Chemical Affinity in terms of Electromotive Force. 5 
performed at the expense of sensible heat*. Twenty such 
combinations have been examined, vide § 209. 
166. An attentive consideration of these and other allied 
phenomena leads to the conclusions that, since only in excep- 
tional cases does the formula hold, 
Ey =Hh= C, ram C, 
(where Hy is the E.M.F. corresponding to the net heat-evolu- 
tion due to chemical change in the cell), the H.M.F. actually 
generated must be due to other causes in addition to the heat of 
chemical change ; and that all the observed phenomena may 
be clearly accounted for if it be assumed that the difference 
of potential set up between the two plates of the voltaic com- 
binations examined 7s due to the superposition of two causes— 
one, the net heat-development or difference in heat of forma- 
tion of the two electrolytes surrounding the two plates respec- 
tively; the other, an action akin to that taking place in a 
thermoelectric combination, and, like that action, expressible 
numerically by the algebraic difference between two constants 
applicable to the two halves of the combination respectively ; 
so that if £, and k, be the two thermovoltaic constants appli- 
eable respectively to the metals (and corresponding with the 
voltaic constants C, and C,), and Hy be the E.M.F. corre- 
sponding with the difference between these heats of formation, 
then H=(Q,—C,=Ey +h, — hy. 5 
If, then, k;—%, does not differ greatly from zero, E and Ey 
are nearly equal; if, on the other hand, £;—/, has a consider- 
able value, E exceeds or falls short of Ey according as that 
value is positive or negative in sign. If k,—, is negative in 
sign and also is numerically greater than Hy, E is of the 
opposite sign to Hy. This case corresponds with the class of 
cells above referred to in which the metal actually acquiring 
the higher potential is the one which would be expected to 
acquire the lower one, were the heat of chemical action the 
sole cause of potential difference in the cell. 
The value of the thermovoltaic constant applicable to any 
given metal necessarily varies in the same way as that of the 
voltaic constant ; 2. e. it is to some extent dependent on the 
nature of the metal surface, and varies with the nature and 
strength of the solution of the salt of the metal employed to 
* A somewhat analogous phenomenon is presented by gravity-diffusion 
cells set up with platinum plates and nitric acid of different degrees of 
concentration. With certain strengths an E.M.F. is set up superior to 
that calculable from the heat of intermixture of the two acid fluids; so 
that when a current flows through a large external resistance, part of the 
work done is performed at the expense of sensible heat; whilst the passage 
of the current reduces the concentrated acid to lower oxides of nitrogen 
(precisely as in a Grove’s cell), thereby absorbing additional heat. 
