Inefficacy of Contact

281

 943.	In dilute nitric acid.—Lead produced no effect at the
first moment;   but afterwards an electric current, gradually
increasing in strength,, appeared; which was able to deflect the
needle 20° or more, the hot metal being negative.    Cadmium
gave the same results as lead.   Tin gave an uncertain result:
at first the hot metal appeared to be a very little negative, it
then became positive, and then again the current diminished,
and went down almost entirely.
 944.	I cannot but view in these results of the action of heat,
the strongest proofs of the dependence of the electric current
in voltaic circuits on the chemical action of the substances con-
stituting these circuits:   the results perfectly accord with the
known influence of heat on chemical action.   On the other
hand, I cannot see how the theory of contact can take cogni-
sance of them, except by adding new assumptions to those
already composing it (862).   How, for instance, can it explain
the powerful effects of iron in sulphuret of potassium, or in
potassa, or in dilute nitric acid; or of tin in potassa or sulphuric
acid; or of iron, copper, tin, etc., in muriatic acid; or indeed
of any of the effects quoted?   That they cannot be due to
thermo contact has been already shown by the results with
inactive metals (919, 929);   and to these may now be added
those of the active metals, silver and copper in dilute nitric
acid, for heat produces scarcely a sensible effect in these cases.
It seems to me that no other cause than chemical force (a very
sufficient one), remains, or is needed to account for them.
 945.	If it be said that, on the theory of chemical excitement,
the experiments prove either too much or not enough, that, in
fact, heat ought to produce the same effect with all the metals
that are acted on by the electrolytes used, then, I say, that
that does not follow.   The force and other circumstances of
chemical affinity vary almost infinitely with the bodies exhibit-
ing its action, and the added effect of heat upon the chemical
affinity would, necessarily, partake of these variations.   Chemi-
cal action often goes on without any current being produced;
and it is well known that, in almost every voltaic circuit, the
chemical force has to be considered as divided into that which
is local and that which is current.   Now heat frequently assists
the local action much, and, sometimes, without appearing to be
accompanied by any great increase in the intensity of chemical
affinity;  whilst at other times we are sure, from the chemical
phenomena, that it does affect the intensity of the force.   The