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five times. During his illustrious career, Thomson, or Kelvin, published over
600 scientific articles and books. It was he who, through brilliant insight and
mathematical calculation, made the first
scientific
computation of the age of
the earth. Kelvin's results seemingly struck a mortal blow to Charles Dar-
win's theory of evolution, for Kelvin's estimate—that the earth was created
less than 100 million years ago—did not seem to allow enough time for the
myriad creatures found on earth to have evolved.
Kelvin's estimate was based on his scientific passion: the study of heat
and heat flow (the absolute temperature scale bears his name). It was known
at that time that each transformation of energy from one form to another re-
sults in the formation of heat, which is then dissipated. This fact, which even-
tually become known as the second law of thermodynamics, is the reason why
perpetual motion machines are impossible—energy is always lost by any ma-
chine, and in every transfer of energy. Kelvin reasoned that the earth and the
sun are both cooling and that, by establishing the initial temperatures of each
of these bodies, as well as their rate of heat loss, he could arrive at an estimate
of their age. Measuring the rate of the sun's heat loss was difficult, but mea-
suring that loss for the earth was not. Kelvin needed just three values: the ini-
tial temperate of the earth, the thermal conductivity of rocks, and the actual
heat flow. He used a value of 3870°C as the initial temperature of the earth,
because this was thought to be a reasonable estimate of the temperature at
which rocks melt to a molten state, and because he assumed that the earth
formed from an originally hot, magmatic body (rather than coalescing from
many cold fragments). Heat flow measurements for various types of rocks
were obtained in the laboratory. All that was wanting was a measure of the ac-
tual rate at which heat was escaping from the earth's interior, and this was
found by measuring the earth's temperature at various depths in underground
mines. Not only did these experiments indicate that heat is indeed flowing
outward from the deep interior of the earth but the initial rate they suggested
was so high that it implied a relatively young age for the earth.
Kelvin's calculations were considered highly authoritative, not so much
for
how
they were done but because of who Lord Kelvin was. Kelvin's conclu-
sion about a relatively young earth was soon supported by others who studied
