RECENT DISCOVERIES IN RADIATION. 493 



carbonate added to redissolve the uranium nitrate, there remained 

 behind an undissolved precipitate which contained a large part of the 

 original activity vs^hich 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, wliich had par- 

 tially 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 diifer- 

 ent 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 alplia type of radiation, while the part which 

 was separated out in the uranium X consisted wholly of the heta 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 

 dlplia particles, while the heta 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. 



