ON COAL. 149 



upon hardness, fracture, &c., such, for instance, as cannel, wliich is a 

 highly hituminous coal, hut very hard, compact, fine-grained, and 

 remarkably free from vegetable structure ; splint coal, &c. 



There are at least three possible methods of accounting for these 

 varieties. 1st. The cause may have existed before the coal was laid 

 down, in the nature of the wood of which the coals were formed. 

 2d. The cause may be sought for in the changes through which the 

 vegetable matter passed in the process of becoming coal. ^ 3d. We 

 may find it in changes to which the coal was subjected after it became 

 coal. 



First. It is possible that the kind of wood may in some degree_ de- 

 termine the variety of coal, as, for instance, the accumulation of pines 

 and other resinous wood may have given rise to the fat coals, while 

 the non-resinous woods to the drier coals. This, I say, is possible, 

 particularly as we know that coniferous trees grew in considerable 

 abundance during the coal period ; but it seems very improbable as a 

 general explanation. 



Second. We have already remarked that wood consists, chemically, 

 of carbon, hydrogen, oxygen, and a small quantity of nitrogen, which 

 may be neglected ; ancl that pit coal consists of the same chemical 

 elements, only in different proportions, the carbon being in excess. 

 It is obvious, then, that in the fermentation process by which wood 

 is changed into coal a portion of the gases, hydrogen and oxygen, 

 escapes. The amount which thus escapes determines the variety of coal. 



The composition of wood is variously stated by chemists ; in fact it 

 is not a definite compound, but consists of the mixture of several 

 proximate principles. It therefore varies much, according to the rela- 

 tive^ abundance of these principles, such as starch, sugar, cellulose, 

 lignire ; in other words, according to the kind or even the age of the 

 wood. For the harder kinds of wood, such as the oak, Liebeg gives 

 the formula, 0,5 H,2 O.,,. For softer kinds of wood, and par- 

 ticularly for succulent Vegetable substances, the proportion of carbon 

 is not so great. Whether, however, the formula which I have 

 adopted be correct for the plants of the coal, or not, would not affect 

 the general correctness of the reasoning upon which my conclusions 

 are based. The composition of bitumen varies also very much, and 

 for the same reason, viz : that it is composed of several proximate 

 principles variously mixed. It is generally given as C. Hj^g, and 

 a variable but small amount of oxygen, from 2 to 4. The composition 

 of cannel coal is given byRegnault as 0.4 H^^g Oj. 



Wood = C36 B.,, 0,, 



Bitumen.... = Coq H^g 0, 

 Cannel coal = C^^ E.^^ 0^ 



It will be seen that the proportion of carbon is greatest in coal and 

 least in bitumen, but that the most striking difference between these 

 substances and wood is the almost entire want of oxygen. Now, ac- 

 cording to Liebeg, wood in the process of decay in the open air forms 

 carbonic acid (0 O2) and water (II 0,) and the carbonic acid is 

 formed by the union of the carbon with the oxygen of the wood, while 

 the water is formed by the union of the hydrogen of the wood with 



