Ellen Gled'dsch — Life of Radium. 113 



The radium formed in the solution can be expressed in 

 terms of its equilibrium amount of emanation q. If a solution 

 of ionium is sealed up, and radium produced at a constant rate, 

 then the amount of emanation after a time t (in days) will be 

 proportional to 



?,-f(i-- ; -') 



And the ratio of the amount of emanation present to the 

 amount in equilibrium with the radium present will be 



q t Xt K ' 



The total amount of emanation after a time t will be equal to: 



t being always the time in days. 



Boltwood's results showed that the growth of radium in the 

 solution was constant for the periods of time over which it was 

 observed. This indicated that the ionium itself changed very 

 slowly, and that it was the immediate parent of radium. The 

 result may be said to justify the use of ionium for the deter- 

 mination of the period of change of radium. The method has 

 further the great advantage that the results are absolutely 

 independent of any radium standard. The results of the sepa- 

 rate measurements are given in terms of radium for the sake 

 of convenience; they might equally well be expressed in 

 arbitrary units. 



It is evident that the justification of using this method lies 

 in the possibility of separating completely the ionium from 

 the minerals. Boltwood in part used ionium solutions pre- 

 pared from very impure materials. But one of his solutions 

 was prepared from a very pure uraninite and was prepared 

 with special care. The growth of radium in this solution 

 gave a value for the constant of change of radium equal to : 

 3-18 X 10 ~\ which corresponds to a half-value period of 1990 

 years. 



This result has been generally accepted and agrees approxi- 

 mately with the values found by more indirect methods. 



Another calculation of the period has been made by Ruther- 

 ford and Geiger from the number of a particles emitted per 

 second per gram of pure radium. The period found was 1760 

 years. It is obvious that the accuracy of this determination 

 depends upon the purity of the radium salt used. Rutherford* 



*Phil. Mag., xxviii, 320, Sept. 1914. 



Am. Jour. Sci. — Fourth Series, Vol. XLT, No. 241. — January, 1916. 

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