Carleton Washburne

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 shore.

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?

Fig. 146.

Fig. 146. Will heating the water make more salt dissolve?

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 butter fat and other substances scattered all through the milk. The butter 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.

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 spaces 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.

Fig. 147.

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 spaces 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 glass is made by dissolving coloring matter in the glass 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.

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 shellac 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 glass 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 glass slide, and examine the specks under a microscope.

Fig. 148.

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 grass 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 constitute precious and semi-precious stones.

Application 64. Explain why you beat fudge as it cools; why the paper around butter 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 pass 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 spaces 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.

Here is an experiment that shows how gases diffuse:

Experiment 85. Take two test tubes with mouths of the same size so that you can fit them snugly against each other when you want to. Fill one to the brim with water and hold your thumb or a piece of cardboard over its mouth while you place it upside down in a pan of water. Take the free end of a rubber tube that is attached to a gas pipe and put it into the test tube a short distance, so that the gas will go up into the tube, as shown in Figure 149. Now turn on the gas gently. When all the water has been forced out of the tube and the gas bubbles begin to come up on the outside, turn off the gas. Put a piece of cardboard, about an inch or so square, over the mouth of the tube so that no air can get into it, and take the tube out of the water, keeping the mouth down and covered. Bring the empty test tube, which of course is full of air, mouth up under the test tube full of gas, making the mouths of the two tubes meet with the cardboard between them, as shown in Figure 150. Now have some one pull the cardboard gently from between the two test tubes, so that the mouths of the tubes will be pressed against each other and so that practically no gas will escape. Hold them quietly this way, the tube of gas uppermost, for not less than one full minute by the clock. A minute and a half is not too much time. Now have some one light a match for you, or else go to a lighted Bunsen burner.

Fig. 149.

Fig. 149. Filling a test tube with gas.

Fig. 150.

Fig. 150. The lower test tube is full of air; the upper, of gas. What will happen when the cardboard is withdrawn?

Take the test tubes apart gently and hold the lower one, which was full of air, with its mouth to the flame. What has the gas in the upper tube done? Now hold the flame to the upper test tube, which was full of gas. What happens? Has all the gas gone out of it?

As you well know, gas is much lighter than air; you can make a balloon rise by filling it with gas. Yet part of the gas went down into the lower tube. The explanation is that the molecules of gas and those of air were flying around at such a rate that many of the gas molecules went shooting down among the air molecules, and many of the molecules of air went shooting up among those of gas, so that the gas and the air became mixed.

Diffusion in liquids. Diffusion takes place in liquids, as you know. For when you put sugar in coffee or tea and do not stir it, although the upper part of the tea or coffee is not sweetened, the part nearer the sugar is very sweet. If you should let the coffee or tea, with the sugar in the bottom, stand for a few months, it would get sweet all through. Diffusion is slower in liquids than in gases, because the molecules are so very much closer together.

Osmosis. One of the most striking and important facts about diffusion is that it can take place right through a membrane. Try this experiment:

Experiment 86. With a rubber band fasten a piece of parchment paper, made into a little bag, to the end of a piece of glass tubing about 10 inches long. Or make a small hole in one end of a raw egg and empty the shell; then, to get the hard part off the shell, soak it overnight in strong vinegar or hydrochloric acid diluted about 1 to 4. This will leave a membranous bag that can be used in place of the parchment bag. Fill a tumbler half full of water colored with red ink, and add enough cornstarch to make the water milky. Pour into the tube enough of a strong sugar solution to fill the membranous bag at its base and to rise half an inch in the tube. Put the membranous bag down into the pink, milky water, supporting the tube by passing it through a square cardboard and clamping it with a spring clothespin as shown in Figure 151. Every few minutes look to see what is happening. Does any of the red ink pass through the membrane? Does any of the cornstarch pass through?

