292 EXCITED RADIO-ACTIVITY elgie-< 
out by the active residue, accompany another very slow change 
occurring in the matter emanation X, the time taken for the 
activity of this product to decay to half value can be deduced 
from general considerations. It will be shown later, in section 
195, that each of the successive changes in radium or thorium, 
which is accompanied by a@ rays, gives rise to about the same 
total amount of ionization. This is merely an expression of the 
fact that the same number of systems must undergo change in 
each successive product, and that each system probably expels 
the same number of « particles with about the same velocity. 
Now it was found experimentally that the ionization current due 
to the active residue was about 1/20,000 of the initial ionization 
due to the emanation, which in its further changes had given 
rise to the slowly decaying active matter. Since the ionization 
current due to the emanation was 20,000 times that due to the 
active matter, its rate of change was 20,000 times faster. But the 
activity of the emanation decays to half value in four days, so that 
the activity of this other active matter would decay to half value 
in about 80,000 days or about 200 years. 
The existence of such a slow change in the emanation X of 
radium probably accounts in part for the radio-activity which 1s 
produced on the walls of the laboratory in which radium prepara- 
tions have been kept in open vessels. The emanation diffuses into 
the air and produces emanation X, which is deposited on the walls 
of the room, and there gives rise to a deposit of very slowly decay- 
ing matter. This activity persists im a room even though no - 
radio-active matter has been kept in it for some time. 
