Chemical Affinity in terms of Electromotive Force. 3 
or ina “gravity ”’ cell) is calculable by taking the difference 
between the constants ; 7. e. by employing the formula 
K=C,—C,, 
where E is the E.M.F. of the combination, and C,; and C, the 
two constants involved, such that if C,—C, is of + sign the 
metal to which C, applies acquires the higher potential, whilst 
if C;—C, is of — sign the metal to which C, applies acquires 
the higher potential. For instance, if C, apply to zine and C, 
to copper in sulphate solution, C;—C,= +1'114, and copper 
acquires the higher potential ; whilst if C, refer to magnesium, 
C,—C,= —°725, and zinc acquires the higher potential. 
165. The value of the voltaic constant thus applicable to any 
given metal is variable, within certain limits, with its surface- 
conditions (according as bright fused metal, electro-deposited, 
amalgamated, &c.), and also with the nature and strength of 
the solution of its salt in which it is immersed, and probably 
also with the temperature, but is independent of the nature of 
the other half of the cell. In order to fix the numerical value 
some convention must be made as regards the zero to be 
adopted as starting-point. For this purpose, and in view of 
the comparisons desired to be instituted between the values of 
C,—C, and the heat-evolutions due to the net chemical 
actions taking place, the most convenient assumption appears 
to be this :—that the potential of an amalgamated zinc plate 
immersed in a solution of a given salt of zinc of given mole- 
cular strength is assumed to be zero, for the purpose of fixing 
the numerical value of the potential of a plate of the metal to 
be examined when immersed in an equally strong solution of the 
corresponding salt of that metal, the two solutions being allowed 
to interdiffuse (e. g. in a “ gravity”’ cell or in a “ Raoult” 
cell). For instance, when plates of electro-copper or cadmium 
are respectively immersed in solutions of the sulphates of these 
metals, and opposed to amalgamated zinc immersed in zinc- 
sulphate solution, the molecular strength throughout being 
1:0 MSO, 100H,0, the potentials of the copper and cadmium 
plates are respectively +1:114 and +-360 volt higher than 
the potential of the zinc plate; which values are accordingly 
the “voltaic constants’’ for electro-copper in 1°0CuSOQO, 
100H,O, and for electro-cadmium in 1:0CdSO, 100H,O 
respectively: whence it results that the H.M.F. of a cell set 
up with electro-copper and electro-cadmium plates and solu- 
tions of the sulphates of these metals of molecular strength 
1:0MSO, 100 H,0 will be 
H=C,—C,=1:114—°360=°754, 
which in point of fact is the case. On the other hand, when 
magnesium is opposed in the same way to zinc in sulphate 
B 2 
