Useful explosions are generally caused by a chemical action which suddenly releases a great deal of heat and combines solid things into expanding gases. But the bursting of a steam boiler, or the "blow out" of an automobile tire, or the bursting of a potato in the oven, although not caused by chemical action, still are real explosions. An explosion is the sudden release of a confined gas.
Application 77. Explain how gasoline makes a motorcycle go, and why it goes "pop, pop, pop." Explain why a paper bag will burst with a bang, when you blow it up and then clap it between your hands; why a Fourth-of-July torpedo "goes off" when you throw it on the pavement.
Inference Exercise
Explain the following:
491. The engine of an automobile is cooled by the water that passes over it from the radiator.
492. When you light a firecracker, the flame travels down the wick until it reaches the gunpowder, and then the firecracker bursts with a bang.
493. If you light a small pile of gunpowder in the open, as you do when you make a squib by breaking the firecracker in two, you merely have a blaze.
494. Air-filled tires make bicycles ride much more evenly than solid tires would.
495. When clay has once been baked into brick, you can never change it back to clay.
496. A photographic negative turns black all over if it is exposed to the light before it is "fixed."
497. The outside of a window shade fades.
498. A vacuum electric lamp globe feels hot instantly when turned on, but if turned off again at once, it immediately feels cold.
499. Coal gas is made by heating coal very hot in an air-tight chamber.
500. White straw turns yellow when it is long exposed to the sunlight.
CHAPTER ELEVEN
SOLUTION AND CHEMICAL ACTION
Section 53. Chemical change helped by solution.
Why does iron have to get wet to rust?
Is it good to drink water with your meals?
When iron rusts, it is really slowly burning (combining with oxygen). If your house is on fire, you throw water on it to stop the burning. Yet if you throw water on iron it rusts, or burns, better than if you leave it dry. What do you suppose is the reason for this?