60 Messrs. J. H. L. Johnstone and B. B. Boltwood on the 



to remove any emanation as formed and to permit the other 

 products (radium A, radium 0) to completely disintegrate. 

 The activity o£ the material forming the films was therefore 

 due to radium alone at the time of ignition, and this activity 

 could be easily determined from the data available. The 

 variation of the activity with the time for two typical films is 

 shown in fig. 2, where the zero time is taken as the moment 

 of ignition. By a simple extrapolation of the curve the 

 initial activity could be obtained with accuracy. 



The calculation of the ratio of the activity of the radium 

 to the activity of the uranium was made by the use of the 

 following equation, where 

 x — the required ratio. 

 Eitij = the activity of the equilibrium amount of emanation 



from one gram of uraninite in div. per min. (in the 



emanation electroscope). 

 Em 2 = the activity of the equilibrium amount of emanation 



from lOc.c. of radium-barium solution in div. per 



min. (in the emanation electroscope) , 

 U = the activity of one gram of uranium oxide in div. per 



min. (in the a-ray electroscope). 

 B = the initial activity of one gram of radium-barium sul- 

 phate (in the «-ray electroscope). 

 W = the weight of sulphate precipitated from 10 c.c. of 



the radium-barium solution. 

 The value of x is given by 



^ WxEn^xBxQ-848 

 x 0'73xEm 2 xU. 



The advantage of this method of calculation lies in the fact 

 that the question of either radium or uranium standards is 

 not involved in the final value. 



The results obtained from the measurement of the radium 

 films are given in Table IV. 



Table IV. 



Film Film. 



Solution. Number. Weight. U. E. W. Em r Ein 2 . X. 



B 101 0-00926 710* 1040 0-0278 853 81-8 0-493 



B 102 0-00711 710* 1050 0-0278 853 81-7 0-497 



B 103 0-00819 710* 1030 0-0277 853 817 0*487 



B "... 104 0-00667 710* 1025 0-0274 853 81*8 0-480 



C 110 0-00548 710* 1025 0-0237 853 71-0 0-478 



C - 113 000300 710* 1050 0*0237 853 70-0 0-495 



Mean value of x =0*488. 

 * The Talue here given is based on a different sensibility of the electroscope 



from that which it possessed in the case of the values given in Table 1. 



