ON THE FOOD OF PLANTS. 115 



" whereas the other pot produced only fifteen ; 

 " the same experiment was made with stock 

 " July flowers, and other plants, with equal 

 " success. The manner in which fixed air acts 

 " in promoting vegetation seems well explained 

 " by Senebier. He first discovered that fresh 

 " leaves exposed to the sun in spring water, or 

 " water slightly impregnated with fixed air, al- 

 " ways produced pure air, as long as this ira- 

 " pregnation lasts; but as soon as it is exhausted, 

 "or if the leaves be placed in water, out of 

 " which this air has been expelled by boiling, 

 " they no longer afford pure air ; from whence 

 " he infers, that fixed air is decomposed, its car- 

 " bonic principle detained by the plant, and its 

 " pure air is expelled ; it appears to me also, 

 " by acting as a stimulant, to help the decomposi- 

 " tion of the wafer." 



Sir Humphry Davy again says, " When a 

 " growing plant, the roots of which are sup- 

 " plied with a proper nourishment, is exposed 

 " in the presence of solar light, to a given 

 " quantity of atmospheric air, containing its due 

 " proportion of carbonic acid, the carbonic 

 " acid, after a certain time, is destroyed, and a 

 " certain quantity of oxygene is formed in its 

 " place. If new quantities of carbonic acid gas 

 " be supplied, the same result occurs, so that 



i 2 



