AERATION OF THE SAP. 29 



the water and the atmosphere. The sprig of mint was ex- 

 posed to the light of the sun for twelve days consecutively: 

 at the end of each day the carbonic acid was seen to di- 

 minish in quantity, the water rising in the jar to supply the 

 place of what was lost, and at the same time the plant ex- 

 haled a quantity of oxgyen, exactly equal to that of the carbo- 

 nic acid which had disappeared. A similar sprig of mint, 

 placed in ajar of the same size,full of distilled water,but with- 

 out having access to carbonic acid, gave out no oxygen gas, 

 and soon perished. When, in another experiment, conducted 

 by means of the same apparatus as was used in the first, ox- 

 ygen gas was substituted in the first jar instead of carbonic 

 acid gas, no gas was disengaged 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 experiment 

 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 deter- 

 mining this decomposition of carbonic acid: but the experi- 

 ments are of so delicate a nature, that this result requires to 

 be confirmed by a more rigid investigation, before it can be 

 admitted as satisfactorily established. 



That the carbon resulting from this decomposition of car- 

 bonic 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 previously con- 

 tained. 



It is in the screen substance of the leaves alone that this 

 process is conducted: a process, which, from the strong ana- 

 logy that it bears to a similar function in animals, may be 

 considered as the respiration of vegetables. The cfiect ap- 



