Ix] EXCITED RADIO-ACTIVITY 259 
expected from the equation. This result, however, is completely 
explained in the hght of later results. The writer’ found that, for 
a short exposure of a body to the thorium emanation, the excited 
activity upon it after removal, instead of at once decaying at the 
normal rate, increased for several hours. In some cases the activity 
of the body increased three to four times its original value in the 
course of a few hours and then decayed with the time. 
Some of the results obtained are shown in the following tables. 
Table I. is for a platinum wire exposed as cathode for 15 minutes ; 
Table II. for aluminium foil with 41 minutes’ exposure to the 
emanation. About 5 minutes elapsed between removal and the 
first observation. 
TABLE I. TABLE I]. 
Time Current Time Current 
0 1 0 1 
75 minutes 1°5 21 minutes 1°6 
24 i 2-1 Sa erton abples 
43 . 2-4 Bi OO 
58 A 2°7 TO.) apn, D2 
78 3 oul 91 ui 2°5 
99 5 3:4 120 r 2:9 
160g e229 
180 ae HnO-9 
22 hours 1:0 
AQ sok 2] 
The initial current is, in each case, taken as unity. In Table II. 
the activity after increasing nearly to three times its original value 
decreases again at about the normal rate, falling to half value in 
about 11 hours. 
With a longer time of exposure to the emanation, the ratio of 
the increase after removal is much less marked. For a long 
interval of exposure, the activity after removal begins at once to 
diminish. In this case, the increase of activity of the matter 
deposited in the last few hours does not compensate for the 
decrease of activity of the active matter as a whole, and conse- 
quently the activity at once commences to decay. This increase of 
activity with time explains the initial irregularity in the recovery 
_+ Phys. Zeit. 3, No. 12, p. 254, 1902. Phil. Mag. Jan. 1903. 
17—2 
