I.—THE AIR.

When we begin to look attentively at the world around us, one of the first things to set us thinking is the air. We do not see it, and yet it is present wherever we may go. What is this air?

Although invisible, it is yet a real, material substance. When you swing your arm rapidly up and down you feel the air offering a resistance to the hand. The air is something which you can feel, though you can not see it. You breathe it every moment. You can not get away from it, for it completely surrounds the earth. To this outer envelope of air, the name of atmosphere is given.

The air is not a simple substance, but a mixture of two invisible gases, called nitrogen and oxygen. But besides these chief ingredients, it contains also small quantities of other substances; some of which are visible, others invisible. If you close the shutters of a room, and let the sunlight stream through only one chink or hole into the room, you see some of the visible particles of the air. Hundreds of little motes, or specks of dust, cross the beam of light which makes them visible against the surrounding darkness, though they disappear in full daylight. But it is the invisible parts of the air which are of chief importance; and among them there are two which you must especially remember—the vapor of water and carbonic acid gas. You will soon come to see why it is needful for you to distinguish these.

Now what is this vapor of water? You will understand its nature if you watch what takes place when a kettle boils. From the mouth of the spout a stream of white cloud comes out into the air. It is in continual motion; its outer parts somehow or other disappear, but as fast as they do so they are supplied by fresh materials from the kettle. The water in the kettle is all the while growing less, until at last, if you do not replenish it, the whole will be boiled away, and the kettle left quite dry. What has become of all the water? You have changed it into vapor. It is not destroyed or lost in any way, it has only passed from one state into another, from the liquid into the gaseous form, and is now dissolved in the air.

Carbonic acid gas is also one of the invisible substances of the atmosphere, of which, though it forms no more than four parts in every ten thousand, yet it constitutes an important ingredient. You will understand how important it is when you are told that, from this carbonic acid in the air, all the plants which you see growing upon the land extract nearly the whole of their solid substance. When a plant dies and decays, the carbonic acid is restored to the air again. On the other hand, plants are largely eaten by animals, and help to form the framework of their bodies. Animals in breathing give out carbonic acid gas; and when they die, and their bodies decay, the same substance is again restored to the atmosphere. Hence the carbonic acid of the air is used to build up the structure both of plants and animals, and is given back again when these living things cease to live. There is a continual coming and going of this material between the air and the animal and vegetable kingdoms.

You know that though you can not see the air you can feel it when it moves. A light breeze, or a strong gale, can be just as little seen by the eye as still air; and yet we readily feel their motion. But even when the air is still it can make itself sensible in another way, viz: by its temperature. For air, like common visible things, can be warmed and cooled.

This warming and cooling of the air is well illustrated by what takes place in a dwelling-house. If you pass out of a warm room, on a winter’s day, into the open air when there is no wind, you feel a sensation of cold. Whence does this sensation come? Not from anything you can see, for your feet, though resting on the frozen ground, are protected by leather, and do not yet feel the cold. It is the air which is cold, and which encircles you on all sides, and robs you of your heat; while at the same time you are giving off or radiating heat from your skin into the air. On the other hand, if, after standing a while in the chilly winter air, you return into the room again, you feel a sensation of pleasant warmth. Here, again, the feeling does not come from any visible object, but from the invisible air which touches every part of your skin, and is thus robbed of its heat by you.

Now, how is it that the atmosphere should sometimes be warm and sometimes cold? Where does the heat come from? and how does the air take it up?

Let us return again to the illustration of the house. In winter, when the air is keen and frosty outside, it is warm and pleasant indoors, because fires are there kept burning. The burning of coal and wood produces heat, and the heat thus given out warms the air. Hence it is by the giving off or radiation of the heat from some burning substance that the air of our houses is made warmer than the air outside.

Now, it is really by radiation from a heated body that the air outside gets its heat. In summer, this air is sometimes far hotter than is usual in dwelling-houses in winter. All this heat comes from the sun, which is an enormous hot mass, continually sending out heat in all directions.

