EXPANSION DUE TO HEAT 19 



14. CHEMICAL EFFECTS OF HEAT 



When the various combustibles united with oxygen and 

 burned quietly, or exploded, we had examples of chemical 

 action. We learned that heat starts chemical action. After 

 the action begins, it gives off heat. In Section 31 we shall 

 study chemical action as a source of heat. 



If we remember that heat is the motion of molecules, and 

 that they not only move, but strike against their neighbors, 

 we can see that this motion, if increased, would make 

 them strike harder, and even unite with some other dis- 

 similar molecules. Thus it is that heat aids chemical action. 



Heat produces several very complex changes in meat and 

 other substances which have had life, and these changes make 

 the substances more suitable for food. Heat also causes 

 chemical changes in food which are very similar to the pro- 

 cesses of digestion. Thus boiling starch slowly changes it 

 into a sugar. See Section 29, Cooking. 



References : 



1. 1703 : 24. Heat Liberates Gases from Compounds. 



2. 1703 : 428. Effect of Heat on Starch. 



3. 1901 : 36. Heat Aids Plant Growth. 



a. 1507 : 53-55. Effects of Heat on Food. 



b. 1708 : 35. Combination Caused by Heat. 



c. 1709 : 2-3. Effect of Heating a Metal in Air. 



d. 1711 : 24-27. Physical and Chemical Effects of Heat. 



15. EXPANSION DUE TO HEAT 



The first effect of heat on a body is to cause its molecules 

 to move faster. Since these hit against their neighbors, and 

 are in turn hit by them, the body as a whole becomes larger, 

 and we say that expansion has taken place. The only ma- 



