THE AIR AND LIFE. 623 



What we know about it is tliat more than a trillion kilograms are 

 contained in the air; that it constitutes nearly half the weight of the 

 minerals of tlie globe and eight-ninths of the weight of water, and that 

 it abounds in the tissues of all living things. At present, however, 

 but one source of oxygen is known, viz, plants, a discovery made by 

 Priestley and explained by Perceval and Senebier. Vegetables have 

 the property, due to their chlorophyll, of decomposing carbonic acid 

 into its elements, carbon, which becomes fixed in their tissues, and 

 oxygen, which, on being freed, diffuses itself through the atmosphere. 

 There are, of course, many chemical reactions which effect a liberation of 

 oxygen, such as the electrolysisof water, the decomposition of chlorate 

 of potassium, or sulphuric acid by heat, but is it from such reactions 

 as these or from others which occur naturally, that this gas is liberated 

 into the atmosphere? We do not know. But the fact being accepted 

 that the composition of the air remains constant, there must be some 

 processes by which the enormous volume of oxygen, absorbed as it is 

 by organic and inorganic combustions at everj^ second in every portion 

 of the globe, is restored sooner or later to the atmosphere. Ts it possible 

 for plants to perform all this chemical labor ? This is a question we ask 

 without being as yet able to answer. Everything seems to indicate 

 however that they do suffice for it. 



Although the proportion of oxygen in the aii* is constant, or practi- 

 cally so, yet it must not be forgotten that certain local conditions tend 

 to increase or diminish its usual ratio. The air loses oxygen in places 

 crowded with living beings, in places where substances oxidize either 

 slowly or rai)idly, as in public halls, for instance, or in mines, and a 

 chemicnl analysis of air quickly shows its condition. Wherever there 

 is consumi)tion of oxygen without sufficient circulation of air the pro- 

 porticm of oxygen is lessened. These local variations, however, do not 

 affect the composition of the whole any more than it is affected by 

 forests, where there is abundant ])roduction of oxygen. 



Let us now proceed to consider nitrogen. This gas was discovered 

 by liutherfoi'd in 1772, and was proved by Lavoisier to enter into the 

 mixture which Ave know under the name of air. It is lighter than air 

 and forms seventy-nine one-hundredths of its bulk. It neither burns 

 nor is it combustible. It does not serve for respiration, nor can it sustain 

 life. It is not toxic, but merely inert, indift'eient, and for respiratory 

 purposes inactive. Our knowledge of its origin is limited. We kjiow 

 that it is produced by certain thermal springs, the sulphurous ones in 

 particular. We know that it is contained in the excretions of animals 

 and that they have absorl)ed it from the air they have breathed. Like 

 oxygen, it seems to be present in air everywhere in the same propor- 

 tions. 



These two elements, oxygen and nitrogen, form the largest and the 

 most essential parts of the air. The elements which we shall now con- 

 sider are only small and variable portions, we might almost say acces- 



