CHAPTER IX. 

 THORIUM, URANIUM, POLONIUM AND ACTINIUM. 



While radium seems, to-day, to be the radio-active ele- 

 ment par excellence, radio-activity is not confined to ra- 

 dium. Among the other radio-active elements, thorium is 

 important. Thorium is a comparatively rare element, and 

 is the fundamental constituent of nearly all the Welsbach 

 gas mantles used for lighting. Its radio-activity is ex- 

 ceedingly small compared with that of radium, but small 

 as it is, it is in some respects more interesting even than 

 that of radium. All thorium compounds are radio-active. 

 All thorium compounds give off alpha-, beta- and gamma- 

 rays but, after this is said, the resemblance between ra- 

 dium and thorium ceases. The radio-activity of radium 

 remains constant no matter how drastic the treatment to 

 which it is subjected. The radio-activity of thorium is 

 not constant. In fact it has been found possible by chemi- 

 cal processes to separate out nearly all its activity and to 

 concentrate it afterwards into a minute quantity of in- 

 tensely active matter. 



If ammonia is added to a solution of thorium, the tho- 

 rium is precipitated in the form of a solid and so sepa- 

 rated from the water in which it was dissolved. But the 

 water now retains the activity of the thorium and the 

 precipitated thorium has lost it. On evaporating this 

 water solution down to dryness and igniting it, a small 

 residue of intensely active matter remains which turns 

 out, weight for weight, to be over a thousand times more 

 (126) 



