DIFFUSION 55 



If a strong (concentrated) solution of a crystalloid 

 is brought into contact with a weak (dilute) solution 

 of the same crystalloid in the same solvent, diffusion 

 proceeds until the solute is equally distributed through 

 the whole of the liquid. This is an expression of 

 the general physical law that all systems tend towards 

 equilibrium. The rate of diffusion varies directly with 

 the difference in concentration of the two solutions, 

 and with the temperature, inversely with the size of 

 the molecule of the solute. If now the two solutions 

 of different concentrations are separated by a colloidal 

 membrane of the nature of a gel, such as vegetable parch- 

 ment, crystalloid solutes will diffuse through the 

 membrane as they would diffuse into a liquid with 

 which they were directly in contact, though less 

 rapidly. The rate of this diffusion through a membrane 

 depends partly on the nature of the membrane, and 

 partly on the size of the molecule of the solute, partly 

 again on the chemical relation between the solute and 

 the membrane. 



The disperse phase of a colloid sol, on the other hand, 

 does not in general pass through a gel membrane at 

 all, or only does so with extreme slowness, and this 

 again depends on the size of the particles, on the nature 

 of the membrane, and on the chemical relations 

 between them. It was this difference which Graham 

 used to distinguish colloids from crystalloids, and he 

 showed that a colloid and a crystalloid in mixed solution 

 could be separated by placing the solution in a parch- 

 ment bag and plunging the bag into water, when the 

 whole of the crystalloid would eventually escape through 

 the membrane, leaving the colloid pure behind. This 

 process is called dialysis. 



We may say, broadly, that crystalloids in solution 



