246 HYDROGEN ION CONCENTRATION 



of the negatively charged ions. But at the same time a potential 

 difference is formed at the phase boundary surface. Still the more 

 adsorbable of the two ions is adsorbed to a greater extent than the 

 other ion, this difference being demonstrable, if not by analysis, 

 then by some other means. Charcoal in contact with dilute HCl 

 solution adsorbs somewhat more H-ions than Cl-ions. While the 

 difference cannot be demonstrated by analysis, it becomes evident 

 from the fact that the charcoal surface has become positively charged 

 towards the solution. The adsorption of the H-ions in excess pro- 

 ceeds to such an extent that the potential due to the separation of the 

 ions just compensates the repulsion tendency of the ions themselves. 

 In this way can we explain the long known fact that a potential 

 difference arises at the boundary surface of any two different phases 

 in general. As the means of studying and recognizing this formation 

 of electric charges we have at our disposal two apparently very 

 different, and intrinsically quite correlated, methods: electric en- 

 dosmosis (electroendosmosis) and electric cataphoresis (electro- 

 phoresis) . 



In entering now upon the discussion of the so-called electr akinetic 

 (Helmholtz) or electroosmotic (von Smoluchowski) 'phenomena, we 

 shall for the time being apparently lose our connecting thi-ead with 

 the preceding chapter, but eventually we shall reestablish the 

 connection. 



71. The earlier history of electroendosmosis 



In 1808 Reuss^^ made the following observation: If two tubes 

 filled with water are partly immersed in wet clay, and a galvanic 

 current is then sent through them by means of metal electrodes, 

 the water rises to a higher level in the tube on the cathode side and 

 falls to a lower level on the anode side. This was the discovery of 

 electroendosmosis. At the same time particles of the clay become 

 loose and travel towards the anode, as shown by the clouding of the 

 water in the tube. This was the discovery of electrophoresis. Both 

 phenomena are, as it were, the expression of the positive charge of 

 the water with respect to the clay. If the clay is thought of as a 

 sohd filter or diaphragm for the water, then only the movement of 



'^ F. Reuss, M6m. de la Soc. imp. des Naturalistes de Moscou. 2, 327 (1S09). 



