HEAT 79 



which man and other animals are dependent for food. 

 Some substances, as was shown in 55, do not require as 

 much heat to change their temperature as others. It 

 was also shown that water per unit of weight requires 

 more heat to change its temperature than any other sub- 

 stance except hydrogen, that is, its specific heat is greater 

 than that of any other substance. So when in the sun- 

 shine dry ground gets warm quicker than moist earth, 

 and the land gets warm faster than the water of lakes, 

 rivers, and oceans. For this reason, land surrounded 

 by water or near a body of water does not become very 

 hot in summer, and is not so cold in winter. In winter 

 the water is giving out the great quantity of heat which 

 it absorbed during the summer and thus moderates the 

 climate and prevents extreme cold weather. For this 

 reason it is possible to grow large quantities of fruit along 

 our Great Lakes. 



Other sources of heat are the chemical processes which 

 man employs for heat production, such as burning wood, 

 coal, gas, etc. These substances all contain carbon, 

 and by the oxidation of carbon much heat is produced. 

 The heat thus produced can be largely regulated by 

 controlling the supply of fuel and the rate of oxidation. 

 Without this means of producing heat civilization could 

 not exist. Our homes 'would not be very happy places 

 if in winter we had to eat sufficient food and wear 

 enough clothing to keep our bodies warm without the 

 aid of fire. 



59. Effects of Heat. In the chapter on Molecules 

 and Atoms 1 we learned that as the molecules of a sub- 

 stance become more active they require more space and 

 so crowd one another apart. Heat makes the molecules 



1 Chapter III. 



