examined for a still longer period. The emanation from 30 milligrams of radium bromide was condensed in a glass tube and then sealed. After a month's interval, the tube was opened and dilute sulphuric acid introduced. The acid dissolved off the active deposit in the tube and on driving off the acid by heat, a radio-active residue was obtained. The activity of this residue, measured by the α rays, steadily increased for a period of 18 months, but the curve of variation of activity with time plotted as in Fig. 93 tends to become more flattened, and is obviously approaching a maximum value.
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Rise of Activity of Radium F measured by the α rays.
Fig. 93.
The explanation of this curve will be considered later in section 236.
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
