ORIGIN. Coal is one of the most important economic minerals, and is of vegetable origin. When vegetable matter accumulates under water it undergoes a slow process of decomposition, gradually giving off its nitrogen, hydrogen, oxy gen. and some carbon, the result of which if car ried far enough is the formation of a mass of car bon. Peat (q.v.), the material so often found underlying swampy tracts in north temperate zones, represents the first stage in the coal-form ing process, and the further stages are obtained by the burial of these vegetable deposits under great loads of sediment, where they become sub jected to pressure. and at times to heat also. This effects a series of changes. especially consoli dation and loss of oxygen, and gives a series of products, whose nature depends on the degree to which the original vegetable matter has been changed. The products are known as lignite, bituminous coal, and anthracite coal; these three These analyses bring out well the general rela tions of the different elements, and the increase in carbon toward the anthracite end of the series; still they give but little information con cerning the commercial value of the coal. The usual custom in making a commercial analysis is to determine the form in which these elements occur—that is, the amount of water, volatile hydrocarbon, fixed carbon, sulphur, and ash. This proximate analysis is also used as the basis of classification of coals. Thus: A proximate analysis like the above is of prac tical value, since it gives us a hetter conception of the coal worth. Thus the freedom of burning increases with the amount of volatile hydro carbons, while the heating power depends on the amount of fixed carbon present. Sulphur is an injurious constituent when the coal is to be used in the manufacture of gas or for metallur gical purposes; while ash is undesirable. since it displaces so much carbon, and if it contains fusible impurities such as iron, lime, or alkalies, it causes clinkering. Moisture retards the heat ing power of the coal until it is driven off. Since the heating power of coal is its most important property, this is often tested by means of an apparatus known as a 'calorimeter.' ( See CALORI 31ETRY. ) The pfinciple of the test depends on the determination of the weight of water which can be converted into steam at 212° F. under atmospheric pressure with one pound of coal. (Sec also HEAT.) In addition to the varieties of coal given above, there may be mentioned semi bituminous coal and cannel coal. The properties
of the different varieties are as follows: Anthracite contains 84 per cent. or more of fixed carbon, and also little ash, sulphur, and moisture. It has great heating power, and burns with a smokeless flame. Owing to its compara-. tive scarcity, it commands a higher price than the bituminous. Anthracite is dense, has a shining lustre, and usually breaks with a smooth con choidal fracture. It is estimated by some geologists that about ten inches of peat is re quired to make one inch of anthracite coal. In the United States anthracite coal is confined chiefly to the eastern edge of the Appalachians in Pennsylvania, where the folding of the rocks has been very intense, and where the coal-seams have been subjected to great pressure. It is also known in Colorado, near Crested Butte, where the bitu minous coals have been locally changed to an thracite by the heat of basalt intrusions. It is mined extensively in England, and large quanti ties are known in China.
Bituminous Coal contains .50 to 75 per cent. of fixed carbon, and 25 to 30 per cent. volatile hydro carbons. It burns with a rather long and smoky flame, and is also much used for steaming pur poses. Many bituminous coals have the prop erty of coking or caking (see CoNE) when hedted to redness. Most of the Carboniferous and many of the Alesozoic coals of the United States are bituminous.
Semi-Bituminous Coal resembles bituminous coal in appearance, but is intermediate between it and anthracite. It contains from 70 to 84 per cent. of fixed carbon, and is considered of su perior value for steaming purposes. This variety is obtained from Pennsylvania, Maryland, and West Virginia.
Cannel Coal is a variety of coal very rich in volatile hydrocarbons, and found sparingly in parts of Kentucky, Ohio, and Indiana. Its chief use is as a gas-enricher, since it yields 8000 to 15,000 cubic feet of gas per ton. Cannel coal is so called because it burns with a bright flame like a candle, and the name parrot coal was given to it in Scotland, from the crackling or chat tering noise it makes while burning. It is very compact in texture and may even have an oily look; certain forms found in England admit of being polished, and ornamental articles have been made from them and sold under the name of jet.
Li-quite, or Broma Coal, is a partially formed coal, containing much moisture and volatile mat ter. It often shows the woody structure of peat and burns very easily, but gives off little heat.