30 Dr. A. S. Eve on tJie Ionization of the 



1909), obtained at Toronto over the land, and over the ice 

 on Lake Ontario, correcting them for the new value, 



e = i-6 xlO" 10 . 





Lead. 



Zinc. 



Aluminium. 



Over ice, Lake Ontario 



Over land, Toronto 



6-4 



9-8 



4-4 



8-2 



4*8 

 7-7 

 2-9 



Difference 



3-4 



3-8 





This indicates a difference of about 3 ions per cm. 3 per sec. 

 within an aluminium electroscope due to the penetrating rays 

 from the earth near Toronto. We have now to consider what 

 would be the effect in free air, where the secondary radiation 

 is less than in a metal electroscope. If radium is placed 

 outside electroscopes of different metals the ionization within 

 is dependent on the nature of the metal employed. Thus four 

 observers have found, to an arbitrary scale : — 





Wright. 



Soddy. 



Bragg. 



Eve. 



100 

 63 

 54 



Lead 



100 



81-5 

 7L5 



100 

 75 



57 



100 

 55 

 49 



Ziuc 



Aluminium 



wherein the variations between observers are no doubt largely 

 due to different dispositions of apparatus. McLennan found 

 that in lead the secondary radiation was twice that due to the 

 primary, but in aluminium one-half. On the other hand, 

 W. Wilson found that in aluminium the secondary radiation 

 gave rise to six times as much ionization as the primary; but 

 his apparatus was not altogether satisfactory for this determi- 

 nation. Some experiments which I am making, not yet 

 completed, seem to indicate that the ionization in an 

 aluminium electroscope is only ten to twenty per cent, more 

 than in free air ; and Bragg finds that the ionization in a 

 cardboard electroscope is about the same as in free air. He 

 advocates the view that the primary 7 rays do not ionize 

 at all, but give rise to secondarv j3 rays, and that these 

 alone produce ionization. From Wright's figures we may 

 therefore conclude that in free air the penetrating rays from 

 the radioactive matter in the earth generate about 



2*5 ions per cm. 3 per sec. .... (5) 



