This element, too, undergoes change, as was discovered by Becquerel. For millions of years, it gives off a-rays; its product is not certain, but it may possibly be radium.

Uranium X

Crookes discovered that it is possible to separate a radio-active substance from uranium. The best way is to add to a solution of uranium nitrate in acetone recently precipitated ferric hydroxide. This precipitate carries with it uranium X, which emits a-, /?-, and y-rays, changing into some unrecognised non-radio-active substance ; its period of half-decay is 22 days. At the same time U-X is continually re-formed from the uranium, as quickly as it disappears; so that there is always equilibrium between the uranium and uranium X.