a platinum dish, and the activity of the residue tested by the electric method. The rate of decay was found to be exactly the same as if the active matter had been left on the wire. In another experiment, an active platinum wire was made the cathode in a copper sulphate solution, and a thin film of copper deposited on it. The rate of decay of the activity was unchanged by the process.
A detailed examination of the physical and chemical properties of the active deposit of thorium has been made by F. von Lerch[1] and some important and interesting results have been obtained. A solution of the active deposit was prepared by dissolving the metal which had been exposed for some time in the presence of the thorium emanation. In most cases the active matter was precipitated with the metal. For example, an active copper wire was dissolved in nitric acid and then precipitated by caustic potash. The precipitate was strongly active. An active magnesium wire, dissolved in hydrochloric acid and then precipitated as phosphate, also gave an active precipitate. The activity of the precipitates decayed at the normal rate, i.e. the activity fell to half value in about 11 hours.
Experiments were also made on the solubility of the active deposit in different substances. A platinum plate was made active and then placed in different solutions, and the decrease of the activity observed. In addition to the acids already mentioned, a large number of substances were found to dissolve the active deposit to some extent. The active matter was however not dissolved to an appreciable extent in ether or alcohol. Many substances became active if added to the active solution and then precipitated. For example, an active solution of hydrochloric acid was obtained by dissolving the deposit on an active platinum wire. Barium chloride was then added and precipitated as sulphate. The precipitate was strongly active, thus suggesting that the active matter was carried down by the barium.
186. Electrolysis of solutions. Dorn showed that, if solutions
of radiferous barium chloride were electrolysed, both electrodes
became temporarily active, but the anode to a greater degree than
the cathode. F. von Lerch has made a detailed examination of
the action of electrolysis on a solution of the active deposit of
- ↑ F. von Lerch, Annal. d. Phys. 12, p. 745, 1903.
