Jatensity of Fadiation 
Radioactivity from Thorium, Radium, and Actinium. 381 
been given by Curieand Danne*. The curves I have obtained 
differ from those given by Curie and Danne in two respects. 
The latter do not give the rapid initial decrease except for expo- 
sures of five minutes’ duration and less, and their curves for all 
exposures under one hour show a rise near the beginning which 
I have observed only when the radiation is measured by the 
B rays. In these experiments on the e rays special precautions 
were taken, by means of a constant current of air through the 
testing vessel, to prevent the results being complicated by any 
trace of emanation which might cling to the radioactive wire. 
Fig, 3.—Variation with time of the excited activity of Radium for different times 
of exposure measured by the 8 rays. 
Time /2 minutes 
The rate of decay of the 8 rays was onvtained by placing 
the radioactive body outside the testing electroscope, whose 
walls of tin 1 mm. thick would completely absorb the 
a rays. or short exposures the radiation rises at first, 
reaches a maximum in about twenty minutes, and then 
decays at the same rate as the a rays in the last change. It 
is evident, then, that only the last of the three changes oc- 
curring in the matter which causes excited radioactivity 
* Comptes Rendus, cxxxvi. p. 364 (1905) 
