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 importance, 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 evidence was forthcoming. Ramsay and Soddy found that the presence of helium could not be detected in a tube immediately after the introduction 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 remains 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 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 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
