102 
longed to thirty-six or forty hours, these leaves will 
have taken up a volume of this gas greater by one- 
fourth than their own bulk *. It farther appears, 
that the organic structure of the leaf is necessary to 
the exercise of this office ; for when the leaf is pre- 
viously reduced to a pulp, it does not diminish the 
volume of atmospheric air, but only changes its oxy- 
gen into carbonic acid t ; a fact which accords with 
an observation of Ingenhousz, who remarked, that 
leaves, when beaten to a pulp, did not, like those 
which were, entire, afford oxygen gas in the sun. 
The leaves of various other plants, of different spe- 
cies, were found, by M. de Saussure, to take up a 
quantity of oxygen, equal or superior in bulk to 
themselves, when they were confined in atmospheric 
air and kept in darkness J ; and this gas was again 
expelled from them when they were exposed to the 
sun II. Thin-leaved plants, however, which possess a 
small extent of parenchymatous structure, do not 
at all vary the bulk of their atmosphere, when they 
are placed alternately in sunshine and in the shade ; 
and, if kept entirely in the shade, they diminish both 
the purity and the volume of their atmosphere . 
343. Granting, therefore, that, in the foregoing 
examples, chemical affinity is, to a certain extent, 
exerted between the vegetable and the elastic fluid 
that surrounds it, yet the almost indiscriminate re- 
ception of such fluids would seem to indicate that 
mechanical causes likewise participate in the pro- 
* Saussure Rech. p. 66. t Ibid. p. 74. 
J Recherches, p. 81. || ib. p. S3. Ib. p. 91. 
