84 BIOLOGY OF PLANTS. 



lime of the atmosphere is carbonic acid. The quantity varies 

 according to the season, but the yearly average remains the 

 same. 



The existence oY this acid in the atmosphere is easily ac- 

 counted for, by the changes which are taking place on the 

 surface of the earth. 



1. Large quantities of carbonic acid are locked up in the 

 rocks, especially in combination with lime, forming carbonate 

 of lime, from which it is constantly liberated by chemical 

 changes. By this means, also, many springs constantly emit 

 it, and often large tracts of land throw it off from all parts of 

 their surface. 



2. In the process of combustion this acid is always formed, 

 and the quantity which is thus emitted into the atmosphere, 

 from all the fires in the world, is very great. 



3. The respiration of animals produces it in such quan- 

 tities, that the respiration of men alone would convert all the 

 oxygen of the atmosphere into carbonic acid, in 303,000 years. 

 But the quantity formed by other animals is probably greater 

 than that formed by the human species. 



4. The decay of vegetables is attended by the absorption 

 of oxygen, decomposition of water, and emission of carbonic 

 acid. This must add greatly to the whole amount. 



The quantity of carbonic acid thrown into the atmosphere 

 cannot be determined with perfect accuracy, although we 

 know how much there is in the air at any one time. Bischof 

 has estimated the quantity, evolved from springs and fissures 

 in the ancient volcanic district of Eifel, to be 100,000 tons or 

 about 27,000 tons of carbon annually. Were the same quantity 

 to be sent up from 500 such spots, (fourteen millions of 

 tons,) it would only be equal to that contained in the coal, 

 which is yearly consumed in Great Britain. As all these 

 causes are constantly operating we should suppose that the at- 

 mosphere would become deteriorated in a short time, and that 

 the relative proportions of oxygen and carbonic acid would be 



