160 S. G. LUSBY AND T. EWING. 
To exhibit the results the common logarithms of the 
activities have been plotted against time in figure 1. fie 
each case a straight line best fits the points, showing that 
the rate of decay, for the interval of time during which 
observations were made, follows an exponential law. The 
results of four experiments are shewn, the times for the 
activity to fall to half value being as follows :— 
1. December 16th,1905... 40°7 minutes 
2.. December (4th, 1905". “Alaa ee 
>. December 20th, 1905.2... 3915. 
4, December 18th, 1905... 419 ,, 
For comparison, values given by Rutherford and Allan! 
for a lead wire exposed in an attic in Montreal, are plotted 
on the same diagram ; in this case the time for the activity 
to fall to half value seems to be 41°7 minutes rather than 
45 minutes as stated in the paper quoted. All these 
estimates of the time for the activity to fall to half value 
agree very Closely with those given by Bumstead in a 
paper in the American Journal of Science for July 1904, 
where the question of atmospheric radioactivity is discussed 
at length. 
1 Phil. Mag., Dec. 1902. 
