PRESENT PROBLEMS IN RADIOACTIVITY. 29 



observed an exactly opposite result. The volume diminished with 

 time to a small fraction of its original value. This diminution of 

 volume was due to the decomposition of the emanation into a non- 

 gaseous type of matter deposited on the walls of the tube, and followed 

 the law of decrease to be expected in such a case, namely, the volume 

 decreased according to an exponential law with the time falling to half 

 value in four days. The helium produced by the emanation must 

 have been absorbed by the walls of the tube. Such a result is to be 

 expected if the particle is a helium atom, for the a particle is pro- 

 jected with a velocity sufficient to bury itself in the glass to a depth 

 of about 1/100 mm. This buried helium would probably be in part 

 released by the heating of the tube, such as occurs with the strong 

 electric discharge employed in the spectroscopic detection of helium. 

 Eamsay and Soddy have examined the glass tubes in which the emana- 

 tion 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 e/m 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 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 dis- 

 tinguished in their behavior from the molecular compounds known 

 to chemistry. In the first place, the radioactive compounds disin- 

 tegrate 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 reac- 

 tions, are here almost entirely without influence. But the most striking 

 feature of the disintegration is the expulsion, in most cases, of a pro- 

 duct 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- 



