vit} RADIO-ACTIVE EMANATIONS 209 
activity. Giesel has obtained an intensely active emanation from 
the “emanating substance.” It has already been pointed out (sec- 
tion 21) that this “emanating substance” is probably the same as 
the actinium of Debierne. The emanation from actinium, like those 
from thorium and radium, possesses the property of exciting activity 
on inactive bodies. However it has not yet been studied as com- 
pletely as the better known emanations of thorium and radium. 
Experiments with large amounts of Radiwm Emanation. 
138. With very active specimens of radium, a large amount 
of emanation can be obtained, and the electrical and photographic 
actions are correspondingly intense. On account of the small 
activity of thorium and the rapid decay of its emanation, the 
effects due to it are weak, and can be studied only for a few 
minutes after its production. The emanation from radium, on the 
other hand, im consequence of the slow decay of its activity, may 
be stored mixed with air in an ordinary gas holder, and its photo- 
graphic and electrical actions may be examined several days or 
even weeks after, quite apart from those of the radium from which 
it was obtained. 
It is, in general, difficult to study the radiation due to the 
emanation alone, on account of the fact that the emanation is 
continually producing a secondary type of activity on the surface . 
of the vessel in which the emanation is enclosed. This excited 
activity reaches a maximum value several hours after the intro- 
duction of the emanation, and, as long as it is kept in the vessel, 
this excited activity on the walls decays at the same rate as the 
emanation itself, 7.e. 1t falls to half its imitial value in about 4 days. 
If, however, the emanation is blown out, the excited activity 
remains behind on the surface, but rapidly loses its activity in the 
course of a few hours. After several hours, the intensity of the 
residual radiation is very small. 
These effects and their connection with the emanation are 
discussed more fully im chapter Ix. 
Giesel? has recorded some interesting observations of the eftect of 
the radium emanation on a screen of phosphorescent zinc sulphide. 
1 Ber, der deutsch. Chem. Ges. p. 3608, 1902. 
R, B.-A. 14 
