On the Delta Radiation emitted by Zinc. 491 



In this connexion it is interesting to see that our results 

 indicate that very probably the same equality would apply 

 to the mobilities o£ positive and negative ions in liquid 

 air. Measurements on the mobilities of ions in air at 

 pressures still higher than those used in this investigation 

 would be required, however, to show whether this surmise 

 were correct or not. 



In closing, we desire to express our appreciation of the 

 services of Mr. P. Blackmail, who assisted us in taking 

 many of the readings in this investigation. 



The Physical Laboratory, 



University of Toronto, 

 May 1st, 1915. 



LII. On the Delta Radiation emitted by Zinc when bombarded 

 by Alpha Rays. By Professor J. (J. McLennan, F.R.S., 

 and 0. G. Found, M.A., University of Toronto*. 



[Plate VII.] 



I. Introduction. 



IN some experiments by V. E. Pound f and described by 

 him in n paper: "On the Secondary Kays excited by 

 Alpha Hays/'' he found that the delta radiation emitted 

 by carbon when bombarded by the alpha rays from polonium 

 increased very considerably when the temperature of the 

 carbon was lowered from room temperature to the temper- 

 ature of liquid air. He also showed th:it 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 bhown by Kiistner J that no photo- 

 electric effect was exhibited by zinc even with wave-lengths 

 as short as \ = 1850 A.U. when the metal was scraped in a 

 vacuum alter extraordinary precautions had been taken to 

 exclude gases, particularly the active ones. Wietlmann and 



* Communicated bv the Author-. Read before the Royal Society 

 of Canada, May 26th, 1915. 

 t Pound, Phil. Mag., November 1912. 

 J Kiistner, Phys. Zeit. p. 68 (191 4). 



