1897.] on Contact Electricity of Metals. 553 



to its outer end a short piece of platinum. The other contact surface 

 was a similar short piece of platinum fixed to the insulated electrode 

 of the electrometer. Hence it will be seen that metallic connection 

 between the two plates was effected by putting the divider at zero and 

 bringing into contact the two pieces of platinum wire. 



Order of Experiment. — The sliding piece of the divider was put 

 to zero, and contact made and broken, and the upper plate raised : 

 then the deflection of the spot of light was observed. These opera- 

 tions were repeated with the sliding piece at ditFerent numbers on 

 the divider scale, until one was found at which the make-break and 

 separation caused no perceptible deflection. The number thus found 

 on the divider scale was the percentage of the electromotive force of 

 the Daniell cell, which was equal to the contact electric difference 

 of the plates in the Volt-condenser. 



[Addendum, November 23, 1880. — Since the communication of 

 this paper to the British Association, I have found that a dry plati- 

 num disc, kept for some time in dry hydrogen gas, and then put into 

 its position in dry atmospheric air in the apparatus for contact elec- 

 tricity, 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 insensible after two or three days.] 



P.S. — On December 24, 1880, one of two 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 be negative 

 to the platinum plate, which had been left undisturbed. The amount 

 of the difference was about • 33 of a volt. The plates were left un- 

 disturbed for seventeen minutes in the condenser, and were then 

 tested again, and the difference was found to have fallen to -29 of a 

 volt. At noon on the 25th they were again tested, and the diflerence 

 found to be * 18. The differences had been tested from time to time 

 since that day, the plates having been left in the condenser undis- 

 turbed in the intervals. The following table shows the whole series 

 of these results : — 



Electric difference between 

 surfaces of a platinum plate in 

 Time. natural condition, and a platinum 



plate after 45 minutes' expoeore 

 to dry oxygen gas. 



Dec. 24, 12.30 p.m '33 of a volt. 



24, 12.47 p.m -29 „ 



25, noon '18 „ 



27, noon '116 „ 



28, 11.20 a.m -097 „ 



31, noon -047 „ 



Jan. 4, 11.0 a.m -042 „ 



11, 11.40 a.m -020 „ 



Mr. Rennie, by whom these experiments were made during the 

 recent Christmas holidays, had previously experimented on a platinum 



