﻿Electrical Changes induced by Ultraviolet Light. 407 



was^made by the method already described, in which the 

 plate was clamped to the electroscope. The measurements 

 in Series B were carried out by the reflexion method with 

 the plate earth-connected. 



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Considered generally the rate of discharge of the elements 

 as given in the table are in the same order as their electro- 

 potentials. This fact was noticed by Elster and Greitel* in 

 the case of the alkali metals ; these were found to have n 

 much more rapid rate of discharge than zinc, so that they 

 would naturally take their position at the top of Table V. 

 There are, however, notable exceptions to the general regu- 

 larity in the above series, amongst which may be noted 

 manganese, iron, chromium, gold, nickel, and cobalt. It is 

 a noteworthy fact that so many of these exceptions should be 

 elements which very readily take up the passive form. TV hen 

 in this form they do not give their correct potentials, so that 

 we may here be dealing with metals which are not giving 

 their normal rate of discharge, but one consequent upon 

 their being in the passive condition. It was found impossible 

 to obtain a quicker rate of discharge by reconverting them 

 into the active condition ; this may be due to the ultraviolet 

 light converting them into the passive form immediately it 

 falls upon them. 



Prof. J. J. Thomson f has shown that sodium, even 



* Elster and Geitel, Wied. Annalen, xliii. p. 225. 



We should like to express our tliauks to Messrs. Johnson & Matthey 

 for the loan of plates of Au, Ir, Pd, Ag, and Pt ; to Messrs. Siemens & 

 Halske for their kindness in presenting us with a plate of tantalum; and 

 to Messrs. Mond & Co. for a plate of nickel, and the Thermit Co. for a 

 plate of pure chromium. 



t Thomson. Phil. Mag. vol. x. p. 584 (Not. 1905). 



