206 Mr. Joule on the Electrical Origin of Chemical Heat. 
the oxides* are by themselves, and at common temperatures, 
non-conductors of voltaic electricity, and therefore refuse to 
yield up their elements. But if on the principles of the theory 
we argue that the heat evolved on tne combination of one equi- 
valent with another is a measure of the intensity of the electric 
current passing between them at the time, we shall have the 
means of eliminating the electromotive force employed other-. 
wise than in separating the elements of the oxides. 
I suppose that there are three forces in operation, of which 
two are against, and one is for, a current engaged in electro- 
lyzing the solution of the sulphate of a metallic oxide. The 
first two are the affinity of the elements of the oxide, and that 
of the oxide for sulphuric acid; and the third, which is in a 
contrary direction to the two others and generally less than 
either, is the affinity of water for sulphuric acid. We elimi- 
nate the two latter forces as follows :— 
Ist. For &ine.—I find that 41 grs., or an equivalent of oxide 
of zinc, evolves 2°82 when dissolved in dilute sulphuric acid. 
This, which is the quantity of heat due to the intensity of cur- 
rent resulting from the difference of the affinities of sulphuric 
acid for the oxides of zinc and hydrogen, leaves, when sub- 
tracted from 13°83, 11°01, the corrected theoretical result, 
which I have given in the 4th column of the table. 
2nd. For Iron.—The black oxide is dissolved with such dif- 
ficulty by dilute sulphuric acid that the heat thereby evolved 
cannot be accurately measured. However, the dissolution of 
the hydrate is easily effected, the quantity of heat generated 
thereby being, per equivalent, 2°74. But we probably ar- 
rive nearer the truth by subtracting from the heat evolved by 
the dissolution of iron in dilute sulphuric acid, that portion 
which is due to the oxidation of the iron. In this way I have 
5°°2—0°-9=4°°3, the quantity due to the solution of protoxide 
of iron in dilute sulphuric acid. This, when subtracted from 
12°*36, leaves 8°-06, the corrected result of theory. 
3rd. For Copper.—The protoxide of copper does not dissolve 
readily in dilute sulphuric acid. Nevertheless by keeping the 
temperature of the surrounding atmosphere equal to that of 
the liquid, I obtained, per equivalent of oxide, 4°-0, a result 
which may, I think, be relied on. Subtracting this from 
9°-97, we obtain 5°97. 
4th. For Hydrogen, little correction is needed+. The li- 
quid used in the experiments made to ascertain the resistance 
to electrolysis of water was mixed with a small quantity only 
* | find that pure water is not at all decomposed by ten pairs of Smee’s 
battery in series, the current being thereby almost if not quite cut off. 
+ On this question I now refer to note * appended to page 205, 