This is an example of diffusion through a membrane. The process is called osmosis, and the pressure that forces the liquid up the tube is called osmotic pressure. It is by this sort of diffusion that chicks which are being incubated get air, and that growing plants get food. It is in this way that the cells of our body secure food and oxygen and get rid of their wastes. There are no little holes in our blood vessels to let the air get into them from our lungs. The air simply diffuses through the thin walls of the blood vessels. There are no holes from the intestinal tract into the blood vessels. Yet the dissolved food diffuses right through the intestinal wall and through the walls of the blood vessels. And later on, when it reaches the body cells that need nourishment, the dissolved food diffuses out through the walls of the blood vessels again and through the cell walls into the cells. Waste is taken out of the cells into the blood and passes from the blood into the lungs and kidneys by this same process of diffusion. So you can readily see why everything would die if diffusion stopped.

Application 65. Explain how the roots of a plant can take in water and food when there are no holes from the outside of the root to the inside; how bees can smell flowers for a considerable distance.

Inference Exercise

Explain the following:

401. A shell in the bottom of a teakettle gathers most of the scale around it and so keeps the scale from caking at the bottom of the kettle.

402. There is oxygen dissolved in water. When the water comes in contact with the fine blood vessels in a fish's gills, some of this oxygen passes through the walls of the blood vessels into the blood. Explain how it does so.

403. Asphalt becomes soft in summer.

404. When the trolley comes off the wire the car soon stops.

405. You cannot see stars in the daytime on earth, yet you could see them in the daytime on the airless moon.

406. Although the carbon dioxid you breathe out is heavier than the rest of the air, part of it goes up and mixes with the air above.

407. On a cold day wood does not feel as cold as iron.

408. To make mayonnaise dressing, the oil, egg, and vinegar are thoroughly beaten together.

409. A solution of iodin becomes stronger if it is allowed to stand open to the air.

410. A drop of milk in a glass of water clouds all the water slightly.

Section 44. Clouds, rain, and dew: Humidity.

Why is it that you can see your breath on a cold day?

Where do rain and snow come from?

What makes the clouds?

There is water vapor in the air all around us—invisible water vapor, its molecules mingling with those of the air—water that has evaporated from the oceans and lakes and all wet places.

This water vapor changes into droplets of water when it gets cool enough. And those droplets of water make up our clouds and fogs; they join together to form our rain and snow high in the air, or gather as dew or frost on the grass at night.

If the water vapor should suddenly lose its power of changing into droplets of water when it cooled,—well, let us pretend it has lost this power but that any amount of water can evaporate, and see what happens:

What fine weather it is! There is not a cloud in the sky. As evening closes in, the stars come out with intense brightness. The whole sky is gleaming with stars—more than we have ever seen at night before.

The next morning we find no dew or frost on the grass. All the green things look dry. As the day goes on, they begin to wilt and wither. We all wish the day were not quite so fine—a little rain would help things wonderfully. Not a cloud appears, however, and we water as much of our gardens as we can. They drink the water greedily, and that night, again no dew or fog, and not the faintest cloud or mist to dim the stars. And the new day once more brings the blazing sun further to parch the land and plants. Day after day and night after night the drought gets worse. The rivers sink low; brooks run dry; the edges of the lakes become marshes. The marshes dry out to hardened mud. The dry leaves of the trees rustle and crumble. All the animals and wood creatures gather around the muddy pools that once were lakes or rivers. People begin saving water and buying it and selling it as the most precious of articles.

As the months go by, winter freezes the few pools that remain. No snow falls. Living creatures die by the tens of thousands. But the winter is less cold than usual, because there is now so much water vapor in the air that it acts like a great blanket holding in the earth's heat.

With spring no showers come. The dead trees send forth no buds. No birds herald the coming of warm weather. The continents of the world have become vast, uninhabitable deserts. People have all moved to the shores of the ocean, where their chemists are extracting salt from the water in order to give them something to drink. By using this saltless water they can irrigate the land near the oceans and grow some food to live on. Each continent is encircled by a strip of irrigated land and densely populated cities close to the water's edge.

