Geology Reference
In-Depth Information
together and continuing to teach the few students who
remained. This was no bad thing since, like Holmes in the early
war years, he too found extra time for research.
Holmes had realised, just before he left for Mozambique, that
one of the potential problems with the uranium-lead method of
dating minerals was in precisely evaluating the amount of lead
that resulted directly from the decay of uranium. If some of the
lead he measured in a sample was what he called 'ordinary' lead
(lead that had been around since formation of the Earth and so
was already present in the mineral when the lead from uranium
started to accumulate) then the age deduced from such a sample
would appear to be much older than it actually was. There were
still many geologists who opposed the idea of an ancient planet
and who considered that all radioactive techniques were totally
unreliable, citing with pleasure the problems that had been
encountered with helium, so they would be only too delighted
to find a similar flaw in the lead method. But how to tell these
two types of lead apart? Just before Lawson left for Vienna the
two friends had been discussing this very problem in the light
of the most recent 'wild miracle'.
By 1913 the nucleus of the atom had been discovered (by
Rutherford in 1911) and so it was known that the atom consisted
of two strongly contrasting portions - a nucleus, surrounded by
an intense electric field of electrons - but it was still considered
that elements could be distinguished, one from another, by their
di¬erent atomic weights. Thus two atoms with di¬erent weights
were considered to be two di¬erent elements. So when Frederick
Soddy, now working in Glasgow on the decay products of
thorium, found that it was impossible to chemically separate
one of thorium's decay products from thorium itself, although
the two had di¬erent atomic weights, he was considerably
perplexed. And the more he examined the problem the more he
found other pairs of elements that were chemically indistin-
guishable from each other, although they too had di¬erent
atomic weights.
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