86 
not be explained without supposing this acid first to 
enter, in solution, into the substance, of the leaf; 
and that it actually does enter, seems to follow from 
the experiments of M. Senebier. He immersed the 
leaves of sedum in boiled water, and placing them 
under the receiver of an air pump, exhausted the 
vessel till the leaves sank. They were then removed 
into water impregnated with carbonic acid, and, in 
seven minutes, they rose from the bottom of the ves- 
sel, and, when exposed to the sun, afforded oxygen 
gas in great abundance ; which proves, he adds, that 
leaves imbibe this acid gas, and again expel it in 
another state, under exposure to the sun. When, 
on the contrary, the leaves were thoroughly exhaust- 
ed of air, and exposed to the sun in boiled water, 
in which no air existed, they did not furnish an atom 
of air *. 
322. Much attention has been employed to ascer- 
tain the part of the leaf in which the carbonic acid 
is contained and decomposed, and the surface from 
which it afterwards issues. As the gas itself is invi- 
sible, its escape can be detected only by placing the 
leaves in water ; or, if they are retained in air, by 
marking the increase of volume which the air may 
receive. This latter method, however, enables us to 
judge only of the fact, but not of the mode in which 
it is accomplished. From experiments, made by 
immersing leaves in water, Dr Ingenhousz supposed 
the air to issue chiefly from the inferior surface of 
the leaves. From the leaves of some fleshy plants. 
* Phys. Veget. torn. iii. ^2-12, 