It is many years before the oceans disappear. But in time they too are transformed into water vapor, and no more life as we know it is possible in the world. The earth has become a great rocky and sandy ball, whirling through space, lifeless and utterly dry.

That which prevents this from really happening is very simple: In the world as it is, water vapor condenses and changes to drops of water whenever it gets cool enough.

How water vapor gets into the air. The water vapor gets into the air by evaporation. When we say that water evaporates, we mean that it changes into water vapor. As you already know, it is heat that makes water evaporate; that is why you hang wet clothes in the sun or by the fire to dry: you want to change the water in them to water vapor. The sun does not suck up the water from the ocean, as some people say; but it warms the water and turns part of it to vapor.

What happens down among the molecules when water evaporates is this: The heat makes the molecules dance around faster and faster; then the ones with the swiftest motion near the top shoot off into the air. The molecules that have shot off into the air make up the water vapor.

The water vapor is entirely invisible. No matter how much of it there is, you cannot see it. The weather is just as clear when there is a great deal of water vapor in the air as when there is very little, as long as none of the vapor condenses.

How clouds are formed. But when water vapor condenses, it forms into extremely small drops of real water. Each of these drops is so small that it is usually impossible to see one; they are so tiny that you could lay about 3000 of them side by side in one inch! Yet, small as they are, when there are many of them they become distinctly visible. We see them floating around us sometimes and call them fog or mist. And when there are millions of them floating in the air high above us, we call them a cloud.

The reason clouds form so high in the air is this: When air or any gas expands, it cools. Do you remember Experiment 31, where you let the gas from a tank expand into a wet test tube and it became so cold that the water on the test tube froze? Well, it is much the same way with rising air. When air rises, there is less air above it to keep it compressed; so it expands and cools. Then the water vapor in it condenses into droplets of water, and these form a cloud.

Each droplet forms a gathering place for more condensing water vapor, and therefore grows. When the droplets of water in a cloud are very close together, some may be jostled against one another by the wind. And when they touch each other, they stick together, forming a larger drop. When a drop grows large enough it begins to fall through the cloud, gathering up the small droplets as it goes. By the time it gets out of the cloud it has grown to a full-sized raindrop, and falls to earth. The complete story of rain, then, is this:

How rain is caused. The surface of the oceans and lakes is warmed by the sun. The water evaporates, turning to invisible water vapor. This water vapor mingles with the air. After a while the air is caught in a rising current and swept up high, carrying the water vapor with it. As the air rises, there is less air above it to press down on it; so it expands. When air expands it cools, and the water vapor which is mingled with it likewise cools. When the water vapor gets cool enough it condenses, changing to myriads of extremely small drops of water. These make a cloud.

A wind comes along; that is, the air in which the cloud is floating moves. The wind carries the cloud along with it. More rising air, full of evaporated water from the ocean, joins the cloud and cools, and the water forms into more tiny droplets. The droplets get so close together that they shut out the sun's light from the earth, and people say that the sky is darkening.

Meanwhile some of the droplets begin to touch each other and to stick together. Little by little the drops grow bigger by joining together. Pretty soon they get so big and heavy that they can no longer float high in the air, and they fall to the ground as rain.

Part of the rain soaks into the ground. Some of it gradually seeps down through the ground to an underground stream. This has its outlet in a spring or well, or in an open lake or the ocean. But the rain does not all soak in. After the storm, some of the water again evaporates from the top of the ground and mixes with the warm air, and it goes through the same round. Other raindrops join on the ground to form rivulets that trickle along until they meet and join other rivulets; and all go on together as a brook. The brook joins others until the brooks form a river; and the river flows into a lake or into the ocean.

Then again the sun warms the surface of the ocean or lake; the water evaporates and mixes with the air, which rises, expands, and cools; the droplets form and make clouds; the droplets join, forming big drops, and they fall once more as rain. The rain soaks into the ground or runs off in rivulets, and sooner or later it is once more evaporated. And so the cycle is repeated again and again.

