Potential Energy of Liquid Surfaces. 457 



the superficial tension developed opposes the continuation of 

 the movement. 



Let us now cite two verifying experiments described by 

 M. Lippmann. 



Experiment I. — Two glasses, both containing mercury 

 covered with acidulated water, are placed side by side ; they 

 are put into electrical communication by means of a cotton 

 wick ; the two masses of mercury communicate with the ex- 

 terior by means of platinum wires which do not touch the 

 acidulated water. After putting these wires in communica- 

 tion with the extremities of a galvanometer-wire, the experi- 

 menter inclines one of the two glasses ; the needle of the 

 galvanometer is immediately deflected, indicating a current 

 passing through the liquid of the inclined glass towards the 

 glass that remains upright. 



By this operation the surface of contact of the mercury 

 and acidulated water in the first glass is enlarged ; this sur- 

 face must therefore be cooled, and the potential energy in it 

 increased ; but, in return, the electrical difference there has 

 diminished ; hence the current just indicated. 



M. Lippmann adds that the electrification is in time dissi- 

 pated ; this is quite natural, since the current arises only from 

 a difference of temperature. 



Experiment II. — Instead of inclining one of the glasses, the 

 surface of the mercury contained in it can be enlarged by 

 depressing it, dipping into it, for example, the end of a glass 

 or wooden rod ; when this is done, we see the galvanometer- 

 needle deflected immediately. 



Thus my second consequence would be found fully con- 

 firmed relatively to the case of the augmentation of the sur- 

 face of contact of two bodies which do not wet one another. 



I cannot avoid recalling a very curious experiment made 

 by M. Becquerel, because it furnishes a double verification of 

 the theory. He puts one of the extremities of a very delicate 

 galvanometer into communication with a platinum capsule, 

 while the other communicates with platinum sponge freshly 

 heated to redness ; he then fills the capsule with concentrated 

 nitric acid, when he observes the following facts : " the instant 

 the immersion is effected the sponge receives from the liquid 

 negative electricity, as if the platinum had been attacked ; it 

 is immediately polarized so as to produce a current in the 

 opposite direction, which continues for some time, diminishes, 

 and becomes ra7"*. Now does not the first current arise 



* Traite de VSlectricitS, tome ii. : see the chapter entitled, " EfFets e"lec- 

 triques produits dans les actions capillaires." 



