PHYSICAL PARADOXES. 



the convertibility of heat into and out of the 

 chemical activity of atoms, and by the possi- 

 bility of heat becoming latent ; but manifest 

 heat must be classed for the present as some 

 undetermined form of vibrations of molecules. 



These vibrations are enormously energetic, 

 more energetic in a hotter body, and less so in 

 one which is colder. In a body which is at a 

 white heat the molecules are vibrating at the 

 rate of more than six hundred millions of 

 millions of times in a second. 



In a solid we may suppose that the mole- 

 cules are so closely packed, and have, compara- 

 tively, so little vibrating energy, that their 

 mutual attractions hold them fairly rigidly in 

 fairly fixed positions. This is only generally 

 true, for it has been found that molecules of 

 even very dense solids have some power of 

 migrating. Still, we must accept it as gener- 

 ally true. 



Now when the solid is made hotter, that 

 means that its molecules, probably by the im- 

 pact of more vigorously vibrating molecules of 

 some other substance, such as a burning gas, 

 are made to vibrate more energetically. They 

 therefore strike one another harder blows, and 

 so drive one another further apart. Thus we 

 get the familiar phenomenon of the expansion 

 of substances under the influence of heat. 



Suppose the substance be a lump of wax, 

 and let it be made hotter still. That means 



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