Page:Popular Science Monthly Volume 67.djvu/31

the magnitude of the α and β 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.^[1]

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 aa, β or γ 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 $e{\lambda _{2}t}-e{\lambda _{1}t}$, where $\lambda _{1}$, $\lambda _{2}$ 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 $\lambda _{1}$, $\lambda _{2}$ corresponding to the two changes. It is seen that the equation is symmetrical in $\lambda _{1}$ and $\lambda _{2}$ and in consequence is unaltered if the values of $\lambda _{1}$ and $\lambda _{2}$ 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

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1. 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.