[MCLENNAN- ON THE DELTA RAYS EMITTED BY ZINC 235 



found] 



in the surface and that within the apparatus. This gradual occlusion 

 of the air by the zinc surface would therefore appear to account for 

 and to be the cause of the gradual development of the electronic cur- 

 rent. 



IV. Summary of Results. 



1. In the present investigation, it has been shown that when a 

 plate of zinc, with a freshly scraped surface, is placed in a highly ex- 

 hausted chamber and bombarded by alpha rays, there is an emission 

 of slow moving electrons or delta rays from it at the rate of three 

 electrons for each alpha particle impact. 



2. It has also been shown that the emission of electrons from 

 such a plate of zinc under bombardment by alpha rays diminished 

 with the lapse of time from the moment when it was placed in the 

 high vacuum. 



3. It has also been shown that initially there is no emission 

 of electrons under bombardment by alpha rays from a surface of zinc 

 deposited from zinc vapour in a high vacuum ; but that as time elapses, 

 an electronic emission is gradually developed under the gradual ab- 

 sorption of air by the surface of the zinc deposit. 



The Physical Laboratory, 



University of Toronto. 

 May 1st, 1915. 



