340 Common Science 



474. A cup does not naturally fall apart, yet after it is broken it 



falls apart even if you fit the pieces together again. 



475. Crayon leaves marks on a blackboard. 



476. A baked potato tastes very different from a raw one. 



477. An air-filled automobile tire is harder at noon than in the early 



morning. 



478. When a live trolley wire breaks and falls to the street, it be- 



comes so hot that it burns. 



479. Glass jars of fruit should be kept in a fairly dark place. 



480. You wash dishes in hot water. 



SECTION 51. Chemical change releases energy. 



Why is fire hot? 

 What makes glowworms glow? 



Why does cold quicklime boil when you pour cold water 

 on it? 



If no energy were released by chemical change, we 

 should run down like clocks, and could never be wound 

 up again. We could breathe, but to do so would do us 

 no more good than it would if oxygen could not combine 

 with things. Oxidation would go on in our bodies, but 

 it would neither keep us warm nor help us to move. A 

 few spasmodic jerks of our hearts, a few gasps with our 

 lungs, and they would stop, as the muscles would have 

 no energy to keep them going. 



The sunlight might continue to warm the earth, as 

 we are not sure that the sun gets any of its heat from 

 chemical change. But fires, while they would burn for 

 an instant, would be absolutely cold ; no energy would 

 be given out by the fuel combining with oxygen. But 

 the fires could not burn long, because there would be 

 nothing to keep the gases and fuel hot enough to make 

 them combine with the oxygen. 



Even during the instant that a fire lasted it would be 



