RADIO-ACTIVITY 167 



previously unknown, were quickly isolated by 

 different observers. Of these three the most 

 famous is the now well-known radium, discovered 

 by M. and Mme. Curie, working with M. Bemont. 

 Radium is obtained from pitch-blende in com- 

 pany with the metal barium ; and the two seemed 

 at first to be connected chemically so intimately 

 that the new substance was for a time called 

 *' active barium." However, a slight difference in 

 the solubilities of some of their salts allows them 

 to be separated gradually by a process of repeated 

 fractionisation, the radium chloride and bromide 

 crystallising out more readily than the correspond- 

 ing compounds of barium. 



These processes of chemical separation were 

 remarkable for their use of the new property of 

 radio-activity as a sole guide in the operations. 

 After each reaction the activities of both the 

 product and the residue were determined. It was 

 thus settled whether the reaction just tried was 

 effective, and in which of the substances separated 

 by the reaction the property of radio-activity had 

 been concentrated. 



The quantity of radium present in pitch-blende 

 is extremely small, many tons of the mineral 

 yielding, after long and tedious work, only a small 

 fraction of a gram of an impure salt of radium. 

 Its extraction is consequently a matter of great 

 labour and high cost. Radium salts of fair purity 

 have now become articles of commerce, though 

 the supply is insufficient to meet the demand ; 

 and radium is at present worth many thousand 

 times its weight in gold. 



An interesting point in these investigations is 

 the extreme sensitiveness of the property of radio- 



