84 gas-jets; mud- volcanoes. 



occurs in the English coal mines, as well as in those 

 of the brown-coal, which is so abundant in some 

 parts of the continent, consists of discharges of 

 carbonic acid with a little marsh-gas, which often 

 pour forth from clefts accidentally opened in 

 mining, and is a proof that this process is going on 

 in the coal-beds, though but slowly. The more the 

 oxygen is withdrawn from the brown-coal, the nearer 

 does the condition of the latter approach to that 

 of common or true coal, which, being of much more 

 ancient origin, generally lies far deeper in the earth. 

 That similar changes have been, and are still, 

 going on in peat-bogs, is certain from the different 

 features of the peat, at different depths in the 

 beds ; though, from their open position at the sur- 

 face, gases, which are given off from the decaying 

 turf, cannot become stored up to add their evidence 

 as to the stages of the process. In common coal 

 there is no, or but very little, oxygen, while it is 

 richer than brown-coal in hydrogen. However car- 

 bonic acid is also still given off from the beds of true 

 coal, under the action of the oxygen of water. And 

 then carburettecl hydrogen (chiefly marsh-gas), the 

 formation of which from the hydrogen of the water 

 is very much favoured in the deeper beds by the 

 influence of high temperature, comes off in greater 

 quantity; so that, indeed, the gases issuing from 

 open clefts may be set on fire, and will then burn as 

 a jet of flame. If a discharge of this inflammable gas 



