Obfervations on inflammable Air. 355 
fame bladder to my month again, I breathed that very 
fame air four times fucceffively. Now there is no 
doubt but that a bird could have breathed it much 
longer: the reafon of which diverfity feems to be the 
following, viz that a fmall bird is in want of a 1'mall 
quantity of air for every time it breathes, whereas a 
man is in want of a much greater quantity ; hence the 
air is rendered more eafiiy noxious, and unfit for refpi- 
ration. From all which it may be concluded, that we are 
in want of a certain quantity of common air neceffary 
for refpiration, and for the fupport of life; and that this 
air, after being infpired, comes out of the lungs lefs fit 
to be breathed a fecon-d time. 
It has been obferyed, that the inflammable air cannot 
be breathed when the lungs are emptied of common air 
as much as poflible ; but that it may be breathed when 
the lungs are in a natural ftate, in which ftate a quantity 
of common air, equal to about forty cubic inches, is 
known to exift in the lungs of an adult perfon. This 
pulmonary air is not infected fo far as to be incapable of 
being breathed various times, and of fupporting ^life. 
After having made a natural expiration I have with force 
expelled from my lungs about thirty inches of air into 
an empty bladder ; and this pulmonary air I have ge- - 
nerally been able to breathe eight times fucceffively, 
but 
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