4 2o THE POPULAR SCIENCE MONTHLY. 



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 con- 

 clude, therefore, that the air in a green wood must contain less car- 

 bonic 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 incon- 

 testable 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 im- 

 perfect methods were employed, that perceptible variations could be 

 proved. Thus, for example, the outbreak of cholera in 1831 was at- 

 tributed to a diminution of oxygen in the air, and here and there ex- 

 periments were made which seemed to confirm the opinion. The 

 hypothesis did not seem improbable, for it was concluded with cer- 

 tainty 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 desei't. 



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 per- 

 haps 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 com- 

 position of the atmosphere in the open air, many persons will not be 



