376 Conduction in Solutions and Gases, 



polarized mercury cathode in acidulated water, there must be 

 on the electrode itself a negative charge : the surface of this 

 electrode in the polarized state may be supposed to be either 

 mercury, or mercury covered with a layer of hydrogen. In 

 the solution adjacent to the electrode, there must be an excess 

 of cations and a deficiency of anions, so as to constitute the 

 other layer of the condenser : these cations may be either 

 mercury cations dissolved from the electrode, or the hydrogen 

 cations of the'solution. 



It was shown in 1870 by Cromwell Fleetwood Varley* that 

 a mercury cathode, thus polarized in acidulated water, shows a 

 tendency to adopt a definite superficial form, as if the surface- 

 tension at the interface between the mercury and the solution 

 were in some way dependent on the electric conditions. The 

 matter was more fully investigated in 1873 by a young 

 French physicist, then preparing for his inaugural thesis, 

 Gabriel Lippmann.f In Lippmann's instrumental disposition, 

 which is called a capillary electrometer, mercury electrodes are 

 immersed in acidulated water : the anode H Q has a large 

 surface, wkile^the cathode H has a variable surface S small in 

 comparison. When the external electromotive force is applied, 

 it is easily seen that the fall of potential at the large electrode 

 is only slightly affected, while the fall of potential at the small 

 electrode is altered by polarization by an amount practically 

 equal to the external electromotive force. Lippmann found 

 that the constant of capillarity of the interface at the small 

 electrode was a function of the external electromotive force, and 

 therefore of the difference of potential between the mercury 

 and the electrolyte. 



Let V denote the external electromotive force: we may, 

 without loss of generality, assume the potential of [ to be zero, 

 so that the potential of H is - V. The state of the system may 

 be varied by altering either V or /S; we assume that these 



* Phil. Trans, clxi (1871), p. 129. 



f Comptes Rendus Ixxvi (1873), p. 1407. Phil. Mag. xlvii (1874), p. 281. 

 Ann. de Chim. et de Phys. v (1875), p. 494, xii (1877), p. 265. 



