J03 
Auction of this effect. This is rendered more pro- 
bable, from the circumstance, that the organic ;orm 
of the leaf is an essential condition in the operation ; 
for, if chemical action alone were concerned, there 
does not appear any good reason why affinity should 
not in part produce a diminution in the air, eve:; al- 
though the texture of the leaf were broken down ; 
a circumstance which cannot be supposed entirely to 
suspend or change, though it may be conceived to 
modify, the action of this power. Jt is farther re- 
markable, that thin leaves, although, in chemical 
composition, they resemble thick ones, exert little or 
no effect of this kind, which shews that it is not so 
much the quality, as the form, of the substance, that 
influences the operation. 
344, Since, then, it appears, that, though chemical 
affinity be present, and allowed to operate, it is yet 
unequal to the explanation of the -phenomena in 
question ; let us, in the next place, direct our view 
to the consideration of mechanical causes. Now, 
mechanically considered, gaseous bodies can only be 
conceived to enter plants to the exclusion of some 
other substance. When, therefore, the vessels or 
cells of plants are already filled, no air can be 
supposed to gain admission, but by the displacement 
of a corresponding bulk of the contained fluids. 
Dr Hales, accordingly, found, that although, by 
means of the air-pump, he could produce a free cur- 
rent of air through sticks of considerable size, yet, 
from young and succulent shoots, placed in similar 
circumstances, and which yielded their fluids very 
