Just as electricity helps chemical changes in plating, it helps changes in a storage battery. But in the storage battery the new compounds formed by "charging" the battery change back again and generate electricity when the poles of the battery are connected with each other by a good conductor.

_APPLICATION 75._ Explain how spoons can be silver plated; how water can be changed into hydrogen and oxygen.

INFERENCE EXERCISE

Explain the following:

471. Clothes dry best in the sun and wind.

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. Gla.s.s 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.

WHY GLOWWORMS GLOW. When a glowworm glows at night, or when the head of a match glows as you rub it on your wet hand in the dark, we call the light _phosph.o.r.escence_. The name "phosphorus" means light-bearing, and anything like the element phosphorus, that glows without actively burning, is said to be phosph.o.r.escent. Match heads have phosphorus in them. Phosph.o.r.escence is almost always caused by chemical change. The energy released is a dim light, not heat or electricity. Sometimes millions of microscopic sea animals make the sea water in warm regions phosph.o.r.escent. They, like fireflies, glowworms, and will-o"-the-wisps, have in them some substance that is slowly changing chemically, and energy is released in the form of dim light as the change takes place. Most luminous paint is phosph.o.r.escent for the same reason,--there is a chemical change going on that releases energy in the form of light.

When you poured the hydrochloric acid on the zinc to make hydrogen, the flask became warm; the chemical change going on in the flask released heat energy.

_APPLICATION 76._ Explain why pouring cold water on cold quicklime makes the slaked lime that results boiling hot; why a cat"s eyes shine in the dark; why a piece of carbon and a piece of zinc placed in a solution of sal ammoniac will make electricity run through the wire that connects them; why fire is hot.

INFERENCE EXERCISE

Explain the following:

481. A baking potato sometimes bursts in the oven.

482. Turpentine is used in mixing paint.

483. Sodium is a metal; chlorine is a poisonous gas; yet salt, which is made up of these two, is a harmless food.

484. When bricklayers mix water with cement and lime, the resulting mortar boils and steams.

485. Green plants will not grow in the dark.

486. Parts of the body are constantly uniting with oxygen.

This keeps the body warm.

487. Water will not always put out a kerosene fire.

488. Delicately colored fabrics should be hung in the shade to dry.

489. A match glows when you rub it in the dark.

490. Candy hardens when it cools.

SECTION 52. _Explosions._

What makes a gun shoot?

What makes an automobile go?

Usually we think of explosions as harmful, and they often are, of course. Yet without them we could no longer run automobiles; gasoline launches would stop at once; motorcycles would no longer run; gasoline engines for pumping water or running machinery would not be of any use; and all aviation would immediately cease. Tunneling through mountains, building roads in rocky places, taking up tree stumps, and preparing hard ground for crops would all be made very much more difficult. War would have to be carried on much as it was during the Middle Ages; soldiers would use spears and bows and arrows; battleships would be almost useless in attacking; modern forts would be of little value; cannon, guns, rifles, howitzers, mortars, and revolvers would all be so much junk.

[Ill.u.s.tration: FIG. 179. The explosion of 75 pounds of dynamite. A "still" from a motion-picture film.]

[Ill.u.s.tration: FIG. 180. Diagram of the cylinder of an engine. The piston is driven forward by the explosion of the gasoline in the cylinder.]

WHAT MAKES AN AUTOMOBILE GO. In all the above cases the explosions are caused by chemical action. When gasoline mixed with air is sprayed into the cylinder of an automobile, an electric spark makes the gasoline combine with the oxygen of the air; the gasoline suddenly burns and changes to steam and carbon dioxid. As you already know, when a liquid like gasoline turns to gases such as steam and carbon dioxid, the gases take much more room. But that is not all that happens. Much heat is released by the burning of the gasoline spray, and heat causes expansion. So the gases formed by the burning gasoline are still further expanded by the heat released by the burning.

Therefore they need a great deal more room; but they are shut up in a small place in the top of a cylinder. The only thing to hold them up in this small s.p.a.ce, however, is a piston (Fig. 180), and the suddenly expanding gases shove this piston down and escape. The piston is attached to the drive wheel of the automobile, and when the piston is pushed down it gives the automobile a push forward. If it were not for the expansion of a gas in the cylinder, this gas being confined to a small s.p.a.ce, the piston would not be pushed down.

An explosion is simply the sudden pushing of a confined gas expanding on its way to freedom. The gasoline vapor and air were the confined gas. Their chemical combining made them expand; by pushing the piston out of its way the newly formed gas suddenly freed itself. This was an explosion, and it gave the automobile one forward push. But the automobile engine is so arranged that the piston goes up into the cylinder again, and is pulled down again, drawing a spray of gasoline and air into the cylinder after it. Then it goes up a second time, an electric spark explodes the gasoline, the piston is forced down violently once more, and so it goes on. There are several cylinders, of course, and the explosions take place within them one after the other so as to keep the automobile going steadily.

HOW A GUN SHOOTS. Pulling a trigger makes a gun shoot by causing an explosion. There is a spring on the hammer of a gun. This drives the hammer down suddenly when you release the spring by pulling the trigger. The hammer jars the chemicals in the cap and causes them to explode. The heat and flame then cause the oxygen in the gunpowder to combine with some of the other elements in the powder to make a gas.

The gas requires more room than the powder and is further expanded by the heat released by the chemical change. The expanding gas frees itself by pushing the bullet out of its way. The bullet gets such a push through the exploding of the gunpowder that it may fly to a mark and pierce it.