In contrast to that which occurs with polonium, radium salts possess a permanent radio-activity which evidences no appreciable diminution after many years. A freshly prepared radium salt in the solid state does not at first possess an activity of constant strength. Its activity increases from the time of preparation until it attains a practically constant limiting value after about one month. The opposite is the case for a solution. When freshly prepared the solution is very active, but when left exposed to the air it rapidly loses activity, and finally reaches a limiting 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:— Activity immediately determined 67 Activity after two hours 20 Activity after two days 0·25 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:— Activity determined immediately 27 Activity determined after 2 days 61 Activity determined after 3 days 70 Activity determined after 4 days 81 Activity determined after 7 days 100 Activity determined after 11 days 100 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:— Limiting activity 800 Initial activity after solution and immediate evaporation 440 Initial activity after the salt has remained dissolved 5 days 120 Initial activity after the salt has remained dissolved 18 days 130 Initial activity after the salt has remained dissolved 32 days 114