[Ill.u.s.tration: FIG. 181. The most powerful explosions on earth occur in connection with volcanic activity. The photograph shows Mt. La.s.sen, California, the only active volcano in the United States.]

There is a slight explosion even when you shoot an air gun. First you compress some air in the upper part of the barrel of the air gun; then you suddenly release it. The only thing in the way of the expanding air is the bullet; so the air pushes this out in front of it.

In Experiment 36, where you stoppered a test tube containing a little water and then held the tube over a flame until the cork flew out, you were causing an explosion. As the water changed to steam, the steam was an expanding gas. It was at first confined to the test tube by the cork. Then there was an explosion; the gas freed itself by blowing out the cork.

Steam boilers have safety valves to prevent explosions. These are valves so arranged that when the steam expands and presses hard enough to endanger the boiler, the steam will open the valves and escape instead of bursting the boiler to get free.

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 pota.s.sium 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 b.a.l.l.s, 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 pa.s.ses 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?

The answer is not difficult. You know perfectly well that iron does not burn easily; we could not make fire grates and stoves out of iron if it did. But when iron is wet, a little of it dissolves in the water that wets it. There is also a little oxygen dissolved in the water, as we know from the fact that fish can breathe under the water. This _dissolved_ oxygen can easily combine with the _dissolved_ iron; the _solution_ helps the chemical change to take place. The chemical change that results is oxidation,--the iron combining with oxygen,--which is a slow kind of burning; and in iron this is usually called _rusting_.[10] But when we pour water on burning wood, the wood _stops_ burning, for there is not nearly enough oxygen dissolved in water to combine rapidly with burning wood; and the water shuts off the outside air from burning wood and therefore puts the fire out.

[Footnote 10: The rusting of iron is not quite as simple as this, as it probably undergoes two or three changes before finally combining with oxygen. But the solution helps all these changes.]

Another chemical change, greatly helped by solution, is the combining of the two things that baking powder is made of, and the setting free of the carbon dioxid (CO_2) that is in one of them. Try this experiment:

EXPERIMENT 104. Put half a teaspoonful of baking powder in the bottom of a cup and add a little water. What happens?

The chemical action which takes place in the baking powder and releases the gas in bubbles--the gas is carbon dioxid (CO_2)--will not take place while the baking powder is dry; but when it is dissolved, the chemical change takes place in the solution.

If you ate your food entirely dry, you would have a hard time digesting it; and this would be for the same reason that baking powder will not work without water. Perhaps you can drink too much water with a meal and dilute the digestive juices too much; certainly you should not use water to wash down your food and take the place of the saliva, for the saliva is important in the digestion of starch. But you need also partly to dissolve the food to have it digest well. Crackers and milk are usually more easily digested than are plain crackers, for the milk partly dissolves the crackers, and drinking one or two gla.s.ses of water with a meal hastens the digestion of the food.

_APPLICATION 78._ Explain why paint preserves wood; why iron will rust more quickly in a wet place than it will either under water or in a dry place; why silver salts must be dissolved in order to plate a spoon by electricity.

INFERENCE EXERCISE

Explain the following:

501. There is dew on the gra.s.s early in the morning.

502. Cold cream makes your hands and face soft.

503. Glowworms and fireflies can be seen on the darkest nights.

504. A lake looks gray on a cloudy day and blue on a clear day.

505. Dried fruit will keep much longer than fresh fruit.

506. If you scratch a varnished surface, you can rub the scratch out completely by using a cloth wet with alcohol.

507. Soda is usually dissolved in a little water before it is added to a sour-milk batter.

508. Iron rusts when it gets wet.

509. Peroxide is usually kept in brown bottles.

510. Dry lye may be kept in tin cans, but if the lye is _moistened_ it will eat the can.