Bronson — Radio-active Measurements. 



187 



stance 

 I 



I 



is given by Rutherford (Radio-activity, p. 258) as 



= e~ At . From curve A, \ is found to be 0*0193, and from 



curve B, 0*0195, where the time is measured in minutes. 

 The mean of these two values gives 35*7 minutes as the time 

 required for the excited activity of actinium to decay to half 

 value. This is a rather shorter time than was found by 

 Debierne (Comptes Rendus, No. 138, p. 491), and verified by 

 Miss Brooks (Phil. Mag., Sept., 1904). This smaller value, how- 

 ever, was confirmed by a large number of observations taken 



© /.<?- 



be 



c 



- 



Zo 40 6o 60 /oo izo 14* l6o - /Bo Zoo zzo 24o 26o ZdO 



Time in seconds. 



by the " rate " method. The values thus obtained varied con- 

 siderably, but their mean was about 36 minutes. 



Fig. 2 shows three logarithmic decay curves for thorium 

 emanation, which on account of its rapid rate of decay has been 

 found so difficult to measure. By this method the observations 

 for the three curves were taken in about thirty minutes. The 

 value of X obtained from curve A is 0*0129, from curve B is 

 0*0127, and from curve C is 0*0128, — a very satisfactory agree- 

 ment for single sets of observations. A mean value of these 

 \'s gives about 54 seconds as the time taken for the emanation 

 to decay to half value, which agrees fairly well with the value 

 51*2 seconds recently obtained by C. Le Rossignol and C. T. 

 Gimingham (Phil. Mag., July, 1904). 



Macdonald Physics Building, 



McGill University, Dec. 20, 1904. 



