DIALYSIS 67 



saturated solutions, and Colloids, which diffuse very slowly and in 

 general have a gluey consistency. ' They appear," he writes, 

 " like different worlds of matter, and give occasion to a corre- 

 sponding division of chemical science." He noticed that crystal- 

 loids passed through animal membranes while colloids did not. 



That is, if a mixture of colloids and crystalloids were put into 

 a simple dialysing drum (such as described in Part II. p. 406), 

 immersed in running water, after a time most of the crystalloidal 

 material would have been dialysed out of the mixture, while the 

 colloidal matter would still remain in the drum. The dialyser 

 furnishes a method for the separation of crystalloids. 



It has now been proved that matter may exist either in a 

 crystalloidal or in a colloidal state, and that by suitable means 

 a colloid may be crystallised and so pass through a membrane 

 previously impermeable to it. The converse process may also 

 take place. 



The solvent is sometimes the factor on which depends the 

 state of the solute. The alkali salts of the higher fatty acids 

 stearic, palmitic, oleic form a true molecular solution in alcohol, 

 but with water they act as colloids. On the other hand, sodium 

 chloride, a typical water-soluble crystalloid, assumes the colloidal 

 state in benzol. 



Von Weimarn and others have prepared colloidal solutions of 

 over two hundred substances usually considered as crystalloids. 

 By proper manipulation, almost any solid can be dispersed through 

 a liquid either as a crystalloid or as a colloid. Consequently, one 

 now speaks of the colloidal state rather than of certain substances 

 as being colloids. 



The difference between a crystalloidal and a colloidal solution 

 depends, in the main, on the size of the particle in the fluid. 



There is some difficulty in expressing the relationship between 

 the colloid and the fluid in which it is. It is not in true solution, 

 but is suspended and dispersed throughout the medium. The 

 colloid may, therefore, be called the dispersed substance or 

 dispersate and the fluid the dispersing medium or dispersant. 



The application of the " Phase Rule " (of W. Gibbs) has helped 

 to clear up several difficulties in physiological physics, and some 

 writers have adopted terminology suitable for use when this rule 

 is discussed. It is sufficient here to say that the dispersed phase 

 is the substance which is suspended or distributed throughout 

 the continuous phase. In nearly every case the former is the 

 colloid and the latter the fluid. 



