376 Miss H. Brooks on the Decay o7 the Eacited 
times of exposure, and from (3) the current at different in- 
tervals after removal may be obtained. 
In fig. 1 are given the curves thus obtained for the initial 
increase in the radiation to a maximum for exposures of 1, 2, 
3, 4, and 6 hours; and the points are also given which were 
obtained by observations on a negatively charged body ex- 
posed to the thorium emanation for a corresponding length of 
time. The agreement between the curves obtained by obser- 
vation and those by calculation from equation (3) for exposures 
of one hour and upwards is very close. For exposures of 
less than one hour the initial increase indicated by the theory 
is not quite realized in practice, as it is almost impossible to 
prevent the deposit on the electrode of small amounts of 
radioactive matter which have already undergone the first 
change. The effect of this, in the case of long exposures, 
is negligible, but when, as in the case of exposures of less 
than one hour, the whole of the resulting activity is very 
small, it becomes an appreciable factor and causes the increase 
after removal to be less than it would otherwise be. 
The arrangements for the experimental observations were 
as follows:—A quantity of thorium hydroxide was placed in 
the bottom of a cylindrical brass vessel with closely fitting 
ends of ebonite, and through a small hole in one of these a 
brass rod was slipped into the vessel. The outside of the 
cylinder was kept connected to the positive pole of a storage- 
battery of 100 volts, and the central rod to the negative pole. 
The electric field was kept on the cylinder during the whole 
course of the experiments, to prevent the collection in the 
vessel of radioactive dust. particles which greatly complicate 
the results, as will be explained later. When a series of 
observations was to be taken, a clean brass rod was intro- 
duced in the place of the central electrode, as rapidly as 
possible, so that the current might not be off for an appre- 
ciable length of time. After an exposure for the required 
interval the rod was removed, placed inside a vessel containing 
a sensitive gold-leaf electroscope, and the radiation from it 
measured at intervals by the rate of collapse of the gold leaves. 
Effect of Dust. 
It has already been mentioned that the curves found by 
experiment for short exposures probably do not really 
represent the decay of the activity excited only in the 
time indicated by the time of exposure. In the course of 
the experiments, the great importance was recognized of the 
necessity of avoiding the deposition on the electrode of matter 
from the gas which had already been made radioactive and 
