distances, and extends beyond the spheres of the
most remote comets."

This conception of the atom as a mere centre of force was hardly such
as could satisfy any mind other than the metaphysical. No one made a
conspicuous attempt to improve upon the idea, however, till just at the
close of the century, when Humphry Davy was led, in the course of
his studies of heat, to speculate as to the changes that occur in the
intimate substance of matter under altered conditions of temperature.
Davy, as we have seen, regarded heat as a manifestation of motion among
the particles of matter. As all bodies with which we come in contact
have some temperature, Davy inferred that the intimate particles of
every substance must be perpetually in a state of vibration. Such
vibrations, he believed, produced the "repulsive force" which (in common
with Boscovich) he admitted as holding the particles of matter at a
distance from one another. To heat a substance means merely to increase
the rate of vibration of its particles; thus also, plainly, increasing
the repulsive forces and expanding the bulk of the mass as a whole. If
the degree of heat applied be sufficient, the repulsive force may become
strong enough quite to overcome the attractive force, and the particles
will separate and tend to fly away from one another, the solid then
becoming a gas.

Not much attention was paid to these very suggestive ideas of Davy,
because they were founded on the idea that heat is merely a motion,
which the scientific world then repudiated; but half a century later,
when the new theories of energy had made their way, there came a revival
of practically the same ideas of the particles of matter (molecules they
were now called) which Davy had advocated. Then it was that Clausius in
Germany and Clerk-Maxwell in England took up the investigation of
what came to be known as the kinetic theory of gases--the now familiar
conception that all the phenomena of gases are due to the helter-skelter
flight of the showers of widely separated molecules of which they are
composed. The specific idea that the pressure or "spring" of gases is
due to such molecular impacts was due to Daniel Bournelli, who advanced
it early in the eighteenth century. The idea, then little noticed, had
been revived about a century later by William Herapath, and again with
some success by J. J. Waterston, of Bombay, about 1846; but it gained
no distinct footing until taken in hand by Clausius in