Coal
The bituminous coal is a substance of vegetable origin, apparently formed from plants by a slow decaying process, going on without access of air, and influenced by moisture, heat, and pressure. As is the case in all vegetable matter generally, it is composed of carbon and hydrogen, with small proportions of oxygen and nitrogen, together with more or less earthy and saline substances, spoken of commonly as inorganic matter. On being heated in the air it is almost consumed, leaving the inorganic components as ashes; but when heated out of contact with the air, viz., subjected to destructive distillation, the volatile hydrogen is driven off, with some carbon, either as a gas or as a tarry liquid, and the residue is coke, containing carbon only, to some extent contaminated with the impurities originally present in the coal.
Anthracite coal, whilst having been formed similarly to the bituminous, has evidently been subjected to some sort of natural distillation, by which it has been deprived of nearly all the hydrogen, nitrogen, and oxygen of the original wood. To some extent it is coke, formed as it were by natural agencies. The specific gravity of anthracite coal is 1.536, and of bituminous 1.280. One cubic foot of anthracite coal weighs 96 pounds; of bituminous, 80 pounds. The space required to stow away one ton of anthracite coal is 40 cubic feet; for bituminous, 44. See COMBUSTION; FUEL.