the Cause and Nature of Radioactivit;/. 393 



a solution to dryness over a large metal plate. Compared in 

 this way, the radioactivity of ThX when first separated almost 

 exactly equals the activity of the nitrate from which it is pro- 

 duced, while the hydroxide retains about two-fifths of this 

 amount. The total activity of the products is therefore 

 greater than that of the original salt ; but this is to be ex- 

 pected, for it is certain that more absorption takes place in 

 the nitrate than in the products into which it is separated. 



Similar difficulties stand in the way of an answer to the 

 second question, whether the nature of the radiations is 

 affected by chemical treatment, for it has been experimentally 

 observed that the penetrating power of these radiations de- 

 creases with the thickness of material traversed. The 

 character of the radiations from ThX and thorium have, 

 however, been compared by the method of penetration power. 

 A large number of comparisons justifies the view that the 

 character of thorium radioactivity is unaltered by chemical 

 treatment and the separation of ThX, although the different 

 types are unequally distributed among the separated products. 

 Determinations of the proportion of rays deviable by the 

 magnetic field in thorium and ThX throws fresh light on the 

 question. The general result is that ThX gives out both 

 deviable and non -deviable rays, and the same applies to the 

 excited activity produced by ThX. But in the experiment 

 in which the excited radiation was allowed to spontaneously 

 decay, by removing ThX as formed, the thorium compound 

 obtained after 23 precipitations was found to be quite free 

 from deviable radiation. This is one of the most striking 

 resemblances between the non-separable radioactivities of 

 uranium and thorium, and warrants the question whether the 

 primary radiation of ThX is not, like that of UrX, composed 

 entirely of cathode-rays. There is, however, no means of 

 deciding this point owing to the excited radiation which 

 always accompanies the primary radiation of ThX, and which 

 itself comprises both types of rays. 



Finally, it may be mentioned that the proportion of de- 

 viable and non-deviable radiation is different for different 

 compounds of thorium. The nitrate and ignited oxide, com- 

 pounds which hardly possess any emanating power, have a 

 higher proportion of deviable radiation than compounds with 

 groat emanating power. This is indirect evidence of the 

 correctness of the view already put forward (Section IX.), 

 that when the emanation is prevented from escaping it 

 augments the proportion of excited radioactivity of the 

 compound. 



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



