378 Miss H. Brooks on the Decay of the Excited 
had begun to undergo its changes before the electrode was 
introduced into the vessel. 
Such an action from the dust in the air of the laboratory 
was quite sufficient to mask entirely, even for exposures of 
only a few minutes, the initial increase in the radiation after 
removal. If air, which had not been previously rendered 
free from dust, was left undisturbed in the presence of the 
thorium emanation for a period of about eighteen hours, and a 
negatively charged rod then introduced into the vessel for 
one minute, the decay of the radiation from the rod after . 
‘removal was the same as that of an electrode which had been — 
exposed for a period of eighteen hours. The amount of excited . 
activity thus obtained is many times that obtained for the 
same time of exposure in dust-free air, or air which has 
been in the presence of the thorium emanation for only a 
short time. As the emanation from thorium decays very 
rapidly, falling to half its value in about a minute, the strength 
of the emanation in a closed vessel should reach a maximum 
in a few minutes, and then remain practically constant. 
Such an increase in the amount of excited activity obtainable 
from it, then, for a given time of exposure must be ascribed 
to the presence in the gas of matter which becomes active 
after exposure to the emanation for a length of time, and is 
driven to the electrodes when an electric field is applied. If 
the air is freed from dust by passing it through a tightly 
packed plug of glass-wool before it enters the emanation 
vessel, or if an electric field is kept on, as in the case of the 
experiments mentioned earlier in the paper, this increase in 
the strength of the emanation is not observed. The rate of 
decay of a body excited in the presence of an “ old’? emana- 
tion also receives a sufficient explanation, if it be assumed 
hat the dust particles suspended in the air of the vessel have 
become radioactive, and are driven to the electrode in the 
electric nell. The resulting activity then manifested on the 
electrode is made up of the rays from these radioactive dust 
particles and rays from the newly excited radioactivity on 
the rod itself; the latter is very small in comparison with the 
former, in the case cited only about 1/20th, so that the rate 
of decay observed is that of radioactive matter which has had 
a long exposure. A number of observations were made in 
which the air was left exposed to the thorium emanation for 
intervals of one half-hour and upwards, and a charged rod 
then inserted for one minute. The rate of decay after re- 
moval was always very nearly the same as that calculated for 
an electrode exposed for the length of time that the gas has 
been left in contact with the thorium. In the following 
table are given a few examples:— 
