246 



Dr. W. M. Bayliss. 



[June 30, 



was - 068 volt instead of - 088 calculated, but in this case the concentration 

 of the outer solution was too low for accurate measurements, and the reading 

 was not made until some time after setting up. 



The reason why the E.M.F. steadily falls for a time after setting up the 

 apparatus is, I think, to be explained by the fact that it is almost impossible 

 to prevent a small diffusion of water through the membrane, owing to the 

 high osmotic pressure within. The difference of concentration must be 

 considerable in order to obtain sufficiently high potential differences for 

 accurate measurement. The dilute layer next the membrane will only 

 slowly diffuse into the rest of the solution and as long as it is contact with 

 the membrane, the conditions are not those corresponding to the concentration 

 of the main bulk. It seems then that the values obtained before the water 

 has had time to pass through are the more reliable. When the osmometer 

 was replaced by an open parchment-paper dish, the E.M.F. fell so rapidly that 

 it was impossible to obtain satisfactory readings. In a special osmometer 

 constructed by cementing parchment-paper over the end of a tube of 8 mm. 

 diameter, it was found that, with solutions of conductivities of 3950 x 10 -6 and 

 1955 x 10 -6 recip. ohms respectively, an E.M.F. of 0149 volt was found, 

 with an osmotic pressure of 288 mm. Hg at 21° C. The calculated value was 

 0*0174 volt, so that 86 per cent, of the theoretical number was obtained. 



It occurred to me that, perhaps, by sending a continuous current of the 

 more concentrated solution through the interior of a parchment-paper tube 

 and one of the dilute solution over the outside, more permanent results might 

 be obtained. The figures were : — 



Conductivity of the stronger solution 5780 x 10 -6 recip. ohms. 



dilute „ 192 xlO" 6 



E.M.F. calculated, - 083 volt ; found, 0-07 volt, or 84 per cent, of theory. 



Since, in these experiments, the osmotic pressure is the sum of that of the 

 ionised and non-ionised salt, while the E.M.F. is due only to the former, it 

 should be possible, by calculation of the osmotic pressure corresponding to 

 the potential difference as measured and subtracting this from the observed 

 osmotic pressure, to determine the relative proportion of the ionised and non- 

 ionised parts of the salt in the osmometer. 



It is worth recording that the E.M.F. can be at once abolished by replacing 

 the outer solution by one of sodium chloride of the same ionic concentration, 

 as estimated by electrical conductivity, as the solution on the other side of 

 the membrane. If the solution thus placed outside is more concentrated in 

 Na" ion than that inside, the sign of the potential difference is reversed, as 

 would be expected. This reversal lasted longer than would be supposed from 



