GASES FROM ORGANIC MATTER. 83 



of the air, in their complete destruction, and con- 

 version into carbonic acid and water. This is one 

 of the commonest causes of the formation of car- 

 bonic acid in the air, and in the upper strata of 

 the earth, so far down as the atmosphere can pe- 

 netrate. But even where the free entrance 

 of air is hindered or altogether shut out, the 

 remains of plants undergo a gradual decomposition, 

 which goes on more or less quickly according as 

 the temperature is higher or lower : but in this 

 case the changes run a different course. 



The substance of wood is, as you are aware, 

 made up of carbon, hydrogen, and oxygen. A part 

 of the carbon goes off, in combination with a due 

 proportion of the oxygen, as carbonic acid ; another 

 part of it unites with hydrogen to form hydro- 

 carbon gas (marsh-gas). This latter compound, 

 however, is not formed so plentifully as the car- 

 bonic acid, at least at first, and where the stages 

 of decomposition are not hastened by the aid of 

 heat, or by such a fine state of division of the 

 remains, as obtains in marshes. For this reason, 

 during the first period of the changes, the amount 

 of oxygen in the woody mass is lessened, and the 

 proportion of hydrogen with respect to the carbon 

 and oxygen is increased. The various sorts of 

 brown-coal are still in tins first stage of change. 

 They contain less oxygen but more hydrogen than 

 does wood. The so-called Choke-damp, which 



g 2 



