XI] AND OF ORDINARY MATERIALS 359 
of small activity through the apparatus. The activity rapidly fell 
to about half value, and this was followed by a very slow decrease 
of the activity with time. This result indicates that about half 
the rate of discharge observed was due to the radiation from the 
emanation and the other half to the excited activity produced by it. 
The apparatus employed by Ebert and Ewers in these experi- 
ments was very similar to that employed by Elster and Geitel, 
shown in Fig. 59. Ebert and Ewers observed that when the wire 
net attached to the electroscope was charged negatively the rate 
of discharge observed was always greater than when it was charged 
positively. The differences observed between the two rates of 
discharge varied between 10 and 20 per cent. This difference in 
the rates of discharge for positive and negative electricity is 
probably connected with the presence of particles of dust or small 
water globules suspended in the gas. The experiments of Miss 
Brooks (section 171) have shown that the particles of dust present 
in the air containing the thorium emanation become radio-active. 
A large proportion of these dust particles acquire a positive charge 
and are carried to the negative electrode in an electric field. This 
effect would increase the rate of discharge of the electroscope when 
charged negatively. In later experiments, Ebert and Ewers 
observed that, in some cases, if the air had been kept in the vessel 
for several days, the effect was reversed, and the electroscope 
showed a great rate of discharge when charged positively. 
J. J. Thomson has observed that the magnitude of the ioniza- 
tion current depends on the direction of the electric field, if fine 
water globules are suspended in the ionized gas. 
In later experiments, Ebert? found that the radio-active emana- 
tion could be removed from the air by condensation in liquid air. 
This property of the emanation was independently discovered by 
Ebert before he was aware of the results of Rutherford and Soddy 
on the condensation of the emanations of radium and thorium. In 
order to increase the amount of radio-active emanation in a given 
volume of air, a quantity of the active air, obtained by sucking the 
air from the soil, was condensed by a liquid air machine. The air 
was then allowed to partially evaporate, but the process was stopped 
1 Phil. Mag. Sept. 1902. 
2 Sitz. Akad. d. Wiss. Munich, 33, p. 133, 1903. 
