VOLTAIC EXCITATION AFFECTED BY TEMPERATURE. 
101 
ducing also only a feeble current. Tin gave the same result, but the current was 
scarcely sensible. 
1954. In dilute sulphuric acid— Copper and zinc, after having produced a first 
positive effect at the hot metal, had that reversed, and a feeble current was produced, 
the hot metal being negative. Cadmium gave the same phenomena, but stronger 
(1918.). 
1955. 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 di- 
minished, and went down almost entirely. 
1956. 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 constituting 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 cognizance of them, except by adding new assumptions to those 
already composing it (1874.). 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, &c. 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 (1931. 1941.) ; 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. 
1957- 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 exhibiting its action, and the added effect of heat upon the 
chemical affinity would, necessarily, partake of these variations. Chemical 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 (1 120.). 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 electric 
current, however, is not determined by the amount of action which takes place, but 
