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56 THE ADVANCE OF PHOTOGRAPHY
It is easy to prove by an experiment that a considerable amount of carbonic acid is contained in the air we exhale. Carbonic acid forms with lime-water an insoluble precipitate, carbonate of lime. If now we blow the exhaled air through a glass tube into perfectly clear lime-water, the latter becomes milky by the formation of carbonate of lime. Hence, by the breathing of human beings and animals, the amount of oxygen in the air is continually being diminished and converted into carbonic acid. The same result is produced on a larger scale by the process of combustion. In this process a combination of wood or coal with oxygen takes place, and the result is again, principally, carbonic acid.
It might be supposed from this fact, that, in the course of time, the amount of oxygen in the air must diminish, while that of carbonic acid would increase. This actually takes place in closed spaces. Leblanc found that, after a lecture in one of the lecture rooms of the Sorbonne at Paris, the air had lost one per cent, of its oxygen.
In the open air no such a diminution of oxygen and increase of carbonic acid gas can be detected, and the reason of this is that the carbonic acid formed by combustion and the exhalations of animals is again decomposed by plants under the influence of light.
Plants absorb the carbonic acid, retaining the carbon and liberating the oxygen ; by which means the oxygen lost by combustion and respiration is made again available .
There was a time when the atmosphere was much richer in carbonic acid gas than now. When the incandescent and fluid masses that once formed our earth gradually solidified, when the aqueous vapours were condensed as seas, the atmosphere contained almost all the carbon of the earth, combined with oxygen, as carbonic acid gas. The air was therefore at that time infinitely richer in carbonic acid than now. When at length the earth had cooled sufficiently for vegetation to be developed,