RECENT DISCOVERIES IN RADIATION. 495 



from the rate at which the activity of the emanation decays ; and, more 

 important still, since it is found to emit both alpha and ieta rays while 

 the emanation emits only alpha rays, it seems necessary to conclude 

 that this film of active matter is a product of the emanation rather 

 than the emanation itself. In fact it appears to bear in all respects 

 the same relation to the emanation which the emanation does to radium. 

 That is, it is the result of the disintegration of the atom of the emana- 

 tion, just as the emanation is the result of the disintegration of the 

 atom of radium. 



In the case of thorium this continuous change from one radio- 

 active substance into another has been followed with certainty through 

 as many as four different stages, thus ; first, thorium produces thorium 

 X; second, thorium X produces an active gas or emanation which is 

 very like the radium emanation; third, the thorium emanation gives 

 rise to a radio-active substance which is responsible for the induced 

 radio-activity which is observable whenever the emanation comes in 

 contact with a solid object; fourth, this induced radio-active matter 

 due to the thorium emanation gradually loses its radiating power, and 

 hence must undergo at least one further change into some other sub- 

 stance. 



The Disintegration of the Atom of Radio-active Substances. 

 We have endeavored to follow step by step the discoveries which 

 have led up to our present knowledge of the nature of radio-activity. 

 These discoveries have seemed to prove conclusively that the atoms 

 of radio-active substances are slowly undergoing a process of disin- 

 tegration, this disintegration being indicated, first by the fact that 

 there is a continuous projection from them of particles of matter, the 

 alpha and beta rays; and second, by the fact that we are able to detect 

 the presence of new and unstable types of matter accompanying the 

 phenomena of radio-activity. Just why these atoms are disintegrating 

 and just how these new types of matter are formed must of course be 

 largely a matter of speculation. Nevertheless, discovery has gone far 

 enough to enable us to form a reasonably plausible hypothesis as to the 

 probable mechanism of radio-active change. In presenting this 

 hypothesis the first remarkable fact to be noted is that the three per- 

 manently radio-active substances thus far discovered, the only ones 

 which can with certainty be classed as elements, namely uranium, tho- 

 rium and radium,* are the substances whose atoms are the three heaviest 



* There are two other substances which must perhaps be added to this list, 

 viz., polonium and actinium. But neither of these has as yet been found to show 

 a distinct spectrum or to show any of the other characteristics of elements; 

 furthermore, the activity of one of them and possibly of both of them slowly 

 decays. Hence it is possible that they, like uranium X and thorium X and the 

 radium emanation, are only stages in the disintegration of radio-active elements. 

 The present indications, however, seem to be that actinium is, like radium, a 

 new and very powerful radio-active element. 



