258 EXCITED RADIO-ACTIVITY [cH 
gas in which it decays. The rate of decay is unchanged whether 
the excited activity is produced on the body with or without an 
electric field. 
The amount of excited activity produced on a body increases 
at first with the time, but reaches a maximum after an exposure 
of several days. An example of the results is given in the following 
table. In this experiment, a rod was made the cathode in a closed 
vessel containing thoria. It was removed at intervals for the short 
time necessary to test its activity and then replaced. 
Time in hours Current 
1°68 6:3 
3°25 10°5 
5°83 29 
9°83 40 
14:00 59 
23°41 77 
29°83 83 
47:00 90 
72°50 95 
96:00 100 
These results are shown graphically in Curve B, Fig. 49. It is 
seen that the decay and recovery curves may be represented 
approximately by the following equations. 
i 
For the decay curve A, 5 dai e7 re, 
0 
I At 
For the recovery curve B, aaa l—e 
0 
The two curves are thus complementary to one another; they 
are connected in the same way as the decay and recovery curves of 
Ur X, and are susceptible of a similar explanation. 
The amount of excited radio-activity reaches a maximum value 
when the rate of supply of fresh radio-active particles balances the 
rate of change of those already deposited. 
170. Excited radio-activity produced by a short ex- 
posure. The initial portion of the recovery curve B, Fig. 49, is 
not accurately represented by the above equation. The activity 
for the first few hours increases more slowly than would be 
