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the first point: In ascribing a constant rate of change to the parent substance--which Becker (loc. cit.) describes as "a simple though tremendous extrapolation"--we reason upon analogy with the constant rate of decay observed in the derived radioactive bodies. If uranium and thorium are really primary elements, however, the analogy relied on may be misleading; at least, it is obviously incomplete. It is incomplete in a particular which may be very important: the mode of origin of these parent bodies--whatever it may have been--is different to that of the secondary elements with which we compare them. A convergence in their rate of transformation is not impossible, or even improbable, so far as we known. As regards the second point: It is assumed that uranium alone of the elements in radioactive minerals is ultimately transformed to lead by radioactive changes. We must consider this assumption. Recent advances in the chemistry of the radioactive elements has brought out evidence that all three lines of radioactive descent known to us--_i.e._ those beginning with uranium, with thorium, and with actinium--alike converge to lead.[1] There are difficulties in the way of believing that all the lead-like atoms so produced ("isotopes" of lead, as Soddy proposes to call them) actually remain as stable lead in the minerals. For one [1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans, Green & Co.). 24 thing there is sometimes, along with very large amounts of thorium, an almost entire absence of lead in thorianites and thorites. And in some urano--thorites the lead may be noticed to follow the uranium in approximate proportionality, notwithstanding the presence of large amounts of thorium.[1] This is in favour of the assumption that all the lead present is derived from the uranium. The actinium is present in negligibly small amounts. On the other hand, there is evidence arising from the atomic weight of lead which seems to involve some other parent than uranium. Soddy, in the work referred to, points this out. The atomic weight of radium is well known, and uranium in its descent has to change to this element. The loss of mass between radium and uranium-derived lead can be accurately estimated by the number of alpha rays given off. From this we get the atomic weight of uranium-derived lead as closely 206. Now the best determinations of the atomic weight of normal lead assign to this element an atomi
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