carbonate added to redissolve the uranium nitrate, there remained behind an undissolved precipitate which contained a large part of the original activity which had been possessed by the uranium nitrate. He called this undissolved precipitate (or better, the portion of it which was responsible for the activity, for when chemically tested, it showed nothing but iron, aluminum and other impurities) uranium X. But he soon afterward discovered that the uranium nitrate, which had partially lost its activity through the separation from it of this unknown substance, uranium X, in the course of a few months had regained completely its original activity, while the uranium X had lost its power to radiate.
A little more than a year ago Rutherford tried the same experiment with thorium and found quite similar results. But more important still, he found that in both cases the rate of loss of activity of the separated substance, that is, of the uranium X or the thorium X, was equal to the rate of recovery of the uranium or the thorium from which the new substance had been extracted. To state this result in a slightly different way, he found that if all the uranium X were removed from a sample of uranium by this process, so that further precipitation by ammonium carbonate would bring down no more uranium X, and if the uranium were then allowed to stand till it had recovered one half of the lost activity, and if then the uranium X was again removed, the amount of this uranium X which could be obtained was now Just one half as much as the amount obtained at first. If the uranium had regained three fourths of its original activity, just three fourths as much uranium X could be obtained from it as at first. This result seems capable of but one possible interpretation, namely, this: the uranium is continually producing, by some change which goes on within itself, some radio-active substance uranium X, which, however, is formed in such minute quantities that it can be detected and measured only by means of its radio-activity. Further, this uranium X itself is unstable, for it undergoes a change by which it loses its activity. Rutherford further found that in this separation of uranium X from uranium the part of the activity which was left behind in the uranium consisted entirely of the alpha type of radiation, while the part which was separated out in the uranium X consisted wholly of the beta type. This seems to show that the first step in the process of radio-active change consists in the expulsion from the uranium atom of the big alpha particles, while the beta particles are expelled only from some product which is formed by the disintegration of the uranium atom.
In all these particulars Rutherford found that thorium and uranium acted essentially alike, the chief difference being that while the uranium X loses one half of its activity in about twenty-two days, it requires but four days for the activity of the thorium X to decay to half its initial value.
