13G 



THE ROYAL SOCIETY OF CANADA 



Screcjiing. 



On the tennis court by placing lead sheets beneath the electro- 

 scope, a small but distinct reduction in the ionization current was 

 obtained. No effect that could be detected was produced by placing 

 similar sheets above the apparatus. Similar results have been obtained 

 by Wulf , and theory and experiment both indicate that the penetrating 

 radiation comes almost entirely from radioactive matter in the ground, 

 that in the air being relatively negligible. 



In the Chemistry Building on the other hand it was easy to screen 

 from above, and also, or independently from below. In this case much 

 of the radiation comes from radioactive matter in the bricks, as Wulf 

 found and noted in the papers above named. 



On this account it is impossible to obtain satisfactory evidence 

 of the decrease of the penetrating radiation with altitude within or 

 near a tall building. 



Magnitude. 



In order to determine the amount of decrease of radiation due 

 to the wood and lead box, the Wulf electroscope and testing vessel 

 were placed within it, and readings were taken when the y rays from 

 14 mg. of radium bromide caused ionization 



(1) through a side of the box. 



(2) with the side removed. 



The radium was two or three metres from the centre of the box. 

 The results are given below. 



From these results it appears that the lead antl wood of the box 

 cut down the very penetrating radiation of radium from 100 to 66. 

 However when the radium was placed in load 2 mm. thick within 

 a thin tinned vessel, 17 cm. in diameter, full of sand, with the radium 

 in the centre of the sand, the diminution due to the side of the box 

 was from 100 to 58. 



