«)02 On Delta Radiation emitted by Zinc. 



85 minutes after the deposit had been made it was much less, 

 on account o£ the removal of the air from the apparatus. 



The numbers given in Table VI. and the curves in fig. 7 

 are also of interest in this connexion, for they show not only 

 that the current gradually diminished with the lapse of time 

 owing to the diminution of the ionization current constituent 

 arising from the gradual reduction of the air-pressure, but 

 also that there was a gradual increase in the electronic 

 stream from the zinc plate with the lapse of time under the 

 bombardment by the alpha rays. 



From what has gone before it is evident that this develop- 

 ment of a delta radiation from the zinc plate arose from the 

 gradual occlusion of air into the surface of the zinc. 



For, as the vapour was deposited on the zinc plate in a 

 high vacuum the surface would not contain any air first. 

 It would not, however, in this state be in an equilibrium 

 condition, and a tendency towards absorption would exist. 

 The result of this would be that so long as air was present 

 in the apparatus, absorption would take place at least until 

 an equilibrium was established between the air occluded 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 current. 



IV. Summary of Results. 



I. In the present investigation it has been shown that 

 when a plate of zinc with a freshly scraped surface is placed 

 in a highly exhausted 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. 



II. 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. 



III. 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 absorption of air 

 by the surface of the zinc deposit. 



The Physical Laboratory, 



University of Toronto. 

 May 1st, 1915. 



