'concerning Volt a' $ Contact Force. 381 



copper ; instantly an avenue for the relief of the strain is 

 provided, positive electricity flows across the junction into 

 the zinc, takes up its abode on the surface facing the oxygen 

 atoms, the oxygen atoms approach slightly nearer all round, 

 the surrounding molecules all round are polarised, their nega- 

 tive poles inwards, their positive poles outwards, a double 

 layer is set up on the zinc, and lines of force appear all 

 through the surrounding dielectric. These lines reach from 

 the zinc free-surface to the copper free-surface, distributing 

 themselves according to the ordinary rules of electrostatic 

 capacity, and their terminations on the copper constitute its 

 negative charge. In other words, the oxygen atoms are 

 slightly further removed than before from the copper, and it is 

 to that extent protected even from its previous small tendency 

 to oxidation ; on the other hand, the oxygen atoms have 

 approached slightly nearer to the zinc. Thus my doctrine is 

 not a doctrine of chemical combination, but of chemical 

 approach. The oxygen atoms approach nearer the zinc, recede 

 further from the copper, and by these slight motions produce 

 the whole Volta effect. 



But this it may be said is nothing but a voltaic cell, except 

 that the electrolyte is replaced by a dielectric, and so the 

 current is momentary, not continuous. Precisely, that is my 

 meaning. 



But then, it will be objected, if a current did pass, there 

 would be actual oxidation ; since for every electric unit that 

 passes in a cell there must be an electrochemical equivalent 

 of zinc oxidised. To this I agree. 



Well then, pursues the objector, consider a condenser of 

 great capacity with its alternate plates made of zinc and 

 copper, very close together, and then short-circuit it at some 

 point ; there will instantly be a great flow o£ electricity across 

 the junction to charge the condenser, and accordingly there 

 will be a perceptible amount of zinc oxidised. Your mere 

 approach of oxygen atoms without combination will no 

 longer serve. 



At this point I appeal to arithmetic. The charge on an 

 atom is of the order 10 -11 electrostatic units. The oxygen 

 atoms immediately in so-called contact with the zinc are 

 constituents of its condensed air-film, a film so dense as to be 

 virtually a liquid, since it is subject to the intense molecular 

 pressure. Not really a liquid, because it is far above the 

 critical point, but not perceptibly differing from a liquid in 

 density. If this be objected to, it is not essential to my 

 argument, because there is plenty of margin ; only it seems 

 to me likely to correspond roughly with the truth. The 



