INTBODTTCTION. 



A great chemist, J. B. Dumas, once said that living organisms are 

 nothing but " condensed air." He thus expressed, in terse terms, the 

 result of the investigations pursued by himself and others into the 

 relations of the atmosphere to living beings. 



My purpose, in the following pages, is to show how closely Dumas's 

 statement agrees with ascertained facts, even more closely than he 

 himself supposed. It is also desirable to briefly instance and illustrate 

 the varied ways in which air influences the general life of the globe. 

 If the ordinary definition of the word were not an impediment to its 

 use in the present case, I would say that I purpose making a general 

 sketch of the " biology " of the atmosphere. More exactly and appro- 

 priately I may use a quite similar term, and say that the subject- 

 matter of this essay is the "natural history" of the air — taking the 

 term in the sense given to it by Geoffroy St. Hilaire — an essay upon 

 the properties of air considered in its relations to living beings, upon 

 its composition, its contents, its origin, its varied modes of action. 



While I shall especially and particularly consider air in its relations 

 to life, I shall also refer briefly to its relations to other subjects, point- 

 ing out those which it would be useful to investigate further in order 

 to increase the scope of our knowledge. 



The study of the atmosphere is truly one of great magnitude; its 

 relations to the remainder of the universe are so varied and impor- 

 tant, the subjects which it suggests are so numerous and take us through 

 so many fields of inquiry, that a comparison suggests itself forcibly — 

 just as the atmosphere surrounds our whole planet and forces itself 

 into the clefts and fissures between its elements and rocks to the depths 

 of the soil, in the same manner does the study of air pertain to all 

 departments of science, to geology as well as astronomy, to physics no 

 less than to chemistry, and to biology in the largest sense of the term. 



While it would be a hazardous enterprise to undertake a complete 

 review of so important a subject, it may prove useful to give a rapid 

 sketch of some features, and that I shall endeavor to do, by showing 

 what air is, physically and chemically considered, what is its origin, 

 what it contains, and of what use it is to life. Doubtless this is but 

 a small part of the subject, but this sketch may contribute to show 

 how vast and varied is that chapter of science that goes under the 

 name of the " study of the atmosphere." 



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