For the "smoke" Mother Nature has still another beautiful plan. She sends the blood-stream flowing through the _lungs_, where it can send off its "smoke" and then get fresh air to carry to the cells in the muscles. When you breathe out, you are sending out the "smoke"; and when you breathe in, you are taking in fresh air.

Our body "smoke" is not brown or blue, like the smoke from a fire; it is a clear, odorless gas, called _carbon dioxid_. This is the same gas that makes the choke-damp of coal mines, which suffocates the miners if the mine is not well ventilated; and the same gas that sometimes gathers at the bottom of a well, making it dangerous for anyone to go down into the well to clean it. And this gas is poisonous in our bodies just as it is in the mine or the well.

You see, then, how important it is that we should live much of our lives in the clear pure air out of doors, and should bring the fresh air into our houses and schools and shops. "Fill up" with it all you can on your way to school, for the best of air indoors is never half so good as the free-blowing breezes outside.

IV. FRESH AIR--HOW WE BREATHE IT

When you are running and breathing hard, and even when you are sitting still and breathing quietly, air is going into your lungs and then coming out, going in and coming out, many times every minute. How does the air get in and out of the lungs? It will not run in of itself; for it is light and floats about, you know. Here, again, Mother Nature has planned it all out. She has made us an air bellows, or air pump, to suck it into the lungs. First we'll see what shape this pump is, and then how it works.

[Ill.u.s.tration: THE CHEST THAT HOLDS THE LUNGS

Back of the lungs is the heart; its position is shown by the broken line. The black line across the chest shows how high the diaphragm rises when we breathe out quietly.]

Stiff rings of bone called _ribs_ run round your body, just like the hoops in an old hoop skirt, or like the metal rings round a barrel.

Here is a picture of the bones of the chest. Perhaps your teacher can show you the skeleton of some animal. You will notice how the rings, or ribs, slant and are joined by hinges behind to the backbone and in front to the breastbone. It looks somewhat like a cage, doesn't it?

Put your hands on the sides of your chest and you can feel your own ribs. Do they slant upward or downward?

This chest-cage is our breathing-machine. Before I tell you how it pumps, I want you to get a pair of bellows and see how they work. When you lift up the handle of the bellows, you make the bag of the bellows larger so that it sucks in air; and when you press the handle down again, the air puffs out through the nozzle.

Our air machine, though it is somewhat different from the bellows in shape, works in exactly the same way. You remember that you found that the ribs slant down and can be moved on hinges. Suppose, now, you place your hands against your ribs and feel the ribs lift as you draw in a long breath. The air will be sucked into your nose just as it was into the bellows when you raised the handle. By lifting your ribs, you have made the chest-cage larger; and the air has rushed into your nose, down your windpipe, and filled your lungs. If you breathe very deeply, you will find that your stomach, too, swells out. This shows that the muscular bottom of the cage, called the _diaphragm_, has been pulled down, making the cage larger still.

In this chest-cage are millions of tiny air bags that make up the lungs; and every time you take a breath, the air bags are puffed out with the fresh air that comes rushing in. By the time you let your ribs sink again, the air has given its oxygen to the blood, and the blood has poured its carbon-dioxid smoke into the air bags for you to breathe out. Nature, with the same bellows, pumps in the oxygen and pumps out the "smoke."

Now, we breathe into our lung-bellows whatever air happens to be around us. So we should take care that the air around us is fresh air.

Unless the air were kept in motion by the heat of the sun, causing breezes and winds, it would become stale and wouldn't do at all for our lung-bellows to use. The air we breathe must be kept moving and fresh if it is to make us feel bright and strong and happy. Mother Nature has given us miles upon miles and oceans upon oceans of this clear, fresh air to breathe--"all outdoors," in fact, as far as we can see around us and for miles above our heads. She sends the winds to move the air about and blow away the dust and dirt; and the sunshine, you remember, not only to warm the air and keep it moving, but to burn right through it and kill the poisons. But this brings us to something else.

You have learned that the air we breathe out would soon smother us, just as smoke would; and now we will see why. If you blow against the window pane on a cold day, the gla.s.s is no longer clear; and when you look at it closely, you see that it is covered with tiny drops of water. This is part of the breath you have just blown out. If the room is cold enough, you can see your breath in the air; that is, the steam in your breath becomes cold and appears as tiny water-drops. You have seen how in the same way, the steam, an inch or so from the spout of the teakettle, cools, making little water-drops that float in the air like clouds. Part of the breath, then, is water; but most of it is a gas, and you can't see it at all as it floats away into the air about you.

