Chemical Composition of Wood and Coal.

445

plants, or of such as contain a limited amount of woody matter, we give in addition Baers' analyses of several woody

substances:

We may compare these results with the following table of the composition of various coals. The first three are from the analyses of Mr. H. Taylor, and the remainder were published by Dr. Playfair and the late Sir Henry De la Beche, but made, we presume, by the former gentleman:

Carbon. Hydrogen. Nitrogen. Oxygen. Ash. Sulphur.

Blaydon burn, bitu-} 78.6

minous coal

It will connect these tables if we give some of Baer's analyses of peat and brown coal, or lignite,-substances which occupy an intermediate position between recent vegetable matter and true coal :

If we take the mean of these tables, we obtain the following average results as to the composition of the three forms of vegetable matter, and which may be regarded as respectively representing recent vegetable matter, that of the tertiary age, and ancient coal. We may observe that in all these tables we

have left out the minor decimals, and now omit the sulphur and nitrogen, as unimportant to our inquiry :—

We thus learn that the principal change which has attended the conversion of recent vegetable substances into coal, has been the loss of half their hydrogen, and nearly all their oxygen. We must remember that the excess of carbon in the coal has not resulted from the replacing of the lost oxygen by carbon introduced from some external source; consequently, where we find eighty per cent. of carbon, in the place of forty-seven per cent., that high amount requires to represent it nearly double the amount of oxygen and hydrogen that are given in the first line of the table. The composition of peat and lignite is so obviously intermediate between recent woods and coal, (the peat, as we should expect from its more modern origin, approaching nearer the former, and the tertiary lignite, the latter,) that no doubt can exist respecting the slow and gradual nature of the chemical processes by which these changes have been effected.

The first table which we have given shows how considerable are the variations in the composition of coals. The fact is, the chemical composition, and consequent quality of coal, largely depends upon the nature of its roof,' or rock which immedi ately covers the coal-seam. The Stigmarian clay, or 'floor,' on which the coal rests, is constant in its nature; but the roof varies enormously. The constancy of the former was one of the ancient conditions on which the growth of the vegetable matter depended. The composition of the latter depended on accidental circumstances, having no relation to the origin of the coal, but much to its quality and composition. Sometimes it is a compact shale or clay, through which scarcely any gases could escape. This is the roof which the coal-owner most admires. At others it is a porous sandstone, through which gases would filter as through a sieve. The component elements of the buried vegetable mass have been gradually released from the mutual bonds that held them together. Thus set free, some of them escaped, whilst those which remained found the new and variable combinations to which are due the several qualities of coal.

We have endeavoured to account for the existence of coal and its variable composition; but we have said nothing of the way in which such vast masses, spread over wide areas, were introduced into the interior of the earth. Two hypotheses long divided the geological world on this point.

A small section, led by Brongniart, was in favour of the notion that coal seams had once been vast peat bogs which had sunk under the ocean. Other geologists had become familiar with the vast rafts of drift wood brought down by the waters of the Mississippi, and whieh, after accumulating in the Gulf of Mexico, ultimately became saturated with water, and sank to the bottom. They inferred that the vegetable material of coal hạd been accumulated in a similar manner. The former of these schools appealed to the widely extended areas and the uniform thickness of coal seams as incompatible with an origin so local in its nature, and likely to be so irregular in the amount of vegetable matter accumulated. Their opponents replied by appealing to the fact that the trees and plants found associated with the coal-measures were not of a class seen growing on the peat bogs; that these were usually devoid of all but a scanty herbaceous or half-shrubby vegetation, and could never have furnished the soil in which the Sigillariæ and Lepidodendra would flourish.

Such was the state of the question when the construction of the Manchester and Bolton railway revealed some fine trees belonging to the coal-measures, standing perpendicularly, and spreading out their huge roots above a thin coal seam, just as they had done when living. This discovery, associated with that of Sir William Logan, that the floor of the coal seam was always a fire-clay, filled with Stigmarian roots and rootlets, led Messrs. Binney and Bowman to the conclusion that the Dixon Fold trees, just referred to had grown where they now stood; that they had, in fact, constituted part of a primæval forest; that a similar forest was represented by each coal seam; and that the thickness of the seam depended upon the time that the forest had flourished, undisturbed by any of the vast forces which altered the levels of land and sea.

