On Contact Electricity of Metals. 133 



due to soaking in oxygen is nearly equal to that o copper ; hence 

 the film of oxygen acts like a liquid film (Chap. VII), but its effect 

 is less permanent. Previous experimenters, except Lord Kelvin, 

 appear to have neglected the existence of these films, and to have 

 looked only to the nature of the body of gas between the plates. Of 

 course I do not here allude to the " double-layer " which has been 

 offered as an explanation of the phenomenon, but to a layer in 

 mechanical and electrical contact with the metal. If the contact- 

 potential of two metals immersed in a gas were the sum of the 

 potential-differences between each metal and a skin of gas close to it, 

 we should have no slope of potential in the body of the gas between 

 the plates. That a slope does exist, however, is proved by Lord 

 Kelvin's earliest experiments with the divided ring of copper and 

 zinc. In this connection Dr. Bottomley's research on contact- 

 electricity in high vacua * is of great importance as showing that 

 the volta-potential of metals is not sensibly different in different 

 gases so long as the metals are not chemically affected, and is not 

 sensibly altered by a great reduction of pressure. The body of the 

 gas may possibly have some influence, but the variations which have 

 frequently been attributed to it may usually be more satisfactorily 

 explained as being due to change in the film in contact with the 

 metallic surface. In the case of a gas which acts vigorously on the 

 metal at ordinary temperatures, the film is permanent, and is 

 probably a solid compound ; in other cases it is not permanent, and 

 hence probably not solid. 



55. In * Nature ' for 1881, Lord Kelvin describes some very 

 important experiments on this subject. As these appear to be but 

 little known I shall give some extracts from his paper before 

 describing my own results. Under the date November 23, 1880,. 

 Lord Kelvin says : " I have found that a dry platinum disc, kept 

 for some time in dry hydrogen gas, and then put into its position in 

 dry atmospheric air in the Volta-condenser, becomes positive to 

 another platinum disc which had not been so treated, but had simply 

 been left undisturbed in the apparatus. The positive quality thus 

 produced by the hydrogen diminishes gradually, and becomes in- 

 sensible after two or three days. P.S. On December 24, 1880, one 

 of the platinum plates in the Volta- condenser was taken out ; placed 

 in dried oxygen gas for forty-five minutes ; taken out, carried by 

 hand, and replaced in the Volta- condenser at 12.30 on that day. It 

 was then found to b$ negative to the platinum plate, which had been 

 left undisturbed. The amount of the difference was about 0'33 of a 

 volt. The plates were left undisturbed for seventeen minutes in the 

 condenser, and were tested again, and the difference was found to 

 have fallen to 0'29 of a volt. At noon on the 25th they were again 

 * ' Brit. Assoc. Report,' 1885. 



