118 CHEMICAL STATICS 



the same valency the power of precipitating an electronegative 

 colloid varies directly as the equivalent conductivity of the salt, 

 that is, as the active mass of the dissociated metal ions. Since 

 electronegative colloids appear to behave, electrically, like the 

 anions of an acid, and electropositive colloids like the cations 

 of a base, their respective affinities for the basic and acid radicals 

 of salts are readily explicable. This fact appears, however, to 

 have been first clearly pointed out by Loeb (45). 



An interpretation of the phenomena attending the precipita- 

 tion of ionic ("electrically active") colloids by electrolytes prac- 

 tically identical with that of Whetham was afterwards brought 

 forward by Bredig (7). 



Modifications of Whetham's theory have been advanced by 

 Billitzer (3) (4) and by Freundlich (15) (16). Billitzer objected 

 to the assumption which was made by Whetham (although as 

 we have seen, it was not at all essential to his hypothesis) that 

 an electrical double layer exists at the surface of colloid particles 

 in aqueous solution and showed that this assumption is inadequate 

 to explain the migration of the particles in an electrical field. 

 The charge on the colloid particles must therefore be supplied 

 by the particles themselves, by giving up oppositely charged 

 ions to the solution, i.e., the colloids must be electrolytes. So far 

 his view is identical with that which is developed above. But 

 he adds to this the assumption that the charge which is carried 

 by the colloid particles is not a full atomic charge. The precipi- 

 tating action of oppositely charged ions he attributes to the 

 electrostatic attraction of the colloid particles by the oppositely 

 charged crystalloid ion. Since the colloid particle is assumed not 

 to possess a full atomic charge, a number of particles must, he 

 considers, be attracted by one crystalloid ion, thus forming a 

 molecular group large enough to be appreciably acted on by 

 gravity. How the colloid particles come to acquire a fraction 

 of an atomic charge Billitzer does not explain, nor does our ex- 

 perience of the behavior of electrolytes afford any legitimate 

 precedent for such an assumption, 



Freundlich believes that the precipitating ion must penetrate 

 (be "adsorbed" by) the surface of the colloid particle and that 

 the surfaces of colloids are permeable to ions possessed of an 

 opposite but not to ions possessed of the same charge as their 

 own. The precipitating powers of various salts are stated to 



