DISCOVERY AND PROPERTIES OF RADIUM 155 



nection between radium and uranium. An intermediate sub- 

 stance has been detected, and it has been called ionium, but it 

 has, according to Professor Soddy, an average life of about 

 30,000 years. It is not possible as yet to establish a direct 

 relation between radium and pure uranium, but it appears that 

 ionium is the parent of radium. While therefore much has been 

 accomplished the history of radium is by no means yet complete. 

 The story is further complicated by the fact that radium is not 

 the only radio-active substance obtainable from uranium 

 minerals. A substance called actinium was early recognised 

 among these products. Further it was discovered by Sir William 

 Crookes that by adding excess of ammonium carbonate to the 

 solution of an active uranium salt, a precipitate is formed which 

 redissolves in excess of the reagent, leaving a small insoluble 

 residue in which all the activity is concentrated. This contains 

 a substance to which the symbol UrX has been assigned. The 

 inactive part of the uranium regains its activity by keeping. 



The following table provides a summary of the changes which 

 have been traced in the uranium-radium series of disintegration 

 products. It should be added by way of explanation that there 

 is reason to believe that at some stages in the process the dis- 

 integration of the active substance may take place in two ways 

 so that a second series of products may arise. 



