INFLUENCE OF ELECTRICAL CONDITION 117 



colloid reacts chemically with the precipitating ion as upon 

 the assumption that the precipitating ion acts merely through 

 altering the electrical condition of the colloidal particles. The 

 former view attributes to the colloids no especial qualities which 

 differentiate them from other chemical systems, the latter view 

 necessarily involves assumptions regarding the nature of col- 

 loidal solutions which have hitherto proved incapable of veri- 

 fication. 



It is not surprising that velocities rather than equilibria deter- 

 mine the ''precipitating-powers" of reagents for colloids, when 

 we recollect, firstly, the enormous part played by the velocity of 

 change in the final physical condition of a colloid, f and secondly 

 the method by which these " precipitating-powers " are measured. 

 Linder and Picton measured the precipitating-power of a salt by 

 titration, running the solution of the salt into the solution of 

 arsenic sulphide until coagulation just began to be perceived. 

 They expressly state that unless the time occupied in the titra- 

 tion be kept approximately the same, serious deviations from the 

 above rule occur, "as a quantity of coagulant insufficient to pro- 

 duce coagulation immediately will do so in the course of time." 

 Under these conditions, what is actually measured is the con- 

 centration of the precipitating agent which is requisite to bring 

 about a given degree of change (visible precipitation) within a 

 given brief period, that is, a velocity and not an equilibrium. 

 Data regarding the degree of precipitation after varying periods 

 and the equilibria which are attained in such reactions as these 

 are lacking, but Linder and Picton have shown that the cation 

 which brings about the precipitation of colloidal arsenic is bound 

 by the colloid and carried down with it. It cannot be washed out 

 with water, but it can be replaced by another metal. Whitney 

 and Ober (80) obtained similar results and, moreover, showed that 

 different metals are carried down by this colloid in equivalent- 

 molecular proportions. 



That these results are susceptible of purely chemical inter- 

 pretation was pointed out by Hardy (22) in 1900 and this view 

 was further developed by him in his communications on globulin 

 referred to above (24). He showed that in a series of salts of 



* According to W. B. Hardy (23), the more slowly the division into two 

 phases occurs the smaller and less curved is the surface of separation. Cf. 

 also Freundlich (15). 



