World 

 of TIME 



{Continued from page 3) 



If the half-life is only a year, or an hour, 

 or, as in the case of some atoms, only a 

 few seconds, it is obvious that such clocks 

 will "run down" in a short time and be 

 of little value. To use such "weak- 

 springed" clocks we have to have an ex- 

 tremely delicate chemical method to an- 

 alyze exactly the number of green atoms 

 and red atoms. Once the number of 

 green ones has fallen below our ability 

 to separate them in the laboratory, the 

 clock is, for all purposes, dead even 

 though there might be some few green 

 atoms still present. The same is true if 

 our ability to detect the "clicks" per unit 

 of time is limited by our laboratory de- 



W e may now depart from "greens 

 and reds" and mention a few actual ex- 

 amples of these atomic changes. Con- 

 sider the mineral called orthoclase. This 

 is one of the most common minerals in 

 granite, the rock which makes up almost 

 all of the earth's crust. Chemically, or- 

 thoclase contains atoms of the element 

 potassium. When a crystal of orthoclase 

 forms, 99.9 per cent of the potassium in 

 it is atomically stable and no changes 

 occur at all. However, less than one- 

 tenth of a percent of the potassium atoms 

 consist of the unstable atom called po- 

 tassium-40. Chemically it is almost 

 identical to the other potassium atoms 

 in the crystal except that its atomic nu- 

 cleus is not stable and tends to break 

 down in time. It breaks down to the 

 element called argon-40, which is very 

 stable and undergoes no further changes. 

 During the breakdown of potassium-40 

 several invisible, high energy rays are 

 given off also. These rays, when passing 

 through a sensitive device such as the 

 well known Geiger counter, produce 



Page 6 JULY 



brief electrical discharges which, when 

 amplified, come out as clicks in a set of 

 ear phones. 



Potassium-40 is an excellent clock for 

 two reasons. First, its half-life is one 

 billion, 320 million years; thus it re- 

 quires a long time to run down. Sec- 

 ond, the mineral orthoclase is very com- 

 mon in the earth's crust so that samples 

 are usually easy to obtain in regions 

 where age determinations are desired. 

 In addition, the black mica mineral 

 called biotite is almost as common in 

 granites as orthoclase. It also contains 

 significant amounts of potassium. Be- 

 cause of the long half-life, the potassium- 

 argon clock is used to measure ages of 

 rocks (and meteorites)) ranging back to 

 four and one-half billion years old — ap- 

 proximately the age of the earth. In 

 using this method the common proce- 

 dure is to measure a sample of potas- 



Chicago Natural History Museum 



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Telephone: 922-9410 



THE BOARD OF TRUSTEES 



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OFFICERS 



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and Assistant Secretary 



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THE BULLETIN 



EDITOR 

 E. Leland Webber, Director of the Museum 



CONTRIBUTING EDITORS 



Paul S. Martin, Chief Curator of Anthropology 



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 Marilyn J. Arado, Associate in Public Relations 



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sium-40 and argon-40 and compute the 

 age from the quotient. 



Within the past ten years much pub- 

 licity has been given to the method of 

 carbon-14 dating. The carbon in a 

 sample of wood, coal, bone, etc., will 

 contain less than one one-hundredth of 

 a percent of the unstable atom carbon- 

 14 when the sample is formed. After a 

 half-life of only 5,700 years, carbon-14 

 converts to the stable atom called nitro- 

 gen-14. In the case of this clock, we 

 know with precision the amount of car- 

 bon-14 that will be incorporated, say, 

 into a tree trunk at the time it was grow- 

 ing. In similar fashion to the potassium 

 atom discussed above, the carbon-14 

 atom, when it breaks down to nitrogen- 

 14, emits a high energy ray which can be 

 detected by a sensitive instrument as an 

 amplified "click." Rather than meas- 

 ure the quotient of carbon-14 to nitro- 

 gen-14 it is simpler to measure the clicks 

 (radioactivity level) to obtain the age of 

 the sample. 



Because of the short half-life of car- 

 bon-14 it is suitable only for relatively 

 young samples. How far back the meth- 

 od can be pushed depends on our ability 

 to measure faint radioactivity levels. At 

 present the method is good only for spec- 

 imens younger than about 30,000 years 

 in age. Geologically, 30,000 years is 

 only "yesterday" to an earth that is 

 about four and one-half billion years 

 old. However, for age determinations 

 of anthropological or archaeological in- 

 terest, carbon-14 dating is a valuable 

 tool. For example, archaeologists reckon 

 the age of the tomb of the Egyptian, 

 Sesostris III, as 1800 B.C. Carbon-14 

 tests give a date of 1670 B.C. The tomb 

 of the Egyptian vizier, Hemaka, has been 

 estimated by archaeologists to have been 

 built sometime between 2750 B.C. and 

 3150 b.c. Carbon-14 tests give a date 

 of 2933 B.C. 



i3o far we have mentioned only the 

 two most popular methods for geologic 

 age determinations. But what happens 

 in those cases where the rocks contain no 

 potassium or carbon? Fortunately, there 

 are other atoms available as clocks to fill 

 in the gaps. A partial list is shown in 

 Table II. 



