PRESENT PROBLEMS OF RADIOACTIVITY 179 



activity, for the possible production of helium from radioactive 

 matter had been predicted on this theory before the experimental evi- 

 dence was forthcoming. Ramsay and Soddy found that the presence 

 of helium could not be detected in a tube immediately after the intro- 

 duction of the emanation, but was observed some time afterwards, 

 showing that the helium arose in consequence of a slow change in the 

 emanation itself or in its further products. 



The question of the origin of the helium produced by the radium 

 emanation and its connection with the radioactive changes occurring 

 in the emanation is one of the greatest importance. The experimental 

 evidence so far obtained does not suffice to give a definite answer to 

 this question, but suggests the probable explanation. There has been 

 a tendency to assume that the helium is the final disintegration pro- 

 duct of the radium emanation, i. e., it is the inactive substance which 

 remains when the succession of radioactive changes in the emanation 

 have come to an end. There is no evidence in support of such a con- 

 clusion, while there is much indirect evidence against it. It has been 

 shown that the emanation which breaks up undergoes three fairly 

 rapid transformations; but after these changes have occurred, the 

 residual matter radium D is still radioactive, and breaks up 

 slowly, being half transformed in probably about forty years. There 

 then occurs a still further change. Taking into account the minute 

 quantity of the radium emanation initially present in the emanation 

 tube, the amount of the final inactive product would be insignificant 

 after the lapse of a few days or even months. It thus does not seem 

 probable that the helium can be the final product of the radioactive 

 changes. In addition, it has been shown that the a particle behaves 

 like a body of about the same mass as the helium atom. The expulsion 

 of a few a particles from each of the heavy atoms of radium would not 

 diminish the atomic weight of the residue very greatly. The atomic 

 weight of the atoms of radium D and E is in all probability of the 

 order of two hundred, since the evidence supports the conclusion that 

 each atom expels one a particle at each transformation. In order 

 to explain the presence of helium, it is necessary to look to the other 

 inactive products produced during the radioactive changes. The a 

 particles expelled from the radioactive product are themselves non- 

 radioactive. The measurement of the ratio ^ shows that they have 

 an apparent mass intermediate between that of the hydrogen and 

 helium atoms. If the a particles consist of any known kind of matter 

 they must be either atoms of hydrogen or of helium. The actual value 

 of ~ has not yet been determined with an accuracy sufficient to 

 give a definite answer to the question. On account of the very slight 

 curvature of the path of the a particles in a strong magnetic or electric 

 field, an accurate determination of ^ is beset with great difficulties. 



