302 
from the embarrassment into which the respiratory 
organs were brought by the respiration of an impure 
air, which rendered them incapable of effecting so 
completely, as usual, the expulsion of the air they re- 
ceived, and consequently led to its more abundant re- 
tention in the lungs. 
598. As it was then our fortune to contend a- 
gainst what was called an absorption of nitrogen by 
the blood, so now we have to combat what has been 
named an evolution of this gas from that fluid. In 
most of the experiments related by Messrs Allen 
and Pepys, when the same person was employed to 
breathe, only a small deficiency occurred in the 
whole bulk of air ; but, in one experiment, in which 
a different person operated, there was an increase of 
eleven cubic inches upon the whole bulk of air em- 
ployed. As, however, this air contained the usual 
proportions of oxygen and nitrogen, no particular 
effect could have been exerted upon either of its 
constituent parts ; and the " excess, therefore, of 1 1 
cubic inches," to use the expression of these able che- 
mists, " no doubt arose from the person not having 
been in the habit of exhausting his lungs, so that 
they contained more when he began, than when he 
left off*." 
599. But in some experiments on the respiration 
of oxygen gas, which contained only 2.5 per cent 
nitrogen, Messrs Allen and Pepys were enabled to 
obtain nearly all the residual gas from the lungs, by 
* Phil. Trans. 1808, p. 256. 
