﻿of Metallic Surfaces in Aqueous Solutions. 311 



composition-cell, require a potential of about 1*7 Daniell's cell to 

 decompose the water ; with potentials of less amount the platinum 

 plates can be charged and discharged like condensers. They have 

 enormous electrostatic capacity. Mercurial surfaces equally admit 

 of polarization with hydrogen. A surface of mercury in dilute sul- 

 phuric acid, when made negative to the water by means of a powerful 

 battery, flattens out. If the mercury be replaced by an amalgam of 

 proper consistency, the flattening out is increased ; the reversal of the 

 current restores the amalgam to its original dimensions. By rever- 

 sing the process, electric currents can be obtained from mechanical 

 force. 



A large vessel on a board has within it two shallow funnels, which 

 are connected by means of glass tubes with similar vessels outside of 

 the large one. Pure mercury is poured into the funnels until they 

 and the outside vessels are one-third filled. By tilting the board, 

 mercury runs into the one funnel and out of the other, and thus 

 the surface in the one is made to increase while that in the other 

 decreases. Dilute sulphuric acid is poured into the larger vessel so 

 as to cover the two funnels ; the latter are connected together 

 through a galvanometer. 



If the mercury is pure and free from polarization, the tilting of 

 the board produces no electric current. On polarizing one of the 

 surfaces with hydrogen by a battery, it gives rise to a current through 

 the galvanometer, and thus shares the polarization over the two sur- 

 faces. If the battery be removed, on augmenting the one surface and 

 diminishing the other, a current of electricity is seen to pass through 

 the galvanometer. 



A convenient method of showing this experiment on a large scale 

 is to procure a gutta-percha trough 4 inches deep and 4 by 2 inches 

 broad. A partition of the same material 2 inches high divides the 

 lower half into two separate chambers : these are partly filled with 

 mercury ; amalgamated platinum plates, hung from a balance -lever, 

 dip into the mercury. On depressing one set of plates the others 

 are elevated, and thus the mercurial surface exposed to the fluid is 

 alternately augmented and diminished to a large amount. Twelve 

 of these arranged in series give a current of rather more potential 

 than one cell of Daniell's battery when the mercury is polarized with 

 hydrogen. The addition of a minute fragment of zinc to the mer- 

 cury maintains the polarization for a very long time, and the power 

 is considerably increased thereby. When a large surface of mercury 

 (25 circular inches) has been polarized with a power of half a Da- 

 niell's cell and is rapidly reduced to the diameter of ^ inch by 

 letting the mercury flow out of the funnel, some bubbles of hydrogen 

 gas appear just as the last of the mercury is running out, the decrease 

 of surface evidently augmenting the potential sufficiently to decom- 

 pose the water : floating a small piece of platinum on the mercury 

 renders this phenomenon much more distinct. 



All attempts to polarize the mercury with oxygen have failed to 

 give a current. By depolarizing the mercury with a battery until no 



