164 



forbidden by its incapacity of uniting with that flui<L 

 We may therefore conclude, that nitrogen gas, when 

 respired, neither suffers any change itself, nor pro- 

 duces any direct operation on the animal system. 



130. The only other gas to which the death of 

 animals, in these circumstances, can be ascribed, is 

 carbonic acid, which, however, when formed by 

 respiration, does not seem destructive to animal life. 

 Dr Goodwyn observes, that when the same air is 

 breathed several times, so as to increase the quan- 

 tity of carbonic acid, its noxious operation is to be 

 attributed not to the presence of this acid, but to 

 the deficiency or absence of oxygen gas * ; and 

 when Spallanzani, by means of an alkaline sub- 

 stance, abstracted this acid as soon as formed by the 

 respiration of birds and quadrupeds, he did not find 

 that they lived longer in a given bulk of air than 

 when it was suffered to remain t In the forego- 

 ing case also, related by Dr Percival, the carbonic 

 acid formed by respiration must have been retained 

 in the cavity, and yet no destructive effect seems to 

 have followed from it. Dr Higgins observes like- 

 wise, that debility, convulsions, and death, follow 

 the successive diminution of the oxygen gas of the 

 air in respiration, long before the whole of that gas 

 is consumed, although the carbonic acid that is ge- 

 nerated be, in the mean time, carefully withdrawn J . 

 Indeed, we might in this, as in former examples, be 



* Connexion of Life with Respiration, p. 66. 



f Memoirs on Respiration, p. 318. 



J Higgius's Minutes of a Society, p. 160. 



