284 Boltwood — Radio-activity of Uranium Minerals. 



proportional to the activity in divisions per minute and the 

 abscissas are taken as the time in minutes from the instant at 

 which the plate was removed from contact with the emanation. 

 The initial activity for radium C (plus radium B) given by the 

 equations is about 10 div. per min., and for radium A + radium 

 B + radium C about 15*7 div. per minute. The initial activity 

 of radium A was therefore about 5*7 div. per min. An inter- 

 esting relation is shown by these numbers if considered in con- 



nection with the ranges in air of the a particles emitted by 

 these products. If it is assumed that the ionization produced 

 by the easily absorbed & radiation from radium B is about 5 

 per cent of the ionization produced by the a particles from 

 radium C,* the activity of radium C alone is found to be 9*5 

 div. per min. The ratio of the activities of equilibrium 

 amounts of radium A and C as determined in this manner is 



*The results obtained by H. W. Schmidt (Annal. d. Phys., xxi, 609, 1906) 

 and Bronson {loc. cit.) afford some basis for this assumption. 



