133 Dr. J. Erskine-Murray. 



After it had been 24 minutes in oxygen it gave in air- 



+ 0-015 volt, 

 and 12 minutes later 



+ 0-010 volt, 



showing that the potential had in 24 minutes in oxygen become 0*110 

 more positive. 



65. It is noticeable that the amount of change in these experiments 

 on silver is to some extent proportionate to the time of exposure to 

 oxygen. Thus, in 63, 15 minutes in oxygen caused a rise of 0'06 

 volt; while, in 64, 24 minutes in oxygen caused a rise of O'll volt, 

 but there is no doubt a limit to the change. 



66. In searching for an explanation of the temperature- 

 variations described in Chapter IX, I compared them with those given 

 above. In the case of copper, the oxygen-film variation is in the same 

 direction as the temperature-variation of copper in air, which suggests 

 the possibility of the latter being caused by an increase of the pro- 

 portion of oxygen in the film at higher temperatures on account of a 

 greater attraction between the elements. The same reasoning holds 

 as regards zinc and tin, but the results for silver are in direct 

 opposition ; while the further experiments described in Chapter X, 

 which show that the temperature- variations exist in cases where air 

 is entirely excluded from the metallic surface, render such an ex- 

 planation very doubtful. Probably, therefore, the temperature- 

 variation is the more general of the two, it being a change in contact- 

 potential of the metals, which, if they are exposed to a gas, may be 

 complicated by alteration of the surface- film. 



IX. Temperature Variations. 



67. A large number of determinations were made of the variation 

 of contact- electricity with the temperature of the conductor. This 

 was done by heating one plate while the other was kept cool, and 

 their potential- difference was measured from time to time as the 

 warm plate was cooling, their temperatures being observed at the 

 same time. In the diagrams, the abscissae represent temperature 

 and the ordinates potential ; each curve, therefore, shows the tem- 

 perature-variation of the contact-potential of a particular metal. 

 For instance, the potential represented by the point which corre- 

 sponds to 16 C. on the gold line is called zero in this and the other 

 chapters of the present communication. Thus a standard gold plate 

 (see 8) at 40 C. is 0'04 volt negative to one at 16 C., and an 

 aluminium plate at 40 C. is O'lO volt, positive to aluminium at 16 C., 

 or 1-20 volts positive to a gold plate at 16 C. 



It must be remembered that unless stated otherwise these varia- 



