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 a* 

 the molecule of caramel. 



Osmose,* or Membrane Diffusion, 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 

 apon 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. 



