of potential at the contact surface of two metals, or of a metal 
and an electrolyte. Nernst explained the difference of poten- 
tial existing between two solutions of the same salt when the 
concentrations differ by the ionic theory. If a strong solution 
of hydrochloric acid is in contact with pure water the acid 
will diffuse into the water. But, since the hydrions and 
chloridions are capable of independent motion — the velocity 
of the hydrion being greater than that of the chloridion — the 
hydrions will travel faster into the water than the chloridions. 
But the hydrions carry a positive charge, while the chloridions 
carry a negative charge ; hence the water becomes positively 
charged owing to an excess of hydrions and the acid solution 
negatively charged owing to an excess of chloridions. In 
such a case as this, however, as the process goes on and the 
water becomes positively charged, an electrostatic repulsion 
will be produced, tending to retard the incoming hydrions 
and to accelerate the chloridions. This will go on until the 
electrostatic repulsion is so great as to cause the hydrions and 
chloridions to move into the weaker solution at the same 
rate. As the diffusion goes on the number of ions in the 
weaker solution will increase, and hence the tendency of the ions 
to diffuse in from the stronger solution will decrease, and the 
electrostatic repulsion necessary to maintain the equal veloci- 
ties of the incoming hydrions and chloridions will diminish. 
Hence the contact difference of potential will, in this case, 
diminish as the concentrations of the two solutions approxi- 
mate to each other. 
It is on this principle that Lippmann and von Helmholtz 
explained the working of the capillary electrometer, and as 
we shall have to consider an analogous explanation of certain 
vital phenomena, it may be as well to glance at the method 
by which the capillary electrometer re-acts to electrical 
forces. The capillary electrometer in its simplest form consists 
of a capillary tube in which mercury and sulphuric acid meet. 
The end of the tube dips into the sulphuric acid, which rises 
to a point where it is in equilibrium with the mercury, which 
descends the tube under a certain pressure. At the meniscus 
there will exist a contact difference of potential : and, since the 
mercury and the sulphuric acid solution are both conductors, 
the difference of potential will lead to an accumulation of 
electricity on the two sides of the bounding surface. The 
mercury is positive to the solution, and therefore the double 
layer of electricity at the bounding surface consists of posi- 
tive electrification on the mercury side and negative electrifica- 
tion on the solution side. If T be the observed surface ten- 
sion of the surface separating two media, and the area of this 
surface is increased by an amount S, the work which is done 
