8 Introduction. 



greater the higher and less the lower the temperature is. 

 From this it follows that ordinarily molecules are not in 

 contact and that there is room in the interior of bodies, 

 however compact they appear to be, not occupied by them. 

 Observations with the ordinary mercurial thermometer 

 prove the same general fact. As the temperature rises 

 a portion of the mercury is forced out of the bulb into the 

 stem showing that there is not room enough there for all 

 of the mercury although the bulb has actually become 

 larger. So, too, when the temperature falls the mercury 

 again returns to the bulb although the bulb has itself be- 

 come smaller than before. 



5. Molecules of Bodies Always in Motion, It follows 

 from what has been said in the last section that with every 

 change of temperature in bodies their molecules move. 

 The general fact is that the molecules of all bodies whose 

 temperature is not absolute zero are in rapid motion no 

 matter whether the body be a solid, a liquid or a gas. The 

 higher the temperature of the body the more rapidly do 

 the molecules in it vibrate, the greater is their rebound 

 after each collision and so the greater is the mean distance 

 between them; this is why most bodies expand with in- 

 crease of temperature and contract when cooling. 



It is the fact of movement among molecules which 

 causes the diffusion of sugar or salt through water after 

 solution takes place, which causes the perfume of flowers 

 to be constantly moving away from them, which gives solid 

 camphor its odor and which causes snow and ice to evapo- 

 rate at temperatures even below freezing. 



The elastic power of air in the bicycle tire is due to the 

 rapid movement of the molecules and their frequent and 

 hard collision against the walls. It is the same fact which 

 gives the steam its power to drive the engine. The larger 

 the amount of air which is pumped into the tire of the 

 bicycle the greater is the number of collisions per square 

 inch of surface per second and so the harder the tire be- 

 comes. Then, again, when the wheel is left in the hot 



