47 



closed air-tight the little space between the stem 

 and the glass. The second aperture he connected 

 with an aspirator, which drew air gradually through 

 the third opening of the receiver. This aspirator 

 was connected with an apparatus for the accurate 

 determination of the amount of carbonic acid. By 

 these means he proved that the air passing through 

 the receiver lost three-quarters of its carbonic acid ; 

 for whereas, before the air entered the receiver, it 

 contained four volumes of carbonic acid in every ten 

 thousand volumes of air, after having been in contact 

 with the leaves, only one volume of it remained. 



The decomposition and assimilation of carbonic 

 acid seems to be only possible under the influence 

 of light and heat : when the sun shines upon a 

 plant, then the leaves absorb the largest quantity 

 of carbonic acid. This vital process is continued 

 only during the day, while at night and in 

 darkness no such assimilation can take place, 

 but the plant, on the contrary, instead of expel- 

 ling oxygen then, expels carbonic acid. A 

 wrong conclusion may be drawn from this 

 observation ; for it would be incorrect to believe 

 that this exhalation of carbonic acid is the product 

 of any chemical process taking place in the plant, 

 or the result of a slow combustion of the carbon, 

 such as occurs in- the animal system. 



The carbonic acid exhaled by plants during 

 the night is derived from the soil through the 

 medium of the roots ; for the circulation of the 



