406 CAUSE OF INFLUENCE OF IONS. I 



(which for many electrolytes lies at a concentration of about m/256) 

 and then drops again with a further increase in concentration. It 

 seems from the writer's experiments that the same phenomenon oc- 

 curs in the case of electrical endosmose through collodion membranes 

 and that the turning point lies near m/512. Powis reports a similar 

 effect of concentration in his observation on the motion of oil drops in 

 an electrical field, and recently published experiments of Kruyt" on 

 "current potentials" demonstrate the same phenomenon. 



It seems to follow from this that the density of the electrical double 

 layer at the boundary of watery phase and membrane increases at 

 first with increasing concentration of an electrolyte up to a certain 

 point which for a number of electrolytes seems to lie at about m/512. 

 If the concentration of the electrolyte rises beyond this point, the 

 density of the charge on the double layer diminishes rapidly with a 

 further increase in the concentration of the electrolyte. 



SUMMARY. 



1. In three previous pubhcations it had been shown that electro- 

 lytes influence the rate of diffusion of pure water through a collodion 

 membrane into a solution in three different ways, which can be under- 

 stood on the assumption of an electrification of the water or the watery 

 phase at the boundary of the membrane; namely, 



(a) While the watery phase in contact with collodion is generally 

 positively electrified, it happens that, when the membrane has re- 

 ceived a treatment with a protein, the presence of hydrogen ions and 

 of simple cations with a valency of three or above (beyond a certain 

 concentration) causes the watery phase of the double layer at the 

 boundary of membrane and solution to be negatively charged. 



(b) When pure water is separated from a solution by a collodion 

 membrane, the initial rate of diffusion of water into a solution is 

 accelerated by the ion with the opposite sign of charge and retarded 

 by the ion with the same sign of charge as that of the water, both 

 effects increasing with the valency of the ion and a second constitu- 

 tional quantity of the ion which is still to be defined. 



11 Kruyt, H. R., KoUoid-Z., 1918, xxii, 81. 



