György Hevesy was born in Budapest on August 1, 1885 and died in Freiburg, Germany, on July 6, 1966. He attended the universities of Budapest, Berlin, and Freiburg. After graduation from Freiburg, he joined Ernest Rutherford's research laboratory at the University of Manchester, England.

The first task Rutherford proposed for him was the separation of the newly discovered radioactive isotope radium-D from ordinary, stable lead with which it occurred. Hevesy discovered that a separation was impossible. From this he concluded that radium-D was another form of lead which was like ordinary lead in all respects except that it was radioactive. The two forms of lead were shortly recognized as isotopes, and radium-D was identified as lead-210.

Hevesy realized the significance of this finding. Suppose that two isotopes of an element are identical in all respects except that one is stable and one, radioactive. If the radioactive isotope is substituted for...