38G FRAGMENTS OF SCIENCE. 
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 Newton. He familiarized the world with 
the conception of molecular force. 
Newton, you know, was preceded by a grand fellow 
named John Kepler a true workingman who, by analyz- 
ing the astronomical observations of his master, Tycho 
Brahe, had actually found that the planets moved as they are 
now known to move. Kepler kuew as much about the motion 
of the planets as Newton did; in fact, Kepler taught New- 
ton 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 toward the 
earth, and that the planets fell toward 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 accom- 
pany attraction. Electricity and magnetism are double or 
polar forces. In the case of magnetism, experience soon 
pushed the mind beyond the bounds of experience, com- 
pelling it to conclude that the polarity of the magnet was 
resident in its molecules. 1 hold a magnetized strip of 
steel by its center, and find that one half of the strip at- 
tracts, and the other half repels, the north end of a mag- 
netic 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 
