114 Dr. J. Erskine-Murray. 



his measurements of the temporary variations of potential due to 

 change of temperature of a copper, iron, or zinc plate ; and (3) his 

 experiments which prove the smallness of the changes produced in 

 the potential-difference of copper and zinc by varying either the 

 pressure or the nature of the gas surrounding them. 



2a. I shall now give a short summary of the results detailed in 

 the present paper : 



(a) Metals covered with non-conducting solid films of wax or 

 glass, except at their point of contact, give nearly the same potential 

 as the bare metals in air. The substitution of wax for air next the 

 metal only causes a small change which may be in the same direction 

 and of approximately equal amounts for metals whose potentials in air 

 are very different ; e.gr., the Yolta-potential-difference between zinc and 

 copper when both are coated with solid paraffin wax is very nearly 

 the same as that between bare zinc and copper. In this connection 

 I may mention that I have measured the potential of sodium coated 

 with wax and glass, and find it to be about 3*56 volts positive to a 

 standard gold plate. 



(6) A metal cleaned by careful polishing and scratching with 

 emery-paper or glass is less positive when its surface is in a sharply 

 scratched condition than when smoothed or burnished, the difference 

 frequently amounting to 0'2 or - 3 of a volt. 



This result is not in opposition to that of M. Pellat mentioned 

 above ( 2 (1)), for the conditions were different, as his plates were 

 washed with alcohol after polishing, while mine were not (see 

 Chap. Ill, and also 39 et seq.). 



(c) The temperature variations, between 15 C. and 60 C., of the 

 Volta-potential of many metals have been determined, both for clean 

 dry metals in air and for metals coated with liquid or non-conducting 

 solid films ; and it has been found that they are of considerable 

 magnitude in both cases. The curves representing the variation of 

 potential with temperature appear as if they should meet at a point 

 below 200 C., at a potential about 0'4 volt positive to a standard 

 gold plate at 16 C. ; this suggests that Volta-potential- differences 

 may possibly vanish at a very low temperature (see Chap. IX). 



(d) A liquid film, even if of extreme thinness, may cause a consider- 

 able change in the potential of a dry polished plate, which continues 

 permanent for many hours and even days after the disappearance of 

 the film. 



Two films of the same liquid opposed to one another on the surfaces 

 of two plates of different metals do not usually give zero potential- 

 difference, as solid conducting films of one material would do, but 

 give nearly the same potential-difference as the dry metals on which 

 they lie (see Chaps. VI and VII). 



(e) A very thin film of oxide on a metal produces only a very 



