Coal

Coal is the name given to various kinds of carboniferous material of vegetable origin found in the crust of the earth. Vegetable matter - peat, lignite, soft coal, anthracite - forms a series of fuels which show a regular gradation in properties, though the plants which produced peat and lignite were not of the same nature as those that gave rise to true coal. Chemically, the transformation of vegetable remains into coal has consisted in the loss of volatile matter, and the consequent increase in the proportion of carbon. This fact is illustrated by analyses, thus:

	Carbon.	Hydrogen.	Oxygen and Nitrogen.
Wood	49.76	6.14	44.80
Sphagnum, chief plant of peat bogs	49.88	6.54	43.58
Black peat	59.70	5.70	34.60
Lignite from Tenditz, Germany.	57.02	5.94	37.04
Lignite from Sardinia	82.26	6.52	11.22
Non-caking bituminous coal, S. Staffordshire	81.39	5.73	12.87
Bituminous coal, Pennsylvania.	85.73	5.49	8.78
Cannel coal, Wigan	83.58	5.77	10.65
Fossil plants from coal-beds, Commentry, France	82.45	4.75	12.80
Welsh anthracite	92.73	3.37	3.90
Ural anthracite	97.46	0.61	1.93

It is noteworthy that in the course of transformation of vegetable matter into coal the diminution of hydrogen is small compared with that of oxygen and nitrogen. This fact is of great economic importance.

Peat is produced from plants which grow in bogs, especially bog-moss or sphagnum. Below the surface of the bog the vegetable structure gradually gives place to a black, somewhat gelatinous mass of peat, which is cut into blocks, dried, and used as fuel. Heather, grasses, sedges, and trees may give rise to peat; occasionally the remains of trees, such as bog-oak and bog-pine, of an ebony-black colour, are found embedded in peat.

Peat consists essentially of water, inorganic matter, vegetable fibre, and humus. It is produced from vegetable matter, probably through the action of micro-organisms, with gradually increasing pressure of superincumbent material. Thus a bed originally a foot thick may be compressed to three inches; and if peat is buried beneath drift or clay the pressure upon it may be further increased, and conditions set up which would eventually transform peat into a kind of coal.

Lignite, or brown coal, is intermediate in composition between peat and coal. It is rightly called fossil wood, for it consists of the remains of trees and shrubs of the Tertiary epoch, which closely resembled those of to-day. It cannot be regarded as coal in process of formation, since the latter is not derived from wood; and it differs from coal in being easily acted on by nitric acid. It is variable in composition, hardness, and colour. Jet is a very hard variety of lignite, which is probably derived from coniferous wood.

True coal is divided into two classes: bituminous coal and anthracite.

Bituminous coal

Bituminous coal varies considerably in composition, but burns always with a luminous and smoky flame, and yields by destructive distillation in closed retorts hydrogen and volatile hydrocarbons, which compose coal-gas, ammonia, water, and tar or bitumen.

Two varieties of bituminous coal are caking and non-caking coal. Caking coal undergoes incipient fusion during burning, becoming pasty, and yielding a variety of coke which is cellular and dissimilar in form from the coal. Non-caking coal shows no signs of fusion when burnt, and does not yield a true coke.

Cannel ( = candle) coal, also called parrot coal, is a highly bituminous kind of coal which takes fire when held in a flame. It is dull black, and shows no banded structure, but has a conchoidal fracture. It is often rich in nitrogen, which is derived from the remains of fish embedded in the coal when it was being laid down under water.

Anthracite

Anthracite, produced from vegetable matter by loss of the maximum amount of volatile constituents, is hard, black, and metallic-looking. It burns without smoke, gives a maximum heat of combustion, and is used for steam-raising.

Precarboniferous coals, such as anthraxolite, schungite, and graphitoid, contain an even less proportion of volatile matter than anthracite. They are metamorphic coals, but contain their carbon in the amorphous state.