Electromotive Forces in the Voltaic Cell. 181 
from those of De la Rive and other older “ chemical theorists;”’ 
but they are (especially in later papers) expressed in so defi- 
nite and decided a manner that they have excited a sharper 
controversy than vaguer and more hesitating writings could. 
This indeed may be regarded as their special merit. The 
main objection which can be taken to them relates to the 
quantitative statements: these are vigorously made, but they 
seem unwarranted by facts accumulated by all other observers, 
though indeed some of his own experiments certainly seem to 
support them. It has also been objected that he misinterprets 
some of his experiments. 
He has got hold of the notion that the heat of combustion 
has some sort of relation to the Volta effect, and there I am 
heterodox enough to agree with him. But what the relation 
is, and how it acts, and what sort of potential-difference you 
ought to expect in accordance with theory, concerning all 
these things I am utterly at variance with him; and I deem 
it prudent not to attempt to represent views which I am 
unable to understand, because it is unlikely that I should do 
them justice. 
them, but in the recombination of the latter; and the E.M.F. of the cur- 
rent so produced is measured by the heat-value of this combination, just 
as the E.M.F. of any galvanic cell is measured by the heat-value of the 
chemical process going on in it. With a so-called contact-action the 
existence of the polarization-current, and obviously of every other cur- 
rent, has nothing whatever to do. The idea then suggested itself to seek 
for the cause of the production of electricity in the experiment of Volta, 
not in the contact of two metals, but in previous chemical actions of the 
surrounding media on their surfaces. I have expressed the opinion that 
so-called contact-electricity is produced by the oxidation of the metal in 
contact by the oxygen of the air, just as in galvanic cells it is evolved by 
oxidation of zinc. Ifthe supposition prove true, and it has proved true, 
the E.M.F. of his metal in contact in air must be measured and expressed. 
by their heats of combustion.” 
He then points out how all Volta tension series are in oxidation order, 
and relates approvingly De la Rive’s view that metalsin air were attacked 
not only by water-vapour but by dry oxygen, and that electricity is pro- 
duced by any kind of chemical action in proportion to the intensity of the 
chemical affinity. Then he gives his numerical theory and supporting 
experiments ; and finally concludes :-—“I believe we are entitled to say 
that no Scheidungskraft exists at the contact of two metals.” The fol- 
lowing must take the place of Volta’s law of the evolution of electricity :— 
“The difference of electric potential between two metals in contact is 
measured by the algebraic sum of the heat-value of the chemical action 
going on at each.” 
In his theory and experiments, and all through the rest of the paper, he 
considers the difference of potential equal to half the difference of heat- 
values, so the above last statement must be a slight numerical slip. 
The above extracts are among the most favourable. It would be easy 
to select passages from this, and from his other memoirs on the subject, 
of a more surprising character. 
