llie Origin and Relations of the (Jarho)b Minerals. 275 



diicecl from some of them. Carbonic acid, carbonic oxide, 

 niti'ogenous and hydro-carbon gases, water and petroleum, are 

 mentioned above as the substances which escape from wood- 

 tissue during its decomposition. Tliat all these are eliminated 

 in the decay of vegetable and animal structures, is now generally 

 conceded by chemists and geologists, although there is a wide 

 dilference of opinion as to the nature of the process. 



It has been claimed that the evolved products enumerated 

 above are the results of the primary decomposition of organic 

 matter, and never of further changes in the residual products ; 

 i. e., that in the breaking-up of organic tissue, variable quanti- 

 ties of coal, anthracite, petroleum, marsh-gas, etc., are formed, 

 but that these are never derived the one from the other. This 

 oj)inion is, however, certainly erroneous, and the formation of 

 any or all the evolved products may take place throughout the 

 entire progress of the decomposition. Marsh-gas and carbonic 

 acid are seen escaping from the surface of pools where 

 recent vegetable matter is submerged, and they are also elimi- 

 nated in the further decomposition of peat, lignite, coal and. 

 carbonaceous shale. Fire-damp and choke-damp, common names 

 for the gases mentioned above, are produced in large quantities 

 in the mines where Tertiary or Cretaceous lignites, or Carboni- 

 ferous coals or anthracites are mined. It has been said that 

 these gases are simply locked up in the interstices of the carbon- 

 aceous matter, and are liberated in its excavation ; but all who 

 have worked coal-mines know that such accumulations are not 

 sufficient to supply the enormous and continuous flow which 

 comes from all parts of the mass penetrated. We have ample 

 proof, moreover, that coal, when exposed to the air, undergoes 

 a kind of distillation, in which the evolution of carbonic acid and 

 hydro-carbon gases is a necessary and prominent feature. 



The gas-makers know, that if their coal is permitted to lie for 

 months or years after being mined, it suffers serious deteriora- 

 tion, yielding a less and less quantity of illuminating gas with 

 the lapse of time. So coking coals are rendered dry, non- 

 caking, and valueless for this purpose, by long exposure. 



Carburetted hydrogen, olefiant gas, etc., are constant associ- 

 ates of the petroleum of springs or wells, and this escape of gas 

 and oil has been going on in some localities, without a2:)parent 



