68 MR. S. W. J. SMITH ON THE NATURE OF ELECTROCAHLLAKY PHENOMENA. 



except that the anion in one is iodine, and in the other chlorine. Until the potenti.-.l 

 difference reckoned from the solution to the electrode reaches a certain value the 

 ..tVcct of the anion upon the surface tension (i.e., in determining the mode of 

 tion from the solution to the mercury) is appreciable ; but this effect gradually 

 diminishes and finally disappears, as is shown by the fact that the form of the curve 

 (beyond 4000) is independent of the nature of the anion. 



From this point of view it is obviously futile to consider that the highest point of 

 the iodide curve corresponds of necessity to zero potential difference between the KI 

 solution and the mercury electrode, since it might equally well be argued that the 

 highest point of the curve obtained with KC1 at the capillary corresponded to zero 

 potential difference between the mercury and the KC1. If there is no appreciable 

 potential difference between the KI and the KC1 both results cannot be true. The 

 potential differences in the two cases (maximum surface tension) must differ by about 

 0-2 volt, or else the potential difference between *nKI and %n KC1 must be about 



0'2 volt. 



In the fcce of evidence that there is a chemical effect of the anion upon the surface 

 tension, and that this effect increases as the potential of the liquid with respect 

 to the 'electrode decreases, it does not seem advisable to say more than that the 

 potential difference (reckoned from the solution to the electrode) is considerably less 

 at the maximum surface tension when the solution is KC1 than when it is KI. The 

 marked depression of the maximum value of the surface tension observable in the 

 case of potassium iodide solutions is one of the characteristic features of the curves 

 dealt with by ROTHMUND the actual fact of the depression was apparently first 

 noticed by GOUY ;* but the depression is really a perfectly general phenomenon. The 

 amount of depression depends upon the concentration of the solution as well as upon 

 its chemical nature. The depression for concentrated solutions of chlorides is very 

 pronounced, and for dilute solutions it can readily be observed that the maximum 

 value of the surface tension rises as the concentration diminishes. It is obviously an 

 effect which does not depend upon the density of the solution. For example, a 

 saturated solution of caustic potash (which is soluble in about half its weight of 

 water) has as high a maximum surface tension as a half-normal solution of potassium 

 chloride. The effect of the ions (apart from the electrostatic effect) upon the surface 

 tension would appear to depend, for a given potential difference, upon their nature 

 and concentration in the solution. Whether the surface tension in the neighbour- 

 hood of the maximum is ever controlled by the electrostatic effect alone, depends (on 

 this view) upon whether, when the potential difference between the solution and the 

 electrode is small, the nature and concentration of the ions is such that their non- 

 electrical effect upon the surface tension can be neglected. 



In the case above considered the curves are identical when the applied E.M.F. 

 exceeds 0'8 volt. The subsequent variation of the surface tension is therefore 



* 'Comptes Rendus,' vol. 114, 1892. 



