52 BIOLOGICAL CHEMISTRY 



disperse system, therefore we can regard the colloidal solution 

 as a disperse system with surfaces of separation between the 

 particles and the solution. The suspended phase is spoken of 

 as the disperse phase. 



" SOLS " AND " GELS " 



Colloids can exist in solution as liquids, when they are called 

 sols, or as jellies, when they are called gels. Sometimes 

 a gel can be turned into a sol, for example gelatine, 

 and such a colloid is called a reversible gel, this is called 

 solation. Some gels cannot be turned into sols ; 

 these are called irreversible gels. The reverse process of 

 turning a sol into a gel is called gelation or coagulation. 



A 

 FIG. ii. Diagram to illustrate relation of phases in colloidal systems. 



If the black be regarded as the more rigid phase, and white as the liquid phase, then A 



represents an ordinary hydrosol, such as that of gold, in which the solid particles are freely 



movable. B represents a gel of an alveolar or honeycomb structure, in which liquid drops 



are imprisoned by more or less solid walls, such as a strong solution of gelatine when cooled. 



From " Principles of General Physiology." W. M. Bayliss (Longmans). 



We have already mentioned that colloids will not diffuse 

 through membranes. The other characteristics of colloids 

 are their viscosity and the ease with which they are precipitated 

 from their solution. 



VISCOSITY OF COLLOIDS 



The high viscosity of colloids can be explained by the 

 hypothesis of Hatschek * that when the suspended particles 

 occupy a large volume of the solution they come into contact 

 and the viscosity is raised by the force required to overcome 

 the friction of the particles against each other. 



If this be so we must believe that the influence of certain 

 reagents on the disperse phase causes a swelling of that 

 phase. For instance, Pauli and Handovsky have found that 

 acid or alkali added to protein solution up to a certain con- 

 centration causes an increase in viscosity, thus indicating that 

 the acid or alkali causes the protein phase to absorb more 



* E. Hatschek, Zeit. f. chem. u. Industrie d. koll., 1911, vol.8, p. 34. 



