//. A. Bum stead — Atmospheric Radio-activity. 



clear day, I have succeeded, in collecting considerable quanti- 

 ties of this slowly decaying radio-activity. Eleven hours after 

 the wire had been put into the testing cylinder, the current 

 through the gas was 35 times that due to the spontaneous ioni- 

 zation and its gradual decay could be followed for several days. 

 Observations extending over various intervals, from 6 hours 

 to 27, showed that the decay was exponential and the average 

 value of A, from five determinations, was 0*0616, which indi- 

 cates a decrease to half in 11*2 hours. 



The observations upon the atmospheric activity can be 

 accounted for fairly well, but not quite satisfactorily, by the 

 assumption that it is entirely due to the excited activities of 

 radium and thorium. In the following tables, the second 

 column gives the calculated values of the ionization upon this 

 assumption, the proportion of the two forms of activity being 

 so chosen that the calculated and observed values shall agree 

 at 1 hour and at 4 hours ; for the present purpose, the 

 decay of the thorium activity is assumed to be exponential (it 

 is not actually so in the earlier stages) while the decay of the 

 radium activity is found from Curve IV, fig. 2.* The third 

 column gives the observed values for corresponding times, and 

 the last column the differences between the calculated and 

 observed values. The first table represents the experiments 

 plotted in Curve I, fig. 2, the second, those plotted in Curve III. 



Assumed, 

 at 1 hour 



at 4 hours 

 t (hours) Calc 



Curve I. 



Ea. activity 

 Th. activity : 

 Ea. activity 

 Th. activity 

 Obs. 



1-0 

 1-5 

 2-0 

 2-5 

 3-0 

 3-5 

 4-0 



18-6 

 11-1 

 6-48 

 3*81 

 2-51 

 1-S9 

 1-57 



18*6 

 Kr8 

 6'20 

 3-69 

 2*45 

 1-84 

 1-57 



17-0 

 1-6 



0-244 



1-324 



Diff. 



o-oo 



+ 0-30 



+ 0-28 

 + 0-12 

 + 0-06 

 + 0-05 



o-oo 



Curve III. 



Assumed, 

 at 1 hour 



at 4 hours -j *?£ 



t (hours) 

 1-0 

 1-5 

 2-0 

 2-5 

 3-0 

 3-5 

 4-0 



Calc 



Ea. activity == 8 *77 

 Th. activity = 1-23 

 activity = 0*12 

 activity = 1*02 

 Obs. Diff. 



10-00 o-oo 



5-95 +0-16 

 3-53 +0-20 

 2-30 +0-03 

 1-67 0-00 



1-32 O'OO 



1-14 0-00 



*Up to 2-5 hours, this Curve IV, and other similar experiments which I 

 have made, give a slightly different result from the empirical formula given 

 by Curie and Danne (C. E., cxxxvi, p. 365, 1903) which is 



1 = 



r -- 



I |_ae 0i 



■(a-l)e fa J 



where a = 4'20, 0i = 2420 seconds, 0„ = 1860 seconds. This formula does 

 not take into account the initial rapidly decaying activity due to radium. 

 In the final, exponential part, my results are in close accord with those of 

 Curie and Danne. The half -value time which they obtain is 27*9 minutes, 

 while the value resulting from several experiments of my own is 27 '7 

 minutes. 



