124 Dr. J. Erskine-Murray. 



negative to a standard gold plate, which value may therefore be 

 taken as the potential of a mass of copper oxide at 15 C. It may 

 be remarked that there are good reasons, which will be given in 

 Chapter IX, for supposing that oxidation does not commence in air 

 unless the temperature of the plate be raised above 80 C. 



26. A plate of copper which had been polished on glass paper 

 and then on wash-leather gave 



+ 0-20 volt, 



with a standard gold plate, and was next held in iodine vapour for a 

 short time. It looked moist at first, but quickly became dry. In 

 this state it gave 



-0-34 volt, 



with the same standard, a change of 54 volt in the negative 

 direction. The surface had a dull colour with a whitish " bloom " 

 on it, the tint resembling that of clean copper, though rather darker. 

 Exposure to iodine vapour thus makes the surface of copper 

 nearly as negative as continued exposure to the atmosphere at a 

 high temperature. 



27. The potential of a clean zinc plate fell about 0*12 volt when 

 it had been exposed to iodine vapour. 



28. The iodine compounds formed on the metallic surfaces were 

 not stable, as the appearance and potential of the plates altered con- 

 siderably during twenty hours' exposure to the atmosphere. 



29. The effects of sulphur fumes, hydrogen sulphide, &c., were 

 tried, but though in most cases considerable changes were observed, 

 the results obtained were not satisfactory. 



V. Atmospheric or Time Effects. 



30. A small variation of the potential is usually found to take 

 place during a short time after the plate has been polished, and before 

 it has settled down to a more or less permanent value. This is partly 

 due to the fact that the plate has been slightly warmed during 

 polishing, and takes some time to come to the atmospheric temperature 

 (see Chap. IX). As regards changes which are not due to variation 

 of temperature, I shall give some typical experiments in the following 

 sections of this chapter. 



31. Two plates of "high conductivity" copper were polished on 

 fine glass-paper, and several experiments by burnishing them by 

 mutual friction were then made (see 16). Forty-seven minutes 

 after the original polishing, their potentials with the standard plate 

 were 



(a) +0-035 volt. 

 (6) + 0-045 



