12 Johnstone and Boltwood — Relative Activity 



completely disintegrate. The activity of 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 figure 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. 



Enij = the activity of the equilibrium amount of emanation 

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

 nation electroscope). 



Em 2 = the activity of the equilibrium amount of emanation 

 from 10 cc. 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 a-ray electroscope). 



w = the weight of sulphate precipitated from 10 cc. of the 

 radium-barium solution. 



The value of x is given by, ' 



W X Em, X R X 0-848 

 0'73 X Em 2 X U 



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. 











Solu- 



Film 



Film 



U 



B 



W 



Em! 



Em 2 



X 



tion 



Number 



Weight 















B 



101 



0-00926 



710 a 



1040 



0-0278 



853 



81-8 



0-493 



B 



102 



0-00711 



710 a 



1050 



0-0278 



853 



81-7 



0-497 



B 



103 



0-00819 



710 a 



1030 



00277 



853 



81-7 



0-487 



B 



104 



0-00667 



710 a 



1025 



0-0274 



853 



81-8 



0-480 



C 



110 



0-00548 



710 a 



1025 



00237 



853 



71-0 



0-478 



C 



113 



0-00300 



710 a 



1050 



0-0237 



853 



70-0 



0-495 







Mean value of x - 



= 0-488 









a The value here given is based on a different sensibility of the elec- 

 troscope from that which it possessed in the case of the values given in 

 table 1. 



