122 Dr. J. Erskine-Murray. 



material of the polishing agent is eliminated by using the same tool 

 to produce different states of surface. Thus a copper plate was 

 scratched very roughly with a steel tool, and gave 



-0-07 volt 



with the standard plate, then burnished slightly with the same tool 

 it gave 



-0-02 volt. 



It had thus risen O05 volt on account of the alteration produced 

 in smoothing its surface. Thus these results also confirm the con- 

 clusion given in 14 



18. It should be noticed that although individual results 

 occasionally appear to conflict, the average value of the potential of 

 any given metal when polished with a certain agent is almost always 

 in proportion to the smoothness of the surface produced. However 

 much one may try, it is impossible to get exactly the same state of 

 surface over again, but by taking the average of a considerable 

 number of experiments with one polishing agent, a general value is 

 got which represents the potential given by the plate in a certain 

 state of polish. It is so arranged that the sharply scratched surfaces 

 come at the head of the table, and each succeeding polishing agent 

 in the list produces a smoother surface. 



19. It should be observed that with soft metals such as tin, 

 different polishers produce but little change in the contact potential. 

 This, though at first sight apparently contrary to the results for 

 harder metals, is really in accordance with them, for different 

 polishers do not produce appreciably different states of polish on a 

 soft metal. This remark applies equally to the fact that a smaller 

 variation was observed with " high conductivity " copper than with 

 the old copper plate, as the old plate was found to be considerably 

 harder than the purer "high conductivity " copper. 



20. It is probable that the variation described in this chapter 

 is not directly due to roughness or smoothness, but rather to altera- 

 tion, either by change in the outer layer of the metal, or in the film 

 of condensed air which no doubt exists on it, of the interface between 

 metal and air. 



IV. Thin Solid Films of Oxides, Iodides, fyc. 



21. Among the earlier experiments were many in which the 

 changes due to films of oxide of different thicknesses were studied ; 

 indeed this was the primary object of investigation suggested by 

 Lord Kelvin. But constant difficulties and ambiguities presented 

 themselves, hindering the interpretation of even the simplest 



