114 CHEMICAL STATICS 



Similar results were obtained with other colloids. Electroneg- 

 ative colloids are precipitated, if at all, by cations; electropositive 

 colloids, by anions. 



Whetham (78) (26) explained these phenomena in the following 

 way: He assumes that at the surface of the colloid particles there 

 exists a double electrical layer. When the particles move towards 

 the anode they are negatively charged, when they move to the 

 cathode they are positively charged. The surface energy of the 

 colloidal phase is reduced by the presence of the electrical double 

 layer and therefore its tendency to contract. The existence of 

 the double layer, therefore, conduces to the stability of a system 

 in which the surface of the colloid is greatly extended, i.e., to the 

 stability of the " colloidal solution." The cations of the added elec- 

 trolyte, in the case of electronegative colloids, or the anions in the 

 case of electropositive colloids, neutralize this charge and therefore 

 increase the energy of the surface of the colloidal phase and its 

 tendency to contract. Hence the finely suspended colloidal par- 

 ticles unite to form large aggregates having a less extended surface, 

 and these aggregates finally become so large as to assume the 

 properties of matter in mass, and hence are carried out of solu- 

 tion by the action of gravity. In this way the dependence of 

 the precipitating-power of an electrolyte upon its dissociation, 

 found by Linder and Picton and by Hardy, and also the reversal 

 in the relative precipitating-powers of the ions of the added 

 electrolyte upon reversion of the sign of the electrical charge 

 carried by the colloid, found an explanation. In interpreting 

 the valency rule discovered by Schultz, Whetham develops his 

 theory as follows: 



"In a solution where ions are moving freely, the probability 

 that an ion is at any instant within reach of a fixed point is, 

 putting certainty equal to unity, approximately represented by 

 a fraction proportional to the ratio between the volume occupied 

 by the spheres of influence of the ions and the whole volume of 

 the solution, and may be written as AC, where A is a constant 

 and C represents the concentration of the solution. The chance 

 that two such ions should be present together is the product of 

 their separate chances, that is (ACy. Similarly the chance for 

 the conjunction of three ions is {ACy, and for the conjunction 

 of n ions is (AC)"." 



"In order that three solutions, containing trivalent, divalent, 



