is S T. Now, the surface of separation between the mercury 
and acid solution with its double layer may be regarded as a 
condenser of which the two armatures are charged to a poten- 
tial difference E, where E is the contact difference of jDotential 
between the mercury and the solution. 
In any condenser of which the plates are kept at a con- 
stant difference of potential, the electrical forces tend to 
increase the capacity of the condenser, and hence, in the case 
of this double layer, there is a tendency for the area of the 
double layer to increase. That is to say, that on account of 
the electrical forces the area of the surface of separation 
between the mercury and the solution tends to increase, so that 
the electrical forces reduce the amount of work which has to 
be done against the surface tension when the area of the sur- 
face of separation is increased. Thus, if T ' is the value the 
surface tension would have, supposing no electrical double 
layer were present, the work done in increasing the area of 
the surface of separation by an amount S would be S T'. 
Therefore, S T, the actual amount of work done, is less than 
ST', the amount of work which would have been done if no 
electrical double layer existed, by the amount of work done 
by the electrical forces owing to the increase in capacity of 
the double layer. Thus, T, the observed surface tension, is 
less than T^, the surface tension if no double layer were 
present. 
"Suppose the contact difference of potential between the 
mercury and the solution be E, the mercury being at the 
higher potential. Then, if an external E M F be applied so 
that the wire X" (leading to the mercury) "is positive, the 
difference of potential between the mercury and the solution 
will be greater than E by the amount of the applied E M F, 
and hence the charges on the double layer will be increased, 
so that the surface tension will be decreased, and to keep the 
meniscus in its sighted position the head of mercury 
must be reduced. If, however, the applied E M F is in such 
a direction that it acts in the opposite direction to the contact 
difference of potential at the meniscus, then the strength of 
the double layer will decrease, and hence the surface tension 
will increase. This increase will go on till the applied E M F 
is exactly equal, and opposite to the contact difference of 
potential, for when this occurs there will be no double laver, 
and hence the surface tension will possess the value which 
it would have if no electrical charges were present. If the 
applied E M F is further increased, then a double layer will 
again be formed, but with the negative charge on the mercury 
side. This inverted double layer will cause a decrease in the 
surface tension, since the presence of such a double laver 
