354 PALAEOZOIC SYSTEM OF ROCKS. 



ness in a retort. By this means the volatile matter is all driven off 

 and may be collected as tar, oil, etc., in condensers, and as permanent 

 gases in gasometers ; and infixed matter is left in the retort as coke. 

 Now, the proportion of these may vary greatly in different coals, and 

 affect the uses to which the coal is applied. For example, when the 

 coal consists wholly of fixed carbon, it is called graphite. This is not 

 usually considered a variety of coal, because it is not readily combusti- 

 ble ; but it is evidently only the last term of the coal series. Its soft, 

 greasy feel, its metallic luster and incombustibility, and its uses for 

 pencils, as a friction-powder, and as a material for crucibles, are well 

 known. 



When the combustible matter of the coal contains ninety to ninety- 

 five per cent fixed carbon, it is called anthracite. This is a hard, brill- 

 iant variety, with conchoid al fracture and high specific gravity. It 

 burns with almost no flame and produces much heat. It is an admi- 

 rable coal for all household purposes, and, with hot blast, may be used 

 in iron-smelting furnaces. 



If the combustible matter contains eighty to eighty-five per cent 

 fixed carbon, and fifteen to twenty per cent volatile matter, it becomes 

 semi-anthracite, or semi-bituminous coal, of various grades. These are 

 free-burning, rapid-burning coals, producing long flame and high tem- 

 perature, because they do not cake and clog. They are admirably 

 adapted for many purposes, but especially for the rapid production of 

 steam, and therefore for locomotive-engines, and hence are often called 

 steam-coals. 



If the volatile combustible matter rises to the proportion of thirty 

 to forty per cent, it becomes full bituminous coals, which always burn 

 with a strong, bright flame, and often cake and form clinkers. This is 

 perhaps the commonest form of coal, and may be regarded as tyjiical 

 coal. 



If the volatile matter approaches or exceeds fifty per cent, then it 

 forms highly -bituminous or fat or fusing coals. This variety is espe- 

 cially adapted to the manufacture of gas and of coke. 



Besides these there are several varieties depending on physical 

 character. Thus cannel or parrot coal is a dense, dry, structureless, 

 lusterless, highly-bituminous variety, which breaks with a conchoidal 

 fracture. There may be also some varieties depending upon the kind 

 of plants of which coal was made, but of this we have no certain evi- 

 dence. 



Origin of these Varieties. — There can be little doubt that these, 

 the true varieties, are produced by slight differences in the nature and 

 degree of chemical change in the process of bituminization. 



It will be seen by the following table, giving approximate formulas, 

 that vegetable matter and coal of various grades have the same general 



