148 LECTURES 



decrease, until, in pure shale or slate, tlie whole becomes incom- 

 bustible. 



Now, wood consists also of combustible matter and ash, but the 

 amount of ash in wood is much less than in coal — the wood of elm 

 contains about 2 per cent.; willow, i half per cent.; beech, ^ per cent.; 

 oak and pine about ^ per cent. The leaves and bark of trees, how- 

 ever, contain much more than this. The fully matured leaves of the 

 beech, willow, and elm contain, severallj'', 6.6_, 8, and 11 per cent, of 

 ash. It is probable, then, that 2 to 3 per cent, is a fair average of 

 the per centage of ash in dry vegetable matter. But even if the coal 

 is perfectly pure, that is, formed of vegetable matter without foreign 

 admixture, we should find a higher proportion of ash than in the 

 wood from which it was formed, for, as we have already seen, wood 

 loses hydrogen and oxygen in the process of change into coal. The 

 weight therefore diminishes, but the absolute amount of ash remains 

 the same^ and consequently the relative amount increases. We may 

 safely say, then, that if coal contains not more than 5 per cent, of ash 

 it may be considered quite pure; but if it contains more than 10 per 

 cent, it is probably impure, i. e., mixed with foreign matter. This 

 foreign matter being evidently the mud or clay upon which the carbo- 

 naceous matter was originally laid down or by which it was after- 

 wards covered. Hence we find the purest coal in the largest seams 

 and in the middle portions equally removed from the floor and roof. 

 As we pass towards the roof of a seam the coal passes by imperceptible 

 degrees into black slate, which is, in fact, mud, more or less mixed 

 with carbonaceous matter. 



So much for the varieties of coal depending upon purity or im- 

 purity, upon the relative proportion of earthy, incombustible, inorganic 

 matter, and of combustible organic matter. 



But, aside from the earthy matter, the combustible or organic matter 

 of coal consi^^ts of two proximate elements mechanically mixed, viz: 

 carbon and bitumen; charcoal is nearly pure carbon; common tar or 

 pitch is very similar both in chemical composition and in general ap- 

 pearance to bitumen. If, then, we conceive a piece of charcoal, care- 

 fully burnt so that the vegetable structure is perfectly retained, to be 

 thoroughly impregnated with pitch or tar, we should have a substance 

 extremely similar to common coal. These two ingredients of coal 

 may also be easily separated from one another. This is constantly 

 done in the process of coking and in the manufacture of illuminating 

 gas. The more volatile bitumen is driven olF in the form of gas or 

 collects in the pipes as coal tar and the carbon remains as coke. 

 Now, the relative proportion of these two ingredients also vary infi- 

 nitely in different coals. We may have a coal of pure carbon, or a 

 coal of pure bitumen, or a coal containing these two in every propor- 

 tion. It is the relative proportion of these which give rise to the 

 principal varieties of coal. A coal of pure carbon is called anthracite ; 

 with a small amount of bitumen, say 10 to 20 per cent., it is called 

 dry coals or semi-bituminous coal; when there is 20 to 30 per cent, of 

 bitumen it is called bituminous or coking coal ; when the per centage 

 is above this and the coal burns with a strong blaze and melts, it is 

 called fat coals. Besides these there are certain varieties depending 



