Ix] EXCITED RADIO-ACTIVITY 255 
Inactive substances is a property of the radio-active emanations, 
and is proportional to the amount of emanation present. 
The phenomenon of excited activity cannot be ascribed to a 
type of phosphorescence produced by the rays from the emanation 
on bodies ; for it has been shown that the activity can be concen- 
trated on the negative electrode in a strong electric field, even if 
the electrode is shielded from the direct radiation from the active 
substance which gives off the emanation. The amount of excited 
activity does not seem in any way connected with the ionization 
produced by the emanation in the gas with which it is mixed. 
For example, if a closed vessel is constructed with two large 
parallel insulated metal plates on the lower of which a layer of 
thoria is spread, the amount of the excited activity on the upper 
plate when charged negatively, is dependent of the distance 
between the plates when that distance is varied from 1 millimetre 
to 2 centimetres. This experiment shows that the amount of 
excited activity depends only on the amount of emanation, emitted 
from the thoria; for the ionization produced with a distance of 
2 centimetres between the plates is about ten times as great as 
with a distance of | millimetre. 
168. Ifa platinum wire is made active by exposure to the 
emanation of thoria, its activity! can be removed by treating the 
wire with certain acids. For example, the activity is not much 
altered by immersing the wire in hot or cold water or nitric acid, 
but more than 80°/, of it is removed by dilute or concentrated 
solutions of sulphuric or hydrochloric acid. The activity has not 
been destroyed by this treatment but is manifested in the solution. 
If the solution is evaporated, the activity remains behind on the 
dish. 
These results show that the excited activity is due to a deposit 
on the surface of bodies of radio-active matter which has definite 
properties as regards solution in acids. This active matter is 
dissolved in some acids, but, when the solvent is evaporated, the 
active matter is left behind. This active matter 1s deposited on 
the surface of bodies, for it can be partly removed by rubbing the 
body with a cloth, and almost completely by scouring the plate 
1 Rutherford, Phil. Mag. Feb. 1900. 
