CHAPTER XIX 

 SOME EFFECTS OF HEAT 



You have noticed that the principal effect of "change of 

 temperature" upon substances is to change the volume. 

 The rule is that increase of heat causes expansion, and 

 decrease of heat causes contraction. Change from solid 

 to liquid or gas means expansion, with the exception of 

 the change of ice into water. The kinetic theory helps 

 us very much in explaining why changes in heat involve 

 changes in volume. You have found that increase in heat 

 means increase in the rate of the movements of molecules; 

 that is, the hotter a substance is, the more rapidly its 

 molecules are moving. And the more rapidly its mole- 

 cules move, the larger the space they tend to occupy. 

 Thus, the heating of water "speeds up" the movements 

 of its molecules; they fly off at the surface more abundantly 

 (evaporation), and thus we get. a rapid transformation 

 from liquid to gas. 



Can there be such a thing as the application of heat to 

 a substance without increase of its temperature? Evi- 

 dently, that depends upon what happens to the heat. If 

 this heat simply increases the rapidity of the motions of 

 the molecules of the substance to which it is applied, then 

 the heat of that substance increases. But if the heat 

 energy applied to a substance is exerted in overcoming 

 the force of cohesion of its molecules, as in transforming 

 a solid to a liquid (or a liquid to a gas), then the heat 

 energy applied to that substance is transformed, and 

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