Page:Radio-activity.djvu/333

 '''183. Active deposit of very slow decay.''' M. and Mme Curie have observed that bodies which have been exposed for a long interval in the presence of the radium emanation do not lose all their activity. The excited activity at first decays rapidly at the normal rate, falling to half value in about 28 minutes, but a residual activity, which they state is of the order of 1/20,000 of the initial activity, always remains. A similar effect was observed by Giesel. The writer has examined the variation of this residual activity, and has found that it increases for several years. The results are discussed in detail in chapter. It will there be shown that this active deposit of slow transformation contains the radio-active constituents present in polonium, radio-tellurium and radio-lead.

Fig. 69.

184. The excited activity from actinium. The emanation of actinium, like that of thorium and radium, produces excited activity on bodies, which is concentrated on the negative electrode in an electric field. Debierne found that the excited activity