380 Miss H. Brooks on the Decay of the Excited 
second change produces no rays, so that the radiation then 
remains constant for a length of time which decreases with 
the increase of the time of exposure. This flat portion of 
the curve of decay lasts for about forty minutes for very 
short exposures. The thirdchange has a period of about twenty- 
eight 28 minutes, and gives rise to a rays, so that the curve of 
Fig. 2.—Decay of excited activity of Radium for different times of 
exposure measured by the a rays. 
60 
Intensity of Fadaion. 
> 
o 
0 i ee aie 
Time (1 Ties. 
decay measured by the @ rays falls regularly to zero several 
hours after removal. In fig. 2 are given several curves for 
different times of exposure. ‘The scale to which these curves 
are plotted shows the relative amounts of activity obtainable 
in the time of exposure indicated. The decay of the a@ rays 
for times of exposure from ten seconds to six days has already 
