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 Faraday. 205 of the nineteenth century, the general opinion was in favour of some such conception as this. Helmholtz attempted to grasp the molecular processes more intimately by assuming that the different chemical elements have different attractive powers (exerted only at small distances) for the vitreous and resinous electricities: thus potassium and zinc have strong attractions for positive charges, while oxygen, chlorine, and bromine have strong attractions for negative electricity. This. differs from Volta's original hypothesis in little else but. in assuming two electric fluids where Volta assumed only one. It is evident that the contact difference of potential between two metals may be at once explained by Helmholtz's. hypothesis, as it was by Volta's; and the activity of the voltaic cell may be referred to the same principles: for the two ions. of which the liquid molecules are composed will also possess: different attractive powers for the electricities, and may be supposed to be united respectively with vitreous and resinous. charges. Thus when two metals are immersed in the liquid, the circuit being open, the positive ions are attracted to the negative metal and the negative ions to the positive metal, thereby causing a polarized arrangement of the liquid molecules near the metals. When the circuit is closed, the positively charged surface of the positive metal is dissolved into the fluid;. and as the atoms carry their charge with them, the positive: charge on the immersed surface of this metal must be perpetually renewed by a current flowing in the outer circuit.

It will be seen that Helmholtz did not adhere to Davy's's doctrine of the electrical nature of chemical affinity quite as: simply or closely as Faraday, who preferred it in its most direct and uncompromising form. "All the facts show us," he wrote, "that that power commonly called chemical affinity can be communicated to a distance through the metals and certain forms of carbon; that the electric current is only another form of the forces of chemical affinity; that its power is in proportion.