68 



DISPERSE SYSTEMS 



As an illustration, attention may be drawn to a disperse system 

 having two phases and only one component, 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 meta- 

 stable. 



Disperse systems may be classified according to the nature of 

 the contact surface between the phases. Taking the three states 

 of matter, solid, liquid and gaseous, five different kinds of contact 

 surface can be produced, as is indicated in the following table, in 

 which are also given examples of the various disperse systems. 



TABLE X. 



Class III. (Emulsoid) is of the most importance in biology, 

 but Class IV. (Suspensoid) has been most studied and is of con- 

 siderable industrial value. Colloids met with in nature are all 

 dispersed in water, having salts in solution. They might therefore 

 be classified according to their degree of dispersion. The degree 

 of dispersion of a phase is the ratio of total surface to volume, or 



