FUNCTION.] DECOMPOSITION OF CARBONIC ACID. 295 



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

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

 riment 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 

 exposed 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 

 access to them, gained carbon; while similar plants, in a 

 situation 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 

 the air-pump nor heat will separate it ; but it appears to in- 

 corporate itself with the tissue, since solar light readily dis- 

 engages it. The inference therefore is, that it is absorbed 

 at night, and combines with the carbon already existing, 

 forming carbonic acid, and that the latter is decomposed by 

 the sun, as has before been shown. 



It has been ascertained from other experiments, that a small 

 quantity of carbonic acid is perpetually evolved by leaves 

 both day and night. Some observations by Burnett, upon this 

 subject, are detailed in the Journal of the Royal Institution, 

 and have led their ingenious author to the opinion, that under 

 the name of respiration two distinct phenomena are con- 

 founded ; and that while respiration, properly so called, which 

 consists in the extrication of carbonic acid, is incessantly in 

 action, digestion, which is indicated by the decomposition of 



