SOG 
sort of analogical evidence, on which the greater part 
of our chemical physiology has been made to rest. 
Until, however, some direct evidence be adduced ? 
that healthy blood undergoes such changes as will 
enable it to yield its nitrogen in a gaseous form, we 
may venture to deny, that the nitrogen gas, afforded 
in these experiments, was derived immediately from 
any natural changes occurring in that fluid. 
604. Rejecting, therefore, the supposition, that the 
large quantity of nitrogen gas, obtained in these ex- 
periments, is derived from any spontaneous changes 
or decompositions going on in the blood itself, we 
are necessarily compelled to consider it as afforded 
by the residual air contained in the lungs. In sup- 
port of this opinion, it may be stated, that the pro- 
duction of nitrogen is always greatest in the first ex- 
piration, and that its quantity progressively diminish- 
es, until, towards the close of the experiment, it is 
deduced almost to nothing * ; circumstances which 
seem plainly to shew that nitrogen is no longer ob- 
tained, when all the residual air, previously existing 
in the lungs, is removed. If this nitrogen were fur- 
nished by the blood, independently of the residual 
air, no reason occurs why it should thus diminish 
and cease to appear, as this air is abstracted ; for the 
function of respiration goes on, and the blood, as 
far as depends on itself, cannot be considered less 
fit to supply nitrogen. The fact, also, that no such 
excess of nitrogen is furnished in natural respiration, 
Phil. Trans, 1809, P 409. 
