A HISTORY OF SCIENCE 



pulsive is exchanged for an attractive force, which de- 

 creases in inverse ratio with the squares of the distances, 

 and extends beyond the spheres of the most remote 

 comets." 



This conception of the atom as a mere centre of force 

 was hardly such as could satisfy any mind other than 

 the metaphysical. No one made a conspicuous at- 

 tempt to improve upon the idea, however, till just at 

 the close of the century, when Humphry Davy was led, 

 in the course of his studies of heat, to speculate as to 

 the changes that occur in the intimate substance of 

 matter under altered conditions of temperature. Davy, 

 as we have seen, regarded heat as a manifestation of 

 motion among the particles of matter. As all bodies 

 with which we come in contact have some temperature, 

 Davy inferred that the intimate particles of every sub- 

 stance must be perpetually in a state of vibration. 

 Such vibrations, he believed, produced the "repulsive 

 force" which (in common with Boscovich) he admitted 

 as holding the particles of matter at a distance from 

 one another. To heat a substance means merely to 

 increase the rate of vibration of its particles ; thus also, 

 plainly, increasing the repulsive forces and expanding 

 the bulk of the mass as a whole. If the degree of heat 

 applied be sufficient, the repulsive force may become 

 strong enough quite to overcome the attractive force, 

 and the particles will separate and tend to fly away 

 from one another, the solid then becoming a gas. 



Not much attention was paid to these very sugges- 

 tive ideas of Davy, because they were founded on the 

 idea that heat is merely a motion, which the scientific 

 world then repudiated ; but half a century later, when 



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