And all this is accounted for by the simple fact that when water evaporates its vapor mingles with the air; and when this vapor is sufficiently cooled it condenses and forms droplets of water.

The barometer. In predicting the weather a great deal of use is made of an instrument called the barometer. The barometer shows how hard the air around it is pressing. If the air is pressing hard, the mercury in the barometer rises. If the air is not pressing hard the mercury sinks. Just before a storm, the air usually does not press so hard on things as at other times; so usually, just before a storm, the mercury in the barometer is lower than in clear weather. You will understand the barometer better after you make one. Here are the directions for making a barometer:

Experiment 87. To be done by the class with the aid of the teacher. Use a piece of glass tubing not less than 32 inches long, sealed at one end. Fill this tube to the brim with mercury (quicksilver), by pouring the mercury into it through a paper funnel. Have the sealed end of the tube in a cup, to catch any mercury that spills.⁷ When the tube is full, pour mercury into the cup until there is at least half an inch of it at the bottom. Now put your forefinger very tightly over the open end of the tube, take hold of the sealed end with your other hand, and turn the tube over. Lower the open end, with your finger over it, into the cup. When the mercury in the cup completely covers your finger and the end of the tube, remove your finger carefully so that no air can get up into the tube of mercury. Let the open end of the tube rest gently on the bottom of the cup, and hold the tube upright with your hand or by clamping it to a ring stand. Hold a yardstick or meter stick beside the tube, remembering to keep the tube straight up and down. Measure accurately the height of the mercury column from the surface of the mercury in the cup. Then go to the regular barometer hanging on the wall, and read it.

Fig. 152.

Fig. 152. Filling the barometer tube with mercury.

The reason your barometer may not read exactly the same as the expensive laboratory instrument is that a little air and water vapor stick to the inside of the tube and rise into the "vacuum" above the mercury; also, the tube may not be quite straight up and down. Otherwise the readings would be the same.

Footnote 7: If mercury spills on the floor or table during this experiment, gather it all into a piece of paper by brushing even the tiny droplets together with a soft brush; squeeze it through a towel into a cup to clean it. It is expensive; so try not to lose any of it.

Of course you understand what holds the mercury up in the tube. If you could put the cup of mercury into a vacuum, the mercury in the tube would sink down into the cup. But the pressure of the air on the surface of the mercury in the cup keeps the mercury from flowing out of the tube and so leaving a vacuum in there. If the air pushes down hard on the mercury in the cup, the mercury will stand high in the tube. This is called high pressure. If the air does not press hard on the mercury in the cup, the mercury stands low in the tube. This is called low pressure.

How weather is forecast. Weather forecasters make a great deal of use of the barometer, for storms are usually accompanied by low pressure, and clear weather nearly always goes with high pressure.

The reason storms are usually accompanied by low pressure is this: A storm is almost always due to the rising of air, for the rising air expands and cools, and if there is much water vapor in it, this condenses when it cools and forms clouds and rain. Now air rises only when there is comparatively little pressure from above. Therefore, before and during a storm there is not so much pressure on the mercury of the barometer and the barometer is low.

Clear weather, on the other hand, is often the result of air being compressed, for compressing air warms it. When air is being warmed, the water vapor in it will not condense; so the air remains clear. But when the air is being compressed, it presses hard on the mercury of the barometer; the pressure is high, and the mercury in the barometer rises high. Therefore when the mercury in the barometer is rising, the weather is usually clear.

Weather men do not have to wait for an actual storm to be reported. If the reports from the west show that the air is rising as it swirls along—that is, if the barometer readings in the west are low—they know that this low-pressure air is approaching them. And they know that low pressure usually means air that is rising and cooling and therefore likely to drop its moisture. In the same way, if the barometers to the west show high pressure, the eastern weather men know that the air that is blowing toward them is being compressed and warmed, and is therefore not at all likely to drop its moisture; so they predict fair weather.

