the Cause and Nature of Radioactivity. 391 



less the emanation succeeds in escaping in the radioactive 

 state, and therefore that de-emanated compounds should 



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possess a greater proportion of excited radioactivity than 

 those with high emanating power. This conclusion was 

 tested by converting a specimen of thorium carbonate with 

 an emanating power five times that of ordinary thoria, into 

 oxide and de-emanating by intense ignition. The energy 

 that before escaped in the form of emanation is now, ail but 

 a few per cent., prevented from escaping. The radioactivity 

 of the oxide so prepared rose in the first three days about 

 thirty per cent, of its original amount, and there thus seem 

 to be grounds for the view that the excited radioactivity will 

 contribute a much greater effect in a non-emanating thorium 

 compound than in one possessing great emanating power. 



Additional confirmation of this view is to be found in the 

 nature of the radiations emitted by the two classes of com- 

 pounds (Section XI.). 



X. Ihe Son-separalle Radioactivity of Thorium. 



It has not yet been found possible by any means to 

 free thorium from its residual activity, and the place of this 

 part in the scheme of radioactivity of thorium remains to be 

 considered. Disregarding the view that it is a separate 



