National Geographic : 1961 Oct
End of a mystery: Au thor Curtis (right) and partner Dr. Jack F. Evernden read the star tling age of African fos sils from a graph. The scientists tested several samples of the rock sandwiching the beds in which Olduvai's earliest known tool makers were found, get ting age readings that ranged from 1,570,000 to 1,890,000 years, for an average of 1,750,000. Mass spectrometer, an instrument for sorting atoms by weight, appears in background. A National Science Foundation grant sup ports the project. can, by means of a magnet, separate electri cally charged particles according to their atomic weights. In other words, the mass spec trometer can separate argon 40 atoms from other argon atoms by their atomic weights. Every sample, however, that comes into our laboratory is contaminated in an unfor tunate way. If you have ever spilled a jar of sugar on a pantry shelf, you will understand the problem. The sugar- many million grains of it-simply goes everywhere, lodging in cracks and crevices so that you can never sweep up the last grain. Heat Releases Captive Atoms The same is true of our samples: Particles of argon 40 from the air- which we call sim ply "air" argon - have lodged, just like grains of sugar, in every cranny of the rock. Since we want only the argon 40 daughter atoms produced within the rock itself, we must re move these other unwanted particles. To do this we put the sample in an elec tronic furnace surrounded by a vacuum (page 591). Then we bake the sample for 12 to 48 hours at about 7500 F., driving off most of the air argon 40 but leaving the precious argon 40 daughter atoms within the rock. Finally we heat the sample again in our crucible to about 2,2000 F. At this point the anorthoclase melts, and the long-imprisoned argon 40 daughter atoms are released. In the form of gas, they go into a tube with porous charcoal, which, when cooled to a tempera ture of 480° below zero Fahrenheit, absorbs the argon 40. Now we are ready for the mass spectrom eter. There each argon 40 daughter atom is ionized-that is, given an electric charge by knocking off one of its electrons. The charged atoms are then shot past a carefully adjusted electromagnet. The magnet pulls the argon atoms out of line and aims them at an elec tronic target, while letting lighter or heavier atoms go wide. As the daughter atoms strike the target, they register on a graph, giving us the amount of argon 40 and, with it, the age of our sam ple. And so we learn the age of this fossil man -1,750,000 years-a giant step toward the solution of one of earth's mysteries. And yet we have only begun. The recent discoveries at Olduvai Gorge, made possible with National Geographic Society support and dated by the potassium-argon process, shed a vast new light not only on man but on the history of his planet. As man explores the frontiers of space and the realms beneath the sea, he stands at still another frontier the fascinating frontier of time. SIX-MONTH INDEX AVAILABLE As one of the privileges of membership in the Society, an index for each six-month volume of the NATIONAL GEOGRAPHIC will be mailed upon request to members who bind their issues as works of reference. The index to Volume 119 (January-June, 1961) is now ready.