The weather man is not ever certain of his forecasts, however. Sometimes the air will begin to rise just before it gets to him. Then there may be a shower of rain when he has predicted fair weather. Or sometimes the air that has been rising to the west, and which has made him predict bad weather, may stop rising; the storm may be over before it reaches his station. Then his prediction of bad weather is wrong. Or sometimes the storm unexpectedly changes its path. There are many ways in which a weather prophecy may go wrong; and then we blame the weather man. We are likely to remember the times that his prophecy is mistaken and to forget the many, many times when it is right.

How snow is formed. The difference between the ways in which snow and rain are formed is very slight. In both cases water evaporates and its vapor mingles with the warm air. The warm air rises and expands. It cools as it expands, and when it gets cool enough the water vapor begins to condense. But if the air as it expands becomes very cold, so cold that the droplets of water freeze as they form and gather together to make delicate crystals of ice, snow is formed. The ice crystals found in snow are always six-sided or six-pointed, because, probably, the water or ice molecules pull from six directions and therefore gather each other together along the six lines of this pull. At any rate, the tiny crystals of frozen water are formed and come floating down to the ground; and we call them snowflakes. After the snow melts it goes through the same cycle as the rain, most of it finally getting back to the ocean through rivers, and there, in time, being evaporated once more.

Hail is rain that happens to be caught in a powerful current of rising air as it forms, and is carried up so high that it freezes in the cold, expanding air into little balls of ice, or hail stones, which fall to the ground before they have time to melt.

Why one side of a mountain range usually has rainfall. When air that is moving along reaches a mountain range, it either would have to stop, or rise and go over the mountain. The pressure of the air behind it, moving in the same direction, keeps it from stopping, and so it has to go up the slopes and over the range. But as it goes up, there is less air above it to push down on it; so it expands. This makes it cool, and the water vapor in it begins to condense and form snow or rain. Therefore the side of mountain ranges against which the wind usually blows, almost always has plenty of rainfall.

It is different on the farther side of the mountain range. For here the air is sinking. As it sinks it is being compressed. And as it is compressed it is heated. If you hold your finger over the mouth of a bicycle pump and compress the air in the pump by pushing down on the handle, you will find that the pump is decidedly warmed. When the air, sinking down on the farther side of the mountain range, is heated, the water vapor in it is not at all likely to condense. Therefore rain seldom falls on the side of the mountains which is turned away from the prevailing winds.

How dew and frost are formed. The heat of the earth radiates out into the air and on out into space. At night, when the earth loses its heat this way and does not receive heat from the sun, it becomes cooler. When the air, carrying its water vapor, touches the cool leaves and flowers, the water vapor is condensed by the coolness and forms drops of dew upon them. Or, if the night is colder, the droplets freeze as they form, and in the morning we see the grass and shrubs all covered with frost.

The cause of fogs. When warm air is cooled while it is down around us, the water vapor in it condenses into myriads of droplets that float in the air and make it foggy. The air may be cooled by blowing in from the warm lake or ocean in the early morning, for at night the land cools more rapidly than the water does. This accounts for the early morning fogs in many cities that are on the coasts.

Likewise when the wind has been blowing over a warm ocean current, the surface of the warm water evaporates and fills the air with water vapor. Then when this air passes over a cold current, the cold current cools the air so much that the moisture in it condenses and forms fog. That is why there are fog banks, dangerous to navigation, in parts of the ocean, particularly off Labrador.

Why you can see your breath on cold days. You really make a little fog when you breathe on a cold morning. The air in your lungs is warm. The moisture in the lungs evaporates into this warm air, and you breathe it out. If the outside air is cold, your breath is cooled; so some of the water vapor in it condenses into very small droplets, and you see your breath.

Here are two experiments in condensing water vapor by cooling the air with which it is mixed. Both work best if the weather is warm or the air damp.

