386 Mr. J. Brown on Helmholtz's Theory of 



slowly, positive to the liquid. The slowness of the charging 

 is well shown in Lippmann's capillary electrometer, where 

 the diffusion is limited by the narrowness of the tube, and 

 the fine thread of mercuiy only slowly creeps to a position of 

 equilibrium different from that of freshly dropped mercury. 



8. " From this I conclude that, when a quickly dropping 

 but otherwise insulated quantity of mercury is in contact by 

 the dropping-point with an electrolyte, the mercury and elec- 

 trolyte can have no difference of potential. If, for example, 

 the mercury were positive, then would each falling drop form 

 on its surface a double layer, taking positive electricity out 

 of the mercury and so making its potential less and less until 

 it became equal to that of the liquid." 



9. Instead of the elaborate theory here described, necessi- 

 tating hypotheses of an admittedly unproved and rather 

 doubtful character, I think it can be shown that there is a 

 much more simple explanation of the action of dropping 

 electrodes. The arrangement constitutes simply a voltaic cell 

 in which the elements are clean mercury, electrolyte, tar- 

 nished mercury. 



10. Suppose a drop with a fresh surface is made to pro- 

 trude a little from the funnel-point immersed in a suitable 

 electrolyte, such as dilute sulphuric acid. Electrolytic action 

 at once takes place : a film of the mercury is oxidized or 

 chemically acted on by the anion of the electrolyte, and there is 

 formed an electric double layer, but with the signs of its com- 

 ponents the reverse of those supposed by Helmholtz, viz. 

 negative in the, uietal, j^ositive in the liquid, the difference of 

 potentials of its components being large because of the clean- 

 ness of the metal ; that is to say, the clean upper mercury is 

 at a loAver potential relatively to the electrolyte than the tar- 

 nished lower mercury, and therefore on joining these two a 

 current flows from the lower to the upper through the con- 

 necting wire, and continues till the upper has become as 

 tarnished as the lower. Let the drop now protrude a little 

 further. A corresponding enlargement of the surface takes 

 place, and a portion of fresh untarnished metal is exposed, 

 causing increased electrolytic action and current, which dies 

 away as the surface of the drop becomes again tarnished. 



Suppose now the drop to increase slowly and continuously 

 till it separates and falls off. As it increases the surface is 

 continually somewhat freshened, and so a constant but moderate 

 current is kept up. Moderate because the amount of fresh 

 surface being exposed is small compared with that of the 

 already tarnished surface. But if the drop increase quickly 

 the more rapid formation of fresh surface causes an increase 



