PRESENT PROBLEMS IN RADIOACTIVITY. 25 



the magnitude of the a and (3 ray activity at any time. It has been 

 deduced that radium D is probably half transformed in 40 years and 

 radium E is half transformed in about 1 year. The evidence at 

 present obtained points to the conclusion that radium E is the active 

 constituent present in Marckwald's radio-tellurium and probably also 

 in the polonium of Mme. Curie.* 



The changes in the active deposit of thorium have been analyzed 

 by the writer, and the corresponding changes in actinium by Miss 

 Brooks. 



The occurrence of a ' rayless change ' in the active deposits from the 

 emanation of radium, thorium and actinium is of great interest and 

 importance. As these products do not emit either a, ft or y rays, 

 their presence can only be detected by their effect on the amount of the 

 succeeding products. The action of the rayless change is most clearly 

 brought out in the examination of the variation of activity with time 

 of a body exposed for a very short interval in the presence of the 

 emanations of thorium and actinium. Let us consider, for simplicity, 

 the variation of activity with time for thorium. The activity 

 (measured by the a rays) observed at first is very small, but gradually 

 increases with the time, passes through a maximum and finally decays 

 according to an exponential law with the time falling to half value 

 in eleven hours. The shape of this curve can be completely explained 

 on the assumption of the two successive changes, the second of which 

 alone gives out rays. The matter deposited on the body during the 

 short exposure consists almost entirely of thorium A. Thorium A 

 changes into B and the breaking up of B gives rise to the activity 

 measured. 



If thorium A does not give out rays, the activity of the body at 

 any time t after removal can be easily shown to be proportional 

 to e~ Xat — e~ A >', where A x , A 2 are the constants of change of thorium A 

 and B, respectively. Now the experimental curves of variation of 

 activity are found to be accurately expressed by an equation of this 

 form. A very interesting point arises in settling the values of A x , A 2 

 corresponding to the two changes. It is seen that the equation is 

 symmetrical in A x and A 2 and in consequence is unaltered if the values 

 of A x and A 2 are interchanged. Now the constant of the change is 

 determined by the observation that the activity finally decays to half 

 value in 11 hours. The theoretical and experimental curves are found 

 to coincide if one of the two products is half transformed in 11 

 hours and the other in 55 minutes. The comparison of the theoretical 

 and experimental curves does not, however, allow us to settle whether 



* The writer has recently found that radium E and radio -tellurium have 

 identical rates of decay. Both lose half of their activity in 150 days. This 

 result shows that the active substance in radio-tellurium is a transformation 

 product of radium. 



