274 THEORY OF COLLOIDAL BEHAVIOR 



tions required should be comparatively low and this was found 

 to be the case. One per cent solutions of casein chloride of pH 

 2.2 were prepared in different concentrations of salts in water of 

 about the same pH. That concentration was determined which 

 causes an almost instantaneous complete precipitation of the 

 protein from the solution so that the supernatant liquid became 

 as clear as water. These concentrations were as follows : 



NaCl about M/8 



NaNO 3 about M/8 



CaCl 2 about M/8 



Na trichloracetate about M/16 



Na 2 SO 4 about M/32 



Though the results are only semi-quantitative, the validity of 

 Hardy's rule and the valency effect are easily recognizable. It 

 is also obvious that the concentrations of electrolytes required 

 for instantaneous, complete precipitation of casein chloride are 

 considerably lower than those required for the precipitation of 

 Na caseinate from their watery solution. 



Hardy's rule, that only that ion of a neutral salt is active in 

 precipitation which has the opposite sign of charge to that of the 

 colloidal ion, and that the efficiency of the ion increases with the 

 valency, is simply the expression of the Donnan effect, as is also 

 the fact that very low concentrations of electrolytes suffice for 

 precipitation. The reader will notice that it is unnecessary to 

 assume that the ions are adsorbed by the casein or that the ad- 

 sorption of ions annihilates the electrical charges on the particles 

 of casein. 



