The Chautauquan, Vol. 04, October 1883

by Chautauqua Literary and Scientific Circle; Chautauqua Institution

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.