tlie Cause and Nature of Radioactivity. 377 



prepared possessing thorium radioactivity in an intense 

 degree. 



The thorium hydroxide which had been submitted to the 

 above process was found to be less than half as radioactive as 

 the same weight of thorium oxide. It thus appeared that a 

 constituent responsible for the radioactivity of thorium had 

 been obtained, which possessed distinct chemical properties 

 and an activity of the order of at least a thousand times 

 as great as the material from which it had been separated. 



Sir William Crookes (Proc. Roy. Soc. 1900, Ixvi. p. 409) 

 succeeded in separating a radioactive constituent of great 

 activity and distinct chemical nature from uranium, and gave 

 the name UrX to this substance. For the present, until 

 more is known of its real nature, it will be convenient to 

 name the active constituent of thorium ThX, similarly. 

 Like UrX, however, ThX does not answer to any definite 

 analytical reactions, but makes its appearance with precipi- 

 tates formed in its solution even when no cpiestion of 

 insolubility is involved. This accords with the view that it 

 is present in infinitesimal quantity, and possesses correspond- 

 ingly great activity. Even in the case of the most active 

 preparations, these probably are composed of some ThX 

 associated with accidental admixtures large in proportion. 



These results receive confirmation from observations made 

 on a different method of separating ThX. The experiment 

 was tried of washing thoria with water repeatedly, and 

 seeing if the radioactivity was thereby affected. In this way 

 it was found that the filtered washings, on concentration, 

 deposited small amounts of material with an activity often of 

 the order of a thousand times greater than that of the 

 original sample. In one experiment, 290 grams of thoria 

 were shaken for a long time with nine quantities, each of 

 2 litres of distilled water. The first washing, containing 

 thorium sulphate present as an impurity, was rejected, the 

 rest concentrated to different stages and filtered at each 

 stage. One of the residues so obtained weighed 6 "4 mg\, 

 and was equivalent in radioactivity to 11*3 grams of the 

 original thoria, and was therefore no less than 1803 times 

 more radioactive. It was examined chemically, and gave, 

 after conversion into sulphate, the characteristic reaction of 

 thorium sulphate, being precipitated from its solution in 

 cold water by warming. So other substance than thorium 

 could he detected by chemical analysis, although of course the 

 quantity was too small for a minute examination. The 

 penetrating power of the radiation from this substance again 

 established its identity with the ordinary thorium radiation. 



Phil. Mag. S. 6. Vol. 4. No. 21. Sept. 1902. 2 



