The law of radioactive decay.
The number of atoms that decay is wholly dependent upon
the number originally present. As the parent decreases, the
daughter increases in the same proportions.
only how much of the daughter element is present, but also how
much of the parent is left too.
By measuring the amount of radium and helium present in
his rock, and knowing the rate at which one decayed to the
other, Rutherford became the very first person ever to date the
true age of a rock. The age he obtained was five hundred mil-
lion years. Five hundred million years! 'Doomsday postponed!',
proclaimed one newspaper reporting on this phenomenon. A
few weeks later, walking through the university grounds with
another small black rock in his hand, Rutherford met the
Professor of Geology; 'Adams', he said, 'how old is the earth
supposed to be?' The answer was the usual one prevailing at the
time - that various methods led to an estimate of 100 million
years. 'I know', said Rutherford quietly, 'that this piece of pitch-
blende is seven hundred million years old.'
Kelvin was not going to be pleased, but someone had to tell
him of this latest wild miracle. Unwittingly Rutherford drew the
short straw. He was about to give a lecture to a packed audience