PRESENT PROBLEMS IN RADIOACTIVITY. 27 



present in the least quantity in radium and pitchblende. Only the 

 slower changing products like the radium emanation and radium D 

 and E exist in sufficient quantities to be examined by the balance. It 

 is possible that the products radium A, B and C may be obtained in 

 sufficient quantity to obtain their spectrum. 



Connection between the a Particles and Helium. 



The discovery of Ramsay and Soddy that helium was produced by 

 the radium emanation was one of the greatest interest and impor- 

 tance, and confirmed in a striking manner the disintegration theory of 

 radioactivity, for the possible production of helium from radioactive 

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

 dence was forthcoming. Ramsay and Sodcly 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 helium is the final disintegration product 

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

 mains when the succession of radioactive changes in the emanation 

 has come to an end. There is no evidence in support of such a 

 conclusion, 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 40 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. Thus it 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 ex< 

 pulsion 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 200, 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 



