ITS SOURCE, THE ATMOSPHERE. 19 



which prevents the increase of carbonic acid, by 

 removing that which is constantly forming ; and 

 there must be some means of replacing the oxygen, 

 which is removed from the air by the processes of 

 combustion and putrefaction, as well as by the 

 respiration of animals. 



Both these causes are united in the process of 

 vegetable life. 



The facts which we have stated in the preceding 

 pages prove, that the carbon of plants must be de- 

 rived exclusively from the atmosphere. Now, car- 

 bon exists in the atmosphere only in the form of 

 carbonic acid ; and, therefore, in a state of com- 

 bination with oxygen. 



It has been already mentioned likewise, that 

 carbon and the elements of water form the prin- 

 cipal constituents of vegetables ; the quantity of 

 the substances which do not possess this composi- 

 tion being in very small proportion. Now, the 

 relative quantity of oxygen in the whole mass 

 is less than in carbonic acid. It is therefore cer- 

 tain, that plants must possess the power of de- 

 composing carbonic acid, since they appropriate 

 its carbon for their own use. The formation of 

 their principal component substances must ne- 

 cessarily be attended with the separation of the 

 carbon of the carbonic acid from the oxygen, 

 which must be returned to the atmosphere, whilst 

 the carbon enters into combination with water or 

 its elements. The atmosphere must thus receive a 



c2 



