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



smaller than the Helmholtz view requires.* If such a view as this were true, the 

 electrostatic effect would be insufficient to account for the observed variation in the 

 surf'atv trnsi.ni. Assuming tin- potential difference, the existence of the electrostatic 

 effect is scarcely o{>en to doubt, it is only the relative importance of the effect that 

 may be questioned. The electrostatic effect apart, the Helmholtz view assumes 

 that the nature of the transition from the solution to the mercury is (through a con- 

 siderable range) independent of the potential difference and of the nature of the 

 solution. 



Several published observations show that there are cases for which this assumption 

 cannot be true. There are many facts in favour of the view that for a given 

 potential difference there is a corresponding condition (of the partly physical, partly 

 chemical kind, pictured by WARBURG) of the space bounded on the one side by the 

 mercury, and on the other by the sensibly homogeneous solution. Obviously the 

 surface tension will depend upon the nature of the transition through the surface 

 layer. The mode of transition may depend only on the chemical nature of the 

 solution and the potential difference across the space in which the transition takes 

 place. On this view the electrostatic effect and the mode of transition for a given 

 solution will l>e determined by the potential difference, and therefore the surface 

 tension will l)e fixed by the potential difference. It remains to determine how the 

 relation between the surface tension and the potential difference depends upon the 

 chemical nature and concentration of the solution. 



It is scarcely necessary now to set forth the arguments against the second hypo- 

 thesis of the Helmholtz theory of the electrometer ; but I shall endeavour to show 

 by consideration of observations of the type held to throw greatest doubt upon the 

 theory, that the first hypothesis gives results in close accord with the facts, and need 

 not therefore be abandoned, even if the second should be proved untenable. 



RELATION BETWEEN THE LIPPMANN-HELMHOLTZ THEORY AND OTHER THEORIES OF 



ELECTROCAPILLARY PHENOMENA. 



It may be well to point out the relation such results bear to the theory of 

 WARBURG, which is, perhaps, the most strongly advocated in opposition to the 

 Ilrlmholtz theory. Strictly speaking, the Warburg theory deals only with the 

 ascending branch of the curve. It ascribes the increase in surface tension to the 

 diminution in the concentration of a mercury salt in the neighbourhood of the 

 capillary meniscus. According to WARBURG the effect of an E.M.F. established 

 IK -tween the terminals of the electrometer is to convert the latter into a kind of 

 concentration cell. Part of the E.M.F. of this cell will presumably be due to a 

 potential difference within the electrolyte. The conclusions drawn later depend upon 

 observations of the descending portions of capillary curves, which are usually much 



* Cf, WARBURG, 'Wied. Ann.,' 1890, vol 41. 



