CHAPTER XIII 

 MOLECULAR MOTIONS AND TEMPERATURE 



IN any of the three forms, solid, liquid, or aeriform, 

 in which we are familiar with matter, we now know 

 that the molecules are in rapid motion. In solids 

 under normal condition the molecules do not move 

 beyond the bounding surfaces of the body. In liquids 

 moving molecules may escape from the surface, as in 

 the familiar case of the evaporation of water from an 

 open vessel. The molecules which have escaped move 

 about in the space above the vessel, unrestricted except 

 for such other surfaces, liquid or solid, as may be in 

 the neighborhood. For all substances which are in 

 an aeriform state such unrestricted motion of the 

 molecules is characteristic. 



The molecular motions of the latter state are com- 

 parable to those of the gnats in a swarm, such as one 

 sometimes meets of a summer evening in the space 

 below a street lamp. Their motions are haphazard 

 and irregular. Individual gnats approach or recede 

 from one another, flying now in one direction and now 

 in another. They picture to us fairly well the natural 

 motions of individual molecules. The entire swarm 

 may be given a motion in the same direction, as by 

 waving them aside with a paper. Even when they 

 have such a motion as a group, they still have their 



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