But, if the sun is always pouring down heat upon the earth, why is the air ever cold? Place a screen between you and a bright fire, and you will immediately feel that some of the heat from the fire place has been cut off. When the sun is shining, expose your hand to its beams for a time, and then hold a book between the hand and the sun. At first, your skin is warmed; but the moment you put it in the shade, it is cooled again. The book has cut off the heat which was passing directly from the sun to your hand. When the atmosphere is felt to be cold, something has come in the way to keep the sun’s heat from directly reaching us.

Clouds cut off the direct heat of the sun. You must often have noticed the change of temperature, when, after the sun has been shining for a time, a cloud comes between it and the earth. Immediately a feeling of chilliness is experienced, which passes off as soon as the cloud has sailed on, and allowed the sun once more to come out.

The air itself absorbs some of the sun’s heat, and the greater the thickness of air through which that heat has to make its way, the more heat will be absorbed. Besides this, the more the rays of heat are slanted the weaker do they become. At noon, for example, the sun stands high in the sky. Its rays are then nearest to the vertical, and have also the least thickness of air to pass through before they reach us. As it descends in the afternoon, its rays get more and more slanted, and must also make their way through a constantly increasing thickness of air. Hence the middle of the day is much warmer than morning or evening.

At night, when the sun no longer shines, its heat does not directly warm the part of the earth in shadow. That part not only receives no heat from it, but even radiates its heat out into the cold sky. Hence night is much colder than day.

Then, again, in summer the sun at noon shines much higher in the sky with us, or more directly overhead, than in winter. Its heat comes down less obliquely and has less depth of air to pass through, and hence is much more felt than in winter, when, as you know, the sun in our part of the world never rises high even at mid-day.

If we were dependent for our warmth upon the direct heat of the sun alone, we should be warm only when the sun shines. A cloudy day would be an extremely cold one, and every night as intensely frosty as it ever is in winter. Yet such is not the case. Cloudy days are often quite warm; while we are all aware that the nights are by no means always very cold. There must be some way in which the sun’s heat is stored up, so that it can be felt even when he is not shining.

In summer the ground gets warmed; in some parts, indeed, becoming even so hot at times that we can hardly keep the hand upon it. In hot countries this is felt much more than in this country. Soil and stones absorb heat steadily, that is to say, soon get heated, and they soon cool again. When they have been warmed by the sun, the air gets warmed by contact with them, and keeps its heat longer than they do; so that even when at night the soil and stones have become ice-cold, the air a little above is not so chilly. On the other hand, when the surface of the ground is cold, it cools the air next it. The ground parts easily with its heat, and a vast amount of heat is in this way radiated at night from the earth outward into the cold starry space. Much more heat, however, would be lost from this cause did not the abundant aqueous vapor of the atmosphere absorb part of it, and act as a kind of screen to retard the radiation. This is the reason why in hot climates, where the air is very dry—that is, contains a small proportion of the vapor of water—the nights are relatively colder than they are in other countries where the air is moister. In like manner, clouds serve to keep heat from escaping; and hence it is that cloudy nights are not so cold as those which are clear and starry.

The atmosphere, then, is heated or cooled according as it lies upon a warm or cold part of the earth’s surface; and, by means of its aqueous vapor, it serves to store up and distribute this heat, keeping the earth from such extremes of climate as would otherwise prevail.

The air lying next to a hot surface is heated; the air touching a cold surface is cooled. And upon such differences of temperature in the air the formation of winds depends.

Hot or warm air is lighter than cold air. You have learned how heat expands bodies. It is this expansion of air, or the separation of its particles further from each other which makes it less dense or heavy than cold air, where the particles lie more closely together. As a consequence of this difference of density, the light warm air rises, and the heavy cold air sinks. You can easily satisfy yourselves of this by experiment. Take a poker, and heat the end of it in the fire until it is red-hot. Withdraw it, and gently bring some small bits of very light paper, or some other light substance, a few inches above the heated surface. The bits of paper will be at once carried up into the air. This happens because the air, heated by the poker, immediately rises, and its place is taken by colder air, which, on getting warmed, likewise ascends. The upward currents of air grow feebler as the iron cools, until, when it is of the same temperature as the air around, they cease.

