188 PROFESSOR E. RUTHERFORD ON THE 



where I is the initial value of the activity, immediately after removal, and 



a = 



By variation of T the curves of variation of activity for any time of exposure can lie 

 accurately deduced from the equation, when the values of the two constants X,, \% are 

 substituted. Miss BROOKS has examined the decay curves of excited activity for 

 thorium for different times of exposure and has observed a substantial agreement 

 between experiment and theory.* 



15. Changes in Actinium. Dr. GIESEL kindly forwarded me a radioactive 

 preparation from pitchblende and called by him the ' emanating substance,' on 

 account of the large amount of emanation it gives out. This had an activity, 

 measured in the usual way, of about 250 times that of uranium. The emanation and 

 excited activity produced by it were kindly examined for me in detail by Miss BROOKS. 

 The emanation was found to have a very rapid rate of decay, its activity falling to 

 half value in a few seconds. The excited activity for a long exposure fell to half 

 value in 41 minutes. DEBiEBNEt has shown that actinium gives off an emanation 

 which loses half its activity in 37 seconds and produces excited activity which falls 

 to half value in 41 minutes. There can be no doubt that the ' emanating substance' 

 of GIESEL and the actinium of DEBIERNE contain the same radioactive constituent. 

 The name actinium will thus be used in this paper to denote the ' emanating 

 substance' of GIESEL. Miss BROOKS investigated the rate of decay of the excited 

 activity of actinium for different times of exposure ; but, for the purpose of elucidation 

 of the changes occurring, we need only consider the curves of decay of excited 

 activity for a short and for a long exposure. For a long exposure the activity decays 

 very nearly according to an exponential law, falling to half value in 41 minutes. The 

 value of the change-constant A. is 2'80 X 10~ 4 (see)" 1 . 



The activity for a short exposure at first increases, rapidly passes through a 

 maximum, and after some time decays according to an exponential law, falling to half 

 value in 41 minutes. The curve of decay (measured by the a rays) for an exposure 

 of 1'5 minutes in the presence of the actinium emanation is shown in fig. 10. 



The maximum is reached about 7 '5 minutes, reckoning from the moment the body 

 is exposed to the emanation. 



The curve is very similar in general shape to the corresponding curve of thorium, 

 and can be analysed in a similar way ; the activity at any time t is proportional to 

 e -v e - A ''. 



These results show that the first change occurring in the active deposit from actinium 

 is a rayless change. Since the activity finally decays according to an exponential law 

 (half value in 41 minutes), one of the constants of the change has a value 2'80X 10~ 4 . 



[* 'Phil. Mag.,' September, 1904.] 

 t 'Comptes Kendus,' 138, p. 411, 1904. 



