Coal

Coal, a compact and usually brittle black rock, containing from 75 to 85 per cent. of carbon, burning readily with a clear flame, and composed of the compressed and altered remains of former vegetation. As this carbon was obtained from the air by the action of green plants under the influence of sunlight, the energy latent in coal has been correctly described as "bottled-up sunshine." Though the "Better-bed" coal of the Newcastle coal-field is almost wholly made up of the sporangia of club-mosses, no trace of organic structure is usually discernible in coal. This is probably explicable by a fermentative process having taken place in the wood-fibre of the plants whilst naturally macerating in water, nitrogenous matter having thus converted this fibre into a morphous dextrinous or mucilaginous matter, with which the siliceous or other earthy impurities constituting the "ash" are most intimately mingled. Wood fibre seems to have undergone a slow oxidation under water, so that the action of air was excluded, losing about 75 per cent. of its weight. much of its hydrogen and oxygen, and almost all its alkaline constituents.

Coal occurs in beds or seams belonging to various geological formations, though mostly to the upper part of that known in consequence as the Carboniferous (q.v.). The plants concerned in its formation differ according to its geological age: those of the Carboniferous and other Palaeozoic formations were mainly club-mosses (Lycopodinae), horse-tails (Equisetaceae), and ferns. In the Trias and Jurassic, cycads predominate; and in the Brown Coal of the Oligocene of Germany, exogenous angiosperms prevail. Though some local patches of coal of great thickness may be due to the drifting out to sea of rafts of vegetable matter, the seams have usually a bed of fire-clay or shale below them which is often penetrated by roots (Stigmaria), continuous with stems in the coal itself, and demonstrates the seam to have been formed by the accumulation of vegetable matter which grew on the spot, the clay or shale being the ancient soil.

Seams vary in thickness from less than an inch to several feet, those between 3 and 7 feet thick being usually most constant, while very thick seams are really each several seams with thin shaley "partings." The "ten-yard" seam of South Staffordshire is made up of eight such seams. The upper part of the Carboniferous System (q.v.), a great series of sandstones and shales, with occasional beds of fire-clay and coal, known as the Coal Measures, has in most countries been much disturbed by subterranean movement, and thrown into folds of which the horizontal and anticlinal (q.v.) portions have been generally denuded, so as only to leave isolated synclinal areas or basins, and the districts over which these areas are worked are known as coal-fields (q.v.). This folding has produced jointing (q.v.), so that coal commonly breaks up into roughly cubical pieces, having two glossy surfaces, or cleats, along the master-joints, the direction in which the main galleries of a mine are usually carried, two more irregular ends and two powdery surfaces of bedding. Some coal breaks down into dust or slack. Faults (q.v.), known locally as heaves or troubles, i.e. dislocation of the strata, are also common in our coal-measures; and in some cases, as in the South Wales coal-field, folding has caused a gradually increasing intensity of the carbonising process, so that the same seams pass from the condition of coking coal in the east, through that of steam coal, to that of anthracite (q.v.) in the most contorted western part of the field. Gases, especially marsh-gas or fire-damp (CH₄), often occur in considerable quantity in coal, and escaping into the workings, especially when atmospheric pressure is reduced, may, on contact with naked lights, produce serious explosions.

Among the chief varieties of coal are lignites, caking coals, cannel coal, bituminous coal, and anthracite (q.v.). Lignite, ox brown coal, mostly Tertiary in age, contains less than 70 per cent. of carbon, much oxygen and ash. Caking or coking coals are so called from softening when heated, so that when air is excluded most of the carbon remains in a compact mass known as cake. Cannel coal, so called from its flaming readily like a candle, with a low carbon percentage, contains so much "disposable hydrogen," i.e. hydrogen beyond that needed to form water with its oxygen, as to be the best gas-making coal. Bituminous coals are misnamed, since they contain no bitumen; but at a low temperature they are partly fused and form hydrocarbons.

We have, perhaps, no exact existing analogue to the formation of coal, the nearest approach to one being probably the mangrove swamps and cummerbunds of low-lying tropical coasts in which arborescent vegetation spreads out into-sea-water. The whole series of coal-measures was laid down during a period of prolonged but intermittent subsidence, the commencement of each seam marking a pause in, and its upper termination the recommencement of, such sinking.