In the case of solid radium salts the capacity for exciting induced radio-activity is largely affected by heating. During heating radium compounds give off a larger amount of emanation than at the ordinary temperature; but on being cooled to the ordinary temperature, not only is their radio-activity much less than before heating, but their capacity for inducing activity is much diminished. During the time that follows the heating, the radio-activity of the product increases, and may even exceed the original value. The induction capacity is also partially re-established; however, after prolonged heating to redness, this capacity is almost entirely destroyed without spontaneous re-appearance afterwards. The induction capacity may be restored to the radium salt by dissolving it in water, and drying it in the oven at a temperature of 120°. This seems to have the effect of leaving the salt in a peculiar physical condition, in which the emanation is given off with much less facility than is the case with the same solid product not heated to a high temperature, and it follows naturally that the salt attains a higher limit of activity than that which it possessed before heating. To transform the salts into the physical condition proper to it before heating, it suffices to dissolve it and to evaporate it to dryness without heating it above 150°.
The following are numerical examples of the above:—
I represent by _a_ the limit of induced activity produced in a closed vessel upon a plate of copper by a specimen of barium-radium carbonate of activity 1600.
Suppose _a_ = 100 for the not heated product. We find—
1 day after heating _a_ = 3·3 4 days after heating _a_ = 7·1 10 days after heating _a_ = 15 20 days after heating _a_ = 15 37 days after heating _a_ = 15
The radio-activity of the product had diminished 90 per cent by heating, but one month afterwards the original value was regained.
The following is an experiment of the same kind made with a barium-radium chloride of activity 3000. The induction capacity is determined in the same manner as before.
Induction capacity of the product after being heated to redness for three hours:—
2 days after heating 2·3 5 days after heating 7·0 11 days after heating 8·2 18 days after heating 8·2 Induction capacity of the unheated substance which has been dissolved and then dried at 150° 92 Induction capacity of the heated substance which has been dissolved and then dried at 150° 105