62 MR. S. W. J. SMITH ON THE NATURE OF ELECTROCAPILLARY PHENOMENA. 



SOLUTIONS OF POTASSIUM CHLORIDE AND POTASSIUM IODIDE. 



1. Tlie Potential Difference between Equally -concentrated Solution*. 



The experiments show that the Helmholtz theory of the electrometer and the 

 Nernst-Planck calculations of the potential differences between solutions cannot 

 both be true. While there are many facts in favour of the view that the Nernst- 

 Planck hypothesis gives the quantitative expression for the potential difference 

 between two solutions, there is one result calculated from the hypothesis which seems 

 to possess greater weight than any of the others, since it would seem to be a conse- 

 quence of almost any form of diffusion hypothesis. This is the result that the 

 potential difference between equally-concentrated solutions of potassium chloride and 

 potassium iodide is so small that in measurements of the type with which we are 

 concerned it can be taken to be zero. 



KOHLRAUSCH has investigated the electrolytic conductivity of solutions of KC1 and 

 KI for different degrees of dilution, and an examination of his numbers shows the 

 relative amount of ionization in equally-concentrated solutions of the two salts may 

 be considered identical when the solutions do not contain more than O'Ol gramme 

 molecule ]>er litre (youth normal). Even when the strengths correspond to a gramme 

 molecule in 2 litres (\ normal) the coefficients of ionization only differ by about 

 two per cent.* Again, according to the most recent values, the ionic velocities of 

 chlorine and iodine are practically identical, t When, therefore, dilute solutions of 

 KC1 and KI of equal strength are brought into contact there can be no tendency 

 of the potassium ions to diffuse, while the chlorine and iodine ions will tend to diffuse 

 with equal velocities across the common surface. Granting the ionic hypothesis we 

 may, therefore, safely assume that no forces which tend to alter the quantity of 

 electricity in the unit of volume act across the surface of separation between the 

 liquids, and that, therefore, no potential difference will arise between the liquids. 



2. The Nature of the Electrocapillary Curves for the same Solutions. 



For the reason given above I have carefully examined the relation between the 

 capillary curves for KC1 and KI in order to determine further the result of the 

 hypothesis that the potential difference between equally-concentrated solutions of 

 KC1 and KI is zero. 



(a.) General character of electrocapillary curves. 



rhe behaviour of the meniscus in the capillary electrometer is in general very 

 different in the " ascending " portion of the curve from what it is in the " descending " 



* KOHLRAUSCH, Wied. Ann.,' vol. 26, 1885. 

 t KOHIJUUSCH, ' Wied. Ann.,' vol. 50, 1893. 



