THE PROBLEMS OF SOLUTION 121 



throwing- light on the nature of colloid solution 

 — perhaps, indeed, of solution in general. It looks 

 as though colloid particles, at any rate, could exist 

 in solution only when charged electrically. If, by 

 the conjunction of more mobile ions, their charge is 

 neutralised, or perhaps reduced to a critical value, 

 an iso-electric point is reached, and coagulation 

 must immediately follow. 



It is probable that these effects depend on 

 changes in the surface of separation between the 

 colloidal particles and the more liquid phase which 

 surrounds them. Such a surface of separation 

 must exhibit thewell-known phenomena of surface- 

 tension, and will possess an amount of available 

 energy proportional to its area, which therefore 

 tends to become as small as possible. A number 

 of separate particles would, in these conditions, 

 tend to coagulate into larger ones, just as small 

 raindrops tend to coalesce into larger ones. If 

 the colloidal particles are electrified, the electric 

 energy is greater when the charge is concentrated 

 on a small area, and, on this account, the area will 

 tend to increase. The effect of the electric charge 

 is thus opposite to that of the natural surface- 

 tension, and diminishes the tendency to coagulate. 

 Thus an electric charge may enable the colloid 

 to dissolve, while neutralisation of the charge may 

 result in coagulation. 



Much discussion has taken place about the 

 nature of liquid colloidal solutions, and their 

 relations with ordinary solutions of mineral salts 

 and other crystalloids. They may either be 

 regarded as ordinary solutions, in which the 

 dissolved particles are similar in kind to those 

 of crystalloid solutions, though of much higher 



