422 PROCEEDINGS OP THE AMERICAN ACADEMY 



II. In the case of mercury — at least in contact with any I'KjuiJ — 

 the superficial tension is independent of the chemical constitution of 

 the liquid, and depends only upon the difference of potential at tln^ 

 surface. The chemical constitution of a liquid affects the superficial 

 tension only by changing the difference of potential. The experi- 

 ments of Bloudlot* make it probable that this law can be extended to 

 the case of platinum. 



III. When, by mechanical means, the surfiice of a liquid is de- 

 formed, the difference of potential at this surface varies in such a 

 direction that the change in superficial tension developed by the vaii.i- 

 tion of potential is opposed to the continuation of the deformation. 

 This agrees with Lenz's law. 



IV. In the case of mercury and dilute sulphuric acid, the super- 

 ficial tension increases nearly proportionally to the difference of 

 potential ; until the difference of potential equals 0.9 Dauiell. Any 

 further increase of the difference of potential causes a diminution of 

 the superficial tension. 



The experiments of Lippmann explain the cause of the variations 

 observed in measurements of the superficial tension. These variations 

 were first noticed by Quincke f and were attributed by him to the 

 presence of impurities. 



Lippmann's experiments show that a close relation exists between 

 contact electricity and superficial tension. This relation may be ex- 

 pressed mathematically as follows : — Let k, as before, represent the 

 coefficient of superficial tension. S represents the area of the surface, 

 and e the difference of potential at the surface; k is a function of e, 

 and is of course independent of S. • 



If, by the action of any force, the surface is increased by d S, the 

 work done is 



d W= — kdS. 



There is at the same time a difference of electrical potential created, 

 the action of which is to oppose the force. An electrical separation 

 takes place across the surface. The quantity, m, of electi-icity sepa- 

 rated is a function of the surface and of the difference of potential : 



m = (}) {S e), 



dm = XdS-\- Tde. 



* Journal de Physique, torn. x. 



t Ann. de Poggendorfl" vol. cxxxix., 1870. 



