n. He familiarised the world with
the conception of _molecular force_.

Newton, you know, was preceded by a grand fellow named John Kepler--a
true working man--who, by analysing the astronomical observations of
his master, Tycho Brahe, had actually found that the planets moved as
they are now known to move. Kepler knew as much about the motion of
the planets as Newton did; in fact, Kepler taught Newton and the world
generally the facts of planetary motion. But this was not enough. The
question arose--Why should the facts be so? This was the great
question for Newton, and it was the solution of it which renders his
name and fame immortal. Starting from the principle that every
particle of matter in the solar system attracts every other particle
by a force which varies as the inverse square of the distance between
the particles, he proved that the Planetary motions must be what
observation makes them to be. He showed that the moon fell towards
the earth, and that the planets fell towards the sun, through the
operation of the same force that pulls an apple from its tree. This
all-pervading force, which forms the solder of the material universe,
and the conception of which was necessary to Newton's intellectual
peace, is called the force of gravitation.

Gravitation is a purely attractive force, but in electricity and
magnetism, repulsion had been always seen to accompany attraction.
Electricity and magnetism are double or _polar forces_. In the case of
magnetism, experience soon pushed the mind beyond the bounds of
experience, compelling it to conclude that the polarity of the magnet
was resident in its molecules. I hold a magnetised strip of steel by
its centre, and find that one half of the strip attracts, and the
other half repels, the north end of a magnetic needle. I break the
strip in the middle, find that this half, which a moment ago attracted
throughout its entire length the north pole of a magnetic needle, is
now divided into two new halves, one of which wholly attracts, and the
other of which wholly repels, the north pole of the needle. The half
proves to be as perfect a magnet as the whole. You may break this
half and go on till further breaking becomes impossible through the
very smallness of the fragments; the smallest fragment is found
endowed with two poles, and is, therefore, a perfect magnet. But you
cannot stop here: you _imagine_ where you cannot _experiment_; and reach
the conclusion ent