level ground is covered by two species of plants (_Astelia pumila_ and _Donatia megellanica_), which by their joint decay compose a thick bed of elastic peat.

"In Tierra del Fuego, above the region of wood-land, the former of these eminently sociable plants is the chief agent in the production of peat.

Fresh leaves are always succeeding one to the other round the central tap-root; the lower ones soon decay, and in tracing a root downwards in the peat, the leaves, yet holding their places, can be observed pa.s.sing through every stage of decomposition, till the whole becomes blended in one confused ma.s.s. The Astelia is a.s.sisted by a few other plants,--here and there a small creeping Myrtus (_M. nummularia_), with a woody stem like our cranberry and with a sweet berry,--an Empetrum (_E. rubrum_), like our heath,--a rush (_Juncus grandiflorus_), are nearly the only ones that grow on the swampy surface. These plants, though possessing a very close general resemblance to the English species of the same genera, are different. In the more level parts of the country the surface of the peat is broken up into little pools of water, which stand at different heights, and appear as if artificially excavated. Small streams of water, flowing underground, complete the disorganisation of the vegetable matter, and consolidate the whole.

"The climate of the southern part of America appears particularly favourable to the production of peat. In the Falkland Islands almost every kind of plant, even the coa.r.s.e gra.s.s which covers the whole surface of the land, becomes converted into this substance: scarcely any situation checks its growth; some of the beds are as much as twelve feet thick, and the lower part becomes so solid when dry that it will hardly burn. Although every plant lends its aid, yet in most parts the Astelia is the most efficient.

"It is rather a singular circ.u.mstance, as being so very different from what occurs in Europe, that I nowhere saw moss forming by its decay any portion of the peat in South America. With respect to the northern limit at which the climate allows of that peculiar kind of slow decomposition which is necessary for its production, I believe that in Chiloe (lat. 41 to 42), although there is much swampy ground, no well characterised peat occurs; but in the Chonos Islands, three degrees farther southward, we have 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 he had 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, a black peaty soil, so penetrated with roots as to allow of an extremely slow and imperfect combustion."

The next stage in the making of coal is one in which the change has proceeded a long way from the starting-point. _Lignite_ is the name which has been applied to a form of impure coal, which sometimes goes under the name of "brown coal." It is not a true coal, and is a very long way from that final stage to which it must attain ere it takes rank with the most valuable of earth's products. From the very commencement, an action has being going on which has caused the amount of the gaseous const.i.tuents to become less and less, and which has consequently caused the carbon remaining behind to occupy an increasingly large proportion of the whole ma.s.s. So, when we arrive at the lignite stage, we find that a considerable quant.i.ty 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 most lignites there is, as a rule, a comparatively large proportion of sulphur, and in such cases it is rendered useless as a domestic fuel. It has been used as a fuel in various processes of manufacture, and the lignite of the well-known Bovey Tracey beds has been utilised in this way at the neighbouring potteries. As compared with true coal, it is distinguished by the abundance of smoke which it produces and the choking sulphurous fumes which also accompany its combustion, but it is largely used in Germany as a useful source of paraffin and illuminating oils. In Silesia, Saxony, and in the district about Bonn, large quant.i.ties of lignite are mined, and used as fuel. Large stores of lignite are known to exist in the Weald of the south-east of England, and although the mining operations which were carried on at one time at Heathfield, Bexhill, and other places, were failures so far as the actual discovery of true coal was concerned, yet there can be no doubt as to the future value of the lignite in these parts, when England's supplies of coal approach exhaustion, and attention is turned to other directions for the future source 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 tertiary lignite, the seam of _surturbrand_, as it is locally called, being a somewhat extensive deposit.

We have now closely approached to true coal, and the next step which we shall take will be to consider the varieties in which the black mineral itself is found. The princ.i.p.al of these varieties are as follows, against each being placed the average proportion of pure carbon which it contains:--

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, the relative proportions of carbon and volatile products are much the same.

Everybody knows a piece of cannel coal when it is seen, how it appears almost to have been once in a molten condition, and how it breaks with a conchoidal fracture, as opposed to the cleavage of bituminous coal into thin layers; and, most apparent and most noticeable of all, how it does not soil the hands after the manner of ordinary coal. It is at times so dense and compact that it has been fashioned into ornaments, and is capable of receiving a polish like jet. From the large percentage of volatile products which it contains, it is greatly used in gasworks.

Caking coal and the varieties of coal which exist between it and anthracite, are familiar to every householder; the more it approaches the composition of the latter the more difficult it is to get it to burn, but when at last fairly alight it gives out great heat, and what is more important, a less quant.i.ty of volatile const.i.tuents in the shape of gas, smoke, ammonia, ash and sulphurous acid. For this reason it has been proposed to compel consumers to adopt anthracite as _the_ domestic coal by Act of Parliament. Certainly by this means the amount of impurities in the air might be appreciably lessened, but as it would involve the reconstruction of some millions of fire-places, and an increase in price in consequence of the general demand for it, it is not likely that a government would be so rash as to attempt to pa.s.s such a measure; even if pa.s.sed, it would probably soon become as dead and obsolete and impotent as 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 is not a "homely" coal. If we use it alone it will not give us that bright and cheerful blaze which English-speaking people like to obtain from their fires.

