0\ VEGKTABLE RESPIRATION. 163 



the leaves are the principal agents. But here a striking difference 

 presents itself; the mode of action is not the same by day as, it is 

 during the night. 



In the night, the green parts act precisely as do the coloured parts; 

 they absorb oxygen and disengage carbonic acid. 



In the day, on the contrary, especially under the direct influence 

 of the solar rays, they decompose carbonic acid, fix the carbon in their 

 tissues, and exliale the oxygen. It was Bonnet who first observed 

 this curious phenomenon. He placed a quantity of leaves in a 

 brook of clear water, penetrated by the rays of the sun ; he soon 

 perceived small bubbles of gas principally on the inferior surface of 

 the leaves. Bonnet at first supposed that these were occasioned by 

 air contained in the water; to be assured of it, he placed the leaves in 

 distilled water, which, consequently, was deprived of air, and as he could 

 not then perceive a single bubble of gas, he confinned himself in an 

 erroneous opinion ; and having neglected to analyse this supposed 

 air, he turned aside from one of the finest discoveries in vegetable 

 physiology. Dr. Priestly, some time after, repeated the same 

 experiment, but like a true chemist, did not fail to make an analysis 

 of the gas produced, and found to his astonishment that it was 

 oxygen. The carbonic acid contained in solution in the water had 

 been decomposed; the leaves had appropriated the carbon and 

 exhaled the oxygen. Hence the reason why Bonnet could not 

 obtain this gas in distilled water, because it contained no carbonic 

 acid for the plant to decompose. But this was not all ; he also failed 

 to prove, that under the influence of the solar rays the plant decom- 

 poses the carbonic acid of the atmosphere as it does that which the 

 water holds in solution. Theodore de Saussure placed this fact 

 beyond a doubt, by an experiment admirable for its simplicity and 

 precision. He took twenty-one periwinkles, as nearly alike as 

 possible; seven of these he analysed, and noted the quantity of 

 carbon contained in each. Seven others were placed under a 

 receiver, into which he had introduced seven centimetres of carbonic 

 acid ; the remaining seven were placed under a second receiver in 

 which the air had been deprived of its carbonic acid. These fourteen 

 plants were suffered to vegetate during six days. He then pro- 

 ceeded to analyse the gas contained under the two receivers. In 

 the first receiver the carbonic acid had entirely disappeared, and the 



