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 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—

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.

For the product not heated $$a$$ = 100.

Induction capacity of the product after being heated to redness for three hours:—

Theory of Interpretation of the Causes of Variations of Activity of Radium Salts after Solution and after Heating.

The facts previously indicated may be in part explained by the theory according to which the energy of radium is produced in the form of an emanation, which is then transformed into the energy of radiation. When a radium salt is dissolved, the emanation produced by it spreads beyond the solution, and causes radio-activity outside the source