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 surrounding 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 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 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 cannot 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 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 Cloëz 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 carbonic acid. They proved the powerful, instantaneous action of solar light on vegetable respiration. If a few leaves of potamogeton or of nayas 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