MK. S. W. J. SMITH ON THE NATURE OF KLECTROCAPILLARY PHENOMENA. 77 



and the meniscus exceeds a certain value, the surface tensions are the same when the 

 potential differences are equal. Conductivity data for KCNS are not available, so 

 that it is impossible to say whether the ionic concentration is strictly the same for 

 the two solutions ; but if we may argue from the values for corresponding solutions 

 of HC1 and HCNS,* it would appear probable that the degree of dissociation of 

 equally concentrated solutions of the salts is practically the same. The ionic velo- 

 cities of the anions would also seem to be about the same. 



THE RELATION BETWEEN THE NATURE OF THE RATION OF THE SOLUTION AND THE 



FORM OF THE ELECTROCAPILLARY CURVE. 



Before the argument previously employed can be applied in the case of salts 

 like KC1 and Na^S possessing different kations, it is necessary to assume that the 

 effect of the kation upon the electro-capillary curve is independent of its chemical 

 nature. The observations upon KC1, NaCl and HC1 described below show that this is 

 probably true. But apart from this there are uncertainties in the above experiments 

 with sodium sulphide from which the others are free. It is difficult to prepare a 

 standard solution of sodium sulphide. The solutions used were only approximately 

 normal and half-normal. But besides this, the degree of dissociation of a normal sodium 

 sulphide solution is probably considerably less than that of a normal potassium chloride 

 solution. Hence the potential difference between the solutions is uncertain. Further, 

 if the Na^S solution is less concentrated than the corresponding KC1 solution, the 

 NajS descending curve will be relatively nearer the KC1 descending curve than it 

 would be if the ionic concentrations were equal (see below). This in itself might 

 suffice to explain why the horizontal distance between the curves is less than that 

 corresponding to the observed E.M.F. of the cell. 



While it seems reasonable to suppose that if the uncertainly concerning the 

 sodium sulphide were eliminated, the agreement would be as close as in the other 

 cases, the experiments seem to leave little doubt as to which of the numbers quoted 

 by NERNST (0'416 volt and zero) is nearer the true value of the potential difference 

 between equally concentrated solutions of KC1 and NaJS. 



Experiments with equally concentrated Solutions of Potassium and Sodium 



Chlorides. 



The following experiments show the probability that for equally concentrated 

 solutions of potassium and sodium salts the kation produces the same effect upon 

 the surface tension. 



* OSTWALD, ' Jouni. f. Prakt. Chemie,' 32, 1886. 



