Permeability 
103 
So far we have considered the surface effects which are attributable 
to purely mechanical phenomena. The conditions at surfaces are 
however actually often more complex than has so far been indicated 
owing to the very general presence of electrical forces at the interface 
between two phases. 
We know for instance that if a metal is immersed in a solution 
of one of its salts a difference of potential between the metal and 
solution results, and that the existence of this potential can be 
explained by the tendency of metallic ions to pass into solution. 
When this takes place the metallic ions give to the solution a positive 
charge, leaving the metal plate correspondingly negatively charged. 
The solution of the metal can therefore only proceed until the mutual 
attraction due to the difference in potential is balanced by the 
tendency of the metal to go into solution; i.e. by the electrolytic 
solution pressure of Ostwald which is proportional to the ratio 
between the concentrations of atoms in the metal and ions in the 
solution. The difference of potential is given by the expression 
where P is the electrolytic solution pressure, p the osmotic pressure 
of the ions in solution, T the absolute temperature and R the gas 
constant. 
The same considerations hold for hydrogen as for a metal and 
are the basis of the well-known electrometric method now so uni¬ 
versally employed in physiology for measurement of hydrogen ion 
concentration. 
In the case of the surface between a solid electrolyte and its 
solution similar considerations hold, though the matter is complicated 
by the presence of two ions. In this case the potential at the interface 
between the solid and liquid phases is given by 
where P\P' are the electrolytic solution pressures of the kation and 
anion respectively and p', p' are the osmotic pressures of kation and 
anion in the liquid phase. 
In general, for the potential difference due to one ion at the inter¬ 
face between any two immiscible phases, we have for the value of 
the potential the expression 
3—6 
