161 



periments differ greatly therefore from those of La- 

 voisier as to the effects produced by the respiration 

 of oxygen on the animal system ; for, while the lat- 

 ter philosopher informs us, that this gas may be re- 

 spired for many days without inconvenience, Mr 

 Davy has shewn that the animal dies long before the 

 whole of it is consumed. Trusting, therefore, to 

 the accuracy of Mr Davy's experiments, as in all 

 respects supported by analogy, we infer, that an ex- 

 cess of oxygen gas in the air that is breathed, is not 

 suited to the due maintenance of the respiratory 

 function : and, on the other hand, the oppressive 

 symptoms which the respiration of impure air occa- 

 sions, as well as the results of Lavoisier's experi- 

 ments (127.X m wn ich nitrogen superabounded, 

 equally instruct us, that a deficiency of this gas is 

 alike unsuited to it. Consequently, we may con- 

 clude, that the atmosphere, as it is naturally com- 

 posed, is best adapted to the economy of the animal 

 system ; but, that this system is, at the same time, 

 so constituted as to be able to bear great variations 

 in the composition of the air without immediate injury 

 to the powers of animal life. 



129. When, however, this variation proceeds to 

 a certain extent, the air is no longer capable of sup- 

 porting vital action ; but different animals, when 

 confined in given volumes of air, possess the power 

 of prolonging this action in very different degrees. 

 Thus we have seen (53. et seq.), that insects, worms, 

 fishes, and the amphibia, live until all the oxygen 

 gas of the air is nearly or entirely consumed ; while 

 birds die in a given quantity of air before they have 

 consumed two-thirds of its oxygen (84.), and a 



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