HEAT AND COLD 



139 



point of fresh water. Ice forms in the ocean only at much 

 lower temperature than in fresh water. Solutions in gen- 

 eral have lower freezing-points than their solvents taken 

 alone. In other words, the act of dissolving is in itself 

 an absorber of heat, and in solutions this extra heat must 

 be overcome before freezing occurs. 



Here again we have a set of phenomena which are clear 

 and understandable only if we keep the kinetic theory 

 in mind. Let us remember that molecular motion (a 

 form of energy) is much greater in liquids than hi solids. 

 Therefore, a substance in changing from a solid to a liquid 

 must thereby acquire more of this form of energy, i, e., of 

 heat. And when it changes from a liquid to a solid, it 

 must give up heat. 



In the study of geography you probably learned that a 

 large lake has an important effect in winter in warming 

 the air which is over it. This is due to the principle you 

 have just been studying; as the water is changed from 

 liquid to ice a great deal of heat is given out, enough to 

 produce a marked effect on the weather of the surrounding 

 region. Thus, Chicago in winter, solely on account of 

 being on Lake Michigan, is often many degrees warmer 

 than other cities farther inland, and even much farther 

 south. It is common in very cold weather to place tubs 

 of water near vegetables stored in cellars. The heat given 

 out by the freezing water may be enough to keep the 

 vegetables from freezing. 



Also in considering the phenomena of heat and cold, 

 or, more accurately, of more heat and less heat, let us 

 constantly remember that our own feelings of heat and 

 cold are purely relative, and are not at all accurate indica- 

 tors of the real state of a substance as to heat. Heat is 



