280 



HYDROGEN ION CONCENTRATION 



troendosmosis. The same results were obtained by Zollner/" 

 Edlund, 51 Haga,52 ^nd Clark." 



The theory of hydrodynamic currents is similar to that of electroen- 

 dosmosis, — it is the reverse of the latter. The movable ionic layer of 

 the double layer is being sheared by the mechanical water current in 

 the capillary. Hence the further end of the capillary becomes enriched 

 in the ions of which this laj^er is made up. This does not occur, 

 to be sure, to an analytically demonstrable extent, but sufficiently 

 to give rise to a potential difference between the two ends of the 

 tube. Figure 30 is an attempt to visuahze this process. The Hquid 

 flows through the capillary A A' into the larger vessel BB'. As long 



as the liquid remained stationary a 

 double layer was being formed in the 

 manner indicated along the wall A. 

 When the liquid is set in motion, it 

 tears along with it the movable layer 

 adjacent to it, while the layer adhering 

 to the wall remains so, as indicated 

 along wall A'. In this way the upper 

 end becomes negatively and the lower 

 end positively charged. 



The value E of the potential differ- 

 ence between the ends of the tube is 

 according to Helmholtz, and including 

 Pellat and Perrin correction for the 



A 



1+ 



1+ 

 1+ 



!+ 

 1+ 

 1+ 



1+ 

 1+ 

 1+ 

 1+ 

 1+ 

 1+ 

 1+ 



neg. 



I 

 +1 

 +1 

 +1 



+ 



+1 



+1 



+1 



+1 



+1 



+ 1 



+ 



+1 



+ 



+ 



A 



\ 



Fig. 30 



dielectric constant (see page 249), as follows: 



E = 



f ■ P-D 



4 TT • 17 X 



(where 77 is the viscosity of the fluid, X its specific electric conduct- 

 ance, P the hydrostatic excess of pressure between the ends of the 

 tube, D the dielectric constant, f the potential of the double la^-er 

 on the capillary wall — or the "adsorption potential"). The E.M.F. 

 of the current of flow is therefore directly proportional to the hydro- 

 static pressure and to the adsorption potential. It is inversely 

 proportional to the viscosity of the liquid, for the more viscous the 



60 F. Zollner, Pogg. Ann. d. Physik. 148, 640 (1873). 



" E. Edlund, Pogg. Ann. d. Physik. 156, 251 (1875) ; Wied. Ann. 1, 184 (1877). 



*2 H. Haga, Wied'. Ann. 2, 326 (1877) : 5, 287 (1878). 



" J. W. Clark, Wied. Ann. 2, 335 (1877). 



