CONDUCTIVITY OF COLLOIDAL MIXTURES 305 



effect upon permeability, will the ions of one prevent in any 

 degi'ee the ions of the other from entering? The bearing of 

 these questions upon any theory of pemieability is obvious. 



That either of these questions might be answered in the 

 affirmative seemed fairly possible in view of the work of several 

 investigators, notably that done by Robertson and Miyake- upon 

 the influence of alkali and alkaline earth salts upon the rate of 

 solution of casein by NaOH. Moreover J. C. Philip^ reports 

 cases where the coagulation of colloidal material in mixtures of 

 chlorides of a monovalent and polyvalent metal is not explainable 

 unless the effect of the two is considered to be antagonistic. 



Method 



li 



In the experiments described here the salts used were the 

 Tested Purity" brand as sold by Eimer and .\mend, and the 

 agar and gelatine were the best commercial brands. Resistance 

 measurements were made in the usual manner with a Wlieat- 

 stone Bridge set-up using a telephone as detector and a small 

 induction coil as the source of current. During the latter part of 

 the work a variation of this method was employed for convenience 

 sake, which in no way had a different effect upon the final values 

 obtained. The cylinders of colloidal material, while being 

 measured for their resistance, were held in a rack similar to that 

 employed bj' Osterhout. In making up solutions double dis- 

 tilled water was used having a specific conductivity of less than 

 2 X 10"'^ reciprocal ohms. Since this factor was practically 

 constant in all the readings, the conductivity of the water was 

 not subtracted from that of the material dissolved or bathed in 

 it. All readings were taken at temperatures between 18° and 

 18.5°C. A fluctuation of 0.5° was shown to be within the 

 lunits of error from other sources. 



Three different classes of colloidal material were used; first 

 15% gelatine in water, second a mixture of 1 gram of agar and 



2 Roberston, T. B. and Miyake, K. The Influence of Alkali and Alkaline 

 Earth Salts upon the Rate of Solution of Casein by Sodium Hydroxide. Journ. 

 Biol. Chem.25:351. 1916. 



' Philip, J. C. Physical Chemistry. London, 1910. 



