Water Clock

UNIVERSITY OF CHICAGO Research Institute requires 30 gal. samples of city of Chicago water at least five yrs. and not more than 20 yrs. old. Phone Midway 3-0800, Ext. 2502, or write 5640 Ellis-av.

For three days the classified ad ran in Chicago newspapers. It brought in no gallons of stale water. A Decatur cistern was tapped for a 29-year-old sample. The water heater of a high-school teacher in Oak Park yielded 30 gallons between five and twelve years old. An undertaker emptied his fire extinguisher and a grocer drained the soda pop cooler he had not cleaned for five years.

The bizarre call for water is part of an experiment being carried on by Chemistry Professor Willard F. Libby. He hopes to develop an atomic time scale for water samples similar to the radioactive carbon 14 calendar, which measures the age of prehistoric relics (TIME, June 2).

Chemist Libby's water clock will be based on the same principle as the carbon 14 calendar. Some ten miles high, in the stratosphere, cosmic rays stream in from outer space. With far more force than an atom-smasher, the cosmic rays collide with nitrogen atoms. The crash produces hydrogen, carbon 14 and a minute amount of radioactive tritium. The atoms of cosmic tritium join molecules of water vapor and fall to the earth in snow and rain.

Tritium has a half-life of 12 1/2 years, i.e., half its radioactivity is dissipated in that time. "If our calculations are correct," says Chemist Libby, "then water 12 1/2 years old should be only half as radioactive as new rainwater or snow."

When he has collected enough samples to calibrate his time clock, Chemist Libby will be able to answer some tough questions. Example: Is it true, as oceanographers believe, that there is no mixing of new water on the sea's surface and "old" brine below 700 feet?

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