262 EXCITED RADIO-ACTIVITY [CH. 
It was found that the intensity of the radiation decreased 
rapidly for the first 10 minutes after removal, but about 15 minutes 
after removal reached a value which is maintained nearly constant 
for an interval of about 20 minutes. It then decays, following an 


ure 
Decay of|Excited aaile 
of Radium— short expo 

+ 
Current. 








Time in Minutes 
Fig. 51. 
exponential law to zero, the intensity falling to half value in about 
30 minutes. With longer times of exposure, the irregularities 
in the curve are not so marked. 
Later, P. Curie and Danne‘ made a detailed investigation of the 
decay of excited activity for times of exposure to the emanation 
from 10 seconds to 6 days. ‘The results are shown graphically 
in Fig. 52, where the ordinates represent the logarithm of the 
intensity of the radiation, and the abscissae time in hours. Curve 
A represents the decay for a long time of exposure. This decay 
_ 1 C. R. 136, p. 364, 1903. 
