Common Science Part 34

The electric bell, like the telephone and telegraph, works on the simple principle that electricity flowing through a wire that is wrapped around and around a piece of iron will turn that piece of iron into a magnet as long as the electricity flows.

THE ELECTRIC MOTOR. The motor of a street car is a still more complicated carrying out of the same principle. In the next experiment you will see the working of a motor.

EXPERIMENT 79. Connect the wires from the laboratory battery to the two binding posts of the toy motor, and make the motor run. Examine the motor and see that it is made of several electromagnets which keep attracting each other around and around.

Motors, and therefore all things that are _moved_ by electricity, including trolley cars and electric railways, submarines while submerged, electric automobiles, electric sewing machines, electric vacuum cleaners, and electric player-pianos, are moved by magnetizing a piece of iron and letting this pull on another piece of iron. And the iron is magnetized by letting a current of electricity flow around and around it.

[Ill.u.s.tration: FIG. 144. A toy electric motor that goes.]

The making of various kinds of electromagnets and putting currents of electricity to work is becoming one of the great industries of mankind. Waterfalls are being hitched up to dynamos everywhere, and the water power that once turned the mill wheels now turns millions of coils of wire between the poles of powerful magnets. The current generated in this way is used for all kinds of work--not only for furnishing light to cities, and cooking meals, heating homes, and ironing clothes, but for running powerful motors in factories, for driving interurban trains swiftly across the country, for carrying people back and forth to work in city street cars, for lifting great pieces of iron and steel in the yards where huge electromagnets are used,--for countless pieces of work in all parts of the globe. Yet the use of electricity is still only in its beginning. Tremendous amounts of water power are still running to waste; there is almost no limit to the amount of electricity we shall be able to generate as we use the world's water power to turn our dynamos.

[Ill.u.s.tration: FIG. 145. An electric motor of commercial size.]

_APPLICATION 62._ Explain how pressing a telegraph key can make another instrument click hundreds of miles away, and how you can hear over the telephone. Is it vibrations of sound or of electricity that go through the telephone wire, or does your voice travel over it, or does the wire itself vibrate?

Explain how electricity can make a car go.

INFERENCE EXERCISE

Explain the following:

371. When a fuse blows out, you can get no light.

372. If you lay your ear on a desk, you hear the sounds in the room clearly.

373. If you touch a live wire with wet hands, you get a much worse shock than if you touch it with dry hands.

374. A park music stand is backed by a sounding board.

375. The clapper of an electric bell is pulled against the bell when you push the b.u.t.ton.

376. A hot iron tire put on a wagon wheel fits very tightly when it cools.

377. Candy will cool more rapidly in a tin plate than in a china plate.

378. When a trolley wire breaks and falls to the ground it melts and burns at the point at which it touches the ground.

379. By allowing the electricity from the trolley wire to flow down through an underground coil of wire, a motorman can open a switch in the track.

380. The bare ends of the two wires leading to your electric lamp should never be allowed to touch each other.

CHAPTER NINE

MINGLING OF MOLECULES

SECTION 41. _Solutions and emulsions._

How does soap make your hands clean?

Why will gasoline take a grease spot out of your clothes?

If we were to go back to our convenient imaginary switchboard to turn off another law, we should find near the heat switches, and not far from the chemistry ones, a switch labeled SOLUTION. Suppose we turned it off:

The fishes in the sea are among the first creatures to be surprised by our action. For instantly all the salt in the ocean drops to the bottom like so much sand, and most salt-water fishes soon perish in the fresh water.

If some one is about to drink a cup of tea and has sweetened it just to his taste, you can imagine his amazement when, bringing it to his lips, he finds himself drinking tasteless, white, milky water. Down in the bottom of the cup is a sediment of sugar, like so much fine gravel, with a brownish dust of tea covering it.

To see whether or not the trouble is with the sugar itself, he may take some sugar out of the bowl and taste it,--it is just like white sand. Wondering what has happened, and whether he or the sugar is at fault, he reaches for the vinegar cruet. The vinegar is no longer clear, but is a colorless liquid with tiny specks of brown floating about in it. Tasting it, he thinks it must be dusty water. Salt, pepper, mustard, onions, or anything he eats, is absolutely tasteless, although some of the things _smell_ as strong as ever.

To tell the truth, I doubt if the man has a chance to do all of this experimenting. For the salt in his blood turns to solid hard grains, and the dissolved food in the blood turns to dustlike particles. His blood flows through him, a muddy stream of sterile water. The cells of his body get no food, and even before they miss the food, most of the cells shrivel to drops of muddy water. The whole man collapses.

Plants are as badly off. The life-giving sap turns to water with specks of the one-time nourishment floating uselessly through it. Most plant cells, like the cells in the man, turn to water, with fibers and dust flecks making it cloudy. Within a few seconds there is not a living thing left in the world, and the saltless waves dash up on a barren sh.o.r.e.

