Page:Radio-activity.djvu/322

 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 concentrated 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 independent 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 1 millimetre.

178. If a platinum wire be 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 be 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 is 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