Prof. Owens on Thorium Radiation. 



373 



the paper causes it to give off a secondary radiation com- 

 parable in its ionizing effects to the more absorbable kind that 

 fails to get through. 



5. Selective Absorption. — The phenomenon of selective 

 absorption has been noticed for Rontgen rays by Prof. J. J. 

 Thomson, and is discussed by Sagnac * in a recent issue of 

 the Journal de Physique. The latter finds that the intensity 

 of the rays, after passing through a particular number of 

 layers of different substances, both as measured electrically 

 and by the action of the rays on a photographic plate, varies 

 with the order of the layers. 



Table IV. 



Thorium Oxide. Thick layer. 



Screening effect of thick Aluminium foil. 



Plates 5 cm. apart. Lower plate 95 volts + 



Layers of foil. 



Time 



of 1G0 div. 



in 



see. 



Current. 









16-0 









1 





187-0 







1000 



2 





2000 







93-5 



3 





2180 







85-8 



4 





2430 







770 



5 



1 



277D 







675 



o 



Fig. 5. — Screening effect of Aluminium sheet. 

 Thorium Oxide. Thick layer. 



2 3 4 5 



Layers of Aluminium sheet. 



Sagnac, Journal de Physique, p. 9, Feb. 17th, 1899. 



