As regards thorium the results are less
satisfactory. The evolution should continue until a stable atomic
condition is finally reached, which, because of this stability, is no
longer radioactive. Thus, for instance, radium would finally be
transformed into helium.[40]
[Footnote 40: This opinion, no doubt formed when Sir William Ramsay's
discovery of the formation of helium from the radium emanation was
first made known, is now less tenable. The latest theory is that the
alpha particle is in fact an atom of helium, and that the final
transformation product of radium and the other radioactive substances
is lead. Cf. Rutherford, op. cit. passim.--ED.]
It is possible, by considerations analogous to those set forth above
in other cases, to arrive at an idea of the total number of particles
per second expelled by one gramme of radium; Professor Rutherford in
his most recent evaluation finds that this number approaches 2.5 x
10^{11}.[41] By calculating from the atomic weight the number of atoms
probably contained in this gramme of radium, and supposing each
particle liberated to correspond to the destruction of one atom, it is
found that one half of the radium should disappear in 1280 years;[42]
and from this we may conceive that it has not yet been possible to
discover any sensible loss of weight.
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