CPiAP. XII. 
DIGESTION. 
375 
that in which nitrogen, hydrogen, or even oxygen, has been 
dissolved, will produce the result. But if a small quantity of 
carbonic acid is dissolved in the water, the green 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 Water Mint 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 beco«mes 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 of plants are placed in the dark, in a 
receiver full of atmospheric air, we find that the quantity of 
oxygen is perceptibly diminished. From this, and many 
other considerations, 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 neither 
B B 4 
