IX | EXCITED RADIO-ACTIVITY 275 
(3) A third change, which is faster than the second, and 
is accompanied by ionizing rays. Half the matter changes in 
about 28 minutes. 
178. Physical and chemical properties of the active 
matter. On account of the slow decay of the activity of emana- 
tion X of thorium, its physical and chemical properties have been 
more closely examined than the emanation X of radium. It has 
already been mentioned that the emanation X of thorium is 
soluble in some acids. The writer! found that the active matter 
was dissolved off the wire by strong or dilute solutions of sul- 
phuric, hydrochloric and hydrofluoric acids, but was only slightly 
soluble in water or nitric acid. The active matter was left behind 
when the solvent was evaporated. The rate of decay of activity 
was unaltered by dissolving the active matter in sulphuric acid, 
and allowing it to decay in the solution. In the experiment, the 
active matter was dissolved off an active platinum wire and then 
equal portions of the solutions were taken at definite intervals, 
evaporated down in 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 emanation X of thorium has been recently made by F. von 
Lerch?, and some important and interesting results have been 
obtained. A solution of emanation X 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 
1 Phys. Zeit. 3, No. 12, p. 254, 1902. 
2 Drude’s Annal. Noy. 1903. 
