ATTENDING THE FOJRMATION OF COAL. &c. 255 



Whenever free oxygen, as that of the atmosphere, has 

 access to the decomposing matter, it is probable that no 

 hydrogen is removed from the plants, except in union with 

 it, and therefore in the form of water. In the change of 

 wood into wood coal, in which the whole ligneous structure 

 is preserved, this would appear to have been pretty con- 

 stantly the case ; for in one specimen of wood coal analysed 

 in Liebig's laboratory, the composition differed from oak 

 wood merely in the loss of 3 atoms of carbonic acid and 1 

 atom of hydrogen ; and in another, which had undergone 

 farther decomposition, by 4 atoms carbonic acid, 5 atoms 

 water, and 2 atoms hydrogen. It is most probable that in 

 both these cases the hydrogen had been removed by free 

 oxygen, and the carbon by the oxygen of decomposed water. 

 A beautiful illustration of the formation of wood coal, by 

 the united action of water, and a limited supply of air, 

 accelerated by a high temperature, was afforded in the 

 analysis of a piece of wood, which had been long kept in the 

 boiler of a steam-engine, and had acquired the appearance 

 of wood coal. It had exactly the composition of the first 

 of the wood coals spoken of above, viz., carbon 33, hydro- 

 gen 21, oxygen 16, having lost 3 atoms of carbonic acid, 

 and 1 atom hydrogen. When vegetable matter decomposes 

 under an entire exclusion of air, or nearly so, as must take 

 place in deep or still water, or when imbedded in such 

 masses of rock as we find the coal formation to be, the 

 changes, no longer influenced by free oxygen, must vary 

 from those already described; and some products of a 

 different character be eliminated as the result of the decom- 

 position. The carbon, still seeking oxygen from every 

 available source, obtains some from the decomposition of 

 the water in contact with the decaying vegetable matter; 

 some from the vegetable itself, whose relative quantity is 

 continually diminishing; and a portion from the salts 

 originally existing within the decaying mass, and in the 



