Page:Popular Science Monthly Volume 12.djvu/436

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in the carbonic acid in the air is observable, no diminution will be observable from vegetation.

It is a universally recognized and incontrovertible fact that the carbonic acid contained in all the vegetable life on earth is derived from the carbonic acid in the air, in water, and the soil. Many conclude, therefore, that the air in a green wood must contain less carbonic acid than that in a city or that of an extensive tract of waste land. But I can assure them that the air in the Sahara, so called, of Munich, formerly called the Dultplatz, contains no more carbonic acid than the neighboring Eschen-grounds. Of this I can give incontestable proof, an argument ad hominem. Dr. Zittel brought me several specimens of air in hermetically-sealed glass tubes, from his travels in the Libyan Desert, from sandy wastes and from oases, on which I could conveniently make experiments at Munich. The amount of carbonic acid does not differ in the least in the air from the barren waste and the greenest oases. The case is just the same with the amount of oxygen in the air. It was formerly thought, when imperfect methods were employed, that perceptible variations could be proved. Thus, for example, the outbreak of cholera in 1831 was attributed to a diminution of oxygen in the air, and here and there experiments were made which seemed to confirm the opinion. The hypothesis did not seem improbable, for it was concluded with certainty that in tropical swamps, which are the home of cholera, the oxygen in the air might have been in course of time diminished by the vast masses of decaying matter. But, since the method of gas analysis has been arranged by Von Bunsen, the amount of oxygen in the air on the summit of Mont Blanc has not been found to differ from that in a city or in the swamps of Bengal. Neither is it greater in forest or sea air than in the air of the desert.

This absence of demonstrable variation, in spite of the production of oxygen by living plants and the absorption of it by the processes of combustion and decay, becomes intelligible when we consider first the mobility, and then the mass of the air encompassing our earth. The weight of this mass is, as the barometer tells us, equal to that of a layer of mercury which would cover the surface of the earth to the depth of 760 millimetres (more than three-quarters of a metre). From the weight of this, several billion kilometres, some idea can be formed of the volume of the air, when we consider that air, even beneath a pressure of 760 millimetres of mercury, is yet 10,395 times lighter than mercury. In masses like these, variations such as those we speak of go for nothing. The amount of carbonic acid and oxygen might perhaps be essentially changed in Paris or Manchester if all organic matter on and in the earth were burning at once.

Even if it is granted, however, in face of these incontrovertible facts, that vegetation exercises no perceptible influence upon the composition of the atmosphere in the open air, many persons will not be