234 THE ROYAL SOCIETY OF CANADA 



One point which is brought out very prominently by these readings 

 is that for the first half hour the current between P and X, under a 

 potential difference of 80 volts was the same whether a magnetic field, 

 as high even as 1 , 1 60 gauss, was applied or not. From this it was mani- 

 fest that during this interval there was practically no emission of 

 electrons from the newly deposited zinc surface, under bombardment 

 by the alpha rays. It will be noted, too, that during the interval the 

 saturation current was about 165 x 10~^ e.s.u., which was about twice 

 as great as that saturation current obtained in the previous experi- 

 ment with the ordinary zinc plate without the fresh deposit. This 

 was very probably due to the air pressure in the apparatus being some- 

 what higher immediately after the deposit had been than it was when 

 the observations were made with the zinc plate in its original condi- 

 tion. Even with the Gaede pump in action the effect of heating the 

 furnace and the charcoal would be to drive off considerable air from 

 the walls of the vessel and the charcoal into the apparatus and as the 

 volume of the apparatus was considerable it would take time to remove 

 this air again. That this interpretation was the correct one is shown 

 by the readings taken in the second period extending from 55 minutes 

 after the deposit had been made up to 85 minutes after that time. 

 These it will be seen show that with increasing positive potentials 

 the current increased and finally reached a maximum of only 78 x 

 10"^ electrostatic units. This would indicate that during the first 

 half hour the ionisation constituent of the current was very consider- 

 able, as it should have been on account of the higher air pressure, 

 while at the end of 85 minutes after the deposit had been made it was 

 much less on account of 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 connection for they show not only that the current 

 gradually diminished with the lapse of time owing to the diminution 

 of the ionisation current constituent arising from the gradual reduc- 

 tion 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 development 

 of a delta radiation from the zinc plate arose from the gradual occlu- 

 sion 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 at 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 



