Page:Radio-activity.djvu/405

 There is thus a good agreement between the calculated and observed values of the activity measured by the β rays.

The results are satisfactorily explained if it is supposed:—

(1) That the change B into C (half transformed in 21 minutes) does not give rise to β rays;

(2) That the change C into D (half transformed in 28 minutes) gives rise to β rays.

222. These conclusions are very strongly supported by observations of the decay measured by the β rays for a long exposure. The curve of decay is shown in Fig. 88 and Fig. 89, curve I.

Fig. 89.

P. Curie and Danne made the important observation that the curve of decay C, corresponding to that shown in Fig. 88, for a long exposure, could be accurately expressed by an empirical equation of the form

I_{t}/I_{0} = αe^{-λ_{3}t} - (α - 1)e^{-λ_{2}t},

where λ_{2} = 5·38 × 10^{-4} (sec)^{-1} and λ_{3} = 4·13 × 10^{-4} (sec)^{-1}, and α = 4·20 is a numerical constant.

I have found that within the limit of experimental error this equation represents the decay of excited activity of radium for a