386 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 

 Brali e, 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 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. I 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 



