produced by the Action of Thorium Compounds. 169 



Connexion between the " Emanation " /rom Thorium and 

 " Excited " Radio-activity. 



In a previous paper * I have shown that compounds of 

 thorium emit some kind of radio-active material or " emana- 

 tion/' which is able to pass through considerable thicknesses 

 of paper and thin layers of metal, and preserves its radiating 

 power for several minutes. These particles diffuse through 

 the gas and become centres of ionization throughout the 

 volume of the gas. The current passing between two charged 

 plates, on one of which is spread thorium oxide, is greatly 

 diminished by directing a slow continuous blast of air between 

 the plates. As the particles have no charge, they may be 

 readily removed from between the plates by a current of air 

 even in a strong electric field. 



There is a very close connexion between this " emanation " 

 and excited radioactivity — in fact, the emanation is in some 

 way the direct cause of the latter. The following facts will 

 serve to show the close connexion that exists : — 



(1) All thorium compounds examined are able to make 

 substances radio-active, but to different degrees. The greater 

 the amount of emanation, the greater the amount of induced 

 radioactivity. As an example, thorium oxide is the most 

 active of all thorium compounds in producing radioactivity 

 and giving out the emanation. A thin layer of thorium oxide 

 gives out very little emanation, and is only slightly effective 

 in producing radioactivity. 



(2) Substances are made radio-active when the active com- 

 pound is covered with several layers of paper or thin metal 

 foil. The emanation also readily passes through paper and 

 thin metal foil. Two or three layers of ordinary foolscap- 

 paper completely cut off' the ordinary radiation given out by 

 thorium compounds, but do not much diminish the amount of 

 induced radioactivity. 



(3) A slow current of air, which quickly removes the 

 emanation as it appears, also diminishes the power of pro- 

 ducing radioactivity. The amount of induced radiation is 

 greater in closed than in open vessels, on account of the dis- 

 turbance of air-currents in the latter case. 



(4) Thorium oxide which had been heated to a sufficiently 

 high temperature gave out very little emanation and produced 

 little radioactivity. 



Speaking generally, it may be said that the presence of the 

 emanation is necessary for the production of radioactivity in 



* Phil. Mag. January 1900. 



