304 



Mr. J. Brown. 



[Nov. 18, 



potential between tbe varnish on tbe zinc and the film on the opposed 

 copper plate did not fall below 0"29D taken while the zinc was hot. 



On cooling for 15 minutes the value rose to 0"35D, and after some 

 hours to 0*46D, an effect easily caused by the passage of moisture 

 into the varnish. Fifteen hours after it had fallen to 0*4D, in- 

 dicating probably the formation of a partially protective coat of 

 oxide under the varnish. 



The commonly accepted value for plates of these metals recently 

 cleaned in air is about 0'8D. 



22. I have also endeavoured to obtain this result, although without 

 much hope of success, by immersing the platinum-zinc quadrants of 

 the electrometer, fig. 1, in naphtha which had been digested with 

 metallic sodium for several months in order to free it if possible from 

 oxidising matters. Metallic sodium was also placed on tbe bottom of 

 the electrometer. The naphtha was contained in a glass jar, aud 

 connected by a syphon to the stopcock S. The difference of poten- 

 tial of the polished metals measured in air was 0'88 D. The naphtha 

 was now ran in for five minutes, when its level was about 0*05 inch 

 over the surface of the quadrants, and about the same distance below 

 the needle. The following observations were then made ; t denoting 

 time elapsed after the metals had become covered : — 



* = 8m. 40m. 1 h. 40 m. 2 h. 40 m< 4h. 7k 12h. 22h. 



66 D G61D 49D 05D 0'49D. 39D. 039D 0-31D. 



Two hours after this last observation the naphtha was partially 

 run out of the instrument, so as to bring its surface below the level 

 of the quadrants, when a measurement gave only 0"23D. It is 

 difficult to see why the thin film of naphtha still present on the 

 plates should possess a more protective power than the bulk of the 

 liquid. 



It may be that there was in the naphtha some small quantity of 

 water or other substance chemically active towards the zinc (the 

 deterioration of sodium or potassium under naphtha is well known) 

 which would soon become exhausted from the thin film, but the 

 hypothesis is open to doubt. 



23. Faraday considered that electrolytic " decomposition and the 

 transference of a current are so intimately connected that one cannot 

 happen without the other" ('Exp. Res.,' vol. 1, p. 252). I think it 

 in the highest degree probable that the same theory applies to the 

 production of the momentary current which gives rise to a difference 

 of potential near the surfaces of two apparently dry metals when 

 placed in contact, and that the electrolyte then active consists of the 

 water supplied from the air and condensed in invisible films on them. 

 Hence the chemical action producing the electrical effect is not only 



