105 
a bulk that exceeds its own absolute volume. Speak- 
ing then mechanically, no other cause of the first en- 
trance of air into vegetables occurs, but that of presr 
sure : and when we consider the vast force which the at?- 
mosphere exerts, in order to preserve an equilibrium, 
we seem to approach a cause adequate, in a certain 
degree, to the purpose assigned it. If, therefore, 
from variations in temperature, from changes in the 
state of the vegetable, or from chemical agency, a 
partial vacuum should, at any time, be created in the 
plant, air would quickly rush in to restore the equi- 
librium ; and thus, on mechanical principles alone, 
the entrance of air may, to a certain extent, be ex- 
plained. This explanation, however, applies only 
to elastic fluids generally; but does not explain why, 
when atmospheric air is employed, its oxygenous 
portion only should enter the leaf, indicating an 
election, which can be accounted for only on the 
principles of chemical affinity. Hence, then, we 
should say, that the air was brought within the 
sphere of attraction, by the operation of mechanical 
pressure, but that the actual attraction of oxygen 
was determined by the superior chemical affinity 
which the vegetable substance possessed for that gas. 
When, however, oxygen is not present, then nitro- 
gen or hydrogen will enter according to their re- 
spective affinities, or, if they be considered to exert 
none, they will, in a certain degree, be forced in by 
mechanical pressure alone. 
347. After the admission of such fluids, different 
chemical affinities may be exerted towards different 
gases. Thus, oxygen may be converted into car T 
