264 EXCITED RADIO-ACTIVITY [CH. 
very rapidly, then passes through a minimum after 8 minutes, and 
increases to a maximum after 40 minutes, and after 2°5 hours 
decays in an exponential law to zerot, As in the case of the excited 
activity from thorium, the rate of decay of the excited activity 
from radium is for the most part independent of the nature of the 
body made active. Curie and Danne (loc. cit.) observed that the 
active bodies gave off an emanation itself capable of exciting 
activity in neighbouring bodies. This property rapidly disappeared 
and was inappreciable 2 hours after removal. In certain substances 
like celluloid and caoutchouc, the decay of activity is very much 
slower than for the metals. This effect becomes more marked 
with increase of time of exposure to the emanation. A similar 
effect is exhibited by lead, but to a less marked degree. During 
the time the activity lasts, these substances continue to give off 
an emanation. . 
It is probable that these divergencies from the general law are 
not due to an actual change in the rate of decay of the true excited 
activity but to an occlusion of the emanation by these substances 
during the interval of exposure. After exposure the emanation 
gradually diffuses out, and thus the activity due to this occluded 
emanation and the excited activity produced by it decays very 
slowly with the time, 
173. Excited radio-activity 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 30 minutes, but a 
residual activity always remains of the order of 1/20,000 of the 
initial activity. This residual activity either does not diminish 
at all, or so slowly that the decrease is not appreciable after 
an interval of six months. 
1 The writer has not observed the rise to a maximum found by Curie and 
Danne for the decay curves of the excited activity due to radium (see Fig. 52), but 
has always obtained curves of decay, for short exposures, similar to that shown in 
Fig. 51. This has been the case whether the excited activity has been produced on 
a body by the action of an electric field or not. In the experiments, a slow current 
of air was always passed through the testing apparatus to remove any emanation 
from the body made active. 
2 Thesis, Paris, 1903, p. 116. 
