In 1896, Becquerel found that compounds of uranium, and also the metal itself, are radio-active.

In the light of what is now known about radio-activity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves, and change into some substance, or substances, different from uranium; in other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating.
This supposition is confirmed by the fact, experimentally proved, that uranium emits [alpha]-rays, that is, atoms of helium, and produces a substance known as uranium X.Uranium X is itself radio-active; it emits [beta]-rays, that is, it gives off electrons.
Inasmuch as all minerals which contain compounds of uranium contain compounds of radium also, it is probable that radium is one of the disintegration-products of uranium.

The rate of decay of radium may be roughly expressed by saying that, if a quantity of radium were kept for ten thousand years, only about one per cent.

of the original quantity would then remain unchanged.

Even if it were assumed that at a remote time the earth's crust contained considerable quantities of radium compounds, it is certain that they would have completely disappeared long ago, had not compounds of radium been reproduced from other materials.