The assumption that the series of radioactive substances is due, not to such molecular changes as chemistry had made familiar, but to a breakdown of the atom seemed to Rutherford in 1913 at least justified by the results of the investigators whose procedure had been dictated by that hypothesis. He set forth in tables these results (since somewhat modified), indicating after the name of each radioactive substance the nature of the radiation through the emission of which the element is transformed into the next-succeeding member of its series. _List of Radioactive Substances_ URANIUM α particles Uranium X β + γ Uranium Y β IONIUM α RADIUM α + slow β Emanation α Radium A α Radium B β + γ Radium C { C{1} α + β + γ { C{2} β RADIUM D } RADIO-LEAD } slow β Radium E β + γ Radium F } Polonium } α THORIUM α MESOTHORIUM 1 no rays Mesothorium 2 β + γ RADIOTHORIUM α Thorium X α + β Emanation α Thorium A α Thorium B slow β Thorium C { C{1} α { C{2} α Thorium D β + γ ACTINIUM no rays Radio-actinium α + β Actinium X α Emanation α Actinium A α Actinium B slow β Actinium C α Actinium D α + γ Even a glance at this long list of new elements reveals certain analogies between one series of transformations and another. Each series contains an emanation, or gas, which through the loss of α particles is transformed into the next following member of the series. Continuing the comparison in either direction, up or down the lists, one could readily detect other analogies. There is some ground for thinking that lead is the end product of the Uranium series. To reverse the process of the transformation and produce radium from the base metal lead would be an achievement greater than the vaunted transmutations of the alchemists. Although that seems beyond the reach of possibility, the idea has stirred the imagination of more than one scientist. "The philosopher's stone," writes Soddy, "was accredited the power not only of transmuting the metals, but of acting _as the elixir of life_. Now, whatever the origin of this apparently meaningless jumble of ideas may have been, it is really a perfect and but very slightly allegorical expression of the actual present views we hold to-day." Again, it is conjectured that bismuth is the end-product of the thorium series. The presence of the results of atomic disintegration (like lead and helium) has proved of interest to geology and other sciences as affording a clue to the age of the rocks in which they are found deposited.