AERATION OF THE SAP. 31 



instead of carbonic acid gas, no gas was disen- 

 gaged in the other jar, which contained a sprig 

 of mint. It is evident, therefore, that the oxygen 

 gas obtained from the mint in the first experi- 

 ment was derived from the decomposition, by 

 the leaves of the mint, of the carbonic acid, which 

 the plant had absorbed from the water. 



Solar light is an essential agent in effecting 

 this chemical change ; for it is never found to 

 take place at night, nor while the plant is kept 

 in the dark. The experiments of Sennebier 

 would tend to show that the violet, or most re- 

 frangible of the solar rays have the greatest 

 power in determining this decomposition of car- 

 bonic acid : but the experiments are of so deli- 

 cate a nature, that this result requires to be con- 

 firmed by a more rigid investigation, before it 

 can be admitted as satisfactorily established. 



That the carbon resulting from this decompo- 

 sition of carbonic acid is retained by the plant, 

 has been amply proved by the experiments of 

 M. Theodore de Saussure, who found that this 

 process is attended with a sensible increase in 

 the quantity of carbon which the plant had j^re- 

 viously contained. 



It is in the green substance of the leaves alone 

 that this process is conducted : a j^rocess, which, 

 from the strong analogy that it bears to a similar 

 function in animals, may be considered as the 

 respiration of vegetables. The effect appears to 



