472. Proofs of photographs that have not been thoroughly "fixed" fade if left out of their envelope. 473. Blowing a match puts it out, yet a good draft is necessary for a hot fire. 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 becomes 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 invisible, for it would give off no light if no energy were released by the chemical change. Only electric lights and heaters would continue to work, and even some of these would fail. The electric motors in submarines and electric automobiles would instantly stop; battery flashlights would go out as quickly as the fire; no doorbells would ring. In short, all forms of electric batteries would stop sending currents of electricity out through their wires, and everything depending upon batteries would stop running. A fire gives out heat and light; both are kinds of energy. And it is the electric energy caused by the chemical change in batteries that runs submarines, electric automobiles, flashlights, and doorbells. Since burning (oxidation) is simply a form of chemical change, it is not difficult to realize that chemical change releases energy.