PRECIPITATION OF PROTEINS BY INORGANIC SALTS 163 



is the same in each of these cases (which is also assumed in Whetham's 

 theory) and equal to A, calling the initial velocities of the respective 

 reactions vi, v 2 and v 3 and the concentrations of the mono- di- and 

 trivalent ions Ci, c 2 and c 3 we have: 



vi is proportional to A*CI 

 V2 is proportional to A 2 C2 

 vs is proportional to A 3 cs 



whence it follows that if Vi = v 2 = v 3 and the velocity-constants of 

 the three reactions are equal (which is also assumed in Whetham's 

 hypothesis) : 



and 



1 : i i = 1 A A2 



Ci C 2 C 3 



- , - and 

 Ci C 2 



i. e., the dilutions of the mono- di- and trivalent ions at which combina- 

 tion proceeds with equal velocity, are related to one another in the 

 same way. Now in the experiments described above, -^ is defined as pi 

 the precipitating-power of the salt, hence: 



PI i P2 : PS = 1 A : A 2 



which is exactly the relation deduced by Whetham. The experimental 

 relations found by Schultz, Linder and Picton, Hardy and others are, 

 therefore, just as explicable upon the assumption that the colloid reacts 

 chemically with the precipitating ion as upon the assumption that the 

 precipitating ion acts in a purely physical way through altering the 

 electrical condition of the colloidal particles. The former view attrib- 

 utes to the colloids in general, and to the proteins in particular no 

 especial qualities which differentiate them from other chemical systems, 

 while the latter view necessitates radical assumptions regarding the 

 nature of colloidal solutions which have hitherto proved incapable of 

 verification. 



It will be noticed, however, that the factor that determines the 

 precipitating-power of a salt is the velocity with which it combines, 

 with the protein and not the final equilibrium which is attained. This 

 is not surprising when we recollect, firstly, the enormous part played 

 by the velocity of change in determining the final physical condition 

 of a colloid, and secondly, the method by which the " precipitating- 

 powers" of salts are measured. Linder and Picton, for example, 

 measured the precipitating-power of salt solutions by titration, running 

 the solution of the salt into the solution of the colloid until precipitation 

 just began to be perceived. They expressly state that unless the time 

 occupied in the titration be kept approximately the same, serious 

 deviations from the "valency rule" occur: "As a quantity of coagu- 

 lant insufficient to produce coagulation immediately, will do so in the 



