38 ROYAL SOCIETY OF CANADA 



The plates used in the experiments measured about 17 x 21 

 cm., and their respective thickness is stated. 



Lead 1.7 ram. 



Silver 4.6 



Tin 4.0 



Zinc 0.45 



Copper 3 . 05 



Iron, steel 1 . 83 



Iron, wrought 6 . 00 



Aluminium 1 . 73 



Carbon 1 . 50 



The zinc and aluminium plates were rather too thin so a few were 

 clamped together as required. 



Effect of Reversing Plates C and D. 



When Y rays pass through the electroscope and when the back 

 plates C and D alone are varied, the resulting changes in the rate 

 of discharge of the electroscope depend simply on the atomic weight 

 of the substance at C, provided that plate is one or two millimetres 



o 



ATOA^/C V\/£IGHT 



Fig. 2. 



thick. For the plate D, unless very thick, does not send back through 

 C sufficient secondary radiation to affect the ionization in the 

 electroscope to a noticeable extent. If the plate D were several 



