CARBON. 175 



" It is consistent with almost universal observa- 

 tion that the same soil is more productive where 

 organic matter is present than where it is absent ; 

 that if a crop be carried off the field, less organic 

 matter is left in the soil than it contained when it 

 began to grow ; that by constant cropping the soil 

 is gradually exhausted of organic matter. The 

 conclusion, therefore, seems to be reasonable and 

 legitimate, that the vegetation which we remove 

 from a field has not derived all its carbon from the 

 air, but has extracted a portion of it immediately 

 from the soil. It is to supply the supposed loss of 

 carbon, as well as other substances, that the prac- 

 tical farmer applies manure to his land. Supposing 

 it to be established that the whole of the carbon 

 contained in plants has originally been derived from 

 the air, we have only to inquire in what state this 

 element exists in the atmosphere in order to satisfy 

 ourselves as to the form of combination in which it 

 is, and has been received into the circulation of 

 plants. Carbonic acid, a compound of carbon and 

 oxygen, is always present in the atmosphere, though 

 in comparatively small quantities; yet it is every- 

 where to be detected, while no other compound of 

 carbon is to be found there in any appreciable 

 quantity. We must conclude that from this gaseous 

 carbonic acid the whole of the carbon contained in 

 plants has been primarili/ derived. But in what 

 state or form of combination do they absorb carbon 

 from the soil ? 



