244 



Dr. W. M. Bayliss. 



[June 30, 



sodium salts of caseinogen were kept within the membrane of an osmometer 

 by electrostatic forces, this membrane itself would be the seat of a consider- 

 able potential difference. 



Hardy* has investigated the question mathematically from different points 

 of view, (1) by consideration of the force acting upon the diffusible ions due 

 to the potential difference between the two oppositely charged sides of the 

 Helmholtz double layer as being equal and opposite to the osmotic pressure 

 of these ions, and (2) by consideration of the work done in moving electricity 

 from the lower potential to the higher. This latter is the method used by 

 Nernst in the calculation of metallic electrode potentials and is based on 

 Helmholtz' theory of contact potential. The same equation is arrived at by 

 both methods : 



E = RT/<? . lognatc 2 /ci, 

 where E is the potential difference between the two sides of the membrane,. 

 E the gas constant, T absolute temperature, q the charge on 1 grm. 

 equivalent of the ion concerned, c 2 the concentration of this ion inside the 

 membrane, and Ci its concentration outside the membrane. Expressed in 

 volts ET/j = 0-0247. 



This equation, as will be noticed at once, has a striking similarity to the 

 well-known one of Nernst expressing the potential at a metallic electrode,, 

 and may in a sense be regarded as an illustration of the way such a potential 

 is produced. In any case it shows how a permanent electromotive force is 

 produced by means of a membrane permeable to one ion only. It is quite 

 different from that concerning the electromotive force of contact of two 

 solutions of different concentration. Here the different rates of migration of 

 the anions and cations are the cause of the potential difference, which only 

 lasts until diffusion has put an end to concentration differences. 



In attempting to apply this equation to actual experiment, I was struck 

 by a result which follows from the above equation as also from that of 

 Nernst on concentration batteries, but which seems to have escaped the 

 notice of writers on the subject. If the outer fluid in the osmometer is 

 distilled water, ci, the denominator of the fraction, becomes zero, and the 

 potential difference therefore infinite. The same applies to the concentration 

 battery when the one solution is water. If the original paper of • Nernstf be 

 consulted, it will be found that this fact had not escaped his notice. He also 

 points out that, theoretically, any diffusion into a vacuum should take place 

 with infinite velocity ; in ordinary cases this would only last for an 

 infinitesimally short time. In actual practice, moreover, there is never an 



* Forthcoming monograph on " Colloids." 



t ' Zeits. f. physik. Chem.,' 1889, vol. 4, p. 139. 



