COLLOIDS 



61 



to behave as suspensoids, and are accordingly precipitated by the elec- 

 trolytes. 



Another property of emulsoids of biological' importance is the pro- 

 tection which they can afford against the precipitating influence of 

 electrolytes on suspensoids. If a colloidal solution of gold is mixed with 

 a trace of gelatin, the subsequent addition of salts will be found to 

 produce no precipitation. The explanation of this is that the emulsoid 

 becomes distributed as a film on the suspensoid particles, thus practically 

 converting them into emulsoids. 



Gelatinization 



One of the best known properties of emulsoids is that of gelatiniza- 

 tion, which has an interesting bearing on many problems of biology. 

 After the gel has set, an enormous pressure is required to squeeze out 

 any water from it, indicating that the water no longer forms the con- 

 tinuous phase but must be enclosed in vesicles formed of more solid 

 material. 



Fig. 16. 



As a gelatin solution cools, the gel at first forms a polarized cone of 

 light, but the very fine particles which are responsible for this effect 

 soon increase in number and size so that they obstruct one another in 

 their Brownian movements and adhere, giving an appearance of fine 

 felt-like threads throughout the solution. A sort of impervious sponge 

 work of the more solid phase is therefore formed, the more fluid phase 

 being inclosed in the meshes. 



If, as in the accompanying diagram, the dispersion medium is repre- 

 sented by white and the dispersoid in black, the relationship between 

 the two in a suspensoid is as in A, and that in a gel as in B. To express 

 any of the dispersion medium in B, it will require a pressure sufficient to 



