SUCCESSION OK CIIAXGES IN RADIOACTIVE BODIES. 1K7 



thorium solutions. Tin- r.-ites of decay ofthe active pnxlticts depended UJXHI conditions, 

 hut he found that, in several cases, rapidly decaying products were obtained whose 

 activity fell to half value in about 1 hour. Allowing for the probability that the 

 product examined was not completely isolated by the electrolysis, but contained also 

 a trace of the other product, this result would indicate that the last change which 

 gives rise to rays is the more rapid of the two. 



The results obtained by VON LERCH* in the electrolysis of solution of the active 

 deposit also admit of a similar interpretation. Products were obtained on the 

 electrodes of different rates of decay, but which lost half their activity in times 

 varying from about 1 hour to 5 hours. This variation is possibly due to the admixture 

 of the two products, but further experiment is necessary to settle this point with 

 certainty. The evidence, as a whole, thus supports the conclusion that the active 

 deposit from thorium undergoes two successive transformations as follows : 



(1) A 'rayless' change for which X, = 175 X 10~ 5 , i.e., in which half the matter 



is transformed in 1 1 hours ; 



(2) A second change giving rise to a, ft and y rays, for which Ag = 2'08 X 10~ 4 , 



i.e., in which half the matter is transformed in 55 minutes, t 



It is, at first sight, a somewhat unexpected result that the final rate of decay ot 

 the active deposit from thorium does not in reality give the rate of change of the last 

 product itself, but of the preceding product, which does not give rise to rays at all. 



A similar peculiarity is observed in the decay of the excited activity of actinium, 

 which is discussed in section 15. 



14. For a long exposure in the presence of a constant supply of thorium emanation, 

 the equation expressing the variation of activity with time is found from equation (8), 

 section 11. 



I' _ Q *a__ e -v _ AI e -v 



M) *XO "V *1 *1 ~~ *2 



' (1 - -083 e- 1 ' 90 " 10 -*). 



2 ^~ A | 



About 4 hours after removal, the second term in the brackets becomes very small, 

 and the activity after that time will decay very nearly according to an exponential 

 law with the time, falling to half value in 1 1 hours. For any time of exposure T, 

 the activity at time t after the removal (see equation 11) is given by 



I, Q^ wT* - be-"' 1 

 I " QT " - b 



* 'Ann. d. Physik,' November, 1903. 



t The ' niyless change ' certainly does not give out a rays, and special experiments showed that no 

 appreciable amount of ft rays were present. On the other hand, the second change gives out all three 

 types of rays. 



2 B 2 



