748 REPORT—1885. 
current may produce, being an expenditure of energy in other than 
chemical directions, must lie outside the range of the above law, inas- 
much as they falsify the assumption of (iv,). Irreversible or frictional 
heat, and likewise reversible or thermoelectric heat, must thus be con- 
sidered separately. If ever the E.M.F. of a cell or a voltameter is 
seriously different from the value calculated on chemical ground it is to 
be examined whether there be not local heatings or coolings in the cell, 
whereby the total E.M.F. may be either diminished or increased, being 
really the sam of two E.M.F.s., a chemical one and a physical or thermo- 
electric one. 
But all this being understood and allowed for, as was really done by 
Thomson in his original paper in 1851, is the law sustained by experi- 
ment for cases other than the Daniell cell and a few other simple ones ? 
For instance, when two substances combine and produce heat, is any 
of the heat a direct result of their union, or are electric currents the 
direct primary result and heat a secondary or derived result? If any ot 
the heat be primarily generated then it is to be surmised that this por- 
tion of the whole combination energy is electrically inactive, having no 
effect on either the positive K.M.F. of a battery or the negative E.M.F. of 
a voltameter. Secondary actions for instance: it has long been a subject 
of debate whether the solution of zinc oxide in acid, or of zine sulphate 
in water, contributed its full quota to the energy of the current and the 
H.M.F. of a cell, or whether the combination of zinc with oxygen was 
more intrinsically effective. 
Joule showed how to attack the question for substances composing 
batteries, by immersing the battery in one calorimeter and its outer circuit 
in another. Whatever heat necessarily and intrinsically appears in the 
cell itself must be heat primarily and directly generated ; but if all the 
heat of the battery can be made to appear in the outer circuit, by making 
the resistance of this circuit high enough, it is proof positive that the 
primary result of the chemical changes is electric current, not heat; and 
so in Joule’s experiments with certain substances it appeared to be. 
More cells have since been tried, with the result that some directly heat, 
while others directly cool, themselves; though the bulk of their energy 
-still seems to take primarily an electro-kinetic form. 
Helmholtz has discussed the whole question from the point of view of 
reversibility, ¢.e. has applied the second law of thermodynamics as well as 
the first. Directly this mode of treatment is suggested it is almost 
obvious that a cell which heats or cools itself must have an E.M.F. 
variable with temperature, according to the law : 
Maes aa 
QdkK = J 7H, 
where H is the reversible heat generated in the cell during the passage of 
a quantity Q of electricity, and 2 is the rate of change of E.M.F. per 
a 
degree at the absolute temperature T. 
Hence, to investigate the sum of the reversible heat coefficients for a 
whole cell, it is only necessary to experiment on the variability of its 
E.M.F. with temperature, and to write 
H T dE 
Sil = 8 
Q J at 
