Radioactivity from Thorium, Radium, and Actinium. 383 
decrease according to an exponential law, half the radio- 
active matter being changed in about forty minutes. There 
is, however, in addition an increase at the beginning for short 
exposures to the emanation, analogous to that observed in the 
case of thorium, and pointing to the existence of an initial 
“‘rayless” change after the concentration of the positive 
carriers on the electrode. The initial rayless change in the 
case of the actinium excited is very rapid, half the matter 
being changed in one minute and a half. For exposures to 
the emanation of a few minutes’ duration, the radiation after 
removal rises to a maximum in about seven minutes and then 
begins to diminish, decaying regularly after a time according 
to the rate determined by Debierne. The equations given forthe 
decay of the thorium-excited activity will be applicable also 
in this case. The coefficients of the primary and secondary 
changes can be determined as before from the equation 
= =e“, This gives \,;=°0077 and A,="000288. Inserting 
these values in the expressions for A, B, and A—5, the 
curves of decay for different times of exposure can be deter- 
mined. Tig. 5 gives the curves obtained by experiment for 
Fig. 5.—Decay of excited activity of Actinium for 
different times of exposure. 
Intensity of Faaation 
T/T7e (2 r7/nules 
times of exposure from one minute to twenty hours, all 
plotted to the scale of the long exposure. These experiments 