If your teacher has a gla.s.s of limewater, and will let you breathe into it through a tube, you will see that your breath soon makes the water look milky. This shows that the gas in your breath is not like the air about you; because air was all over the top of the limewater, yet did not change it at all. The milky look is caused by carbon dioxid, one of the poisons in your breath.

When some people come close to you, you want to turn away your head, because you do not like the smell of their breath. Even when one is quite well, the breath has a queer "mousey" odor, so that we never like to breathe the breath of another person. This disagreeable odor comes not only from the lungs but from the teeth.

We are always breathing out poisons into the air. One of these you can see in the milky limewater, and others you can smell when you happen to come close to anyone else.

[Ill.u.s.tration: PROVING THAT THE BREATH IS NOT LIKE THE AIR]

If you blow on your fingers, you feel that your breath is much warmer than the air. If people are crowded together in rooms with doors and windows shut, their breath soon heats and poisons the air, until they begin to have headache, and to feel dull and drowsy and uncomfortable.

If they should be shut in too long, without any opening to let in the fresh air, as in a prison cell, or in the hold of a ship during a storm, the air would become so poisonous as to make them ill, and would even suffocate them and kill them outright. Even the bees found this out thousands of years ago; and in their hives in hot weather they station lines of worker-bees, one just behind another from the door right down each of the main pa.s.sages, whose business it is to do nothing but keep their wings whirring rapidly, so that they fan a steady current of fresh air into every part of the hive.

[Ill.u.s.tration: DUSTING--HOW SHALL WE DO IT?]

How does Mother Nature get rid of these poisons from our breath? Of course, you say, "She uses the wind and the sunshine." Yes, the winds can whisk up the poison and blow it away so fast, and the sunshine can burn up the horrid smell so quickly, that even the air above big cities, and in their streets, is quite clean enough for us to breathe, except where the people are very closely crowded together and very dirty. Mother Nature wants all of us to help in keeping the air clean.

This we can do by keeping ourselves and our houses clean, and by being careful not to leave sc.r.a.ps of waste, or dirty things, in the streets and cars and parks and other public places. And you children ought to be very careful about your school yard and the halls and the cla.s.srooms, where you spend so much of your time.

IN SCHOOL

I. BRINGING THE FRESH AIR IN

The only place where air is absolutely sure to be fresh is out of doors. There, as we have seen, the sun and the winds keep it so all the time. But, unluckily, we cannot spend all our time outdoors, either when we are little or after we have grown up. So we must try in every way that we can to bring the outdoors indoors--to get plenty of fresh air and light into the houses that we live in, especially the bedrooms we sleep in and the schoolrooms we study in when we are children, and the offices or shops we work in when we are grown up.

After you have your lungs and your blood well filled with air, either by walking briskly to school or by chasing one another about the school playground, you will suddenly hear the bell ring, and you march indoors and sit down at your desks. Here, of course, the air cannot blow about freely from every direction, because the walls and doors and windows are shutting you in on every side. The room, to be sure, is full of air; but if the doors and windows are shut, this air has no way of getting outside, nor can the fresh, pure air out of doors--even though it be moving quite fast, as a wind or a breeze--get inside.

[Ill.u.s.tration: A CLa.s.sROOM ALMOST AS GOOD AS THE OUT-OF-DOORS

Notice the windows open top and bottom, and the high windows under the roof. Why are these good?]

We must let the fresh air come in and the stale air go out. This is one of the things that windows are for; and this is why they are hung upon pulleys and made to slide up and down easily. Of course, even when the windows are not open, they are letting in light, which, you remember, is a deadly enemy to germs and poisons.

Bright sunlight is best for purifying the air of a room, but even ordinary daylight has a good deal of germ-killing power. Therefore, a room that is well lighted is not only much pleasanter to live in, but much healthier, than one that is dull and gloomy. You see why we need plenty of windows and doors: we must let in the breezes and the sunshine, and let out the poisons and the dirt. Then, too, we must make the air in the building move about in order to keep it fresh; for if the air is not fresh, we soon grow tired and sleepy and have headaches. That is why your teacher keeps the windows open at the top a foot or so. You can easily see that when there are twenty or thirty of you breathing out poisons, and each one of you needing about four bushels of fresh air every minute, the old air ought to be going out and the fresh air coming in all the time.

