( 186 ) 
Starting with the one pure metal and a solution, containing only 
the first metallic salt, it is found that on addition of the second 
metallic salt a small portion of the second metal will be separated 
and dissolved in the first one until the metallic phase is again in 
equilibrium with the electrolyte. 
This equilibrium requires that a, ==, or for dilute solutions: 
YB ae ag de RTG MD 
ue je 
ny, Pr ns Pa: 
ny Pp No P. 
or also Vie — El 
Pi Pr 
in which P?, and P, are the partial solution tensions of the two 
metals in the homogeneous metal phase. P, and P, are not constant 
here, but vary with the composition of the electrode. 
This formula was obtained by Nernst’) and verified by Oae ®) 
by means of the example Hg + AgNO, 2 HeNO, + Ag. 
The electrode now contains both metals, as may also happen in 
the case of non-homogeneous mixtures (D in fig. 1). The difference, 
however, is that there the metals form 2 phases and here only one. 
If the electrolyte is a mixture of fused salts or a solution in which 
the total concentration of the two cations is constant, then, at a 
constant temperature and pressure, the system will still be monovariant 
ny Ng nj Ng 
and the relation WP :WP, or Vp, :Vp, may still be variable. Once 
ny Ne 
however, the relation Vp,:V/p,, that is the composition of the electro- 
n Neo 
lyte, having been given, WP, VP, or the composition of the metal 
phase, is also determined and consequently also a. 
At each temperature a series of two such coexisting phases are 
possible. The potential difference continuously changes with their 
composition. 
In order to trace the general course of this a-line it must first be 
ascertained how P, and P, depend on the composition of the electrode. 
If, in the metal phase, there are « atoms of J/, and 1—v atoms 
of JV, and « is small, the lowering of the solution tension may be 
taken as proportional to the number of dissolved molecules of the 
second metal, which is analogous to the lowering of the vapour 
pressure in liquid mixtures. If we call the solution tension of the 
pure metal M, P,, then P, = P;(1—2). 
For small concentrations P, is proportional to the concentration 
1) Z. fiir phys. Ch. 22, 539. 
*) Z. fiir phys. Ch. 27, 285. 
