Common Science Part 35

INFERENCE EXERCISE

Explain the following:

381. A kind of lamp bracket is made with a rubber cup. When you press this cup against the wall or against a piece of furniture and exhaust the air from the cup, the cup sticks fast to the wall and supports the lamp bracket.

382. You can take a vaseline stain out with kerosene.

383. If the two poles of an electric battery are connected with a copper wire, the battery soon becomes discharged.

384. Electric bells have iron bars wound around and around with insulated copper wire.

385. Piano keys may be cleaned with alcohol.

386. Linemen working with live wires wear heavy rubber gloves.

387. Crayon will not write on the smooth, glazed parts of a blackboard.

388. Varnish and sh.e.l.lac may be thinned with alcohol.

389. Filtering will take mud out of water, but it will not remove salt.

390. Explain why only one wire is needed to telegraph between two stations.

SECTION 42. _Crystals._

How is rock candy made?

Why is there sugar around the mouth of a syrup jug?

How are jewels formed in the earth?

You can learn how crystals are formed--and many gems and rock candy and the sugar on a syrup jug are all crystals--by making some. Try this experiment:

EXPERIMENT 83. Fill a test tube one fourth full of powdered alum; cover the alum with boiling water; hold the tube over a flame so that the mixture will boil gently; and slowly add boiling-hot water until all of the alum is dissolved. Do not add any more water than you have to, and keep stirring the alum with a gla.s.s rod while you are adding the water. Pour half of the solution into another test tube for the next experiment. Hang a string in the first test tube so that it touches the bottom of the tube. Set it aside to cool, uncovered. The next day examine the string and the bottom of the tube.

EXPERIMENT 84. While the solution of alum in the second test tube (Experiment 83) is still hot, hold the tube in a pan of cold water and shake or stir it until it cools. When white specks appear in the clear solution, pour off as much of the clear part of the liquid as you can; then pour a little of the rest on a gla.s.s slide, and examine the specks under a microscope.

[Ill.u.s.tration: FIG. 148. Alum crystals.]

In both of the above experiments, the hot water was able to dissolve more of the alum than the cold water could possibly hold. So when the water cooled it could no longer hold the alum in solution. Therefore part of the alum turned to solid particles.

When the string was in the cooling liquid, it attracted the particles of alum as they crystallized out of the solution. The force of adhesion drew the near-by molecules to the string, then these drew the next, and these drew more, and so on until the crystals were formed.

But when you kept stirring the liquid while it cooled, the crystals never had time to grow large before they were jostled around to some other part of the liquid or were broken by your stirring rod.

Therefore they were small instead of large. Stirring or shaking a solution will always make crystals form more quickly, but it will also make them smaller.

HOW ROCK CANDY IS MADE. Rock candy is made by hanging a string in a strong sugar solution or syrup and letting the water evaporate slowly until there is not enough water to hold all the sugar in solution.

Then the sugar crystals gather slowly around the string, forming the large, clear pieces of rock candy. The sugar around the mouth of a syrup jug is formed in the same way.

You always get crystallization when you make a liquid too cool to hold the solid thing in solution, or when you evaporate so much of the liquid that there is not enough left to keep the solid thing dissolved.

When you make fudge, the sugar forms small crystals as the liquid cools. When a boat has been on the ocean, salt crystals form on the sails when the spray that has wet them evaporates.

But crystals may form also in the air. There is always some moisture in the air, and when it becomes very cold, some of this moisture forms crystals of ice. If they form up in the clouds, they fall as snow.

If they form around blades of gra.s.s or on the sidewalk, as the alum crystals formed on the string, we have frost.

Still another place that crystals occur is in the earth. When the rocks in the earth were hot enough to be melted and then began to cool, certain substances in the rocks crystallized. Some of these crystals that are especially hard and clear const.i.tute precious and semi-precious stones.

_APPLICATION 64._ Explain why you beat fudge as it cools; why the paper around b.u.t.ter becomes encrusted with salt if it is exposed to the air for some time.

INFERENCE EXERCISE

Explain the following:

391. Dynamos have _copper_ brushes to lead the current from the coils of wire to the line wires.

392. A megaphone makes the voice carry farther than usual.

393. Copper wire is used to conduct electricity, although iron wire costs much less.

394. A flute gives notes that differ in pitch according to the stops that are opened.

395. There are usually solid pieces of sugar around the mouth of a syrup jar.

396. You can beat eggs quickly with a Dover egg beater.

397. When ocean water stands in shallow open tanks for some time, salt begins to form before the water has all evaporated.

398. In a coffee percolator the boiling water goes up through a tube. As this water drips back through the ground coffee beans, it becomes brown and flavored, and the coffee is made.

399. Kerosene will clean off the rim of soap and grease that forms in bathtubs.

400. Beating cake frosting or candy causes it to sugar.

SECTION 43. _Diffusion._

How does food get into the blood?

Why can you so quickly smell gas that is escaping at the opposite side of a room?

On our imaginary switchboard the DIFFUSION switch would not be safe to tamper with. It would be near the SOLUTION switch, and almost as dangerous. For if you were to make diffusion cease in the world, the dissolved food and oxygen in your blood would do no good; it could not get out of the blood vessels or into the cells of your body. You might breathe all you liked, but breathing would not help you; the air could not get through the walls of your lungs into the blood. Plants would begin to wither and droop, although they would not die quite as quickly as animals and fishes and people. But no sap could enter their roots and none could pa.s.s from cell to cell. The plants would be as little able to breathe through their leaves as we through our lungs.

If gas escaped in the room where you were, you could not smell it even if you stayed alive long enough to try; the gas would rise to the top of the room and stay there. All gases and all liquids would stay as they were, and neither would ever form mixtures.

It would not make so much difference in the dead parts of the world if diffusion ceased; the rocks, mountains, earth, and sea would not be changed at all at first. To be sure, the rivers where they flowed into the oceans would make big s.p.a.ces of saltless water; and when water evaporated from the ocean the vapor would push aside the air and stay in a layer over the ocean, instead of mixing with the air and rising to great heights. But the real disaster would be to living things. All of them would be smothered and starved to death as soon as diffusion ceased.