EXPLOSIVES. Dynamite, gunpowder, and most explosives are mixtures of solids or liquids that will combine easily and will form gases that expand greatly as a result of the combination. One of the essentials in explosives is some compound of oxygen (such as the manganese dioxid or potassium chlorate you used to make oxygen in Experiment 93) which will easily set its oxygen free. This oxygen combines very swiftly with something else in the explosive, releasing heat and forming a gas that takes much more room. In its effort to free itself, this expanding gas will blast rocks out of the way, shoot cannon balls, or do any similar work. But if gunpowder does not have to push anything of much importance out of its way to expand, there is no explosion. That is why a firecracker merely fizzes when you break it in two and light the powder. The cardboard no longer confines the expanding gas; so there is nothing to burst or to push violently out of the way. 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?