ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 283 



E denotes that the lower end of the capillary is positive in respect 

 to the upper end. The potential f of the wall of the capillary was 

 calculated from the corrected Helmholtz equation. 



It is of particular interest to observe that notable hydrodynamic 

 potentials were obtained only in very dilute solutions of electroly1:e. 

 While in 10~^ N KCl solutions of 20-40 milhvolts were obtained, 

 no measurable potentials were observed in 10~^ to 10"^ N solutions. 



We have already stated on various occasions that the presence of 

 an excess of neutral salts diminishes potentials (diffusion-, interphase 

 boundary-, membrane-potentials), and the same was the case in 

 electroendosmosis experiments, where an excess of neutral salt 

 caused a diminution in the amount of transported water. But in all 

 of these cases an excess of salt of an entirely different order of magni- 

 tude was involved. Hydrodynamic potentials may, in general, be 

 properly observed only in electrolyte solutions of concentrations of 

 the order of magnitude of 10"'' to 10~'* normal. The cause for this is 

 the fact that the current potential depends upon the conductivity 

 of the solution. By increasing the concentration of the salt, in the 

 first place the adsorption potential f is greatly diminished; secondly, 

 the current potential E produced by a given adsorption potential and 

 a given pressure is very profoundly depressed. It must be recalled 

 that in the equation given above (page 280) the conductivity X is 

 in the denominator of the fraction. 



By calculating from the current potential E the potential of the 

 wall f towards the solution, with the help of the corrected Helmholtz 

 equation, the values given in the above table were obtained by 

 Kruyt. It is notable that the values of the f potential do not vary as 

 much with the electrolyte concentration as those of the E potential. 

 This corresponds to the fact that the phenomena of endosmosis and 

 cataphoresis, which show a greater paralleHsm with the f potential, 

 do not depend to so great an extent upon the electrolyte concentra- 

 tion as hydrodynamic potentials do (since the specific conductivity 

 does not appear in the equation for endosmosis) . 



It will be stated only in a corollary way that in the course of sohd 

 particles falling through a layer of water a potential difference 

 develops between the upper and the lower ends of the water column. 

 This is the potential of falling particles. The phenomenon is the re- 

 verse of the hydrodynamic potential, and the two are entirely 

 analogous. 



