Section III., 1915 [219] Trans. R.S.C. 



On the Delta Rays Emitted by Zinc when Bombarded 

 by Alpha Rays. 



By Professor J. C. McLennan, F.R.S. and C. G. Found, M. A., 

 University of Toronto. 



(Read May Meeting, 1915). 



I. Introduction. 



In some experiments by V. E. Pound^ and described by him 

 in a paper, "On the secondary rays excited by alpha rays," he found 

 that the delta radiation emitted by carbon when bombarded by the 

 alpha rays from polonium increased very considerably when the tem- 

 perature of the carbon was lowered from room temperature to the 

 temperature of liquid air. He also showed that this increase in the 

 delta radiation from carbon as its temperature was lowered was due 

 to an increase in the amount of air occluded in the surface of the 

 carbon. 



Numerous observers have also found that the amount of a gas 

 occluded in the surface of metals determines to a very considerable 

 extent the intensity of the photo-electric effect exhibited by such 

 metals when stimulated by ultra-violet light. Indeed, it was shown by 

 Kiistner^ that no photo-electric effect was exhibited by zinc even 

 with wave-lengths as short as A = 1850 A°.U., when the metal was 

 scraped in a vacuum after extraordinary precautions had been taken 

 to exclude gases, particularly the active ones. Wiedmann and HaU- 

 wachs^ have shown, too, that the removal of occluded gases from potas- 

 sium by repeated distillation in a very high vacuum caused its photo- 

 electric effect to disappear completely with light which included wave- 

 lengths down to A = 3,40(JI A°. U. The results of Kiistner and Wied- 

 mann and Hallwachs have also been confirmed by Fredenhagen.'* 



In addition, Hughes^ has shown that the contact difference of 

 potential between zinc or bismuth both distilled in vacuo and plati- 

 num is exceedingly small when the surfaces of the zinc or bismuth 

 consist of a fresh deposit of the distilled metals. If traces of air, how- 



1 Pound. Phil. Mag. Nov. 23 and 24, 1912. 



2 Kustner. Phys. Zeit. p. 68, 1914. 



* Wiedmann and Hallwachs. Verh. d. Deutsch. Phys. Ges. p. 107, 1914. 



* Fredenhagen. Verh. d. Deutsch. Phys. Ges. p. 201, 1914. 

 » Hughes. Phil. Mag. Sept. 1914, p. 337. 



