National Geographic : 1954 Jan
84 Life's Built-in Atomic Clock, Radioactive Carbon, Dates Ancient Remains Carbon dioxide absorbed from air by plants contains radioactive carbon. When animals or humans eat the plants, the isotope enters their bodies. During life it gives off an average 16 rays a minute per gram of carbon. At death its radioactivity begins to slow down at a fixed rate. After 5,600 years, the element's half life, eight rays a minute escape; four after the next 5,600; and so on. Hence the count reveals how long ago the death occurred. Dr. Willard F. Libby, seen at University of Chicago, developed radiocarbon dating. Mummy mask from 650 B. C . and textiles from 1500 B. C., dated by older methods, serve as checks on his work. had no way of knowing how much digitoxin was actually used by the body and how much was unused and excreted. "We grew foxglove in the greenhouse, and the plants absorbed radiocarbon. "In cooperation with Dr. E. M. K. Geiling, Department of Pharmacology, University of Chicago, digitoxin which contained radiocar bon was isolated from these plants. The Uni versity of Chicago group is utilizing the iso lated radioactive drug in medical studies designed to give new information on the fate of this important heart stimulant in humans. Additional unextracted radioactive digitalis plants have also been made available through the Atomic Energy Commission to other medi cal groups for use in heart studies." New clues to one of Nature's greatest mys teries, the process of photosynthesis, by which green plants manufacture sugars and starches, also are being uncovered with the aid of tagged carbon atoms. If scientists could duplicate the process, they might be able to create almost unlimited supplies of food and fuel. Plants make sugars and starches from water and from carbon dioxide taken out of the air, with the aid of sunlight. Radioactive carbon atoms added to the carbon dioxide are being traced all the way through the process of pho tosynthesis until they turn up in the plants' sugars. Several chemical compounds are formed in the process; some are produced by the plant in the amazingly short time of five seconds, and others make their appearance within 90 seconds. Radioactive Insects Reveal Travels Insects tagged with radioisotopes and then released can be easily identified among thou sands of others caught later in traps. This is much easier than the old method of marking insects with colored dyes and is useful in learn ing how far and fast insects travel. Flies, tagged by feeding them on radioactive phosphorus, were found to travel as far as 28 miles from the point where they were re leased in Oregon farm country. They flew about four miles in the first 24 hours. Traps set in barnyards caught several times as many of the tagged flies as those set in open fields. Flights of yellow-fever mosquitoes in Nigeria have been traced in the same way.