
360 RADIO-ACTIVITY OF THE. ATMOSPHERE [CH. 
before the point of volatilization of the emanation was reached. 
This process was repeated with another quantity of air and the 
residues added together. Proceeding in this way, he was able to 
concentrate the emanation in a small volume of air. On allowing 
the air to evaporate, the ionization of the air in the testing vessel 
increased rapidly for a time and then slowly diminished. Ebert 
states that the maximum was reached earlier for the emanation 
which had been liquefied for some time than for fresh air. The rate 
of decay of activity of the emanation was not altered by keeping 
it at the temperature of liquid air for some time. In this respect 
it behaves like the emanations of radium and thorium. 
J. J. Thomson’ found that air bubbled through Cambridge tap 
water showed much greater conductivity than ordinary air. The 
air was drawn through the water by means of a water pump into a 
large gasometer, when the ionization current was tested with a 
sensitive electrometer. When a rod charged negatively was intro- 
duced into this conducting air it became active. After an exposure 
for a period of 15 to 30 minutes in the conducting gas, the rod, 
when introduced into a second testing vessel, increased the saturation 
current in the vessel to about five times the normal amount. Very 
little effect was produced if the rod was uncharged or charged 
positively for the same time. The activity of the rod decayed 
with the time, falling to half value in about 40 minutes. The 
amount of activity produced in a wire under constant conditions 
was independent of the material of the wire. The rays from the 
rod were readily absorbed in a few centimetres of air. 
These effects were, at first, thought to be due to the action of 
the small water drops suspended in the gas, for it was well known 
that air rapidly drawn through water causes a temporary increase 
in its conductivity. Later results, however, showed that there 
was a radio-active emanation present in Cambridge tap water. 
This led to an examination of the waters from deep wells in 
various parts of England, and J. J. Thomson found that, in some 
cases, a large amount of emanation could be obtained from the 
well water. The emanation was released either by bubbling air 
through the water or by boiling the water. The gases obtained by 
boiling the water were found to be strongly active. A sample of 
1 Phil. Mag. Sept. 1902. 
