A HISTORY OF SCIENCE 



erties of these constituents, he would have replied, with 

 equal facility, that these are among the most important 

 elements; that oxygen might almost be said to be the 

 life - giving principle, inasmuch as no air - breathing 

 creature could get along without it for many moments 

 together; and that nitrogen is equally important to 

 the organism, though in a different way, inasmuch as 

 it is not taken up through the lungs. As to the water- 

 vapor, that, of course, is a compound of oxygen and 

 hydrogen, and no one need be told of its importance, 

 as every one knows that water makes up the chief bulk 

 of protoplasm; carbonic-acid gas is also a compound 

 of oxygen, the other element this time being carbon, 

 and it plays a quite different role in the economy of 

 the living organism, inasmuch as it is produced by the 

 breaking down of tissues, and must be constantly ex- 

 haled from the lungs to prevent the poisoning of the 

 organism by its accumulation ; while ammonia, which 

 exists only in infinitesimal quantities in the air, is a 

 compound of nitrogen and hydrogen, introducing, 

 therefore, no new element. 



If one studies somewhat attentively the relation 

 which these elements composing the atmosphere bear 

 to the living organism he cannot fail to be struck with 

 it; and it would seem a safe inductive reasoning from 

 the stand-point of the evolutionist that the constitu- 

 ents of the atmosphere have come to be all-essential to 

 the living organism, precisely because all their com- 

 ponents are universally present. But, on the other 

 hand, if we consider the matter in the light of these 

 researches regarding the new gases, it becomes clear 

 that perhaps the last word has not been said on this 



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