314 PHYSIOLOGY. BOOK II. 



that in which nitrogen, hydrogen, or even oxygen, have been 

 dissolved, will produce the result. But if a small quantity of 

 carbonic acid is dissolved in the water, the gi'een parts, stimu- 

 lated by the sun, disengage oxygen. Various ingenious means 

 have been contrived to prove this fact, and to show that the 

 quantity of oxygen given out is proportioned to the quantity 

 of carbonic acid decomposed. One of the prettiest experiments 

 is the following, by De Candolle: — He placed in the same 

 cistern two inverted glasses, of which one (A), as well as the 

 cistern itself, was filled with distilled water, and had a plant 

 of Watermint floating in it; the other glass (B) was filled with 

 carbonic acid. The water of the cistern was protected from 

 the action of the atmosphere by a deep layer of oil. The ap- 

 paratus was exposed to the sun. The carbonic acid in the 

 glass B diminished daily, as was obvious from the water rising 

 in it ; and at the same time there rose to the top of the glass 

 A a quantity of oxygen, sensibly equal to the quantity of car- 

 bonic acid absorbed. During the twelve days that the experi- 

 ment was continued, the Mint plant remained in good health ; 

 while, on the contrary, a similar plant, placed under a glass, 

 filled with distilled water only, had disengaged no oxygen, 

 and exhibited manifest signs of decomposition. The same 

 experiment having been tried, only employing oxygen in the 

 place of carbonic acid, no gas was disengaged in the glass that 

 contained the Mint plant." 



" This is sufficient to show[that the green parts of plants ex- 

 posed to the sun decompose carbonic acid. By others, not 

 less ingenious, it has been ascertained that the carbon which 

 is the result becomes fixed in the plant itself. It has been 

 found that Periwinkles, growing where carbonic acid had ac- 

 cess to them, gained carbon ; while similar plants, in a situ- 

 ation cut off from the access of carbonic acid, not only gained 

 no carbon, but lost a part of what they previously possessed. 



" If the green parts are placed in the dark, in a receiver full 

 of atmospheric air, we find that the quantity of oxygen is per- 

 ceptibly diminished. From this, and many other consider- 

 ations, we are forced to conclude that oxygen is absorbed by 

 plants at night. This gas does not, however, remain in the 

 system of a plant in an elastic state, for neitlier the air-pump 



