60 MR. S. W. J. SMITH ON THE NATURE OF ELECTROCAPILLAUY PHENOMENA. 



where M,X, and M 2 X 2 signify two solutions which have been examined in the capillary 

 electrometer ; but the E.M.F. found cannot be applied to test the Helmholtz theory 

 of the electrometer unless we know the value of the potential difference between the 

 two solutions. 



The state of our knowledge of the potential differences between liquids is not 

 satisfactory. Within recent years, however, NEKNST,* starting from the dissociation 

 hypothesis, has given a theoretical investigation for the case in which the two liquids 

 are solutions of the same salt, but of unequal concentration. PLANCK! has extended 

 the investigation to the case in which the liquids are solutions of different salts. In 

 many cases the values found experimentally agree very closely with those calculated ; 

 but it must be borne in mind that in the experiments the potential difference between 

 the liquids only formed part of the E.M.F. actually measured. For example, in 

 testing the formula as applied to two solutions of potassium chloride of different 

 concentrations, it is necessary to introduce a fresh hypothesis in order to calculate the 

 difference between the potential difference between mercury, covered with calomel, 

 and the stronger solution, and the potential difference between mercury, covered with 

 calomel, and the weaker solution.^ In order to show the nature of the agreement 

 between the calculated values and those found experimentally, some results for KC1 

 solutions are given below. In the first column are given the concentrations in gram 

 equivalents per litre ; in the second the observed E.M.F.s of cells of the type ; 



HgCl KC1 KC1 HgCl 



dilute 



concentrated 



Hg, 



and in the third the calculated values of these E.M.F.s. 



These observations form part of a series of experiments which will be described 

 later. 



* NEUXST, 'Zeits. f. Physik. Chemie,' 4, 1889. 



t PLANCK, ' Wied. Ann.,' 39, 1890, and 40, 1890; cf. also NEGBAUR, ' \Vied. Ann.,' 44, 1891. 



J NERXST, ' 7eits. f. Physik. Chemie,' 4, 1889. 





