in the preceding formula. One may also take as the constant the time necessary for the intensity of the radiation to diminish to one-half.
It is very remarkable that these constants of time seem to be invariable under widely different circumstances. Thus it is that the emanation from radium diminishes to one-half during each period of four hours, whatever may be the conditions of the experiment, and whatever the temperature between -180° and +450°. The rate of its disappearance is the same whether the emanation be in the gaseous state (room temperature) or condensed (-180°). The properties of the emanation from radium thus give us an invariable standard of time which is independent of all agreements as to the unit.
The time-constants of the radioactivity serve to characterize in a precise manner the nature of the different radioactive energies.
The following are the times necessary for the activity to fall to one-half of its value:
| Emanation of | Time. |
|---|---|
| Radium | 4 days |
| Thorium | 1 minute 10 seconds |
| Actinium | Several seconds |
| Induced radioactivity of | |
| Radium | 1 hour (after beginning of the loss). 28 minutes (for times more than two hours after the beginning of the loss) |
| Thorium | 11 hours |
| Actinium | 36 minutes |
Thus J. J. Thomson and Adam found recently that the emanation from water from certain sources falls to one-half for each period of four days, and that this emanation causes induced activity in solids which falls to one-half in about forty minutes. The supposition is that the emanation contained in the water is due to radium.
Ordinary thorium extracted from monazite sand is slightly radioactive. Thorium from pitchblende is strongly radioactive (thorium with actinium of Debierne). The radioactivity in the two cases is not due to the same substance, for the time-constants of the emanation and of the radioactivity are different.
Certain radioactive substances like actinium have never been separated in a pure state, and it may be supposed that the very active substances that have been studied contain only traces of them. The chemical reactions of substances cannot be recognized with certainty when they are found solely in a diluted condition, mixed with other substances. Under those circumstances one element may carry another down with it in a precipitation, and the action of the reagent is not the same as when we have pure compounds. Hence the chemical reactions will not serve to characterize a radioactive substance. It may, however, be recognized under all circumstances by the time constants of the emanation it emits, and of the induced radioactivity excited by that emanation upon solids.
Nature of the Emanation.—According to Rutherford, the emanation of a radioactive substance is a radioactive, material gas which escapes from it. In fact, the emanation from radium acts in many ways like a gas.
When we put in communication two glass reservoirs, one containing the emanation and the other none, the emanation diffuses into the second and, when equilibrium is established, it is found that the emanation is divided between the two reservoirs according to their respective volumes. One of the two may be heated to 350°, while the other remains at the room temperature, and it is found that in this case, also, the emanation is divided between the two reservoirs as if it were a perfect gas obeying the laws of Marriotte and Gay-Lussac.