Electromotive Forces in a Voltaic Cell. 381 



by a sufficient E.M.F., it is of small or no moment whether 

 the ion be really set free, or be dissolved, or otherwise kept in 

 the liquid. But when the electrode is of such a metal that an 

 ion can combine with it, that ion retains its charge, and 

 accordingly very little B.M.F. is sufficient to decompose the 

 liquid under such circumstances. 



Without this statement it might be objected, that when a 

 piece of zinc is plunged into acidulated water, since it has a 

 strong attraction for positive electricity, it ought to attract 

 the positively charged hydrogen atoms up to itself, instead of 

 oxygen as commonly supposed. I suppose Helmholtz would 

 admit that it must do this to begin with, when isolated from 

 other metals, on his theory, but that it has no way of sepa- 

 rating and liberating atoms because it cannot get rid of their 

 charges. If it be kept at a sufficiently negative potential 

 artificially it can indeed dispose of these charges, and it then 

 does attract and liberate hydrogen. But contact with copper 

 raises it to a positive potential, and it then attracts negatively 

 charged oxygen, rather than hydrogen, and combines with it ; 

 while the negatively electrified copper seizes hydrogen atoms, 

 tears their charges away from them, and sets them free. 



I must confess, however, that I feel a difficulty here. The 

 natural tendency of zinc is, by hypothesis, to attract positive 

 and repel negative atoms ; but, while a very feeble positive 

 electrification applied to it is sufficient to reverse this tendency, 

 a comparatively strong negative electrification is needed to 

 enable it to exert that force which by the hypothesis is sup- 

 posed to be natural to it. 



In considering a Daniell cell, zinc /Z11SO4 / CuS0 4 / copper, 

 Helmholtz obtains the energy producing the current, by point- 

 ing out that electricity is removed from copper, which only 

 attracts it feebly, and given to zinc which attracts it strongly. 

 Zinc goes into solution and becomes positively charged, while 

 an equivalent of equally positive copper comes out ; and since 

 this results in a gain of energy, it follows that electricity must 

 do more work in going to zinc, than in going to copper. 



But if zinc attracts electricity so much more strongly than 

 copper, why is it so easy to drive electricity across a copper/zinc 

 junction ? And why does no energy manifestation result at 

 the junction from such an operation ? 



The answer probably is, because of the large charge already 

 existing at the junction ; the zinc has pulled as much positive 

 electricity out of the copper as it wants, and there exists at 

 the contact an electrical double layer, whose existence makes 

 it quite easy for extraneous electricity to flow either way 

 across the junction. 



But, then, if zinc pulls so much electricity out of a piece of 



