Electromotive Forces in a Voltaic Cell. 383 



its numerical exactitude. This is the datum required for 

 Helmholtz's theory. 



It may thus be said that while the writer's view rests on 

 two admitted facts which it does not attempt to explain, but, 

 assuming them, explains other things from them ; Helmholtz's 

 theory rests on one unverified hypothesis, which, being granted, 

 everything else follows ; except indeed the numerical equality 

 of atomic charges, which has to be regarded as experimentally 

 , demonstrated in both theories. 



According to my view, the zinc of a cell pulls up the nega- 

 tively charged oxygen atoms (or S0 4 radicals, whatever the 

 real ions may be) to itself, not because they are electrified, but 

 because they are oxygen. The electrical current produced 

 is a secondary result caused by the chemical action. 



According to Helmholtz, the zinc pulls the oxygen atoms 

 because it is itself electrified by contact with copper, and 

 because they are oppositely electrified by contact with hy- 

 drogen. The chemical action resulting is a secondary result 

 caused by the electric forces. 



Helmholtz's is thus a true " contact theory," and is in many 

 respects like Sir W. Thomson's. Sir William postulates an 

 attraction of zinc for copper, and from this explains the Volta- 

 effect and the production of a current. I am unable to picture 

 to myself exactly hoiv the attraction of zinc for copper results 

 in a difference of potential when they are put into contact; 

 but undoubtedly such a force, if granted, would put a supply 

 of energy at disposal which could account for the Volta-effect. 

 And a difference of potential thus set up may result in an 

 electrical decomposition of water and maintenance of current, 

 just as in Helmholtz's theory. 



I must confess that I am unable to feel quite comfortable 

 about energy considerations with either of these theories of 

 the voltaic cell ; because it seems as if the metallic junction 

 were, after all, driving the current, whatever be said about 

 chemical energy. But in this I must certainly be wrong ; 

 that is to say, there must be some thorough way of reconciling 

 these, by such men advocated, views with energy considerations, 

 though I am unable satisfactorily to perceive it. 



Both these contact-theories, in explaining the Volta-effect, 

 ignore the existence of the oxidizing medium surrounding the 

 metals. My view explains the whole effect as a result of this 

 oxygen bath, and of the chemical strain by it set up. The 

 other theory which likewise took account of the atmosphere 

 is the old chemical one ; but this leaves the aspect of the 

 matter exceedingly vague. The strictures applied to it by 

 Prof. Helmholtz (Faraday Lecture, 1881) are well deserved: — 



" The so-called chemical theory of Volta's fundamental 



