CRYSTALLINE AND MOLECULAR FORCES. 259 



vents them from flying straight oft' into space ? Supposing it to be 

 ascertained that from a part of the earth's rocky crust a firmly-fixed 

 and tightly-stretched chain started toward the sun, we might be in- 

 clined to conclude that the earth is held in its orbit by the chain 

 that the sun twirls the earth around him as a boy twirls a bullet at 

 the end of a string round his head. " But why should the chain be 

 needed?" asks the speculative mind. "It is a fact of experience that 

 bodies can attract each other at a distance, and without the interven- 

 tion of any chain. Why should not the sun and earth so attract each 

 other ? and why should not the fall of bodies from a height be the result 

 of their attraction by the earth ? " Here, then, we have one of those 

 higher thoughts of speculation which grow out of the fruitful soil of 

 observation. Having started with the savage and his sensations of 

 muscular force, we pass on to the observation of force exerted between 

 a magnet and rubbed amber, and the bodies which they attract, and 

 rise 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 a 

 familiar one 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 insen- 

 sible. 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, in the case of Newton, the guess was supplemented 

 by being brought to 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 estab- 

 lished the law of variation with the distance, thus placing in our hands 

 a principle which enables us to determine the date of astronomical 

 events in the far historic past or in the distant future. 



But, on his way to this great result, Newton found room in his 

 ample mind for other conceptions, some of which, indeed, constituted 

 the necessary stepping-stones to his result. The one which here con- 

 cerns us most is this : Newton proved that not only did the sun at- 

 tract the earth, and the earth attract the sun, as a whole, but that 

 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 late of certain disturbers of 

 the public peace named Democritus, Epicurus, and Lucretius. These 



