226 POTENTIAL DIFFERENCES AND ANOMALOUS OSMOSIS 



diffusion had been going on for 1 hour; and when these values were 

 plotted as ordinates over the original pH as abscissae, the curves 

 obtained were found to be similar to the osmotic rate curves. 

 This confirms the view expressed by Girard, Bernstein, Bar tell, 

 and Freundlich that these cases of anomalous osmosis are in reality 

 cases of electrical endosmose where the driving force is a p.d. between 

 the opposite sides of the membrane, 



3. The question arose as to the origin of these p, d, and it was found 

 that the p,d, has apparently a double origin. Certain features of the 

 P.D. curve, such as the rise and fall with varying pH, seem to be the 

 consequence of a Donnan equilibrium which leads to some of the free 

 HNO3 being forced from the solution containing salt into the outside 

 solution containing no (or less) salt. This difference of the concen- 

 tration of HNO3, on the opposite sides of the membrane leads to a 

 P.D, which in conformity with Nernst's -theory of concentration cells 

 should be equal to 58 X (pH inside minus pH outside) millivolts at 

 18°C, The curves of the values of (pH inside minus pH outside) 

 when plotted as ordinates over the original pH as abscissae lead to 

 curves resembling those for the P. d. in regard to location of minimum 

 and maximum, 



4. A second source of the p.d, seems to be diffusion potentials, which 

 exist even if no membranes are present and which seem to be 

 responsible for the fact that the rate of diffusion of negatively 

 charged water into the salt solution increases with the valency of the 



' cation and diminishes with the valency of the anion of the salt, 



5. The experiments suggest the possibility that the establishment 

 of a Donnan equilibrium between membrane and solution is one of 

 the factors determining the Helmholtzian electrical double layer, at 

 least in the conditions of our experiments. 



