CAUSES OF THE MOTION OF JUICES. 393 



The table shows that the diffusive activity of hydro- 

 chloric acid through water is 98 times as great as that of 

 caramel (see p. 66, Exp. 29). In other words, a mole- 

 cule of the acid will travel 98 times as far in a given 

 time as 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 mutual attraction of the molecules of the dif- 

 ferent liquids or dissolved substances) are complicated 

 with those of imbibition or capillarity, and of chemical 

 affinity. The adhesive or other force which the septum 

 is able to exert upon the liquid molecules supervenes 

 upon the mere diffusive tendency, and the movements 

 may suffer remarkable 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 pro- 

 ceed to the same result as were the membrane absent. 

 Molecules 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 liquids 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 interposed. 



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 filled to the neck, and immersed the bladder in a 



* From a Greefc word meaning impulsion. 



