Page:Radio-active substances.djvu/90

 activity which may be considerably less than the original. These variations of activity were first observed by M. Giesel. They are easily accounted for by the emanation theory. The diminution of the activity of the solution corresponds to the loss of the emanation which escapes into space; this diminution is much less when the solution is contained in a sealed tube. A solution which has lost its activity in air recovers a greater activity in a sealed tube. The time of increase of the activity of the salt which, after solution, has been recently obtained in the solid state, is that during which the emanation is being newly stored in the solid radium.

The following are some experiments on this subject:—

A solution of barium-radium chloride left exposed to the air for two days becomes 300 times less active.

A solution is enclosed in a stoppered vessel; the vessel is opened, the solution poured into a dish, and the activity determined:—

A solution of barium-radium chloride, which has been kept open to the air, is enclosed in a sealed glass tube, and the radiation of this tube determined. The following results were observed:—

The initial activity of a solid salt after preparation is feeble in proportion as the time of solution was long. A greater proportion of activity is then transmitted to the solvent. The following figures give the initial activity with a chloride whose limiting activity is 800, and which was kept for a given time in solution; the salt was afterwards dried, and its activity immediately determined:—