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t rutherford in 1904； in which some atoms decayfrom one element into another at a rate predictable enough that you can use them as clocks。 ifyou know how long it takes for potassium…40 to bee argon…40； and you measure theamounts of each in a sample； you can work out how old a material is。 holmes鈥檚 contributionwas to measure the decay rate of uranium into lead to calculate the age of rocks； and thus鈥攈ehoped鈥攐f the earth。
but there were many technical difficulties to overe。 holmes also needed鈥攐r at leastwould very much have appreciated鈥攕ophisticated 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 鈥渄uck soup。鈥潯n fact； it would take years。
patterson began work on the project in 1948。 pared with thomas midgley鈥檚 colorfulcontributions to the march of progress； patterson鈥檚 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鈥攁nything much younger would obviously give you misleadingly youthfuldates鈥攂ut 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鈥檚 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鈥攔ather a large one； but correct as it turned out鈥攚as 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鈥檚 measurement technique proved finicky in the extreme andneeded much refinement。 above all； there was the problem that patterson鈥檚 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鈥攖heworld鈥檚 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）鈥斺�渁 figure that stands unchanged 50 years later；鈥潯s 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鈥攁nd not surprisingly； hediscovered； since for forty years every study of lead鈥檚 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鈥攖hat鈥檚 what makes it so dangerous鈥攁nd 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鈥攕till do； in fact；since lead never goes away鈥攁nd that about 90 percent of it appeared to e fromautomobile exhaust pipes； but he couldn鈥檛 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 the foundation of ice core studies； on which much modernclimatological work is based。
what patterson found was that before 1923 there was almost no lead in the atmosphere； andthat since that time its level had climbed steadily and dangerously。 he now made it his life鈥檚quest to get lead taken out of gasoline。 to that end； he became a constant and often vocalcritic of the lead industry and its interests。
it would prove to be a hellish campaign。 ethyl was a powerful global corporation withmany friends in high places。 （among its directors have been supreme court justice lewispowell and gilbert grosvenor of the national geographic society。） patterson suddenly foundresearch funding withdrawn or difficult to acquire。 the american petroleum institutecanceled a research contract with him； as did the united states public health service； asupposedly neutral government institution。
as patterson increasingly became a liability to his institution； the school trustees wererepeatedly pressed by lead industry officials to shut him up or let him go。 according to jamielincoln kitman； writing in the nation in 2000； ethyl executives allegedly offered to endow achair at caltech 鈥渋f patterson was sent packing。鈥潯bsurdly； he was excluded from a 1971national research council panel appointed to investigate the dangers of atmospheric leadpoisoning even though he was by now unquestionably the leading expert on atmospheric lead。
to his great credit； patterson never wavered or buckled。 eventually his efforts led to theintroduction of the clean air act of 1970 and finally to the removal from sale of all leadedgasoline in the united states in 1986。 almost immediately lead levels in the blood ofamericans fell