An Hourglass of Great Precision - Four Revolutions in the Earth Sciences - page 31

An Hourglass of Great Precision
How Old Is the Earth Supposed to Be?
In 1903, Rutherford carried out an experiment that his biographer said demon-
strated “remarkable ingenuity—even for Rutherford.” 1 He separated the alpha and
beta particles and measured the velocity of the alphas, finding that they travel at
the fantastic speed of 24,000 kilometers per second, or about 54 million miles per
hour. Rutherford observed that the alpha particles have about twice the mass of a
hydrogen atom, from which he could deduce that they were almost certainly he-
lium. Since the energy of a moving object is one-half its mass times its velocity
squared, Rutherford could calculate the energy of the alphas, which he found to be
far greater than that of the beta and gamma rays combined. 2
Pierre Curie (1859-1906) had discovered that one gram of radium gives off
about 100 calories of heat per hour—enough to raise the temperature of a gram of
water from the freezing to the boiling point. Rutherford had discovered why. As
the alpha particles fly outward, they collide with other atoms. Since energy must
be conserved, the collision transforms the enormous kinetic energy of the alphas
into heat.
In a 1905 lecture at Yale, Rutherford said that “the weight of the evidence points
to the conclusion that the alpha particle from radium is an atom of helium.” 3 Then,
“if the rate of production of helium from known weights of the different radio-ele-
ments were known . . . it should thus be possible to . . . determine the age of the
mineral” (187-188). He recognized that some of the gaseous, unreactive helium
would have escaped from minerals, but even sothe method would allow “a minim-
umlimitfortheageofthemineral” (188).In1907,RutherfordandThomasRoyds,
a graduate of Manchester University, where Rutherford had moved by this time,
proved that the alpha particles were indeed nuclei of helium atoms.
By measuring the amount of uranium and helium in a mineral and by knowing
the half-life of uranium, Rutherford had the amount of parent atom, the amount of
daughter atom, and the rate at which parent changed to daughter: he had an hour-
glass. He applied the method to a sample of a uranium mineral called fergusonite,
finding that it was at least 600 million years old (189). Another uranium mineral
gave about the same age. Rutherford said, “When the data required for these cal-
culations are known with more definitiveness [this] will prove one of the most re-