OF THE BRAIN, ETC. 19 



brain has ceased to perform its office ; and I now 

 have the honour of communicating an account of 

 these experiments to the Society. 



It has been shown, by Messrs. Allen andPepys*, 

 first, that every cubic inch of carbonic acid re- 

 quires exactly a cubic inch of oxygen gas for its 

 formation ; secondly, that when respiration is 

 performed by a warm-blooded animal in atmo- 

 spheric air, the nitrogen remains unaltered, and 

 the carbonic acid exactly equals, volume for 

 volume, the oxygen gas which disappears. 



There is therefore reason to believe, that the 

 watery vapour, which escapes with the air in 

 expiration, is not formed from the union of hy- 

 drogen with oxygen in the lungs, but that it is 

 exhaled from the mucous membrane of the mouth 

 and pharynx, resembling the watery exhalation 

 which takes place from the peritonaeum, and 

 other membranous surfaces, when exposed ; and 

 this conclusion appears to be confirmed by the 

 experiments of M. Magendie, lately communi- 

 cated to the National Institute of Paris. 



These circumstances are of importance in the 

 present communication, which they render more 

 simple, as they show, that in order to ascertain 

 the changes produced on the air in respiration, 

 it is only necessary to determine the quantity of 

 carbonic acid given out from the lungs. This 

 becomes an exact measure of the oxygen con- 

 sumed ; and the nitrogen of the air and the watery 



* Phil. Trans. 1807, 1808, 1809. 

 c 2 



