Balloons are sometimes, as I have told you, filled with heated air. This heated air is lighter than the cool air around it, and so the balloon rises, or, rather, is pushed up. Now observe why the heated air is lighter than the cool air. It is because the heat swells the air, orexpandsit, as it is commonly expressed. The heat, in expanding it, makes it thinner, and of course it is lighter.
Experiment with a bladder.
You can see by a little experiment that heat swells or expands air. Lay a bladder, partly filled with air, before the fire. The heat will fill out the bladder, making it plump and hard, for it will expand the air that is in it; and if the bladder is already filled with air before you lay it down on the hearth, the swelling air will burst the bladder.
Roasting apples.
You remember that I told you about putting an apple under the jar of an air-pump. If the apple is shriveled, the moment that you begin to pump the air from around it the apple begins to swell out, because the air in it swells or expands. In this experiment the air in the apple expands because the pressure of the air around it is lessened by its becoming thinner. Now the air in the apple can be made to expand in another way—by applying heat. If you observe an apple put down to the fire for roasting, you see that it swells. If it happens to be rather wilted, the swelling of it will be very manifest; it will become as plump asit would in the air-pump when the air is pumped out. This is because the air in it is expanded by the heat. And when it sputters, it is the expanded air that throws out some of the juice through the broken skin.
Popping of roasting chestnuts.
Why pricking them prevents it.
You know that, if you roast chestnuts, they pop open with quite a noise, and sometimes fly half across the room. This is owing to the expansion of the air in the chestnut by the heat. This air is shut up in the tight skin of the chestnut; and when it is considerably swelled by the heat, it makes the skin give way all at once, and so produces the popping noise. This is because of the springiness or elasticity of the air. That I have explained before. If you prick a hole in the skin of the chestnut before you put it down to the fire, there will be no popping, for the air will gradually escape from this hole as fast as it is expanded. This hole is to the chestnut what the safety-valve is to a steam-engine. The engine will not burst while the steam can go out by the valve, and so the chestnut, with a hole for the air to get out, does not burst. In the case of both the apple and the chestnut, there is steam mixed with the air. The steam comes from the moisture in the apple and the chestnut, and this has the same springiness that air has, and so helps to produce the effect. I shall tell you about steam in another chapter.
Heated air always rises, for the same reason that a light gas rises. It is pushed up by the cold air, which is heavier. In warming a room, the cold air is constantly pushing the warmed air up, and the air is always warmer in the upper part of the room than it is near the floor. So, also, it is warmer in the galleries ofa church than it is in the body of the house, as you perhaps have sometimes noticed.
Paper wind-mills on a stove-pipe.
The toy wood-sawyer.
Around a stove-pipe, the motion of the heated air as it goes up is very manifest. Light things are often seen flying up in the current of the air about the pipe. Sometimes, for amusement, little paper wind-mills are fastened to a stove-pipe, the heated air whirling them around as it strikes them in going up. I have seen a very curious toy, in which a wood-sawyer is made to work by the whirling of a little paper wind-mill. Whenever there is a strong current of hot air, the wind-mill turns quite rapidly, and this sets the sawyer to working his paper saw most furiously. The little figure goes through the motions of sawing very perfectly. It has sawed into the middle of the log, but never gets any farther.
The stream or current of air about a stove-pipe is made by the cooler air, which pushes up that which is warm. As fast as the air is heated by the pipe, cooler air takes its place by pushing it up out of the way; and then this air, coming thus near the pipe, gets heated, and is pushed up in its turn by some more air. As this is constantly going on, there is a constant upward current of air; and the hotter the pipe is, the more rapid is the current, because it heats the air so quickly and so much.
Why heated air goes up.
You know, in a house heated by a furnace, how the heated air comes up from the registers. This air is pushed up. As soon as the air around the furnace is heated, cool air comes in to push it up out of the way, and then this cool air is heated and is pushed up by more cool air, and so on. The heated air escapes fromthe pressure of the cool air by going up in the large tin pipes. The cool air is always driving the warm up, just as it is with the air about a stove-pipe.
Why a great fire makes the wind rise.
Whenever a great fire occurs, after it has continued some time, the wind rises, as it is expressed; though perhaps it blew very gently at first, now it blows very hard. What is the reason of this? It is because the air just about the fire becomes much heated, and therefore very light. The cold air all around rushes therefore toward the fire, just as it does toward a stove or a fire-place in a room, and pushes the light heated air up. In doing this it becomes itself heated, and is pushed up by other cold air, and so on. In this way the air all around the fire is set in motion toward it, and the hotter the fire the more brisk is this motion—that is, the harder does the wind blow. I shall tell you something about the way in which heat makes winds in another chapter.
Questions.—Why is heated air lighter than cool air? What experiment shows that heat expands air? Tell about the shriveled apple. Why do chestnuts often pop open when they are roasted? How can you prevent their popping? Give the comparison of the safety-valve. In warming a room, what is done to the heated air? What is said about the galleries of a church? What is said about the air around a stove-pipe? Tell about the paper wind-mills and the wood-sawyer. How is the current of air about a stove-pipe made? What makes the hot air come up from the registers of a furnace? Why does the wind rise in a great fire?
Questions.—Why is heated air lighter than cool air? What experiment shows that heat expands air? Tell about the shriveled apple. Why do chestnuts often pop open when they are roasted? How can you prevent their popping? Give the comparison of the safety-valve. In warming a room, what is done to the heated air? What is said about the galleries of a church? What is said about the air around a stove-pipe? Tell about the paper wind-mills and the wood-sawyer. How is the current of air about a stove-pipe made? What makes the hot air come up from the registers of a furnace? Why does the wind rise in a great fire?