THORIUM, URANIUM, POLONIUM AND ACTINIUM. 131 



greater. Like radium and thorium, it gives rise to an in- 

 tensely active emanation which, however, loses its activity in 

 a few seconds. A minute quantity only has been extracted, 

 but it has been sufficient, nevertheless, to demonstrate its 

 powerful radiations. A piece of paper placed in the same 

 vessel with a trace of ac s tinium becomes in a short time 

 radiantly active and is capable of lighting up a screen of 

 zinc sulphide by its mere presence. 



These then are the radio-active elements : radium, tho- 

 rium, uranium, polonium, and actinium. Each one has 

 properties which differentiate it from all the others. 



We append a tabular statement of the main radio-active 

 changes which uranium, thorium, and radium undergo, to- 

 gether with some of the properties of the changing bodies. 



