400 Dr. R. A. Lehfeldt on Electromoti 



re 



But van't HofPs factor as determined by means of the 

 electrolytic conductivity is 2*45 for deeinormal, 1*97 for 

 normal solution : hence the osmotic pressure may be reckoned 

 to exceed that calculated from the laws of gases by about 

 11 per cent, in the former case, 38 per cent, in the latter. 



The curve for zinc sulphate shows the same general trend, 

 but differs in certain respects. I could not obtain measurer 

 ments with mercury cells of concentration much below 

 docinormal. Goodwin met with the same difficulty, and: 

 attributed it to some secondary reaction of the Hg 2 S0 4 . I 

 am inclined to think that it is mainly due to the appreciable; 

 solubility of that salt, which prevents it from constituting an 

 ideal depolarizing electrode (see Goodwin's work on TiCl 

 loc. cit.). In either case the difficulty remains ; but it it be 

 permissible to associate Goodwin's measurements with PbS0 4 

 as depolarisator with mine, as shown on the diagram, it will 

 be found that the curve bends sharply down to the right, as 



it should do, since for limiting dilution the value of ,~ — 



should be 0'058 volt for unit difference in common logarithm 

 (2/3 of the value for the chloride, since there are only two 



dE 

 ions) ; while in the straight part of the curve -=— has the 



abnormally low value of 00272 volt. On account of this 

 sharp curvature the integration constant in (7) cannot be 

 neglected, and the approximate estimate of the osmotic 

 pressure given in the preceding paragraph fails. 



The few observations I have made with concentration-cells 

 of the ordinary type were for the sake of verifying tlie, 

 previous work of Moser, Wright & Thomson, and Lussana; 

 This they do satisfactorily; but I do not consider that, at 

 present, the measurements are sufficiently exact to justify 

 a calculation of HittorFs transference ratios in this way. It 

 may be seen from the curve that x increases for ZnCJ 2 from,' 

 about 0'6 in dilute solution to more than unity in strong 

 solution, in agreement with the results of migration experi- 

 ments ; but before calculating the values in detail it will be 

 necessary to extend the experimental data, and to take into 

 theoretical account the successive ( a stufenweise ") dissociation 

 which undoubtedly occurs in this salt. 



(viii.) Calculation of the Osmotic Pressure. 

 To calculate the osmotic pressure we have 

 reE=jYdP. 



