Affinities of the Elements of Water. 299 
sity.* It is therefore quite independent of the magnitude of the surfaces which are 
in action, as (for example) the measure of gravity at our earth’s surface is inde- 
pendent of mass. By establishing a measure of it, we of course obtain a measure of 
the forces producing it when they can be assigned ; and though these are in gene- 
ral mixed in considerable complexity, yet it is often possible to separate their 
values. For instance, in the common voltaic cells, if we conceive a plate of pure 
(or amalgamated) zinc immersed in dilute sulphuric acid, along with, but not 
touching, a plate of copper, no sensible action takes place, notwithstanding the 
existence of very energetic affinities tending to produce it, till an electric current 
is permitted to circulate by connecting the metals. Then the zinc is oxidized 
at the expense of a portion of the water present, whose equivalent hydrogen is 
evolved as gas. The forces which tend to favour this must be regarded as 
exciting the current, those which are opposed to it as antagonists or negative. 
We have, therefore, 
= = 
1. Affinity of zine for oxygen. 5. Affinity of hydrogen for oxygen. 
2. i: sulphuric acid for oxide 6. = copper for oxygen. 
of zine. Te S zine for hydrogen. 
3. ss copper for hydrogen. 8. 5 sulphuric acid for water. 
4. e water for sulphate of zinc. r 
Of these 3 is probably insensible, and 7 feeble; and it is known that 2, 4, and 
8 have no appreciable effect. If, therefore, we represent the affinity of zinc for 
oxygen by z.0, &c., we have 
E = 2.0 — h.o —cu.0. (a) 
But if, as in Daniell’s battery, instead of permitting the hydrogen to be evolved, 
we present to it, at the place of its evolution, a substance from which it can obtain 
oxygen, such as sulphate of copper, that change increases the intensity of the cur- 
rent by ho. — cw.o, and the total effect is 
E= 2.0 —2¢uU.0. (b) 
* It seems impossible to disturb the molecules of a body from their normal condition without 
exciting electricity ; compression, friction, fracture, heat, magnetism, chemical action, vaporiza- 
tion, and congelation, are familiar examples: and, on the other hand, electricity can be made to 
reproduce several of these at pleasure. 
