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 per- 

 petually renewed by a current flowing in the outer circuit. 



It will be seen that Helmholtz did not adhere to Davy'ss 

 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,f 

 "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. 



* In his celebrated memoir of 1847 on the Conservation, .o.Huergy. 

 t Exp. Ties., 918. 



