202 
EXCITED RADIO-ACTIVITY 
[CH. 
Ay, Ay In the equation for g, the theoretical value of the activity for 
any time 7’ after exposure is shown in the followimg table. 
observed experimental values are also shown. 
activity is taken as unity. 

Time in 
minutes 
15 
30 
60 
120 
220 
305 

Theoretical value 
of activity 
Observed value 
of activity 

"22 
“38 
“64 
‘90 
1-00 
‘97 

23 
37 
63 
WH 
1:00 
“96 

The 
The maximum 
The curve drawn from zero is thus almost in exact agreement 
with the equation, taking \, =°75. 
It may thus be concluded that the matter emanation X under- 
goes at least two changes :— 
(1) A change which is not accompanied by ionizing radia- 
tions, but in which the amount of matter undergoes change 
according to an exponential law with the time, falling to half 
value in 55 minutes. 
(2) 
A second change, accompanied by the emission of rays, in 
which half the matter undergoes change in 11 hours. 
The existence of such a well-marked change in the matter 
emanation X of thorium, not accompanied by the emission of” 
ionizing rays, 1s very interesting. 
It will be shown later that 
there is strong evidence of a change of a similar character in the 
emanation X of radium. 
It may be supposed that the change 
consists In a rearrangement of the components of the atom 
which is not of such a violent character as to cause a portion of 
Since there is only one changing system 
involved, it is to be expected that the law of change would be the 
same as for a monomolecular change in chemistry. 
the atom to be expelled. 
177. Secondary changes in emanation X of radium. 
The decay curves of the activity produced on a rod by a short 
exposure to the radium emanation are of a very different character 
