432 Mr. J. Larmor on the Molecular 



be investigated, it is to be expected that much light will be 

 thrown upon them by their relations to other purely surface- 

 distributions of energy, of which the best known is that lead- 

 ing to capillary phenomena. 



We are not required to explain the manner in which this 

 double layer at the surface of contact of two dissimilar sub- 

 stances is brought about. We may illustrate it by the rather 

 crude hypothesis that each molecule of an electrolyte consists 

 of a positively charged cation radical and a negatively charged 

 anion radical held together by electrical forces, but partly also 

 by their forces of chemical affinity, so as to be analogous to a 

 magnetic molecule with north and south poles ; that along the 

 surface of the electrode these molecules are all turned into the 

 same direction (polarized) by reason of the greater chemical 

 affinity of one of their constituents for the matter of the elec- 

 trode ; and that they thus form a double sheet analogous to a 

 magnetic shell. This illustration will at any rate show that it 

 is possible to give an account of the matter which shall be in 

 unison with the commonly received ideas of electrical and 

 chemical action, without having to speculate on the deeper 

 question of the relation of the material atom to its electrical 

 charge. 



The electro-capillary observations of Lippmann quoted 

 above, and the later ones of Koenig for various electrolytic 

 fluids, show that, for one definite amount of polarization, each 

 of these fluids in contact with mercury shows a maximum 

 surface-tension. As we have seen that the existence of an 

 electrical double layer on the surface must diminish the sur- 

 face-tension, it follows that the critical value *905 D for Lipp- 

 mann's acidulated water is that difference of potential which, 

 applied from without, just neutralizes the naturally existing 

 double electrical layer on the surface. It would seem there- 

 fore that the natural contact-difference of potential between 

 Lippmann's mercury and acidulated water is *905 D, and that 

 an absolute measure of a contact electromotive force has thus 

 been obtained. 



Appendix. 



The result that the mechanical effect of the electrification 

 on a charged condenser with thin uniform dielectric, whether 

 flexible or not, is equivalent to a uniform negative surface- 

 tension, has been derived in § 2 from the Principle of Energy 

 without the use of any analysis. 



The same result will of course follow from direct calculation 

 of the mutual forces exerted by the charged elements of the 

 surfaces on one another. As it forms a good example of the 



