392 CAUSE OF INFLUENCE OF IONS. I 



tive effect of the two oppositely charged ions. Perrin,^ however, 

 states that in the case of electrical endosmose only one of the oppo- 

 sitely charged ions of an electrolyte influences the transport of liquid 

 through the membrane; namely, the one with the same sign of charge 

 as that of the water (or with the opposite sign of charge from that of 

 the membrane). 



II. 



Perrin's^''^ view of the influence of electrolytes on the amount of 

 liquid transported in electrical endosmose rests on the assumption 

 that the sign of the electrification of the double layer is primarily 

 determined by the H and OH ions. He assumes that the positive 

 electrification of a membrane bounded by liquid containing a mono- 

 valent acid is due to the adsorption of a layer of hydrogen ions by 

 the membrane. This membrane layer of adsorbed hydrogen ions is the 

 fixed stratum of the double layer and the next stratum of the liquid — 

 the watery phase — contains a corresponding excess of negative ions. 

 The stratum of negative ions is sufficiently far removed from the 

 fixed layer so as to be able to undergo the tangential displacement on 

 which the phenomena of electrical osmosis are supposed to depend. 



Perrin assumes that the negative electrification of a membrane 

 under the influence of a monovalent base is due to the OH ions situ- 

 ated in that stratum of the liquid which is in immediate contact with 

 the membrane, while a corresponding excess of positive ions exists in 

 the opposite stratum of liquid (the watery phase, in our terminology). 

 When another electrolyte is added to a weak acid or weak alkaline 

 solution the charge of the membrane is, according to Perrin, influ- 

 enced only by one of the two ions of the electrolyte added; namely, the 

 one with the opposite sign of charge from that of the membrane. 



"When a liquid electrifies a membrane with a certain sign the addition to this 

 liquid of a polyvalent ion of the same sign does not increase the electrification, 

 while the addition of a polyvalent ion of the opposite sign diminishes this electrifi- 

 cation considerably. The influence of bivalent ions is inferior to that of trivalent 

 ions and that of trivalent ions is inferior to that of tetravalent ions. In the case 

 of the ions with high valencies the diminution of the charge can result in the 

 complete reversal of the sign of the charge." 



''Perrin, J., Notice sur les titres et travaux scientifiques de M. Jean Perrin, 

 Paris, 1918, 36-37. 



