could be tested and verified with the greatest ease; and, as usual, the
discovery being made, it seems surprising that earlier investigators--in
particular so sagacious a guesser as Kepler--should have missed it.

Galileo himself must have been to some extent a student of light, since,
as we have seen, he made such notable contributions to practical
optics through perfecting the telescope; but he seems not to have added
anything to the theory of light. The subject of heat, however, attracted
his attention in a somewhat different way, and he was led to the
invention of the first contrivance for measuring temperatures. His
thermometer was based on the afterwards familiar principle of the
expansion of a liquid under the influence of heat; but as a practical
means of measuring temperature it was a very crude affair, because the
tube that contained the measuring liquid was exposed to the air, hence
barometric changes of pressure vitiated the experiment. It remained for
Galileo's Italian successors of the Accademia del Cimento of Florence
to improve upon the apparatus, after the experiments of Torricelli--to
which we shall refer in a moment--had thrown new light on the question
of atmospheric pressure. Still later the celebrated Huygens hit upon the
idea of using the melting and the boiling point of water as fixed
points in a scale of measurements, which first gave definiteness to
thermometric tests.

TORRICELLI

In the closing years of his life Galileo took into his family, as
his adopted disciple in science, a young man, Evangelista Torricelli
(1608-1647), who proved himself, during his short lifetime, to be a
worthy follower of his great master. Not only worthy on account of his
great scientific discoveries, but grateful as well, for when he had
made the great discovery that the "suction" made by a vacuum was really
nothing but air pressure, and not suction at all, he regretted that
so important a step in science might not have been made by his
great teacher, Galileo, instead of by himself. "This generosity of
Torricelli," says Playfair, "was, perhaps, rarer than his genius: there
are more who might have discovered the suspension of mercury in the
barometer than who would have been willing to part with the honor of the
discovery to a master or a friend."

Torricelli's discovery was made in 1643, less than two years after the
death of his master. Galileo had observed that water will not rise in
an exhausted tube, suc