Ix | EXCITED RADIO-ACTIVITY 273 
from those observed from thorium. In the first place, there is 
a rapid decay of the activity to less than 1/8 of the initial value, 
then a very slow variation for about 20 minutes, and then a gradual 
decay according to an exponential law. It is not possible to explain 
this variation of the activity on the assumption of two changes. It 
is necessary to suppose that there are three, the second of which is 
a change not accompanied by ionizing rays. 
Some evidence will first be considered of the decay of activity 
of a body exposed for several days in the presence of the radium 
emanation. P. Curie and Danne (loc. cit.) state that the law of 
decay of the activity of such a body is expressed accurately by 
the equation 
I = ae“ — (a— 1) 67%, 
I, 
where Ny = TLo0? No = zs)50 and a= 4:20. 
Curie and Danne do not state definitely whether the law of 
decay holds for the first ten minutes after removal. The shape of 
the decay curve for short exposure suggests from theoretical con- 
siderations that there should be in addition a small but rapid 
initial drop of activity during the first ten minutes after removal. 
Such a rapid initial drop of activity has been experimentally 
observed by the writer. 
It seems probable that the equation of P. Curie and Danne 
applies for the decay of excited activity starting from a time about 
ten minutes after removal. During that short interval the un- 
changed deposited matter rapidly passes through the first change, 
for half the matter is changed in about three minutes. At the 
time at which the measurements of Curie and Danne began, 
probably nearly all of the deposited matter had gone through the 
first change. 
Since the decay of activity after that time can be expressed by 
two exponentials, it is probable that there are two further changes 
occurring. The view that the first of these changes is a change 
unaccompanied by ionizing rays, followed by another change with 
the emission of rays, will be found to be in very close agreement 
with the results of P. Curie and Danne. It has been shown 
that, after a short exposure for a time dt to the emanation, durmg 
Ph, Tek 18 
