be the negative charge on the zinc. When the charge has reached a certain value, solution of the zinc ceases ; this takes place when the potential difference due to the charges attains the value —
86 X 10-« , P
TTo = . In »
n p
for then it exactly counterbalances the effect of the solution pressure of the metal.
Potential Difference between a Metal and a Liquid. — When there is a potential difference V between two con- denser plates of area S which are at distance d apart, and when the insulator is the light ether, the quantity of electricity on the condenser is given by —
VS
If we know q, F", and S for one plate, which is polarised in an electrolyte, then d can be calculated, d being the distance between the polarised plate and the nearest layer of ions. Thus, Helmholtz (i) found in the case of polarised platinum 0*4 x 10" to 0*8 x lO" cm., which corresponds almost with molecular dimensions.
If we use a liquid metal, for instance mercury, the surface tension comes into play — that is to say, the surface of separation between mercury and an electrolyte in contact with it tends to -Electrolyte decrease. If the contact surface is chai^ged, + Hg
the electricity tends to bring about the fio. 44.
opposite effect. The electricity strives to spread itself over as large a surface as possible, and since it is bound at the surface it tends to increase this. Con- sequently, if we have an electrical double-layer at the contact between mercury and an electrolyte, the surface tension — i.e, the force with which the surface tends to diminish itself — is determined by the natural surface tension of the metal diminished by the force with which the electric charge
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