310 
they may, although individually their action be small, 
produce nevertheless, by their combined operation, 
an effect of considerable magnitude and power. We 
confess, however, that another circumstance, arising 
out of this peculiar structure, offers, in our opinion, a 
better explanation of this supposed fact. It is well 
known, that a strong attraction or adhesion is ex- 
erted between air and the surfaces of all bodies, so 
that from many bodies it is detached with considera- 
ble difficulty. The more, therefore, that we increase 
the extent of surface between the air and the bodies 
with which it is brought into contact, the more do 
we augment the effect of this attractive force. Now 
the surface of the cells of the human lungs, inde- 
pendently of that of the bronchi, is estimated by Dr 
Keill at 21907 square inches (114.), which is ten 
times greater than that of the whole body. If, there- 
fore, we call to mind, that no part of the air in these 
cells can be at a greater distance from their surface 
than half their diameter, or the T ^ part of an inch, 
and consider, also, the great extent over which this 
attraction operates, we offer no apparent violence to 
probability in believing that such a force, acting to 
such an extent, may constrain the elasticity of the air, 
and considerably reduce its volume. 
610. That moistened animal substances exert a 
strong attraction towards gaseous fluids, may be in- 
ferred from certain facts which occur in the re- 
spiration of fishes. There can be little doubt, 
but that the oxygen gas, which is converted into 
carbonic acid by the gills and skins of these ani- 
mals (550.), is attracted by the animal substance, and 
