301 
the nitrogen gas ; and in their experiments on human 
respiration (571.)? tne deficiency in bulk was so ex- 
tremely small as to preclude all idea that nitrogen is 
necessarily retained in the system, ft otwithstanding, 
also, this small deficiency in the whole bulk of air, yet 
its constituent parts, (estimating the carbonic acid as 
oxygen) maintained their usual proportions, the nitro- 
gen gas amounting always to ^y^, which could hard- 
ly have happened, if any portion of that gas had 
been abstracted from the general mass, and retained 
permanently in the system. In the late experiments, 
also, of M. Berthollet, no deficiency, but rather an 
excess of nitrogen appeared in the air expelled from, 
the lungs *, Lastly, nitrogen gas is not absolutely 
necessary to the exercise of this function ; for we 
have seen (127*) that animals live very well in pure 
oxygen ; and both M. Lavoisier, and Messrs Allen 
and Pepys, found, that hydrogen gas might, also, be 
made to supply its place. These facts, therefore, 
entitle us to repeat, that nitrogen gas exerts no direct 
agency, nor suffers any apparent change, during the 
exercise of the respiratory function. 
597. We, however, before remarked (107.)> that 
Drs Priestley ajid Henderson, Mr Davy and others, 
from finding a smaller volume of nitrogen to be, in 
some cases, expelled, than had been previously taken 
into the lungs, concluded that a portion of this gas 
was absorbed in respiration. This opinion we con- 
tested, both on anatomical and chemical grounds, 
and endeavoured to shew (92.), that the error arose 
* Mem. d'Arcueil, torn. ii. p. 459. 
