ORIGIN OF COAL AND ITS VARIETIES. 353 



the carbon and hydrocarbon known to us are probably of organic origin. 

 Carbon probably existed at first o:ily as carbonic acid, and has been re- 

 duced from that condition only by organic agency. 



Varieties of Coal. — The varieties of coal depend upon the purity, 

 upon the degree of bituminization, and upon the proportion of fixed 

 and volatile matter. 



Varieties depending upon Purity. — Coal consists partly of organic 

 or combustible and partly of inorganic or incombustible matter. On 

 burning coal, the organic, combustible matter is consumed, and passes 

 away in the form of gas, while the inorganic, incombustible is left as 

 ash. Now, the relative proportion of these may vary to any extent. 

 We may have a coal of only one or two per cent ash. We may have a 

 coal of five, ten, fifteen, twenty per cent ash ; the coal is now becoming 

 poor. We may have a coal of thirty or forty per cent ash ; this is 

 called ~bo7iy coal, or shaly coal ; it is the valueless refuse of the mines. 

 We may have a coal of fifty or sixty per cent ash ; but it now loses 

 the name as well as the ready combustibility of coal, and is called 

 coaly shale. Finally, we may have a coal of seventy, eighty, ninety, 

 ninety-five per cent ash ; and thus it passes, by insensible degrees, 

 through black shale into perfect shale. This passage is often observed 

 in the roof of a coal-seam. 



Now, all vegetable tissue contains incombustible matter, which, on 

 burning, is left as ash. The amount of ash in vegetable matter is on an 

 average about one to two per cent. But as, in the process of change 

 from wood to coal, much of the organic matter is lost (p. 355 et seq.), 

 and the relative amount of ash is thereby increased, we may say that, 

 if a coal contains five per cent or less of ash, it is absolutely pure — 

 i. e., its ash comes wholly from the plants of which it is composed ; 

 but if a coal contains more than ten per cent ash, it is probably impure 

 — i. e., mixed with mud at the time of its accumulation. 



Varieties of Coal depending on the Degree of Bituminization. — 

 The previously-mentioned varieties consist of pure and impure coals ; 

 these consist of perfect and imperfect coals. We find the vegetable 

 matter, accumulated in different geological periods, in different stages 

 of that peculiar change called bituminization. Brown coal and lignite 

 are examples of such imperfect coal. They are always comparatively 

 modern. 



Varieties depending upon the Proportion of Fixed and Volatile 

 Matter. — Coal, even when pure and perfectly bituminized, consists 

 still of many varieties, having different uses, depending upon the pro- 

 portion of fixed and volatile matters. These are the true varieties of 

 coal. 



In pure and perfect coal, then, the combustible matter is part fixed 

 and part volatile. These may be easily separated by heating to red- 

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