Radioactivity from Thorium, Radium, and Actinium. 379 
Length of exposure | Observed ratio of | Calculated ratio | 
of the gas to the maximum toori- | of maximum to 
emanation. ginal radiation. original radiation. 
Quevhouri ).\osae 3°9 33 
iwi, HOUTS, <<. «ess oe 1-9 1°82 
Ua NM RRS 1:38 1°35 
SUR | iy) ge queetacen | 13 113 
By means of this property which dust possesses of becoming 
radioactive, some information can be gained concerning the 
change that takes place after the positively charged carrier 
has become attached to a solid body. If the air in the ema- 
nation vessel is dust-free or has just been introduced, when 
an electric field is applied, no radioactivity is manifested on 
the positive electrode, it is all confined to the negative. 
When, however, air which is not dust-free has been closed up 
in the presence of the emanation for a time and an electric 
field then applied, a certain percentage of the whole activity 
obtainable from the gas appears on the positive electrode. 
When the gas has been left undisturbed for a length of time, 
to give to every dust particle its maximum amount of activity, 
almost equal amounts of the active particles are carried to the 
positive and negative electrodes. This appearance of radio- 
activity on the positive electrode when exposed to “old” 
emanation has been noticed before by Sella*, who ascribed 
it to a change in the nature of the emanation. 
Excited Activity from Radium. 
The same effect is to be observed in the case of the radium 
emanation; but owing to the more rapid decay of the excited 
activity from radium, the error due to the presence of dust 
is not so great. 
In a corresponding series of experiments on the decay of 
the excited activity from the radium emanation for different 
times of exposure, the rate of decay of the a, 8, and y rays 
was measured separately. It has already been shownf that 
the decay of the « rays from a body made active by a short 
exposure to the emanation from radium, gives evidence that 
three changes occur after the positive carriers are deposited on 
the solid body. The first change gives rise to a rays only and 
is very rapid, half the matter being changed in about three 
minutes. This change causes a rapid initial decrease in 
the intensity of the radiation for the first ten minutes. The 
* Il Nuovo Cimento, [5] iv. p. 181 (1902). 
+ E. Rutherford, ‘ Radioactivity,’ p. 274. 
