254. EXCITED RADIO-ACTIVITY [CH. 
plate placed in the bottom of the tube, while the negative pole 
was connected with the four plates. A mass of thoria was placed 
in the bottom of the tube under the plate A, and the current due 
to the emanation determined at each of the four plates. After 
passing a current of air of 0:2 cm. per second, for 7 hours along the 
tube, the plates were removed and the amount of excited activity 
produced on them was tested by the electric method. The follow- 
ing results were obtained. 


Relative current Relative excited 
due to emanation activity 
Plate A ... 1 1 
PN 0 rtd YS) “43 
seg Cpe au. 18 16 
yp Oy eee ‘072 061 





Within the errors of measurement, the amount of excited 
activity 1s thus proportional to the radiation from the emanation, 
z.e. to the amount of emanation present. The same considerations 
hold for the radium emanation. The emanation in this case, on 
account of the slow loss of its activity, can be stored mixed with 
air for long periods in a gasometer, and its effects tested quite 
independently of the active matter from which it is produced. 
The ionization current due to the excited activity produced by the 
emanation is always proportional to the current due to the emana- 
tion for the period of one month or more that its activity is large 
enough to be conveniently measured by an electrometer. 
If at any time during the interval, some of the emanation 1s 
removed and introduced into a new testing vessel, the ionization 
current will immediately commence to increase, rising in the course 
of four or five hours to about twice its original value. ‘This increase 
of the current is due to the excited activity produced on the walls 
of the containing vessel. On blowing out the emanation, the 
excited activity is left behind, and at once begins to decay. 
Whatever its age, the emanation still possesses the property of 
causing excited activity, and in amount always proportional to its 
activity, z.e. to the amount of emanation present. 
These results show that the power of exciting activity on 