This is the principle on which our fire-places are constructed. The fire is not kindled on the hearth, for, in that case, it would not get a large enough draft of air underneath, and would be apt to go out. It is placed some way above the floor, and a chimney is put over it. As soon as the fire is lighted, the air next it gets warmed, and begins to mount, and the air in the room is drawn in from below to take the place of that which rises. All the air which lies above the burning coal gets warmer and lighter; it therefore flows up the chimney, carrying with it the smoke and gases. You will understand that though a bright blazing fire is a pleasant sight in winter, we do not get all the heat which it gives out. On the contrary, a great deal of the heat goes up the chimney; and, except in so far as it warms the walls, passes away and warms the outer air.

What happens in a small way in our houses takes place on a far grander scale in nature. As already pointed out, the sun is the great source of heat which warms and lightens our globe. While the heat of the sun is passing through the air, it does very little in the way of warming it. The heat goes through the air, and warms the surface of the earth. You know that in summer the direct rays of the sun are hot enough to burn your face, and yet, if you put even a thin sheet of paper over your head, enough to cut off these rays, the sensation of burning heat at once goes off, although the same air is playing about you all the time.

Both land and water are heated by the sun’s rays, and the same change in the air then takes place which we find also at our firesides. The layer of air next the warmed earth becomes itself warmed. As it thereby grows lighter it ascends, and its place is taken by colder air, which flows in from the neighborhood to take its place. This flowing in of air is wind.

One of the most important ingredients in the air is the vapor of water. Let us try to see, first of all, how it gets into and out of the air. And in this case, as before, you will find that great questions in science often admit of being simply and readily illustrated by the most familiar things.

You may have noticed that on very cold nights the windows of sitting-rooms, or crowded public halls, are apt to be found streaming with water on the inside.

Now, in such cases, where does the moisture come from? Certainly not out of the glass. It is derived from the vapor of water present in the air. This word vapor is often used to describe some kind of visible mist or fog. But these visible forms of moisture are not properly vapor in the sense in which the term is used in science. The aqueous vapor of the air is always invisible, even when the air is saturated with it, and only when it passes back into the state of water do you actually see anything.

When the invisible vapor dissolved in the air becomes visible, as in mists, clouds, dew, or rain, it is said to be condensed, and this process of liquefaction is called condensation.

The quantity of vapor which the air can contain varies according to temperature, warm air being able to hold more than cold air.

As the air is cooled, its power of retaining vapor diminishes. When it becomes colder than the temperature at which it is able to keep its supply of vapor dissolved, the excess of vapor is condensed and becomes visible. The temperature at which this takes place is the point of saturation, or dew-point.

Perhaps you may ask how it is that the vapor so universally present gets into the atmosphere, and where it comes from. If you pour a little water into a plate, and set it down in the open air, you will note in the course of a day or two, that the water has sensibly diminished. The air has drunk up part of it, and will drink up the whole, if the water is allowed to stand long enough. What takes place from a small quantity of water goes on from every surface of water on the face of the earth, from every brook and river and lake, and from the great sea itself. Water is constantly passing off into vapor, which is received and retained by the air. This process is called evaporation, and the water which passes off into vapor is said to evaporate.

Since warm air can hold more vapor than cold air, evaporation must be more vigorous in sunshine than at night, and during summer than during winter.

On a dry, bracing day, evaporation goes on rapidly, because the air has not nearly got all the quantity of vapor it can hold in solution. On a damp day, however, when the air contains about as much vapor as it can hold at that particular temperature, evaporation is quite feeble, or ceases altogether. This varying capacity of the air for vapor is the reason why laundresses find so much difference between days, in the ease with which they can have their clothes dried.

