concerning Voltds Contact Force. 383 



fitting plates of zinc and copper is of a moderate thinness, z, 

 say the hundredth part of a millimetre, or something of that 

 order. 



Hence, when the plates are approached from infinity to this 

 distance z, and are put into contact to establish the state of 

 polarisation, there will be a rush of charge out of the molecular 

 condensers into the newly formed condenser ; for they are 

 condensers in series, and every unit gained by one must be 

 lost by the other. 



But since the molecular condensers are to retain their 

 original potential-difference, this supply of electricity can 

 only be given by a change in capacity, i. e. a change in their 

 molecular thickness x ; such that 



dx 1 1 



or 



x 2 10- 



dx = 



10- 



the hundred-thousandth part of molecular magnitude. 



Thus the diminution in thickness of the superficial double 

 layer, or the approach of the oxygen atoms, in an extreme 

 case, has only to be a hundred-thousandth part of their whole 

 distance. This can hardly be called chemical combination : 

 indeed it is a scarcely perceptible chemical approach. 



Put in another way it amounts to this : that if the oxygen 

 atoms varied their distance from the zinc and the copper re- 

 spectively (one decreasing, the other increasing) by so much as 

 a tenth of 1 per cent., the electricity so set free or trans- 

 ferred from one surface to the other would be able to charge 

 an ordinary condenser of the same area in the intervening 

 space to a density of 100 c.G.s. ; which is considerably more 

 than ordinary air can stand. 



The only way to get a perceptible amount of real oxidation, 

 that is an approach over the whole molecular distance, is 

 either to bring the condenser plates themselves within molecular 

 distance of each other, which would squeeze out the films and 

 modify the whole action, or else to bring them within 

 molecular distance of different parts of one and the same 

 liquid conductor (so that the whole strain is thrown upon the 

 molecular film-condensers and none upon any intervening 

 material) ; and this is exactly what is done in an ordinary 

 voltaic cell. 



- ; [To be continued.] 



