THORIUM, URANIUM, POLONIUM AND ACTINIUM. 129 



thorium emanation. No similiar radium X has yet been 

 discovered. The radio-activity of a solid thorium salt is 

 then due to the radio-activity of the thorium, the thorium 

 X, the emanation, and the emanation X, all condensed 

 within it. 



THE RADIO-ACTIVITY OF URANIUM. 



Uranium was the first element discovered to possess 

 radio-activity, the power of continuously emitting pene- 

 trating rays. Since radium was extracted from uranium 

 minerals, it was at first imagined that the activity of ura- 

 nium was due to radium impurity. This was a mistake. 

 The radio-active power of uranium, however, is very small; 

 1,500,000 times less than pure radium. It possesses, how- 

 ever, a special interest in the fact of its simplicity. Like 

 thorium, and unlike radium, it continuously gives rise to a 

 solid disintegration product, uranium X, though this differs 

 from thorium X in emitting beta- instead of alpha-rays. 

 The uranium from which the uranium X has been extracted 

 emits only alpha-rays. Just as in all radio-active processes, 

 the decay of the activity of the uranium X and the revival 

 of the activity of the pure uranium take place at the same 

 rate, so C A u,t the activity of a uranium compound at any one 

 instant is due to a balance between the opposing forces of 

 decay and restoration. Half of this process of decay and 

 restoration takes place in twenty-two days. Unlike both 

 radium and thorium the uranium X gives rise to no emana- 

 tion and consequently to no emanation X. As a result of 

 this fact bodies in the neighbourhood do not become radio- 

 active. The active uranium X decays, apparently, directly 

 into some inactive product. Uranium has been used to 

 test the constancy of radio-active processes. For over five 

 years the activity of a sample of uranium has been tested at 

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