304 
lungs at the beginning of the experiment, or whether 
a portion of oxygen is not actually exchanged for 
nitrogen, when pure oxygen is respired V 
601. The determination of this question, it is just- 
ly observed, will be much influenced by the actual 
capacity of the chest, and the quantity of air which 
it is able to contain. We have already given various 
estimates of this kind, from which we inferred (89,), 
that, after a natural expiration, or the final expiration 
that is made at death, the lungs contain from 109 to 
118 cubic inches of air ; that, after a forced expira- 
tion of air during life, or after that state which suc- 
ceeds to drowning, the quantity of air is much small- 
er ; while, in violent death from hanging, it some- 
times amounts (90.) to 250 or 270 cubic inches. 
From a particular method, employed by Messrs Al- 
len and Pepys, they. make the volume of air in the 
lungs, after complete expiration at death, to occupy 
about 1O8 cubic inches ; but the mean of their ex- 
periments, on the respiration of oxygen, would make 
it equal to 1 83 : we are, therefore, say they, " al- 
most compelled to allow, that, when pure oxygen is 
respired, a portion of nitrogen is given off by the 
blood f." 
602. In subsequent experiments of a similar nature 
on a Guinea pig, that animal was made to breathe, 
for one hour and twelve minutes, without apparent 
inconvenience, 1060 cubic inches of oxygen gas. 
The gas, before respiration, contained 97.40 inches 
* Phil. Trans. 1809, p. 40$. -J- Ibid. p. 412. 
