PRESENT PROBLEMS OF RADIOACTIVITY 181 



such as occurs with the strong electric discharge employed in the 

 spectroscopic detection of helium. Ramsay and Soddy have examined 

 the glass tubes in which the emanation had been confined for some 

 time, to see if the buried helium was released by heat. In some cases, 

 traces of helium were observed. 



Accurate measurements of the value of ^ for the a particle, and 

 also an accurate determination of the relative volume of the emana- 

 tion and the helium produced by it, would probably definitely settle 

 this fundamental question. 



Certain very important consequences follow on the assumption 

 that the a particle is, in all cases, an atom of helium. It has already 

 been shown that the radio-elements are transformed into a succession 

 of new substances, most of which in breaking up emit an a particle. 

 On such a view, the atom of radium, thorium, uranium, and actinium 

 must be supposed to be built up in part of helium atoms. In radium, 

 at least five products of the change emit a particles, so that the 

 radium atom must contain at least five atoms of helium. In a similar 

 way, the atoms of actinium and thorium (or if thorium itself be not 

 radioactive, the atom of the active substance present in it) must be 

 compounds of helium. These compounds of helium are not stable, but 

 spontaneously break up into a succession of substances, with an 

 evolution of helium, the disintegration taking place at a definite but 

 different rate at each stage. Such compounds are sharply distin- 

 guished in their behavior from the molecular compounds known to 

 chemistry. In the first place, the radioactive compounds disintegrate 

 spontaneously and at a rate that is independent of the physical or 

 chemical forces at our control. Changes of temperature, which exert 

 such a marked influence in altering the rate of molecular reactions are 

 here almost entirely without influence. But the most striking feature 

 of the disintegration is the expulsion, in most cases, of a product of 

 the change with very great velocity - - a result never observed in 

 ordinary chemical reactions. This entails an enormous liberation of 

 energy during the change, the amount, in most cases, being about one 

 million times as great as that observed in any known chemical reac- 

 tion. In order to account for the expulsion of an a or /? particle with 

 the observed velocities, it is necessary to suppose that their particles 

 exist in a state of rapid motion in the system from which they escape. 

 Variation of temperature, in most cases, does not seem to affect the 

 stability of the system. 



It is well established that the property of radioactivity is inherent 

 in the radio-atoms, since the activity of any radioactive compound 

 depends only on the amount of the element present and is not affected 

 by chemical treatment. As far as observation has gone, both uranium 

 and radium behave as elements in the usual accepted chemical sense. 



