2, which lived 2,000,000 years and begot Ionium, which lived
200,000 years and begot Radium, which lived 1850 years and begot Niton,
which lived 3.85 days and begot Radium A, which lived 3 minutes and
begot Radium B, which lived 26.8 minutes and begot Radium C, which lived
19.5 minutes and begot Radium D, which lived 12 years and begot Radium
E, which lived 5 days and begot Polonium, which lived 136 days and begot
Lead.

The figures I have given are the times when half the parent substance
has gone over into the next generation. It will be seen that the chemist
is even more liberal in his allowance of longevity than was Moses with
the patriarchs. It appears from the above that half of the radium in any
given specimen will be transformed in about 2000 years. Half of what is
left will disappear in the next 2000 years, half of that in the next
2000 and so on. The reader can figure out for himself when it will all
be gone. He will then have the answer to the old Eleatic conundrum of
when Achilles will overtake the tortoise. But we may say that after
100,000 years there would not be left any radium worth mentioning, or in
other words practically all the radium now in existence is younger than
the human race. The lead that is found in uranium and has presumably
descended from uranium, behaves like other lead but is lighter. Its
atomic weight is only 206, while ordinary lead weighs 207. It appears
then that the same chemical element may have different atomic weights
according to its ancestry, while on the other hand different chemical
elements may have the same atomic weight. This would have seemed
shocking heresy to the chemists of the last century, who prided
themselves on the immutability of the elements and did not take into
consideration their past life or heredity. The study of these
radioactive elements has led to a new atomic theory. I suppose most of
us in our youth used to imagine the atom as a little round hard ball,
but now it is conceived as a sort of solar system with an
electropositive nucleus acting as the sun and negative electrons
revolving around it like the planets. The number of free positive
electrons in the nucleus varies from one in hydrogen to 92 in uranium.
This leaves room for 92 possible elements and of these all but six are
more or less certainly known and definitely placed in the scheme. The
atom of uranium, weighing 238 times the atom of hydrogen, is the
heaviest known and therefore the ultimate