the Radiations from Radioactive Substances. 23 



be the case. The results obtained were only approximate on 

 account of the initial rapid decay after removal. 



Summary of Results. 



From the differences observed in the behaviour of the 

 radioactive substances most closely studied, viz. uranium, 

 thorium, and radium, it will be seen that radioactivity is a 

 very complicated phenomenon. Both uranium and radium * 

 emit negatively charged particles with high velocities, 

 similar in all respects to cathode-rays. In addition, uranium, 

 radium, and thorium emit rays uon-deviable by a magnetic 

 field, which are readily absorbed by gases and thin layers of 

 metal. These non-deflectable rays differ from one another 

 in penetrating power and cannot consequently be ascribed to 

 any radioactive impurity common to all these substances. In 

 addition, thorium and radium possess the remarkable pro- 

 perty of continuously emitting radioactive emanations which 

 behave in all respects like radioactive gases. The emanations 

 from thorium and radium differ greatly in their rates of 

 decay of radiating power. The presence of an emanation 

 gives rise to the complicated phenomenon of "excited" 

 radioactivity. The nondeviable "excited" radiations due 

 to thorium and radium, although apparently of the same pene- 

 trating power, decay at very different rates. 



" Excited " radioactivity is not confined to radium and 

 thorium, for Elster and Geitel t have recently shown that a 

 negatively -charged wire, exposed in the open air, free from 

 all possible contamination by radioactive substances in the 

 laboratory, becomes strongly radioactive. This excited radio- 

 activity J decays at a different rate from that due to the 

 emanations of thorium and radium, and also is of greater 

 penetrating power. 



Macdonald Physics Building, 

 McGill University, March 6, 1902. 



* It has recently been found that thorium also emits some rays deviable 

 by a magnetic field. The excited radiations produced by thorium and 

 radium also possess the same property. (Rutherford and Grier, Amer. 

 Phys. Soc. April 21, 1902.) 



t Phys. Zeit. 1901 . 



% Rutherford & Allen, Phys. Zeit. 10, 1902. 



