d
rubbed amber and the bodies which they attract, rising, by an unbroken
growth of ideas, to a conception of the force by which sun and planets
are held together.

This idea of attraction between sun and planets had become familiar in
the time of Newton. He set himself to examine the attraction; and
here, as elsewhere, we find the speculative mind falling back for its
materials upon experience. It had been observed, in the case of
magnetic and electric bodies, that the nearer they were brought
together the stronger was the force exerted between them; while, by
increasing the distance, the force diminished until it became
insensible. Hence the inference that the assumed pull between the
earth and the sun would be influenced by their distance asunder.
Guesses had been made as to the exact manner in which the force varied
with the distance; but Newton supplemented the guess by the severe
test of experiment and calculation. Comparing the pull of the earth
upon a body close to its surface, with its pull upon the moon, 240,000
miles away, Newton rigidly established the law of variation with the
distance. But on his way to this result Newton found room for other
conceptions, some of which, indeed, constituted the necessary
stepping-stones to his result. The one which here concerns us is,
that not only does the sun attract the earth, and the earth attract
the sun, as wholes, but every particle of the sun attracts every
particle of the earth, and the reverse. His conclusion was, that the
attraction of the masses was simply the sum of the attractions of
their constituent particles.

This result seems so obvious that you will perhaps wonder at my
dwelling upon it; but it really marks a turning point in our notions
of force. You have probably heard of certain philosophers of the
ancient world named Democritus, Epicurus, and Lucretius. These men
adopted, developed, and diffused the doctrine of atoms and molecules,
which found its consummation at the hands of the illustrious John
Dalton. But the Greek and Roman philosophers I have named, and their
followers, up to the time of Newton, pictured their atoms as falling
and flying through space, hitting each other, and clinging together by
imaginary hooks and claws. They missed the central idea that atoms
and molecules could come together, not by being fortuitously knocked
Against each other, but by their own mutual attractions. This is one
of the great steps taken by Newto