256 HYDROGEN ION CONCENTRATION 



conductance as of an electrophoresis of suspended particles of molec- 

 ular dimensions. Let us take again a U-tube, open at both ends, filled 

 with a solution of an electrolyte, and with an electric current passing 

 through it. The anions in moving towards one end of the tube will 

 carry along with them a certain amount of water, while the cations 

 moving to the other end will carry along with them a certain, but 

 different, amount of water. The water should, therefore, rise at one 

 end. But since hydrostatic differences of pressure cannot be main- 

 tained, the water pulled upward at every moment by electric forces 

 must immediately sink downward again because of gravity. There- 

 fore the only recognizable manifestation of the var5dng capacity of 

 ions to bind water is the varying motihty of the ions and the changes 

 of concentration resulting from it in the electrolyte solution, which is 

 numerically expressed as the transport number of the electrolyte. 

 In electroendosmosis experiments the presence of the diaphragm 

 permits, under favorable conditions, of the formation of hydrostatic 

 pressure differences, and the displacement of the water can thus be 

 directly observed. Hence, from the purely quaUtative standpoint 

 the analogy between both phenomena is quite complete, and 

 Smoluchowski drew our attention to it. Whether this theory is also 

 acceptable on a quantitative basis is not yet entirely clear. 



In fact, it will be shown in section 78 that under appropriate con- 

 ditions, namely, through the introduction of a diaphragm with 

 narrow pores, and whose walls have no potential towards the solution, 

 it is possible to demonstrate experimentally the displacement of 

 water in an electrolyte solution by the passage of an electric current. 



The first purposeful investigations which rendered possible the 

 correlation of ionic adsorption and of endosmosis were made by 

 Perrin. 



75. Perrin's experiments 



Perrin^^ carried out his experiments shown in figure 29. The dia- 

 phragm is represented by the glass tube M which is filled with the 

 powdered substance. A and B are the electrodes, and the motion of 

 the water is observed by the change of level in the almost horizontally 

 ascending tube G. The electric field maintained is, as a rule, such 

 that between the electrodes a potential gradient of 10 volts per 



" Jean Perrin. Jour. chim. phys. 2, 601 (1904), and 3, 50 (1905). 