[Ill.u.s.tration: VENTILATION

Watch the candle flames. Which way is the air moving, and why?]

That is also why your teacher gives you a recess, so that you can run out of doors and get some fresh air. Then she can throw open all the windows and doors and have the air in the room clean and fresh when you come back again. So when recess comes, don't hang about in the hallways or on the stairs or in the bas.e.m.e.nt, but run right out of doors into the playground and shout and throw your arms about and run races to fill your lungs full of fresh, sweet air and stretch all your muscles, after the confinement and sitting still. Don't saunter about and whisper secrets or tell stories, but get up some lively game that doesn't take long to play, such as tag or steal-sticks or soak-ball, or duck-on-a-rock or skipping or hopscotch. These will blow all the "smoke" out of your lungs and send the hot blood flying all over your body and make you as "fresh as a daisy" for your next lesson.

When you come back into the schoolroom after recess, the air will seem quite fresh and pure; but unless you keep the windows open, it will not be long before your head begins to be hot, and your eyes heavy, and you feel like yawning and stretching, and begin to wonder why the lessons are so long and tiresome. Then, if your teacher will throw open all the windows and have you stand up, or, better still, march around the room singing or go through some drill or calisthenic exercises, you will soon feel quite fresh and rested again.

In the mild weather of the spring or early fall, all you need to do to keep the air fresh in the schoolroom is to keep the windows well open at the top. But in the winter, the air outdoors is so cold that it has to be heated before it is brought in; and this, in any modern and properly built schoolhouse, is usually arranged for. The fresh air is drawn in through an opening in the bas.e.m.e.nt and is either heated, so that it rises, or is blown by fans all over the building. This sort of fresh air, however, is never quite so good as that which comes directly from outdoors; so it is generally best to keep at least two or three windows in each room opened at the top as well, and never to depend entirely upon the air that comes through the heating system.

Sometimes this may mean a little draft, or current of uncomfortably cool air, for one or two of you who sit nearest the windows; but your teacher will always allow you to change your seat if this proves very unpleasant. If you have plenty of warmth in the room you sit in, unless the air outside is very cold, this "breeze" won't do you any harm at all; on the contrary, it will be good for you. Instead of catching cold from a draft like this, it is from foul, stuffy, poisonous air, loaded with other people's breaths and the germs contained in them, that you catch cold.

[Ill.u.s.tration: GARDENS TAKE US OUT OF DOORS]

In fact, staying indoors is usually the reason why people are sick.

They don't go out into the clean fresh air for fear they'll be too cold! It seems a pity we can't just live out of doors all the time.

Perhaps we shall some day; for doctors are finding out that fresh outdoor air and good food are the very best medicines known, and the only "Sure Cures." They are pleasant to take, too. Many cities are providing outdoor schools for children who have weak lungs or are not strong in other ways. Perhaps some day all school children will be allowed to study in the open air at least part of every school day.

II. HEARING AND LISTENING

Now you are all ready to go to work. What are you going to work with?

Books? pencils? paper? Yes, but you have something better than those and all ready for use. It is that little kit of tools that are sometimes called our "Five Senses." You remember that we have already talked about one of them, the sense of touch in the skin. Now which one are you going to use first this morning? If your teacher talks to you, I hope it will be the one we call the sense of hearing. Suppose we try to find out something about this sense of hearing, and begin with a little experiment.

Take a piece of cork in your hand and lift it up high and then let it drop into a large basin or tub of water. What happens? The cork strikes and then goes bob-bob-bobbing up and down on its own waves.

Now watch the little waves all around the cork. Where do they stop?

They don't stop until they touch the edge of the pan; and no matter how big the pan is, the waves go on and on until they reach the edge.

We can see these waves of water, and so we easily believe that they are there. Now there are, just as truly, waves of air all around us.

We cannot see the waves, because they are too small and roll too quickly. But some of these, when they roll against our ears, make us hear. They make what we call _sound_. You have heard about sending messages through the air, without telegraph wires. Wireless messages are often sent to ships out in the middle of the ocean. This is done by starting tiny electric waves, which travel through the air much as the waves of water are traveling across the ocean beneath. Of course there must be a machine, called a _receiver_, to catch the waves and "hear" the message.

Mother Nature has given each of you two very delicate little receivers to catch the sound waves and carry them to your brain. You know what they are--you can name them. But how are these wonderful little machines made?