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POPULAR SCIENCE MONTHLY. 



detail. The residual activity at first mainly consists of ft rays, and 

 the activity measured by them does not change appreciably during 

 the period of one year. The a ray activity is at first small, but increases 

 uniformly with the time for the first few months that the activity has 

 been examined. These results receive an explanation on the hypoth- 

 esis that radium C changes into a product D which emits only /? 

 rays. D changes into a product E which emits only a rays. This 

 view has been confirmed by separating the a ray product by dipping 

 bismuth-plate into the solution containing radium D and E. The 

 probable period of these changes can be deduced from observations of 



