688 RICE ART. L 



65. Lippmann's Work on Electrocapillarity and Its Connection 

 with Gibbs' Equation [690] 



The paragraph marked (IV) makes a brief reference to 

 electrocapillarity, and in it Gibbs derives equation [689] which, 

 under the conditions that govern the use of the capillary electrom- 

 eter, reduces to a simpler form without the second term on the 

 right-hand side, and this is shown to be equivalent to [690] which 

 is the well-known equation due to Lippmann. The fact that 

 the tension in an interface between mercury and acidulated 

 water is dependent on the electric conditions was first discovered 

 by Varley (Phil. Trans., 161, 129 (1871)). Two or three years 

 later Lippmann began a fuller investigation of the phenomenon. 

 He derived the equation which goes by his name, and designed 

 the capillary electrometer to test his conclusions.* The essence 

 of his experiment is the use of an electrolytic cell consisting of 

 sulphuric acid solution and mercury electrodes; the anode has a 

 large surface exposed to the solution, the cathode a very small 

 surface (actually the section of a capillary tube). A current is 

 passed, and if it is not too large the density of the current per 

 unit area of the anode is very small, while the current density 

 at the cathode is so great that the cathode surface becomes 

 highly polarized while little or no effect is produced at the anode 

 surface, and the current is stopped by the reverse E.M.F. set up. 

 A new state of equilibrium is produced which varies as the 

 applied E.M.F. from the external source is increased up to a 

 limit beyond which the current cannot be stopped and equi- 

 librium becomes impossible. The theory which he gave for his 

 results is essentially the theory of a charged surface — purely 

 electrical with no hypothesis as to the physical occurrences at a 

 mercury electrode. A charged conductor like a body of mer- 

 cury has its charge on the surface. Looking at the surface ten- 

 sion as if it were due to tangential attractions in the surface, the 

 conclusion that a surface charge should reduce the surface ten- 

 sion by reason of the mutual repulsions of its parts is very 



♦ Comptes Rendus, 76, 1407 (1873); Phil. Mag., 47, 281 (1874); Ann. 

 chim. phys., 6, 494 (1875) and 12, 265 (1877); Comptes Rendus, 95, 686 

 (1882). 



