310 



Mr. J. Brown. 



[Nov. 18, 



quent disappearance of the difference of potential near them, and a 

 decrease of the former produces a subsequent decrease of the latter. 



In the case where the decrease of chemical action is due to drying, 

 it is, perhaps, sufficiently in correspondence with the theory to sup- 

 pose that when the film is partially removed, the effect it produces 

 should also partially disappear, although we may not be in a position 

 to specify the particular nature of the molecular process involved, or 

 whether there is included with it anything of the kind described next 

 following. 



In the case, however, of the hindrance of chemical action by the 

 formation of an intervening oxide or other compound layer, there 

 appears at first sight a difficulty, since it may be said that after the 

 separation and disposal of the two opposite charges on the two films 

 resident on the metals in contact as above described, the mere forma- 

 tion of intervening oxide on other layers would tend more to keep 

 these charges apart than to allow them to combine. 



It must, bowever, be remembered that in this case the films them- 

 selves are also in contact, and that a conducting connexion between 

 them may reasonably be supposed to provide a continuous leak, by 

 which the difference of potential suffers a constant reduction below 

 the normal, and would, therefore, disappear altogether, if the leak 

 were not greatly overpowered by continuous electrolytic combination 

 at the surface of the metal. 



The lateral resistance of the film must of course be very great, in 

 order that so much difference of potential may be observable ; but it 

 is obvious that unless the film were of infinite conductivity some 

 difference would still exist. 



Where the metals are not in immediate contact the films on (or 

 moisture in the pores of) the supports may readily provide the neces- 

 sary electrolytic connexion, considering the very small quantities of 

 electricity involved. I have observed that when the vulcanite piece, 

 M, fig. 1, carrying the quadrants, became damp from remaining some 

 days inclosed with the water in the vessel below it, the potential 

 difference was considerably decreased. 



The existence of "local action " or local currents in the film would 

 seem also not improbable, and would account for some of the pheno- 

 mena connected with tarnishing, otherwise difficult to explain. — 

 November 1, 1886.] 



33. I may here refer to an experiment made prior to these last but 

 related to them. It was based on the supposition that the molecules 

 of water in the condensed film on a metal must (in order to oxidise 

 it) have their oxygen atoms turned and attracted towards it, but if 

 we could reverse this arrangement by attracting the hydrogen atoms, 

 we might be able to retard the oxidising action. Possibly the fact 

 that the potential for a charged conductor is constant throughout 