We have already glanced at the changes which probably took place. Vast regions of land covered with vegetation, apparently presented almost uniformly level surfaces, but little raised above the waters of the ocean. Indeed, it is not improbable that these were vast saline marshes planted in a shallow sea, but little disturbed by tides; that the huge Sigillariæ grew out of the spongy vegetable mass which encompassed their roots, like the Mangroves in the Keys of Florida, or like the river vegetation in the deadly lagoons of Western Africa. Such an origin would explain the myriads of aquatic shells, whether marine or freshwater is as yet uncertain, as well as the remains of fish, that are constantly found associated with the beds of coal. As the plants grew, vegetable mud accumulated about their roots, forming a peaty soil. At length the land sank to a lower level, and became

submerged beneath the ocean. Aqueous currents now flowed over the depressed forest. These were sometimes gentle, bringing with them mud so finely comminuted that, falling upon the vegetation covering the submerged land, it preserved, with the minute accuracy of the electrotype, every detail of surface and outline: whilst doing this, it also sealed up, almost hermetically, the gases set free by the slow decomposition of the subjacent vegetable mass.

There were seasons

But these seas were not always tranquil. of storm and tempest; the currents then brought down coarser materials; the plants were covered with rough grits and sandstones, admired by neither miner nor physiologist, however adapted to the purposes of the builder and the architect. In the course of ages, the land once more reached the surface of the water; the fine mud in which the plants with Stigmarian roots luxuriated, was again deposited; a living vegetation once more resumed its reign, to be buried as its predecessor had been. The processes that we have just described were gone through anew; again and again repeated, until those vast reservoirs of fuel were stored up, destined by the loving providence of God to constitute one of His chiefest blessings to the yet uncreated human race.

Dependent as our vast commercial interests are upon a continued supply of this precious fuel, it becomes a question of no small moment, both to manufacturers and to senators, what the amount of that supply is likely to be in future years. Nothing can be more discordant than the conclusions at which even practical men have arrived on this point. It appears that the average quantity of coal annually raised in Great Britain is eighty millions of tons, whilst the whole world beside only raises about thirty millions. This quantity will certainly increase year by year, rather than become diminished. A curious illustration of the uncertain value of testimony on this point is afforded by the evidence which Mr. Hugh Taylor gave, in 1829, before a select committee of the House of Lords, respecting the duration of the great northern coal-field of Northumberland and Durham. He calculated that the consumption from that field would proceed at the rate of 3,500,000 tons per annum, and that its duration might be safely estimated at more than seventeen centuries; but already, in little more than thirty years, that annual consumption has risen from three and a half to seventeen millions of tons. In 1846, Mr. Greenwell estimated its duration at three hundred and thirty-one years; whilst Mr. Hall, wisely recognising the steady increase in the demand, concludes that the coal-field in question would probably become exhausted in two hundred and fifty-six years.

When the writer of this article first visited Lancashire, twenty-five years ago, the coal raised from the Lancashire and Cheshire fields was estimated at three-and-a-half millions of tons per annum. It is now found to be nearly eleven millions. It is obvious that all previous calculations made on this subject will be vitiated that do not recognise this rapid increase of consumption in the past, as typical of what will take place in the future; and were mining to be carried to no greater depth than is at present customary, the existing coal-fields would soon be worked out. Illustrations of what is likely to take place are already being afforded by the eastern coal-field of Lancashire. Some years ago that field was not only equal to the wants of its own district, but exported largely to other towns. This is no longer the case; the Ashton, Hyde, and Dukinfield district now receives large supplies from Yorkshire. Oldham, in like manner, receives a considerable amount from other quarters to supply the local consumption. The same may be affirmed even to a larger extent of the towns of Rochdale, Heywood, Bury, Bacup, Rawtenstall, and Bolton, which, in addition to the supplies yielded by their own vicinity, draw largely from the valuable coal-fields of Wigan and its neighbourhood. These, especially the three belts near to the town of Wigan, are probably not to be equalled in Great Britain for quality of coal, number of seams, nature of roofs, freedom from water, and general facilities for working. But, notwithstanding this, few persons who knew Wigan twenty years ago, and who know it now, would have imagined that coals could have become so scarce in that short space of time. If the demand for coal increases during the next eighty years at the rate of the last twenty, we believe that the rich Wigan deposits will be exhausted for all practical purposes.

Of course we are fully alive to the fact that there are vast extensions of the coal-measures underlying the Permian, Triassic, and Oolitic rocks of England; and that, were it possible to work all these at unlimited depths, they would supply the world to the end of time. But, unfortunately, no such possibility exists; and in estimating the probable future supply, the main question to be solved is, the depth to which these coal seams can be profitably followed. Several elements enter into this problem; one of the most important being that of temperature. The heat increases as we descend, at the rate of about one degree to every sixty or seventy feet; it is obvious, then, that a depth of two thousand five hundred feet would bring our miners into a tropical temperature, and arrest their working powers; but the impediments springing from this source can be largely counter