Experiment 88. Put the bell jar on the plate of the air pump and begin to pump the air out of it. Watch the air in the jar. If the day is warm or damp, a slight mist will form.

As part of the air is pumped out, the rest expands and cools, as warm air does when it rises and is no longer pressed on so hard by the air above it. And as in the case of the rising warm air, the water vapor condenses when it cools, and forms the mist that you see. This mist, like all clouds and fog, consists of thousands of extremely small droplets.

Inference Exercise

Explain the following:

411. A gas-filled electric lamp gets hotter than a vacuum lamp.

412. You can remove a stamp from an envelope by soaking it in water.

413. We see our breath on cold days and not on warm days.

414. The electric arc is exceedingly hot.

415. Rock candy is made by hanging a string in a strong syrup left open to the air.

416. Dishes in which candy has been made should be put to soak.

417. Moisture gathers on eyeglasses when the wearer comes from a cold room into a warm one.

418. Sprinkling the street on a hot day makes the air cool.

419. You cannot see things in a dark room.

420. Where air is rising there is likely to be rain.

Section 45. Softening due to oil or water.

Why does fog deaden a tennis racket?

How does cold cream keep your face from becoming chapped?

Let us now imagine that animal and plant substances have suddenly lost their ability to be softened by oil or water.

All living things soon feel very uncomfortable. Your face and hands sting and crack; the skin all over your body becomes harsh and dry; your mouth feels parched. The shoes you are wearing feel as if they had been dried over a radiator after being very wet, only they are still harder and more uncomfortable.

A man driving a horse feels the lines stiffening in his hands; and the harness soon becomes so dry and brittle that it cracks and perhaps breaks if the horse stops suddenly.

The leaves on the trees begin to rattle and break into pieces as the wind blows against them. Although they keep their greenness, they act like the driest leaves of autumn.

I doubt whether you or any one can stay alive long enough to notice such effects. For the muscles of your body, including those that make you breathe and make your heart beat, probably become so harsh and stiff that they entirely fail to work, and you drop dead among thousands of other stiff, harsh-skinned animals and people.

So it is well that in the real world oil and water soften practically all plant and animal tissues. Of course, in living plants and animals the oil and water come largely from within themselves. Your skin is kept moist and slightly oily all the time by little glands within it, some of which, called sweat glands, secrete perspiration and others of which secrete oil. But sometimes the oil is washed off the surface of your hands, as when you wash an article in gasoline or strong soap. Then you feel that your skin is dry and harsh.

And when you want to soften it again you rub into it oily substances, like cold cream or vaseline.

In the same way if harness or shoes get wet and then are dried out, they can be made properly flexible by oiling. You could wet them, of course, and this would soften them as long as they stayed wet. But water evaporates rather quickly; so when you want a thing to stay soft, you usually apply some kind of oil or grease.

Just as diffusion and the forming of solutions are increased by heat, this softening by oil and water works better if the oil or water is warm. That is why you soak your hands in warm water before manicuring your nails.

Application 67. Explain why women dampen clothes before ironing them; why crackers are put up in waterproof cartons; why an oil shoe polish is better than one containing water.

Inference Exercise

Explain the following:

421. You can shorten your finger nails by filing them.

422. You can do it more quickly after washing them than before.

423. After a flashlight picture is taken, the smoke soon reaches all parts of the room.

424. A jeweler wears a convex lens on his eye when he works with small objects.

425. Shoemakers soak the leather before half-soling shoes.

426. Lightning often sets fire to houses or trees that it strikes.

427. The directions on many bottles of medicine and of preparations for household use say, "Shake well before using."

428. If you set a cold tumbler inside of one that has just been washed in hot water, the outer one will crack in a few minutes.

429. A dry cloth hung out at night becomes wet, while a wet cloth hung out on a clear day dries.

430. Putting cold cream or tallow around the roots of your finger nails will help to prevent hangnails.