The effect was particularly noticeable in some experiments with a copper plate, which was made active by leaving it a short time in a solution of the active deposit from radium. This active solution was obtained by placing an active platinum wire in dilute hydrochloric acid. On placing the copper plate in a testing vessel for a few minutes, and then removing it, activity was observed on the walls of the vessel amounting to about one per cent. of the activity of the copper plate.
It was found that this effect was not due to the emission of an emanation from the active body, but must be ascribed to a slight volatility of radium B at ordinary temperatures. This was proved by observations on the variation of the activity of the matter deposited on the walls of the vessel. The activity was small at first, but rose to a maximum after about 30 minutes, and then decayed with the time. The curve of rise was very similar to that shown in Fig. 87, and shows that the inactive matter radium B was carried to the walls and there changed into C, which gave rise to the radiation observed.
The product B only escapes from the body for a short time after removal. This is a strong indication that its apparent volatility is connected with the presence of the rapidly changing product radium A. Since A breaks up with an expulsion of an α particle, some of the residual atoms constituting radium B may acquire sufficient velocity to escape into the gas, and are then transferred by diffusion to the walls of the vessel.
Miss Brooks observed that the activity was not concentrated on the negative electrode in an electric field but was diffused uniformly over the walls of the vessel. This observation is of importance in considering the explanation of the anomalous effects exhibited by the active deposit of radium, which will be discussed in the following section.
228. Effect of the first rapid change. We have seen that
the law of decay of activity, measured by the β or γ rays, can be
explained very satisfactorily if the first 3-minute change is disregarded.
The full theoretical examination of the question given
in sections 197 and 198 and the curves of Figs. 72 and 73 show,
however, that the presence of the first change should exercise an
