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in thestratosphere it is beneficial； since it soaks up dangerous ultraviolet radiation。 beneficial ozoneis not terribly abundant； however。 if it were distributed evenly throughout the stratosphere； itwould form a layer just one eighth of an inch or so thick。 that is why it is so easily disturbed；and why such disturbances don鈥檛 take long to bee critical。
chlorofluorocarbons are also not very abundant鈥攖hey constitute only about one part perbillion of the atmosphere as a whole鈥攂ut they are extravagantly destructive。 one pound ofcfcs can capture and annihilate seventy thousand pounds of atmospheric ozone。 cfcs alsohang around for a long time鈥攁bout a century on average鈥攚reaking havoc all the while。
they are also great heat sponges。 a single cfc molecule is about ten thousand times moreefficient at exacerbating greenhouse effects than a molecule of carbon dioxide鈥攁nd carbondioxide is of course no slouch itself as a greenhouse gas。 in short； chlorofluorocarbons mayultimately prove to be just about the worst invention of the twentieth century。
midgley never knew this because he died long before anyone realized how destructivecfcs were。 his death was itself memorably unusual。 after being crippled with polio；midgley invented a contraption involving a series of motorized pulleys that automatically raised or turned him in bed。 in 1944； he became entangled in the cords as the machine wentinto action and was strangled。
if you were interested in finding out the ages of things； the university of chicago in the1940s was the place to be。 willard libby was in the process of inventing radiocarbon dating；allowing scientists to get an accurate reading of the age of bones and other organic remains；something they had never been able to do before。 up to this time； the oldest reliable dateswent back no further than the first dynasty in egypt from about 3000b。c。 no one couldconfidently say； for instance； when the last ice sheets had retreated or at what time in the pastthe cro…magnon people had decorated the caves of lascaux in france。
libby鈥檚 idea was so useful that he would be awarded a nobel prize for it in 1960。 it wasbased on the realization that all living things have within them an isotope of carbon calledcarbon…14； which begins to decay at a measurable rate the instant they die。 carbon…14 has ahalf…life鈥攖hat is； the time it takes for half of any sample to disappear1鈥攐f about 5；600 years；so by working out how much a given sample of carbon had decayed； libby could get a goodfix on the age of an object鈥攖hough only up to a point。 after eight half…lives； only 1/256 of theoriginal radioactive carbon remains； which is too little to make a reliable measurement； soradiocarbon dating works only for objects up to forty thousand or so years old。
curiously； just as the technique was being widespread； certain flaws within it becameapparent。 to begin with； it was discovered that one of the basic ponents of libby鈥檚formula； known as the decay constant； was off by about 3 percent。 by this time； however；thousands of measurements had been taken throughout the world。 rather than restate everyone； scientists decided to keep the inaccurate constant。 鈥渢hus；鈥潯im flannery notes； 鈥渆veryraw radiocarbon date you read today is given as too young by around 3 percent。鈥潯heproblems didn鈥檛 quite stop there。 it was also quickly discovered that carbon…14 samples can beeasily contaminated with carbon from other sources鈥攁 tiny scrap of vegetable matter； forinstance； that has been collected with the sample and not noticed。 for younger samples鈥攖hose under twenty thousand years or so鈥攕light contamination does not always matter somuch； but for older samples it can be a serious problem because so few remaining atoms arebeing counted。 in the first instance； to borrow from flannery； it is like miscounting by a dollarwhen counting to a thousand； in the second it is more like miscounting by a dollar when youhave only two dollars to count。
libby鈥檚 method was also based on the assumption that the amount of carbon…14 in theatmosphere； and the rate at which it has been absorbed by living things； has been consistentthroughout history。 in fact it hasn鈥檛 been。 we now know that the volume of atmosphericcarbon…14 varies depending on how well or not earth鈥檚 magnetism is deflecting cosmic rays；and that that can vary significantly over time。 this means that some carbon…14 dates are more1if you have ever wondered how the atoms determine which 50 percent will die and which 50 percent willsurvive for the next session； the answer is that the half…life is really just a statistical convenience…a kind ofactuarial table for elemental things。 imagine you had a sample of material with a half…life of 30 seconds。 it isntthat every atom in the sample will exist for exactly 30 seconds or 60 seconds or 90 seconds or some other tidilyordained period。 each atom will in fact survive for an entirely random length of time that has nothing to do withmultiples of 30； it might last until two seconds from now or it might oscillate away for years or decades orcenturies to e。 no one can say。 but what we can say is that for the sample as a whole the rate ofdisappearance will be such that half the atoms will disappear every 30 seconds。 its an average rate； in otherwords； and you can apply it to any large sampling。 someone once worked out； for instance； that dimes have ahalf…life of about 30 years。
dubious than others。 this is particularly so with dates just around the time that people firstcame to the americas； which is one of the reasons the matter is so perennially in dispute。
finally； and perhaps a little unexpectedly； readings can be thrown out by seeminglyunrelated external factors鈥攕uch as the diets of those whose bones are being tested。 onerecent case involved the long…running debate over whether syphilis originated in the newworld or the old。 archeologists in hull； in the north of england； found that monks in amonastery graveyard had suffered from syphilis； but the initial conclusion that the monks haddone so before columbus鈥檚 voyage was cast into doubt by the realization that they had eaten alot of fish； which could make their bones appear to be older than in fact they were。 the monksmay well have had syphilis； but how it got to them； and when； remain tantalizinglyunresolved。
because of the accumulated shortings of carbon…14； scientists devised other methods ofdating ancient materials； among them thermoluminesence； which measures electrons trappedin clays； and electron spin resonance； which involves bombarding a sample withelectromagnetic waves and measuring the vibrations of the electrons。 but even the best ofthese could not date anything older than about 200；000 years； and they couldn鈥檛 date inorganicmaterials like rocks at all； which is of course what you need if you wish to determine the ageof your planet。
the problems of dating rocks were such that at one point almost everyone in the world hadgiven up on them。 had it not been for a determined english professor named arthur holmes；the quest might well have fallen into abeyance altogether。
holmes was heroic as much for the obstacles he overcame as for the results he achieved。
by the 1920s； when holmes was in the prime of his career； geology had slipped out offashion鈥攑hysics was the new excitement of the age鈥攁nd had bee severely underfunded；particularly in britain； its spiritual birthplace。 at durham university； holmes was for manyyears the entire geology department。 often he had to borrow or patch together equipment inorder to pursue his radiometric dating of rocks。 at one point； his calculations were effectivelyheld up for a year while he waited for the university to provide him with a simple addingmachine。 occasionally； he had to drop out of academic life altogether to earn enough tosupport his family鈥攆or a time he ran a curio shop in newcastle upon tyne鈥攁nd sometimeshe could not even afford the 锟�5 annual membership fee for the geological society。
the technique holmes used in his work was theoretically straightforward and arose directlyfrom the process； first observed by ernest rutherford in 1904； in which some atoms decayfrom one element into another at a rate predictable e