104 NATURE AND LIFE. 



Priestley's experiments seem to point out, but that they dis- 

 charge this important duty in a few hours, and in the most 

 thorough way ; that this singular operation is not due at 

 all to vegetation, but to the effect of sunlight ; that it does 

 not begin until the sun has been some time above the hori- 

 zon ; that it ceases entirely during the darkness of night ; 

 that plants shaded by high buildings or by other plants do 

 not complete this function, that is, they do not purify the 

 air, but that, on the contrary, they exhale an injurious at- 

 mosphere, and really shed poison into the air about us ; 

 that the production of pure air begins to diminish with the 

 decline of day, and ceases completely at sunset ; that all 

 plants corrupt the surrounding air during the night ; and 

 that not all portions of the plant take part in the purifica- 

 tion of the air, but only the leaves and green branches." 



How do this transformation of impure air into pure air 

 under the influence of sunlight, and the reverse process 

 during darkness, take place ? Senebier, the countryman 

 and friend of Bonnet, gives us the answer. Applying to 

 the problem the late discoveries of Lavoisier, he showed 

 that the impure air absorbed and decomposed in the day- 

 time by plants is nothing more than the carbonic acid thrown 

 off by a burning candle or a breathing animal, and that the 

 pure air which results from this decomposition is oxygen. 

 He proved besides that the gas released by vegetables dur- 

 ing the night is also carbonic acid, and consequently that 

 the respiration of plants in the night-time is the reverse of 

 that in the daytime. He also demonstrated that heat can- 

 not supply the place of light in these processes. Thus the 

 nature of the phenomenon was explained, but it remained 

 to be learned what relation exists between the volume of 

 carbonic acid absorbed and that of the oxygen released. 

 Another Genevese, Theodore de Saussure, proved that the 

 quantity of oxygen released is less than that of carbonic 

 acid absorbed, and at the same time that a part of the oxy- 



