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 singuiar 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 ora 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, Théodore 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- 