Probably we had better let the SOLUTION switch alone, after all.

Instead, here are a couple of experiments that will help to make clear what happens when anything dissolves to make a _solution_.

EXPERIMENT 80. Fill a test tube one fourth full of cold water.

Slowly stir in salt until no more will dissolve. Add half a teaspoonful more of salt than will dissolve. Dry the outside of the test tube and heat the salty water over the Bunsen burner. Will hot water dissolve things more readily or less readily than cold? Why do you wash dishes in hot water?

EXPERIMENT 81. Fill a test tube one fourth full of any kind of oil, and one fourth full of water. Hold your thumb over the top of the test tube and shake it hard for a minute or two.

Now look at it. Pour it out, and shake some prepared cleanser into the test tube, adding a little more water. Shake the test tube thoroughly and rinse. Put it away clean.

When you shake the oil with the water, the oil breaks up into tiny droplets. These droplets are so small that they reflect the light that strikes them and so look white, or pale yellow. This milky mixture is called an _emulsion_. Milk is an emulsion; there are tiny droplets of b.u.t.ter fat and other substances scattered all through the milk. The b.u.t.ter fat is _not_ dissolved in the rest of the milk, and the oil is _not_ dissolved in the water. But the droplets may be so small that an emulsion acts almost exactly like a solution.

[Ill.u.s.tration: FIG. 146. Will heating the water make more salt dissolve?]

But when you shake or stir salt or sugar in water, the particles divide up into smaller and smaller pieces, until probably each piece is just a single molecule of the salt or sugar. And these molecules get into the s.p.a.ces between the water molecules and bounce around among them. They therefore act like the water and let the light through. This is a solution. The salt or sugar is _dissolved_ in the water. Any liquid mixture which remains clear is a solution, no matter what the color. Most red ink, most blueing, clear coffee, tea, and ocean water are solutions. If a liquid is _clear_, no matter what the color, you can be sure that whatever things may be in it are dissolved.

[Ill.u.s.tration: FIG. 147. Will the volume be doubled when the alcohol and water are poured together?]

EXPERIMENT 82. Pour alcohol into a test tube (square-bottomed test tubes are best for this experiment), standing the tube up beside a ruler. When the alcohol is just 1 inch high in the tube, stop pouring. Put exactly the same amount of water in another test tube of the same size. When you pour them together, how many inches high do you think the mixture will be? Pour the water into the alcohol, shake the mixture a little, and measure to see how high it comes in the test tube.

Did you notice the warmth when you shook the tube?

If you use denatured alcohol, you are likely to have an emulsion as a result of the mixing. The _alcohol_ part of the denatured alcohol dissolves in the water well enough, but the _denaturing_ substance _in_ the alcohol will not dissolve in water; so it forms tiny droplets that make the mixture of alcohol and water cloudy.

The purpose of this experiment is to show that the molecules of water get into the s.p.a.ces between the molecules of alcohol. It is as if you were to add a pail of pebbles to a pail of apples. The pebbles would fill in between the apples, and the mixture would not nearly fill two pails.

The most important difference between a solution and an emulsion is that the particles in an emulsion are very much larger than those in a solution; but for practical purposes that often does not make much difference. You dissolve a grease spot from your clothes with gasoline; you make an emulsion when you take it off with soap and water; but by either method you remove the spot. You dissolve part of the coffee or tea in boiling water; you make an emulsion with cocoa; but in both cases the flavor is distributed through the liquid. Milk is an emulsion, vinegar is a solution; but in both, the particles are so thoroughly mixed with the water that the flavor is the same throughout. Therefore in working out inferences that are explained in terms of solutions and emulsions, it is not especially important for you to decide whether you have a solution or an emulsion if you know that it is one or the other.

HOW PRECIOUS STONES ARE FORMED. Colored gla.s.s is made by dissolving coloring matter in the gla.s.s while it is molten. Rubies, sapphires, emeralds, topazes, and amethysts were colored in the same way, but by nature. When the part of the earth where they are found was hot enough to melt stone, the liquid ruby or sapphire or emerald, or whatever the stone was to be, happened to be near some coloring matter that dissolved in it and gave it color. Several of these stones are made of exactly the same kind of material, but different kinds of coloring matter dissolved in them when they were melted.

Many articles are much used chiefly because they are good _emulsifiers_ or good _solvents_ (dissolve things well). Soap is a first-rate emulsifier; water is the best solvent in the world; but it will not dissolve oil and gummy things sufficiently to be of use when we want them dissolved. Turpentine, alcohol, and gasoline find one of their chief uses as solvents for gums and oils. Almost all cleaning is simply a process of dissolving or emulsifying the dirt you want to get rid of, and washing it away with the liquid. Do not forget that heat helps to dissolve most things.

_APPLICATION 63._ Explain why clothes are washed in hot suds; why sugar disappears in hot coffee or tea; why it does not disappear as quickly in cold lemonade; why you cannot see through milk as you can through water.