MATTER AND FORCE. 59 



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 attrac- 

 tions. This is one of the great steps taken by Newton. 

 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 solu- 

 tion 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, 

 34 



