308 
this function, but no excess of nitrogen is ever a& 
forded in other cases, unless its place be supplied by 
an equal or superior bulk of some other gas. Thus, 
when nearly pure oxygen was breathed, a bulk of 
that gas disappeared, equal to that of the nitrogen 
obtained ; and when a mixture of oxygen and hy- 
drogen gases, in the proportion of T \^ of the former to 
TO of the latter, was respired, then, also, a bulk of 
hydrogen disappeared, equal to that of the nitrogen 
obtained ; so that when nearly pure oxygen gas is 
respired, say Messrs Allen and Pepys, a portion of 
it is missing 'at the end of the experiment, and its 
place is supplied by a corresponding quantity of ni- 
trogen ; and the same thing, they add, takes place 
when an animal is made to breathe a mixture of hy- 
drogen and oxygen gases *. 
607. These facts appear to afford evidence, that, 
in this supposed evolution of nitrogen from the blood, 
nothing more than a mechanical substitution of one 
gas for another takes place ; and as in these experi- 
ments oxygen and hydrogen caused the expulsion of 
nitrogen by occupying its place, so we have no doubt 
but that, when atmospheric air was again breathed, 
these gases were in turn expelled, and the lungs re- 
covered their former proportion of nitrogen gas. 
This must, we think, of necessity happen ; for we have 
seen that oxygen suffers no loss of bulk in the lungs 
(567. ), but is only changed into carbonic acid ; and 
Mr Davy long since proved (104.), that hydrogen 
gas, in its respiration, undergoes no apparent alter a- 
Pbil. Trans. 1809, p. 427- 
