Word: lead
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Dates: during 1930-1939
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Because they know how long it takes for uranium to "die," scientists can tell how old a deposit is from the proportion of live uranium and inactive uranium lead found side by side within it. Mineral deposits have been dated back nearly two billion years by means of the uranium-lead timepiece...
...Harvard finding concerns ordinary, or common, lead, which may be called the "older brother" of uranium lead. Ordinary lead is as old as earth itself, and was included in the molten mass of the earth before it cooled down...
...valuable new secret about ordinary lead, which has been kept from scientists by a peculiar feature, is that the relative proportions of the isotopes of this metal vary from sample to sample. Isotopes are atoms of the same element which differ in weight...
...Ordinary lead has four isotopes, weighing 204, 206, 207, and 208 atomic units. The Harvard finding shows that the relative abundance of these isotopes may vary as much as fifteen per cent. Scientists had always believed that the isotopes of ordinary lead had a certain fixed ratio...
...Ordinary lead's peculiar isotope distribution dates back millions of years and was probably caused by the early "contamination" of ordinary lead in its active elements uranium and thorium, Dr. Nier believes. This theory has yet to be thoroughly investigated...