MR. JOHN LEIGH ON THE FORMATION OF COAL. 251 



oxidation, have for ages been undergoing a slow process of 

 distillation, by which a portion of their more volatile con- 

 stituents has been eliminated, the carburetted hydrogen 

 and the carbonic acid escaping at every crevice, forming 

 the fire-damp and choke-damp of the miner. The greater 

 the extent to which this has taken place, the farther 

 the coal is removed, in structure and properties, firom the 

 pristine vegetable matter from which it originated, till, at 

 length, the carbon and ash alone remain, a mass of anthra- 

 cite, which only burns like coke, and gives no gas. The 

 oils which saturate the rocks in which it lies own the same 

 source. 



Thus we see that coal originates in vegetation; that, 

 vitality departed, its elements are left to the play of their 

 own affinities, exalted during ages of entombment by a 

 temperature sufficient to carry off, by gentlest distillation 

 and subsequent infiltration, the more volatile results of the 

 new arrangements of those elements, undisturbed by the 

 more active exercise of atmospheric agency; that coal, 

 therefore, must differ from the original vegetable matter of 

 which it was composed, by the loss of portions of its ingre- 

 dients; that the remaining constituents, then, must exist in 

 a different proportion and different relation to each other 

 to what is found in fresh vegetable matter; and analysis 

 proves this — analysis shows that coal is wood, minus a cer- 

 tain number of atoms of carbonic acid, water, and light 

 carburetted hydrogen. And this gaseous elimination may 

 proceed from the disunion and recombination of the ele- 

 ments of water in the vegetable matter. 



It is in the elimination of oxygen, however, that coal 

 chiefly differs from the original woody matter whence it is 

 derived; for the relative proportion of carbon and hydrogen 

 are not greatly different in coal and in wood. Woody 

 fibre is a compound of 36 atoms of carbon, 22 atoms 

 hydrogen, and 22 atoms oxygen. Cannel coal (some 



