and there will be equilibrium of the metallic phases /” to B with 
the electrolytes A to B. a rises or falls (as in fig. 3). 
It may be expected here as in II, that the line AGAA will toa 
large extent run close to the a-axis of J/, and that in consequence 
the concentration in M‚-ions in G and K will be very small. 
When no solid mixtures of the two metals are possible AC and 
BF coincide with the z-axis. # then lies above JB. 
If there is more than one compound, the sudden change of potential 
DE is repeated for such compound. Hrrscukowrrscn (Le.) has noticed 
these sudden rises with Zn,Cu, Zn,Ag, Zn Sb,, Cu,5n, Ag,Sn and 
has regarded them as evidence of the existence of these compounds. 
We should, however, be careful when drawing such conclusions 
as to the composition of alloys from measurements of potential dif- 
ference, for an alloy, obtained by melting together the two components 
and rapidly cooling the mass, is a badly defined substance and often 
contains more than two phases which are not at all in equilibrium. 
When they are brought into contact with an electrolyte consisting of 
a salt of the less noble metal, the unstable compounds in the alloy 
may be converted into the more stable ones and this reaction, which 
is caused by a short circuited element (unstable compound, electrolyte, 
stable compound), continues until only the two phases, which are 
really in equilibrium, remain. During this period the ZMF observed is 
not necessarily constant. 
The constant cells. 
As already stated, there is no equilibrium between the two elec- 
trolytes of a cell; they tend to form a homogeneous mixture by 
diffusion. The potential difference between two electrolytes is, howe- 
ver, generally very small and when the diffusion is small, it will 
change very little. As, moreover, the ZM F of a cell consists of 
the sum of the potential differences between the two electrolytes and 
between electrolytes and the electrodes, an apparent equilibrium and 
consequently a constant 4 M/F may be secured by making the dif- 
fusion as small as possible. 
To attain this it is necessary that there should be equilibrium 
between the electrodes and their electrolytes. But, in a constant 
cell, that equilibrium must not be modified when the current is allowed 
to flow and an interchange between the phases takes place in con- 
sequence. At constant ¢ and p the system must be invariant. 
If the electrode consists of a single metal, the concentration of the 
ions of the metal in the electrolyte must be kept constant. In case 
1 
nen te dS OO 
