=231. Variation of the β ray activity.= The residual activity consists of both α and β rays, the latter being present initially in an unusually large proportion. The proportion of α to β rays from the platinum plate, one month after removal, was at the most one-fiftieth of that from a thin film of radium bromide in radio-active equilibrium. Unlike the α ray activity, the activity measured by the β rays remains constant after the active deposit is about one month old, and, in consequence, the proportion of α to β rays steadily increases with the time. The experiments showed that the intensity of the β rays did not vary much, if at all, over a further period of eighteen months. The want of proportionality between the α and β rays shows that the two types of rays arise from different products. This conclusion is confirmed by experiments, to be described later, which show that the products giving rise to α and β rays can be temporarily separated from one another by physical and chemical means. [Illustration: Fig. 94.] If observations of the active deposit are begun shortly after its formation, it is found that the activity, measured by the β rays, is small at first, but increases with the time, reaching a practical maximum about 40 days later. Experiments were made on a platinum plate, which was exposed for 3·75 days in a vessel containing the radium emanation. The observations of the β ray activity began 24 hours after removal. The results are shown in Fig. 94, where the time was measured from the middle of the time of exposure to the emanation. Similar results were obtained for a negatively charged wire exposed to the emanation. The curve, if produced back to the origin, is seen to be very similar to the recovery curves of Ur X, and other active products, and can be expressed by the equation $$ \frac {I_t} {I₀} = 1 − e^{–λt} $$, where _I₀_ is the maximum activity. The activity reaches half its final value in about six days, and the value of λ is equal to ·115 (day)⁻¹. We have shown in section 203 that a rising curve of this character indicates that the β ray activity arises from a product which is supplied at a constant rate from a primary source. Before discussing in detail the explanation of these curves, showing the rise with time of the α and β ray activity, further experimental results will be considered.