THE STORY OF NINETEENTH-CENTURY SCIENCE 



changes that occur in the intimate substance of matter 

 under altered conditions of temperature. Davy, as \ve 

 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 substance 

 must be perpetually in a state of vibration. Such vibra- 

 tions, 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 sep- 

 arate and tend to fly away from one another, the solid 

 then becoming a gas. 



Not much attention was paid to these very suggestive 

 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 the new 

 theories of energy had made their way, there came a 

 revival of practically the same ideas of the particles of 

 matter (molecules they were now called) which Davy 

 had advocated. Then it was that Clausius in Germany 

 and Clerk Maxwell in England took up the investigation, 

 of what came to be known as the kinetic theory of gases 

 the now familiar conception that all the phenomena 

 of gases are due to the helter-skelter flight of the show- 

 ers of widely separated molecules of which they are 

 composed. The specific idea that the pressure or 

 "spring" of gases is due to such molecular impacts was 



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