126 I)r. J. Erskine-Murray. 



34-. An old zinc plate which had probably not been polished for 

 many years gave 



+ 0-37 volt 



with the standard gold plate. It was next polished on glass-paper, 

 and gave 



+ 075 volt 



with the same standard. This shows that the ultimate effect of 

 exposure is to make the potential of zinc fall. I have found that in 

 some cases this fall is preceded by a slight rise, as in the case of tin 

 ( 32, 33) ; but the ultimate effect is in the negative direction. 



35. The potential of a silver plate which had been polished on 

 glass-paper remained constant for an hour. The experiment was not 

 continued further. 



36. Aluminium becomes gradually negative in air. In one case 

 the potential fell about 0*18 volt in a week. The time-change of this 

 metal, mainly on account of its large temperature-variation (see 

 Chap. IX), is rather difficult to determine. 



37. An iron plate which had not been cleansed for two months 

 rose O05 volt when polished. Hence the effect of exposure had been 

 to. make its potential fall slightly. 



38. I have included these results, for which, as for many others 

 like them, I can as yet give no definite explanation, in the hope that 

 they may serve as a basis for future experiment and generalization. 

 The great difficulty in all such experiments on contact electricity is 

 to define the circumstances and,, with .them, the causes of any given 

 variation. 



VI. Very Thin Liquid Films on Metals. 



39. In the earlier experiments it was found that, as a rule, copper 

 polished on glass-paper or emery-cloth was about 0*20 volt negative to 

 the standard alcohol- washed gold plate. 



On comparing this value with M. Pellat's results* I found that his 

 value for copper was about 0'20 positive to the same standard. I at 

 once determined to try his method of cleansing the plate so as to find 

 if the difference (0'40 volt) were due to that alone. For this purpose 

 a copper plate was carefully polished on glass-paper. It gave, with 

 the standard, 



0-20 volt. 



It was then washed with alcohol and allowed to dry in air. It now 



gave 



+ 0-22 volt 



with the same standard. The change due to the treatment is there- 

 fore -f 0'42 volt, and continues permanent many hours. 

 * Ann. Chim. Phys.,' 1881. 



