( 193 ) 
the electrolyte consists of a solution of the metallic salt, the presence 
of this salt in a third phase of constant composition, such as a solid 
hydrate, is required. These conditions are satisfied in the original 
form of the Clark cell, which contains on one side Zn and a saturated 
solution of ZnSO,, 7H,O and on the other side mercury and a 
saturated solution of He, SO,. 
If the electrode consists of two metals forming only one phase 
liquid solution, solid solution or compound), the current will neces- 
sarily cause a change in the equilibrium, because the ratio of the 
metals in the electrolyte is generally different from that in the elec- 
trode. The equilibrium will only then become invariant when a 
second metallic phase appears. 
If there is no solvent, and the electrolyte therefore consists of a 
mixture of the two fused metallic salts, its composition is completely 
defined by the presence of 3 phases of the 3 components (M,, M, 
and the common acid radicle). If, however, there is also a fourth 
component in the form of a solvent, a fourth phase must be present 
to make the equilibrium invariant such as’ the crystals of one of the 
two salts. The choice between the two salts is not an arbitrary one, 
but is regulated by the required relation of the concentration of the 
cations and the solubility of the two salts. 
From this it follows, that on passing the current, only that metal, 
the salt of which is present in a second constant phase, can dissolve 
or deposit on the electrode (which consists of a mixture of the two 
metal phases). The ratio of the quantities of the two metallic phases 
must be regulated accordingly. 
An example, from among the commonly used normal elements 
is the Weston cell in which the Cd-electrode consists of a mixture 
of a liquid phase (Hg with 5°/, Cd) and a solid one (Hg with 14 °/, 
Cd)') while the surrounding electrolyte consists of a solution of 
© 
Ap 5 
Cd SO, and traces of Hg, SO,, saturated with Cd SO, a H,O. 
The Clark cell in which a zine amalgam with 10—15°/, of zine 
is used, is clearly a similar combination. 
1) Byl, lee. 
