The radioactivity of the emanation diminishes in accordance with an exponential law.

The constant of time which characterises this decrease is easily and exactly determined, and has a fixed value, independent of the conditions of the experiment as well as of the nature of the gas which is in contact with the radium and becomes charged with the emanation.

The regularity of the phenomenon is so great that it can be used to measure time: in 3985 seconds[36] the activity is always reduced one-half.
[Footnote 36: According to Professor Rutherford, in 3.77 days .-- ED] Radioactivity induced on any body which has been for a long time in presence of a salt of radium disappears more rapidly.

The phenomenon appears, moreover, more complex, and the formula which expresses the manner in which the activity diminishes must contain two exponentials.
To find it theoretically we have to imagine that the emanation first deposits on the body in question a substance which is destroyed in giving birth to a second, this latter disappearing in its turn by generating a third.

The initial and final substances would be radioactive, but the intermediary one, not.