42 OF THE ASSIMILATION OF CARBON. 



of carbonic acid, which immediately supply its 

 place. 



The most exact and most recent experiments of 

 De Saussure, made in every season for a space of 

 three years, have shown, that the air contains on an 

 average 0*000415 of its own volume of carbonic acid 

 gas; so that, allowing for the inaccuracies of the 

 experiments, which must diminish the quantity ob- 

 tained, the proportion of carbonic acid in the atmo- 

 sphere may be regarded as nearly equal to lo^jgth part 

 of its weight. The quantity varies according to the 

 seasons ; but the yearly average remains continually 

 the same. 



We have no reason to believe that this proportion 

 was less in past ages ; and nevertheless, the im- 

 mense masses of carbonic acid which annually flow 

 into the atmosphere from so many sources, ought per- 

 ceptibly to increase its quantity from year to year. 

 But we find that all earlier observers describe its 

 volume as from one-half to ten times greater than 

 that which it has at the present time ; so that we can 

 hence at most conclude that it has diminished. 



It is quite evident that the quantities of carbonic 

 acid and oxygen in the atmosphere, which remain 

 unchanged by lapse of time, must stand in some fixed 

 relation to one another; a cause must exist 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 re- 

 moved 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, carbon 

 exists in the atmosphere only in the form of carbonic 

 acid, and therefore in a state of combination with 

 oxygen. 



