146 PROCESS OF NUTRITION. CHAP. III. 



fined atmospheres evolve carbonic acid gas in the 

 shade, or during the night ; and the vitiated state 

 of their atmospheres after experiment is owing to 

 that evolution. 



Process of But in this alternate inhalation and extrication 

 t!on. a *" of carbonic acid, is any part of it assimilated to the 

 plant? or is the quantity extricated always equal 

 to the quantity inhaled ? From the continual in- 

 crease of the carbon of the vegetating plant, the 

 assimilation of carbonic acid is unquestionable ; 

 and the supply inhaled by the leaves indispensable 

 to vegetation. For if the carbonic acid that is 

 evolved in the night is withdrawn from the artificial 

 atmosphere as soon as it is formed, the leaves 

 wither and the plant dies. Into a receiver con- 

 taining atmospheric air deprived of its carbonic 

 acid, in which a Pea had been made to vegetate, 

 Saussure introduced a small quantity of lime, 

 placing the receiver over lime water, and exposing 

 the apparatus to the sun. On the second day the 

 atmosphere of the plant had diminished in volume. 

 On the third day the lower leaves began to fade ; 

 and on the fifth or sixth day the stem was com- 

 pletely stript of its leaves ; the atmosphere when 

 examined was found to be vitiated, containing 

 only T VV f oxygeue. But there had been an ab- 

 sorption of carbonic acid by the lime, and conse- 

 quently a formation of that acid, the component 

 parts of which could have been derived only 

 from the plant. The elaboration of carbonic acid 



