260 EXCITED RADIO-ACTIVITY [CH. 
curve, for the active matter deposited during the first few hours 
takes some time to reach its maximum activity, and the initial 
activity 1s, m consequence, smaller than would be expected from 
the equation. 
The increase of activity on a rod exposed for a short interval in 
the presence of the thorium emanation has been further investigated 
by Miss Brooks. The curve Cin Fig. 50 shows the variation with 
time of the activity of a brass rod exposed for 10 minutes in the 
emanation vessel filled with dust-free air. The excited activity 
after removal increased in the course of 3:7 hours to five times its 
initial value, and afterwards decayed at the normal rate. 




Intensity of Radiation 









1 
1 
1 
! 
o Ee 40 80 120 160 200 240 280 320 
Time in Minutes 
Fig. 50. 
171. Effect of dust on the distribution of excited activity. 
Miss Brooks, working in the Cavendish Laboratory, observed that 
the excited activity due to the thorium emanation appeared in 
some cases on the anode in an electric field, and that the distribu- 
tion of excited activity varied in an apparently capricious manner. 
This effect was finally traced to the presence of dust in the air of 
the emanation vessel. For example, with an exposure of 5 minutes 
