354 HOW CROPS GROW. 
The table shows that the diffusive activity of chlor- 
hydric acid through water is 98 times as great as that of 
caramel, (see p. 73, Exp. 29). In other words, a molecule 
of the acid will travel 98 times as far in a given time as 
the molecule of caramel. 
Osmose,* or Membrane Diffasion.—When two miscible 
liquids or solutions are separated by a porous diaphragm, 
the phenomena of diffusion (which depend upon the mu- 
tual attraction of the molecules of the different liquids or 
dissolved substances), are complicated with those of im- 
bibition or capillarity, and of chemical affinity. The ad- 
hesive or other force which the septum is able to exert 
upon the liquid molecules supervenes upon the mere dif: 
fusive tendency, and the movements may suffer remarka- 
ble modifications. 
If we should separate pure water and a solution of 
common salt by a membrane upon whose substance these 
liquids could exert no action, the diffusion would proceed 
to the same result as were the membrane absent. Mole- 
cules of water would penetrate the membrane on one side 
and molecules of salt on the other, until the liquid should 
become alike on both. Should the water move faster than 
the salt, the volume of the brine would increase, and that 
of the water would correspondingly diminish. Were the 
membrane fixed in its place, a change of level of the liq- 
uids would occur. Graham has observed that common 
salt actually diffuses into water, through a thin membrane 
of ox-bladder deprived of its outer muscular coating, at 
very nearly the same rate as when no membrane is inter- 
posed. 
Dutrochet was the first to study the phenomena of 
membrane diffusion. He took a glass funnel with a long 
and slender neck, tied a piece of bladder over the wide 
opening, inverted it, poured in brine until the funnel was 
¢ From a Greek word meaning impulsion. 
