INTO MINERAL COAL. 363 



The inflammable gases which stream out of clefts in the strata 

 of mineral coal, or in rocks of the coal formations, always con- 

 tain carbonic acid, according to a recent examination by Bischoff, 

 and also carburetted hydrogen, nitrogen, and olefiant gas ; the 

 last of which had not been observed, until its existence in these 

 gases was pointed out by Bischoff. The analysis of Jire-damp, 

 after it had been treated with caustic potash, showed its consti- 

 tuents to be — 



Light carburetted hydrogen 91*3^ 

 Olefiant gas 

 Nitrogen gas 



100-00 10000 100-00 



The evolution of these gases proves that changes are constantly 

 proceeding in the coal. 



It is obvious from this, that a continual removal of oxygen in 

 the form of carbonic acid is effected from layers of wood-coal, 

 in consequence of which the wood must approach gradually to 

 the composition of mineral coal. Hydrogen, on the contrary, is 

 disengaged from the constituents of mineral coal in the form of 

 a compound of hydro-carbon ; a complete removal of all the 

 hydrogen would convert coal into anthracite. 



The formula C 86 H 22 29 , which is given for wood, has been 

 chosen as the empirical expression of the analysis, for the pur- 

 pose of bringing all the transformations which woody fibre is 

 capable of undergoing under one common point of view. 



Now, although the correctness of this formula must be doubted, 

 until we know with certainty the true constitution of woody fibre, 

 this cannot have the smallest influence on the account given of 

 the changes to which wood fibre must necessarily be subjected 

 in order to be converted into wood or mineral coal. The theore- 

 tical expression refers to the absolute quantity, the empirical 

 merely to the relative proportion, in which the elements of a body 

 are united. Whatever form the first may assume, the empirical 

 expression must always remain unchanged. 



