86 MR. S. W. J. SMITH ON Till- NATUKE OF ELECTROCAPILLARY PHENOMENA. 



KI ind KC1 respectively is rendered evident if a drop of phenol phthalein solution is 

 added to the solution in each experiment. The solution turns pink very much more 

 rapidly in the case of the KI than in the case of the KC1, showing apparently tlmt 

 tin- combination of the mercury with the iodine is very much more rapid than with 

 the chlorine, of the solution into which it is dropped, under the conditions described. 



SUMMARY. 



1. The Lippmann-Helmholtz theory of the capillary electrometer contains two 

 assumptions. 



2. The first assumption would apply to any electrolytic cell. A deduction from it, 

 which would apply to any cell having a large and a small electrode, is that the 

 variation of the potential difference at the capillary electrode of an electrometer is the 

 same as that of the applied electromotive force. 



In order to trace the relation between surface tension and potential difference on 

 the view that this first assumption is correct, it is necessary to eliminate the possible 

 effect of depolarization upon the form of the electrocapillary curve i.e., the curve 

 which shows the relation between the surface tension and the applied electromotive 

 force. A direct method of examining the depolarization current is described and 

 applied. An estimate of the magnitude of the depolarization effect is given, and the 

 circumstances under which the effect may become appreciable are discussed. 



3. The second assumption of the Lippmann-Helmholtz theory, that the electro- 

 capillary phenomena are controlled by a simple variation of the electrostatic surface 

 energy, leads to two conclusions, each of which is beset with difficulties. 



(a.) The form of the electrocapillary curve is remarkably dependent upon the 

 nature and concentration of the electrolyte, and depolarization is quite insufficient to 

 account for the dependence. 



(b.) The conclusion that the potential difference between the solution and the 

 capillary electrode is zero when the surface tension has its maximum value, leads to 

 the necessity for assuming large potential differences between certain solutions. 



4. The hypothesis that the potential difference between equally concentrated 

 solutions of potassium chloride and iodide is negligible possesses a high degree of 

 probability. It has been shown by previous observers that if this hypothesis be true 

 the points of maximum surface tension on the electrocapillary curves for the above 

 solutions cannot have the significance which HELMHOLTZ'S theory gives them. 



It is shown in the paper that the first hypothesis of the Lippmann-Helmholtz 

 theory is in striking accord with this hypothesis concerning the potential difference 

 between KC1 and KI when the very definite " descending " branches of the electro- 

 capillary curves are considered. 



5. If both the hypotheses just mentioned be true, we get the result that the 

 surface tension of mercury (for a certain range of potential differences) in two solutions 