After sunset, when the sky is clear, you know that the grass gets wet with dew. In the morning you may see mists hanging over woods, and streams, and hills, and gradually melting away as the sun mounts in the sky. At all times of the year you may watch how clouds form and dissolve, and form again, ever changing their size and shape as they move through the air. Now these are all examples of the condensation of vapor. Let us see how the process takes place.

Condensation, as we have seen, results from a cooling of the air. When vapor is condensed, it does not at once take the form of running water. The cold glass brought into the warm room has first a fine film of mist formed upon it, and then by degrees the clear drops of water come. In reality mist is made up of exceedingly minute particles of water, and it is the running together of these which makes the larger drops. So in nature on the great scale, when condensation occurs the vapor first appears as a fine mist. This is always the result of cooling; so that, whenever you see a mist or cloud forming, you may conclude that the air in which it lies is being cooled.

Dew is the name given to the wetness which we notice appearing in the evening, or at night, upon grass, leaves, or stones, or even sometimes on our hair. In the morning you have, no doubt, often watched the little dewdrops sparkling upon the foliage and the delicate threads of gossamer. Now this wetness does not come out of the leaves or stones, nor out of your hair. It is all derived from the air by condensation, exactly as we see the film of mist form upon the cold tumbler in the warm moist air of a room. In fact, that film of mist was really dew, and all dew is formed in the same way, and from the same cause.

At night, when the sky is clear, the earth radiates heat rapidly; that is to say, it gives off into cold space a great part of the heat which it has received from the sun during the day. Its surface consequently becomes cold, as you may have felt when you put your hand upon leaves or stones after nightfall. The layer of air next the cooled ground is chilled below its point of condensation, and the excess of vapor is deposited as dew upon the grass, twigs, stones, and other objects. Hence it is that the temperature at which this condensation begins to take place is called the dew-point.

Another way in which a cold surface of the earth may produce condensation is shown by what takes place among mountains. When a warm moist wind blows upon a chill mountain top, the air is cooled, and its vapor becomes visible in the form of a mist or cloud. You can often see that the cloud is quite solitary, and even shapes itself to the form of the ground, as if it were a sort of fleecy cap drawn down over the mountain’s head. This is often well marked in the morning. As day advances, the ground, warmed by the sun, no longer cools the air, and hence the mist is gradually re-absorbed into the atmosphere. But by and by, at the coming on of night, when the ground is once more cooled by radiation, if there should be vapor enough in the air, the mist will re-form, and the mountain put on his cap again.

Cold air, as well as cold ground, condenses the vapor of warmer air. If you watch what goes on along the course of a river, you will often see examples of this kind of condensation. The ground on either side of the river parts with its heat after sundown sooner than the river itself does, and consequently cools the air above it more than the air above the river is cooled. So when this colder air from either side moves over to take the place of the warmer damp air lying on and rising from the river, condensation ensues in the form of the mist or river-fog, which so commonly hangs at night and early morning over streams.

A cloud is merely a mist formed by the cooling of warm moist air, when it loses its heat from any cause, such as expansion during ascent, or contact with currents of cooler air. If you watch what goes on in the sky, you may often see clouds in the act of forming. At first a little flake of white appears. By degrees this grows larger, and other cloudlets arise and flock together, until at last the sky is quite overcast with heavy clouds, and rain begins to fall. The vapor which is thus condensed in the air has all been obtained by the evaporation of the water on the earth’s surface. It rises with the warm air, which losing its heat as it ascends, and coming too in contact with colder layers of the atmosphere, can not hold all its vapor, and is obliged to get rid of the excess, which then condenses into cloud.

On a summer morning the sky is often free from cloud. As the day advances, and the earth gets warmed, more vapor is raised; and as this vapor, borne upward by the ascending air-currents, reaches the higher and colder parts of the atmosphere, it is chilled into the white fleecy clouds which you see forming about mid-day and in the afternoon. Toward evening, when less evaporation takes place, the clouds cease to grow, and gradually lessen in size until at night the sky is quite clear. They have been dissolved again by descending and coming in contact with the warm air nearest to the earth. Again, you have often noticed that clouds move across the sky. They are driven along by upper currents of air, and of course the stronger these currents are the faster do the clouds travel. In this way the sky is sometimes completely overcast with clouds which have come from a distance.

