Ix] EXCITED RADIO-ACTIVITY 261 
the amount of excited activity to be observed on a rod depended 
on the time that the air had been allowed to remain undisturbed 
in the emanation vessel beforehand. The effect increased with th= 
time of standing, and was a maximum after about 18 hours. The 
amount of excited activity obtained on the rod was then about 
20 times as great as the amount observed for air freshly introduced. 
The activity of this rod did not increase after removal, but with 
fresh air, the excited activity, for a 5 minutes’ exposure, increased 
to five or six times its initial value. 
This anomalous behaviour was found to be due to the presence 
of dust particles in the air of the vessel, in which the bodies were 
made radio-active. These particles of dust, when shut up in the 
presence of the emanation, become radio-active. When a nega- 
tively charged rod is introduced into the vessel, a part of the 
radio-active dust 1s concentrated on the rod and its activity 1s 
added to the normal activity produced on the wire. After the air 
in the vessel has been left undisturbed for an interval sufficiently 
long to allow each of the particles of dust to reach a state of radio- 
active equilibrium, on the application of an electric field, all the 
positively charged dust particles will at once be carried to the 
negative electrode. The activity of the electrode at once com- 
mences to decay, since the decay of the activity of the dust particles 
on the wire quite masks the initial rise of the normal activity 
produced on the wire. 
Part of the radio-active dust is also carried to the anode, and 
the proportion increases with the length of time during which the 
air has been undisturbed. The greatest amount obtained on the 
anode was about 60°/, of that on the cathode. 
These anomalous effects were found to disappear if the air was 
made dust-free by passing through a plug of glass wool, or by 
application for some time of a strong electric field. 
172. Decay of excited activity from radium. The excited 
activity produced on bodies by exposure to the radium emanation 
decays much more rapidly than the thorium excited activity. For 
short times of exposure’ to the emanation the decay curve is very 
uregular. This is shown in Fig. 51. 
1 Rutherford and Miss Brooks, Phil. Mag. July, 1902. 
