3 HEAT 



and the food is preserved. Hot or even warm food should not 

 be put into a refrigerator and the door should not be left open, 

 admitting warm air from outside. 



Picnickers carry hot drinks and cold drinks in straw-covered 

 bottles ; the straw keeps the hot drink hot because it prevents 

 the escape of heat ; it keeps the cool drink cool because it pre- 

 vents the entrance of heat from the outside. The " vacuum " 

 bottles so much used to prevent change in the temperature of 

 liquids utilizes a double-walled bottle with a vacuum between 

 the walls. In this vacuum there is practically no matter 

 to conduct heat, therefore the temperature changes very little 

 in a long time. 



Effect of heat. A third effect of heat has a vital influence on 

 our lives because to it we owe the changes which take place 

 when food is cooked. The doughy mass which goes into the 

 oven comes out a light spongy loaf; the hard, tough beet comes 

 out the soft succulent vegetable; the small, hard, indigestible 

 rice grain comes out the swollen, fluffy, and digestible grain. 

 If heat did not bring about a change in the nature of substances, 

 many of our present foods would be useless because they cannot 

 be digested in a raw state. 



Coal may be heated in such a way that it will break up into 

 illuminating gas and into substances that yield beautiful dyes. 

 Wood may be treated in such a way that it will form charcoal. 

 Whenever heat causes such changes in the nature of a sub- 

 stance, it is said to produce a chemical change in that sub- 

 stance. Of course chemical changes can take place without 

 heat, but some of the most striking of these changes, such as 

 the breaking up of coal into illuminating gas and coke, take 

 place only with the application of heat. 