You are well aware that rain always comes from clouds in the sky. When the sky is clear overhead, no rain falls. Only when it gets overcast does the rain come. You can watch a dark rain-cloud gather itself together and discharge a heavy shower upon the earth. When a cold glass is brought into a warm room, you will remember that the film of mist formed upon the glass is found by degrees to gather into drops, and trickles down the cold surface. Now the mist on the glass and the cloud in the sky are both formed of minute particles of water, separated by air. It is the running together of these particles which gives rise to these drops. In the one case, the drops run down the cold glass. In the other case, they fall as drops of rain through the air. Rain, therefore, is thus a further stage in the condensation of the aqueous vapor of the atmosphere. The minute particles of the cloud, as condensation proceeds, gather more moisture round them, until at last they form drops of water, too heavy to hang any longer suspended in the air. These then fall to the earth as rain-drops.

But there is another important form in which the moisture of the clouds may descend to the surface of the earth. When the weather is cold enough, there fall to the ground not drops of rain, but flakes of snow.

If you bring snow indoors, it soon melts into water. If you expose this water for a time it evaporates. Snow, water, and aqueous vapor are thus only different forms of the same substance. We say that water can exist in three forms—the gaseous, the liquid, and the solid. Snow is an example of the solid condition.

On a frosty night pools of water are covered with a hard, transparent crust, of what is called ice. You may break this crust into pieces, but if the cold continues, a new crust will soon be formed with bits of the old one firmly cemented in it. And the greater the cold the thicker will the crust be, until perhaps the whole of the water in the pools may become solid. If you take a piece of this solid substance, you find it to be cold, brittle, and transparent. Brought into a warm room it soon melts into water, and you may drive off the water as before into vapor. Ice is the general name given to water when it is in the solid state, such forms as snow and hail being only different appearances which ice puts on. Whenever water becomes colder than a certain temperature it passes into ice, or freezes, and this temperature is consequently known as the freezing-point.

The upper layers of the atmosphere are much colder than the freezing-point of water. In the condensation which takes place there, the clouds do not resolve themselves into rain. The vapor of the up-streaming currents of warm air from the earth’s surface is condensed and frozen in these high regions, and passes into little crystals, which unite into flakes of snow. Even in summer the fine white cloudlets which you see floating at great heights are probably formed of snow. But in those countries, such as ours, where in winter the air even at the surface is sometimes very cold, the snow falls to the ground, and lies there as a white covering, until returning warmth melts it away.

Besides rain and snow, the moisture of the air takes sometimes the form of hail, which consists of little lumps of ice like frozen rain; and of sleet, which is partially melted snow. But rain and snow are the most important, and it is these two forms which we must follow a little further.

Before doing so, let us gather together the sum of what has been said about the aqueous vapor of the air. We have learned that, as every sheet of water on the face of the globe evaporates, the air is full of vapor; that this vapor is condensed into visible form, and appears as dew, mist, and cloud. We have learned further, that the vapor of which clouds are formed is resolved into rain and snow, and, in one or other of these forms, descends to the earth again. There is thus a circulation of water between the solid earth beneath and the air above. This circulation is as essential to the earth in making it a fit habitation for living things, as the circulation of blood is in keeping our bodies alive. It mixes and washes the air, clearing away impurities, such as those which rise from the chimneys of a town. It moistens and quickens the soil, which it renders capable of supporting vegetation. It supplies springs, brooks, and rivers. In short, it is the very mainspring of all the life of the globe. So important a part of the machinery of the world deserves our careful consideration. Let us next attend, therefore, to what becomes of the rain and the snow after they have been discharged from the air upon the surface of the earth.