COLLOIDAL STATE 73 



matter, and give occasion to a corrcspoiuliiio- (li\ jsion ol" clicinical 

 science." The process of separating crystalloids i'roni colloids by 

 means of a membrane is called dialysis^ (See.Part II., Experiment 



26.) 



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. As an illustration, attention may be 

 drawn to a disperse system having two phases and only one com- 

 ponent, e.g., a fine mist of liquid water suspended in water vapour. 

 The dispersed, internal or non-continuous phase is composed 

 of the droplets of water ; the continuous or external phase, or 

 dispersion medium, is the water vapour. The stability of this 

 dispersion depends on two factors, {a) the temperature of and 

 {b) the diameter of the droplets. (Such a system is called divariant.) 

 The smaller the droplets, the greater is the ratio of surface to 

 mass and the higher is the vapour pressure. All the droplets 

 will not be of the same size, and therefore the larger droplets will 

 tend to become larger still at the expense of the smaller ones. 

 The system is, on this account, said to be metastable. 



