THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



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[FIFTH SERIES.] 



FEBRUARY 1900. 



XI. Radioactivity produced in Substances by the, Action 

 of Thorium Compounds. By E. Rutherb^ORD, M.A., 

 B.Sc, Macdonald Professor of Physics, McGitl University y 

 Montreal *. 



THORIUM compounds under certain conditions possess 

 the property of producing temporary radioactivity in 

 all solid substances in their neighbourhood. The substance 

 made radio-active behaves, with regard to its photographic 

 and electrical actions, as if it were covered with a layer 

 of radio-active substance like uranium or thorium. Unlike 

 the radiations from thorium and uranium, which are given 

 out uniformly for long periods of time, the intensity of the 

 excited radiation is not constant, but gradually diminishes. 

 The intensity falls to half its value about eleven hours after 

 the removal of the substance from the neighbourhood of 

 the thorium. The radiation given out is more penetrating 

 in character than the similar radiations emitted by uranium 

 and thorium and the radio-active derivatives from pitchblende, 

 radium f, and polonium %. 



Attention was first drawn to this phenomenon of what may 

 be termed " excited radioactivity" by the apparent failure of 

 good insulators, like ebonite and paraffin, to continue to 

 insulate in the presence of thorium compounds. 



The apparatus first used is shown in fig. 1. 



Two insulated plates, B and C, were placed parallel to one 

 another. In a shallow square depression LM in the plate C, 



* Communicated by Prof. J. J. Thomson, F.R.S. 



t Curie, C. B. 1898, p. 175. t Curie, ibid. 26 Dec. 1898 



Phil. Mag. S. 5. Vol. 49. No. 297. Feb. 1900. M 





