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s’s contributionwas to measure the decay rate of uranium into lead to calculate the age of rocks； and thus—hehoped—of the earth。
but there were many technical difficulties to overe。 holmes also needed—or at leastwould very much have appreciated—sophisticated gadgetry of a sort that could make veryfine measurements from tiny samples； and as we have seen it was all he could do to get asimple adding machine。 so it was quite an achievement when in 1946 he was able toannounce with some confidence that the earth was at least three billion years old and possiblyrather more。 unfortunately； he now met yet another formidable impediment to acceptance： theconservativeness of his fellow scientists。 although happy to praise his methodology； many maintained that he had found not the age of the earth but merely the age of the materials fromwhich the earth had been formed。
it was just at this time that harrison brown of the university of chicago developed a newmethod for counting lead isotopes in igneous rocks （which is to say those that were createdthrough heating； as opposed to the laying down of sediments）。 realizing that the work wouldbe exceedingly tedious； he assigned it to young clair patterson as his dissertation project。
famously he promised patterson that determining the age of the earth with his new methodwould be “duck soup。” in fact； it would take years。
patterson began work on the project in 1948。 pared with thomas midgley’s colorfulcontributions to the march of progress； patterson’s discovery of the age of the earth feelsmore than a touch anticlimactic。 for seven years； first at the university of chicago and then atthe california institute of technology （where he moved in 1952）； he worked in a sterile lab；making very precise measurements of the lead/uranium ratios in carefully selected samples ofold rock。
the problem with measuring the age of the earth was that you needed rocks that wereextremely ancient； containing lead… and uranium…bearing crystals that were about as old as theplanet itself—anything much younger would obviously give you misleadingly youthfuldates—but really ancient rocks are only rarely found on earth。 in the late 1940s no onealtogether understood why this should be。 indeed； and rather extraordinarily； we would bewell into the space age before anyone could plausibly account for where all the earth’s oldrocks went。 （the answer was plate tectonics； which we shall of course get to。） patterson；meantime； was left to try to make sense of things with very limited materials。 eventually； andingeniously； it occurred to him that he could circumvent the rock shortage by using rocksfrom beyond earth。 he turned to meteorites。
the assumption he made—rather a large one； but correct as it turned out—was that manymeteorites are essentially leftover building materials from the early days of the solar system；and thus have managed to preserve a more or less pristine interior chemistry。 measure the ageof these wandering rocks and you would have the age also （near enough） of the earth。
as always； however； nothing was quite as straightforward as such a breezy descriptionmakes it sound。 meteorites are not abundant and meteoritic samples not especially easy to gethold of。 moreover； brown’s measurement technique proved finicky in the extreme andneeded much refinement。 above all； there was the problem that patterson’s samples werecontinuously and unaccountably contaminated with large doses of atmospheric lead wheneverthey were exposed to air。 it was this that eventually led him to create a sterile laboratory—theworld’s first； according to at least one account。
it took patterson seven years of patient work just to assemble suitable samples for finaltesting。 in the spring of 1953 he traveled to the argonne national laboratory in illinois；where he was granted time on a late…model mass spectrograph； a machine capable of detectingand measuring the minute quantities of uranium and lead locked up in ancient crystals。 whenat last he had his results； patterson was so excited that he drove straight to his boyhood homein iowa and had his mother check him into a hospital because he thought he was having aheart attack。
soon afterward； at a meeting in wisconsin； patterson announced a definitive age for theearth of 4；550 million years （plus or minus 70 million years）—“a figure that stands unchanged 50 years later；” as mcgrayne admiringly notes。 after two hundred years of trying；the earth finally had an age。
his main work done； patterson now turned his attention to the nagging question of all thatlead in the atmosphere。 he was astounded to find that what little was known about the effectsof lead on humans was almost invariably wrong or misleading—and not surprisingly； hediscovered； since for forty years every study of lead’s effects had been funded exclusively bymanufacturers of lead additives。
in one such study； a doctor who had no specialized training in chemical pathologyundertook a five…year program in which volunteers were asked to breathe in or swallow leadin elevated quantities。 then their urine and feces were tested。 unfortunately； as the doctorappears not to have known； lead is not excreted as a waste product。 rather； it accumulates inthe bones and blood—that’s what makes it so dangerous—and neither bone nor blood wastested。 in consequence； lead was given a clean bill of health。
patterson quickly established that we had a lot of lead in the atmosphere—still do； in fact；since lead never goes away—and that about 90 percent of it appeared to e fromautomobile exhaust pipes； but he couldn’t prove it。 what he needed was a way to parelead levels in the atmosphere now with the levels that existed before 1923； when tetraethyllead was introduced。 it occurred to him that ice cores could provide the answer。
it was known that snowfall in places like greenland accumulates into discrete annual layers（because seasonal temperature differences produce slight changes in coloration from winter tosummer）。 by counting back through these layers and measuring the amount of lead in each； hecould work out global lead concentrations at any time for hundreds； or even thousands； ofyears。 the notion became