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militates against the notion of its proceeding directly 
from the blood. 
605. Assuming, then, that the nitrogen in ques- 
tion is primarily derived from the air contained in 
the lungs, can we suppose this gas first to enter the 
blood, and to be subsequently expelled from it ? for 
such operations must take place if nitrogen be really 
given off from the blood. Now air can be conceived 
to enter the blood only by the force of mechanical 
pressure, by absorption, or by the exertion of che- 
mical affinity. If it entered by the first mode, the 
structure of the cells and vessels would, probably, be 
irrecoverably injured, and the presence of the air in 
the vessels would speedily destroy (103.) life ; if by 
the second mode, the air gained admission, it would 
be carried, by the absorbents, into the venal circula- 
tion (95.), which few will suppose to happen ; and, 
lastly, it cannot be considered to enter by the exer- 
tion of chemical attraction, not only because the cells 
and vessels are interposed between the blood and air, 
but because no affinity subsists (1 10.) between nitro- 
gen gas and blood. Denying, therefore, the possibi- 
lity of nitrogen gaining admission into the blood Ves- 
sels by any mode that has been hitherto suggested, 
and knowing no way in which this gas can be sup- 
plied by that fluid, without supposing it previously to 
enter into it, we are necessarily obliged to deny also 
the evolution of nitrogen from the blood. 
606. We think, too, that the circumstances re- 
quired to accomplish this supposed evolution of ni- 
trogen, oppose the belief that it ever takes place ; for 
it not only does not occur in the ordinary exercise of 
