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set in motion in such a way that geologists always know that they are dating
the true formation of a given rock: These special conditions occur when
molten magma cools and solidifies. Any lava has a great deal of potassium,
and the argon heing produced by radioactive decay is not trapped because of
the heat and liquidity of the lava. When the lava cools, however, crystals of
feldspar and other minerals form, each containing potassium-40 that is de-
caying to argon-40 at a rate such that half of the potassium will be trans-
formed into argon in 1.25 billion years. The moment the crystal forms, the
clock is set at 0.
Recently, the K/Ar method has been improved. Samples with potas-
sium crystals are irradiated with fast neutrons in an atomic reactor, which
converts a fraction of potassium-39 to argon-39. The ratio of the two argons
is then examined. This technique is the most precise of all dating methods,
but it can be used only on rocks a million years of age or older. In younger
rocks there just has not been enough time to cause measurable accumula-
tions of the daughter isotopes.
By the earliest part of the twentieth century, much of the theory be-
hind radiometric age dating had been worked out. What was necessary to
apply the theory, however, was a reliable set of instruments. The physicists
needed to measure tiny quantifies of specific isotopes with great precision. It
was now up to the instrumentalists to put theory into practice.
The machine necessary to measure various isotopic quantities is known
as a mass spectrograph. The first of these elegant machines was designed by
the physicist J. J. Thompson of the Cavendish labs in Cambridge, England.
His crude prototype was soon redesigned by a colleague in the same lab,
F. W. Anston, who named his creation the "positive ray spectrograph." With
this machine, Aston began measuring the isotopic compositions of many
materials. In 1927 he turned his attention to the radioactive decay of lead
isotopes, and in so doing launched the field of research that eventually gave
us some of the most profound insights into not only the age, but also the for-
mation, of the solar system.
Mass spectroscopes have speciated and multiplied over the years, and
now they are quite task-specific. They are among the most common of major
