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metamorphosis-these elements leaving the carbon-would more readily assume their simplest and more permanent state of combination. But conditions of temperature and pressure complicate the reaction, with the result that CO2 and hydrocarbons of the marsh-gas series are formed, leaving an accumulation of carbon and mineral matter in the residue.
Now the bulk of woody fibre (the substance of which the vegetable deposits were chiefly composed) is known as cellulose, and the formula of this substance is CH100, and we can represent the complete reaction of the fermentation and decay of this substance by the following equation;
2 (C,H,O) = 5 CO, + 5 CH, + Cg
This theory is supported by the fact already alluded to, that these two gases, CO, and CH4, are found in the coalseams of the present day.
The most complete conversion of woody fibre into carbon is seen in the substance known as graphite or "blacklead," while intermediate stages in the decomposition yield lignite, bituminous coal, and anthracite.
The gradual conversion of woody fibre into peat, coal, and graphite is shown in the following table:1
1 Roscoe and Schorlemmer, "Treatise on Chemistry," vol. i., p. 576.
Now coals may be broadly classed into three principal varieties, viz., lignite, bituminous coal, and anthracite, and these are examples of the material cellulose or woody fibre, in different stages of carbonization or decomposition. “Brown coal or lignite belongs to a different and more recent geological period than coal proper, being found in the tertiary formation. It consists of the remains of trees and shrubs, as ash, poplar, and others which exist at the present time upon the surface of the earth. It possesses a brown colour, and often exhibits a characteristic woody structure. Its specific gravity varies from 1·15 to 1:30. Earthy brown coal is another brown friable material, sometimes forming layers in beds of lignite, but it is not a true coal, inasmuch as a considerable portion of it is soluble in ether and benzine, and often even in alcohol, whereas true coal is nearly if not quite insoluble in these liquids." Lignite contains a larger amount of hydrogen than any other variety of coal, and it is consequently more easy to ignite, and burns with more flame than any of the other varieties.
Common lignite is of interest to the gas manufacturer by reason of its strong resemblance to coal, for which it might easily be mistaken. Anthracite belongs to the oldest coal formations. In its purer form it consists of little else than carbon, water, and mineral matter, and represents the last stage of the coal-forming process. It has a high degree of lustre, but is not suitable for gas-making.
Anthracite is found in large quantities in America, and in this country it is found in the South Wales coal-field. The following is an analysis of Welsh anthracite :
The bituminous coals consist of a large number of varieties, differing considerably from one another in their chemical composition, as also in their products of decomposition by heat. They have the common property of burning with a smoky flame when placed in the fire, and yielding, on distillation, volatile hydrocarbons, tar or bitumen, whence their name is derived. Coal-gas is usually obtained from bituminous coals.
Bituminous coals are generally found in secondary formations; the most marked varieties being caking coal, splint coal, cherry coal, and cannel coal. Of the above varieties, caking coal is the one which is most used as a gas producer. It is found in almost all English coal-fields, but the Northumberland and Durham districts are the most noted producers. This variety of coal is moderately compact, and possesses a lustre which varies from a resinous to a velvety or greyish-black appearance. Its specific gravity is usually about 1.25. It derives its name, “caking coal," from the fact that on the application of heat it melts and cakes (or cokes) together into a pasty mass.
Splint, or hard coal, is that dull coal which is often found mixed with samples of caking coal, and which seems to have the property of preventing the charge from swelling in the retort. Splint coal is found in the coal-measures in the vicinity of Glasgow; its colour varies from black to a brownish-black.
Cherry coal is very similar in appearance to caking coal; it is, however, more brittle, and does not fuse on heating, each piece of coke retaining the shape of the original coal. Cherry coal is found in the upper strata of the Glasgow coal-beds, and also in the Staffordshire and Lancashire district. Cannel coal is also known as " parrot coal," and is found in Scotland, Lancashire, Warwickshire, and North Wales, and is greatly prized for its gas-producing qualities, as the different kinds of cannel yield varying quantities of
gas, ranging from 10,000 to 15,000 cubic feet of gas per ton, of a quality of from 20 to 45 or 50 candles. As a rule the coke from cannel is inferior to that produced by common coals, although this distinction cannot always be sharply drawn. The Wigan cannel, for instance, yields a coke similar to that from "caking coal;" it is therefore called "caking cannel."
"In the term cannel are included many kinds of coal, the extreme members of which offer more differences in
practical results than the other class of coals. Broadly classed under this head are all hard, non-laminated coals having a homogeneous character. When having a flaky fracture and an earthy appearance, it is often called shale.
"For gas-manufacturing purposes all hard coals, homogeneous in appearance, and yielding gas of 20 candlepower, and upwards, are classed as cannels. The richer descriptions are dull and brown; the poorer, or secondary cannels, are generally bright and black; the third class, producing gas of a quality of about 20 candles, are dull and black. True cannels do not cake, but yield on distillation a similar residue (with the exception of some cracks and fissures) to coal. These cracks, however, indicate the laminated structure, which was not visible in the original cannel. (Any sample of coal will, on examination, be found either to consist of laminations of varying thickness and of different kinds of coal, or to be homogeneous in appearance, giving no sign of lamination or stratification.) The ash in cannel coals is greater than that in caking coals, and often shows the laminated structure more clearly than the coke.
Cannel coals as a rule yield more tar, and of a lighter specific gravity, than caking coals.
"Whilst the physical difference between common coal and cannel is well marked, the same cannot be said of the
chemical differences between the various cannels and the caking coals; certain cannels having precisely the same elementary composition as some caking coals. In common cannels, yielding about 10,000 cubic feet of 21 candle gas per ton, the carbon is about 13 times as much as the hydrogen. In the highest members, such as Boghead, or Australian shale, the carbon is from 7.2 to 77 times the hydrogen. The structure of cannel is suggestive of its having been originally in a highly divided form as an organic mud, and it leaves on combustion an ash as bulky as the coal, the richest cannels generally containing most of this earthy admixture" (L. T. Wright).
The following tables give the chemical composition of some of these different varieties of coal: