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.

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?

[Ill.u.s.tration: FIG. 149. Filling a test tube with gas.]

[Ill.u.s.tration: FIG. 150. The lower test tube is full of air; the upper, of gas. What will happen when the cardboard is withdrawn?]

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 gla.s.s tubing about 10 inches long. Or make a small hole in one end of a raw egg and empty the sh.e.l.l; then, to get the hard part off the sh.e.l.l, 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 pa.s.sing 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 pa.s.s through the membrane? Does any of the cornstarch pa.s.s 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 pa.s.ses 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.

[Ill.u.s.tration: FIG. 151. Pouring the syrup into the "osmosis tube."]

_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 sh.e.l.l 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 pa.s.ses 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 gla.s.s 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 gra.s.s 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 gra.s.s. 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 sh.o.r.es 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 s.p.a.ce, 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.