EXCRETION 203 



it will be found that great changes have occurred in the con- 

 stitution of the two liquids. At the commencement, 

 although their total tensions were equal, the proportions in 

 which the various salts were distributed in A, and therefore 

 their partial tensions, were very different to their proportions 

 and partial tensions in B. At the end of the experiment 

 each of the several salts is equally divided between A and 

 B, supposing the volume of A to equal that of B. This 

 experiment shows that the molecules of substances in solu- 

 tion are free to move. They behave like gases. Gases diffuse 

 through a membrane until their partial tensions are the same 

 in the two spaces which the membrane separates. The aether 

 in which physicists picture gases as dissolved offers no re- 

 sistance to the migration of their molecules ; neither does the 

 solvent water, for example prevent the movement of salts 

 which are distributed through it. 



One other illustration of the phenomena of osmosis will 

 suffice to give an idea of the laws by which they are governed. 

 In the case just cited the membrane was permeable to all the 

 salts in solution. When the phenomena of osmosis were 

 first investigated, a distinction was drawn between substances 

 which will pass through membranes crystalloids and sub- 

 stances which cannot pass colloids. We have already had 

 occasion to note that, whereas albumin is a colloid which does 

 not diffuse, its hydrate, peptone, is a crystalloid which does. 

 The term " crystalloid " indicates that substances which can 

 be crystallized are diffusible. Substances which are diffusible 

 are therefore allied to those which crystallize. The nature 

 of the membrane used to test diffusibility was not at first 

 taken into account. Now a distinction is drawn between 

 membranes which are permeable to all diffusible substances, 

 and membranes which are permeable to the solvent, but im- 

 permeable to the substances which it dissolves. The latter 

 are termed " hemipermeable." Imagine now that water is 

 separated from a solution of sugar by a membrane which stops 

 sugar, but is permeable to water. Water will pass through 

 the membrane into the solution of sugar. The level of the 

 solution will rise. Pressure will be needed, and a very con- 

 siderable pressure, to prevent its rising to prevent endos- 

 mosis, that is to say. The force needed to resist osmosis is 



