8 II. A. Bum stead — Atmospheric Radio-activity. 



actinium. The following tables will make this clear ; they are 

 like the two previously given except that, in the calculated 

 column, it has been assumed that actinium and thorium activi- 

 ties are present instead of radium and thorium. 





Curve I. 







Curve III. 





Assumed 









Assumed 









, 1 , ( Actin. activity 

 at 1 hour j Th actiyify 



=17-6 

 = 0-96 



•* * h - 1 T"!; ity 



= 9-11 



= 0-89 



, A , S Actiu. activity 

 at4hours lTh. activity " 



= 0-78 

 = 0-79 



«*?»" {jgZSflg* 



= 0-40 

 = 0-74 



t hours. 



Calc. 



Obs. 



Diff. 



t hours. 



Calc. 



Obs. 



Diff. 



l'O 



18-6 



18-6 



o-oo 



l'O 



10-00 



10-00 



o-oo 



1-5 



11-4 



10-8 



+ 0-60 



1-5 



6*27 



5-95 



+ 032 



2*0 



7-11 



6-20 



+ 0-91 



2-0 



4-05 



3-53 



+ 0-52 



2-5 



4-56 



3-69 



+ 0-87 



2-5 



2-72 



2-30 



+ 0-42 



3-0 



3-04 



2-45 



+ 0-59 



3-0 



1-92 



1-67 



+ 0'2o 



3'5 



2-12 



1-84 



+ 0-28 



3-5 



1-43 



1-32 



+ 0-11 



4-0 



1-57 



1-57 



000 



4-0 



1-14 



1-14 



o-oo 



The differences are much larger than on the assumption of 

 radium and thorium, and are quite beyond any possible experi- 

 mental errors; and, to get even such an agreement, it is neces- 

 sary to assume a smaller amount of thorium activity than the 

 observations at later times than four hours will permit. Upon 

 a priori grounds also, it is improbable that much of the effect 

 can be due to actinium ; the radium emanation is known to be 

 present in the ground and decays so slowly (half-value in four 

 days) that there is ample time for it to diffuse widely through 

 the atmosphere ; the actinium emanation, on the other hand, 

 decays with great rapidity (half-value in four seconds),* and 

 unless it were present in the ground in relatively enormous 

 quantities, and not far from the surface, its effects in the 

 open air could not predominate over those of the radium ema- 

 nation. There is no reason to suppose that it does ; any sub- 

 stitution of actinium for radium in the calculated values will 

 cause them to depart more widely from the observed values. 

 But, as previously remarked, if it should turn out that a small 

 amount of a more rapidly decaying activity is present (as there 

 is some reason to suspect), it is not impossible that some 

 actinium activity may also be found. Mr. H. M. Dadourian, 

 to whom I am indebted for assistance in many of the present 

 experiments, is now engaged in an attempt to ascertain more 

 definitely whether such an activity is present or not. 



It will be observed that the general slope of Curve III is 

 less rapid than that of Curve I, and that a considerably greater 

 proportion of the thorium activity appears to be present. 

 This, I think, may be accounted for from the known proper- 



* Debierne, loc. cit. 