It is a significant fact, and one which proves that the various kinds of coal which are found are nothing but stages begotten by different degrees of disentanglement of the contained gases, that where, as in some parts, a ma.s.s of basalt has come into contact with ordinary bituminous coal, the coal has a.s.sumed the character of anthracite, whilst the change has in some instances gone so far as to convert the anthracite into graphite.

The basalt, which is one of the igneous rocks, has been erupted into the coal-seam in a state of fusion, and the heat contained in it has been sufficient to cause the disentanglement of the gases, the extraction of which from the coal brings about the condition of anthracite and graphite.

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 in pa.s.sing, has nothing of lead about it at all, is best known in the shape of that very useful and cosmopolitan article, the black-lead pencil. This is even purer carbon than anthracite, not more than 5 per cent. of ash and other impurities being present. It is well-known by its grey metallic l.u.s.tre; the chemist uses it mixed with fire-clay to make his crucibles; the engineer uses it, finely powdered, to lubricate his machinery; the house-keeper uses it to "black-lead" her stoves to prevent them from rusting. An imperfect graphite is found inside some of the hottest retorts from which gas is distilled, and this is used as the negative element in zinc and carbon electricity-making cells, whilst its use as the electrodes or carbons of the arc-lamp is becoming more and more widely adopted, as installations of electric light become more general.

One great source of true graphite for many years was the famous mine at Borrowdale, in c.u.mberland, but this is now almost exhausted. The vein lay between strata of slate, and was from eight to nine feet thick. As much as 100,000 is said to have been realised from it in one year. Extensive supplies of graphite are found in rocks of the Laurentian age in Canada.

In this formation nothing which can undoubtedly be cla.s.sed as organic has yet been discovered. Life at this early period must have found its home in low and humble forms, and if the _eozoon_ of Dawson, which has been thought to represent the earliest type of life, turns out after all not to be organic, but only a deceptive appearance a.s.sumed by certain of the strata, we at least know that it must have been in similarly humble forms that life, if it existed at all, did then exist. We can scarcely, therefore, expect that the vegetable world had made any great advance in complexity of organism at this time, otherwise the supplies of graphite or plumbago which are found in the formation, would be attributed to dense forest growths, acted upon, after death, in a similar manner to that which awaited the vegetation which, ages after, went to form beds of coal. At present we know of no source of carbon except through the intervention and the chemical action of plants. Like iron, carbon is seldom found on the earth except in combination. If there were no growth of vegetation at this far-away period to give rise to these deposits of graphite, we are compelled to ask ourselves whether, perchance, there did not then exist conditions of which we are not now cognisant on the earth, and which allowed graphite to be formed without a.s.sistance from the vegetable kingdom. At present, however, science is in the dark as to any other process of its formation, and we are left to a.s.sume that the vegetable growth of the time was enormous in quant.i.ty, although there is nothing to show the kind of vegetation, whether humble mosses or tall forest trees, which went to const.i.tute the ma.s.ses of graphite. Geologists will agree that this is no small a.s.sumption to make, since, if true, it may show that there was an abundance of vegetation at a time when animal life was hidden in one or more very obscure forms, one only of which has so far been detected, and whose very ident.i.ty is strongly doubted by nearly all competent judges. At the same time there _may_ have been an abundance of both animal and vegetable life at the time. We must not forget that it is a well-ascertained fact that in later ages, the minute seed-spores of forest trees were in such abundance as to form important seams of coal in the true carboniferous era, the trees which gave birth to them being now cla.s.sed amongst the humble _cryptogams_, the ferns, and club-mosses, &c. The graphite of Laurentian age may not improbably have been caused by deposits of minute portions of similar lowly specimens of vegetable life, and if the _eozoon_ the "dawn-animalcule," does represent the animal life of the time, life whose types were too minute to leave undoubted traces of their existence, both animal life and vegetable life may be looked upon as existing side by side in extremely humble forms, neither as yet having taken an undoubted step forward in advance of the other in respect to complexity of organism.

[Ill.u.s.tration: FIG 30.--_Lepidodendron_. Portion of Sandstone stem after removal of bark of a giant club-moss]

There is but one more form of carbon with which we have to deal in running through the series. We have seen that coal is not the _summum bonum_ of the series. Other transformations take place after the stage of coal 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 carbon into the condition of a diamond cannot be stated. Diamond is the purest form of carbon found in nature. It is a beautiful object, alike from the results of its powers of refraction, as also from the form into which its carbon has been crystallised. How Nature, in her wonderful laboratory, has precipitated the diamond, with its wonderful powers of spectrum a.n.a.lysis, we cannot say with certainty. Certain chemists have, at a great expense, produced crystals which, in every respect, stand the tests of true diamonds; but the process of their production at a great expense has in no way diminished the value of the natural product.

The process by which artificial diamonds have been produced is so interesting, and the subject may prove to be of so great importance, that a 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 considered thoroughly established, that the diamond is nothing but carbon crystallised in nearly a pure state, and many chemists have since been engaged in the hitherto futile endeavour to turn ordinary carbon into the true diamond.

Despretz at one time considered that he had discovered the process, which consisted in his case of submitting a piece of charcoal to the action of an electric battery, having in his mind the similar process of electrolysis, by which water is divided up into the two gases, hydrogen and oxygen. He obtained a microscopic deposit on the poles of the battery, which he p.r.o.nounced to be diamond dust, but which, a long time after, was proved to be nothing but graphite in a crystallised state.

This was, however, certainly a step in the right direction.