926 



Mr. D. C. H. Florancc on 



with the position of the screen. In position I. the large 

 Bcreen does more than cut out the direct radiation— it adds 

 the secondary radiation scattered 1'roni all parts of its volume. 

 This quantity will decrease as the solid angle subtended by 

 the screen at the radium decreases. On the other hand, as 

 the screen approaches the electroscope it would be expected 

 that the secondary radiation emitted would have an increased 

 effect. Results point to the general conclusion that the pro- 

 line! ion of the secondary radiation is the chief cause for the 

 variation in the value of the absorption coefficient determined 

 under different experimental conditions. The initial rapid 

 change of the absorption coefficient is no doubt due to the 

 rapid absorption of the soft portion of the primary radiation. 



In connexion with the secondary y-rays a few experiments 

 were performed to test the quality and the amount of the 

 primary y-rays passing through various materials. 



In this case a lead electroscope '6 cm. thick was used. 

 The thickness of absorbing screen was *G24 cm. of lead, 

 which was placed against the electroscope. The 20 mgrs. of 

 radium was surrounded by "-08 cm. of lead. (See tig. 1.) 



Fiff.]. 



£lectf?oscop£ 



E is the electroscope in the position of direct radiation. 



E, and E 2 are positions of the electroscope for the measure- 

 ment of secondary radiation. 



The radium cannot be placed symmetrically as regards the 

 radiator as it is required to have E : as near the position E as 

 possible without intercepting the direct radiation. 



