506 Boltwood — Life of Radium. 



This would appear to justify the conclusion that the rate of 

 change of ionium is relatively slow and that it is the immediate 

 substance from which radium is formed. 



It is obvious that the significance of the value found in 

 these experiments for the rate of disintegration of radium 

 depends chiefly on the matter of the complete separation of the 

 ionium. That the separation of the ionium was essentially 

 complete in the preparation of Solution 5 appears highly 

 probable, as it has been found* that by similar, and even more 

 complicated chemical operations, the amount of ionium to be 

 expected from the disintegration theory can be separated from 

 a pure uranium mineral. The method which has been used 

 has also the further advantage that the result obtained is 

 wholly independent of any standard of radium salt or of any 

 hypothesis or assumption other than the fundamental theory of 

 disintegration and the conclusion that the emanation is a meas- 

 ure of the radium present. Except for convenience there is 

 no advantage in expressing the results of the separate measure- 

 ments in terms of the standard radium solution. They could 

 just as well be expressed in terms of divisions per minute in 

 the electroscope."!" 



It is therefore very significant that the rate of disintegration 

 of radium as determined in this direct manner should agree so 

 closely with the rate which has been predicted by Rutherford 

 from distinctly complex theoretical considerations, and this 

 agreement is a further corroboration of Rutherford's extraor- 

 dinary ability in such matters. It affords me much pleasure to 

 acknowledge my indebtedness to Professor Rutherford for his 

 constant interest and encouragement during the course of this 

 investigation. 



Summary. 



Results obtained on the growth of radium in preparations of 

 ionium separated from uranium minerals indicate that the dis- 

 integration constant of radium has a value of approximately 

 348 X 10 4 (year)" 1 . 



The half-value period of radium is therefore about 2000 

 years. 



New Haven, Conn., March 24, 1908. 



*This Journal, xxv, 269, 1908. 



f The rate of change of uranium, calculated from the rate of change of 

 radium and the amount of radium associated in a mineral with one gram of 

 uranium, is given as 1*16 x 10 -10 (year) -1 . This is equivalent to a half- value, 

 period of 6 x 10 9 years for uranium. These values are of course dependent 

 on the purity of the radium standard. 



