296 THE POPULAR SCIENCE MONTHLY. 



not begin until the sun has been some time above the horizon ; 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 atmosphere, 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 surround- 

 ing air during the night, and that not all portions of the plant take 

 part in the purification 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 daytime 

 by plants is nothing more than the carbonic acid thrown off by a burn- 

 ing 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 during 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 car- 

 bonic acid absorbed, and at the same time that a pai*t of the oxygen 

 retained by the plant is replaced by nitrogen thrown off; and supposed 

 that this nitrogen was furnished by the substance of the plant itself. 

 This function of the green portions of vegetables is, moreover, performed 

 with great rapidity and energy. Boussingault, who has made some 

 remarkable experiments on this subject, filled a vessel of water with 

 vine-leaves, placed it in the sun, and sent a current of carbonic acid 

 through it ; on its passing out, he collected nothing but pure oxygen. 

 It is calculated that a leaf of nenuphar gives out in this way during 

 the summer more than 66 gallons of oxygen. 



In 1848 Cloez and Gratiolet contributed new facts. They showed 

 that aquatic plants follow the same course during the day as others, 

 but that at night they are at rest, and give rise to no release of car- 

 bonic acid. They proved the powerful, instantaneous action of solar 

 light on vegetable respiration. If a few leaves of potamogeton or of 

 nay as are put into a gauge full of water saturated with carbonic acid, 

 as soon as the apparatus is placed in the sun, an immense number of 

 light bubbles, of almost pure oxygen, are seen to detach themselves 

 from the surface of the leaves. The shadow of a slight cloud, crossing 

 the sky, suffices to check their disengagement at once, followed by 



