Geoscience Reference
In-Depth Information
liable methods of determining the age of the various geological formations” (190).
The first attempts to measure geologic ages by radioactivity gave results many
times greater than either Kelvin's 100-million-year upper limit or the geological
hourglasses.
Rutherford knew that helium was not the end product of uranium decay but only
anintermediatestepinthedecaychain.Rutherfordcalculatedthatthefinalproduct
of uranium decay would have an atomic weight of 206.5, close to the measured
atomic weight of lead, at 206.9. The Yale chemist Bertram Boltwood had noted
that minerals high in uranium also contain lead, which is chemically quite unlike
uranium. These facts suggested that lead is the final product of uranium decay,
prompting Rutherford to say that the “percentage of lead in radioactive minerals
should be a far more accurate method of deducing the age of the mineral.” Why?
Because unlike gaseous helium, “the lead formed in a compact mineral has no pos-
sibility of escape” (192). But he thought that it would take “many years to prove
or disprove experimentally that lead is the final product of radium” (192-193). On
this occasion, Rutherford turned out to be wrong.
HistalksatYalewerenotthefirsttimeRutherfordhadreportedanagemuchlar-
ger than Kelvin's accountancy would allow. As his biographer reports, a year or so
earlier, Rutherford was walking the McGill campus, carrying in his pocket a spe-
cimen of pitchblende, the same uranium mineral that Becquerel and Marie Curie
had used in their experiments. Meeting a colleague, Rutherford asked, “Adams,
how old is the earth supposed to be?” The answer came back at Kelvin's then still
prevalent figure of 100 million years. “I know,” said Rutherford quietly, “that this
piece of pitchblende is 700 million years old.”
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