, L 123 ] 
fiate with more propriety than 
that glafs exifts actually in our 
body, becaufe, by the action of 
fire, our body may be changed in 
a conitituent part of that fubftance; 
and that fat exifts in grafs and other 
vegetables, becaufe in the organs of 
an animal feeding upon thefe herbs 
they are partly changed into fat, 
Thus, when we feed upten vegeta- 
bles, we do not in reality take in 
fixed air, exifting as fuch in the fub- 
{tance of that food, and only let loofe 
or extricated in our bowels; but it is 
more probable, that fuch food, un- 
dergoing in our ftomach and intef- 
tines a kind of fermentation, yields 
really fixed air, not extricated, but 
generated bythe act of fermentation. 
As we have feen now, that com- 
mon air is far from being an un- 
alterable fluid, only to be i 
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