Effect on Thin Films of Platinum. 423 



This was at first attempted by arranging a screen at S of the 

 same material as the Avails of the vessel, so that the relative 

 amounts of light falling on the film and the vessel could 

 be altered. This was abandoned, as it was quite sufficient 

 for the present purpose to know how a varies with the 

 thickness. 



A quartz tube was cemented to the side tube to let ultra- 

 violet light into the vessel. Another side piece, F, had a 

 ground joint for the purpose of rotating the film 0. The 

 rod carrying the quartz plate for the film was insulated at 

 G by an amber plug. The contact between the film and the 

 rod was made as in a former paper, by getting a thick deposit:, 

 of platinum at an edge of the quartz-plate, laying tinfoil 

 •over this so as to allow part of the thick platinum film to be 

 uncovered, and clamping this prepared edge of the quartz to 

 the rod. The films to be tested were deposited over the 

 whole of one side of the quartz plate, and only the centre 

 of the plate was exposed to the light. The plate was of 

 dimensions 10 mm. x 8 mm. x 1 mm. 



The inside of the vessel was coated with soot from burning- 

 camphor in one series of experiments, and in another series 

 consisted of zinc. The platinum film was joined to a pair 

 of quadrants of a Dolezalek electrometer whose sensitiveness 

 was 2000 divisions per volt, when the needle was charged to 

 100 volts. The current from the film was measured by 

 charging the walls of the vessel to a high positive potential, 

 and that from the walls by charging them to a high negative 

 potential, so that the maximum effects were obtained in each 

 ease. 



The following results were obtained : — 



Table I. 



Thickness of 

 film. 



Current from 



film. 



Current from the walls 

 of the vessel. 



50 seconds. 



8 



51 



80 „ 



22 



46-5 



130 „ 



235 



41 



240 „ 



150 



16 



The thickness of the film was measured in terms of the 

 length of time to deposit it. The quartz plate was taken 

 •out of the vessel in each case and placed in a discharge-tube 

 with a platinum cathode to have the film formed or thickened. 



