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?
CHAPTER XIV.
CHIMNEYS.
Smoke is not drawn up a chimney, but is pushed up.
You hear people sometimes say of a chimney that it draws well, as if the smoke were in some way drawn up the chimney. This is not so. It is pushed up. Smoke is mostly heated air and gas. What you see in the smoke is something from the wood that is carried up in the heated air, in the same way that down or any light thing is carried up by the heated air around a stove-pipe. It is this part of the smoke which you can see that makes the soot. The heated air is pushed up the chimney by the cooler air in the room. It is done in this way: The air close to the fire is heated; the air next to it presses it up, and then gets heated itself, and is pressed up by some more air that comes in its turn to be heated, and so on. In this way there is a constant stream of air up the chimney, just as there is around a stove-pipe.
The air in a room where there is a fire is ever pushing toward the fire; and air is coming into the room, too, in every way that it can get in, to take the place of that which goes up the chimney. It comes through the door when it is opened, and through every crack and crevice. If you hold a light near the fire-place, the flame will bend toward it, because the air is pressing that way. If you hold it near a crack, the air that is coming in will blow it toward you.
A lady in trouble from a smoking fire-place.
If there are two rooms connected by folding-doors, with a fire-place in each, when a fire is made in one alone, cold air will come down the other chimney; for, as the air in the room, as I have told you, is all moving toward the fire, the cold air comes in wherever it can get in to take its place. A lady of my acquaintance was once in great trouble because she did not understand this. Her house was filled with smoke. It happened in this way. There were two rooms connected by folding-doors. A fire had been built in one fire-place, and, after this was well agoing, a fire was built in the other; but the moment this second fire was lighted, the smoke puffed out into the room. How was this? It was pushed out by the cold air coming down the chimney. The lady sent for a neighbor who understood about such things, and he relieved her of the trouble at once. He shut the folding-doors, and opened a window in the room where the fire-place smoked, and now the smoke went directly up the chimney. After the fire had been burning for a little time, and had warmed the chimney, the folding-doors were opened, and both fires burned well.
The reason of all this, I suppose, is plain to you. While the folding-doors were open, there was a movement of the air in both rooms toward the fire first kindled, and so the cold air came down the chimney where there was no fire. When the fire, therefore, was kindled in the second fire-place, this cold air, coming down, blew the smoke out, and would not let it go up to warm the chimney. But when the doors were closed between the rooms, there was a stop put to all this. The movement of the air toward the fire first made was now confined to that one room. There could no air come from the other room now. And then opening the window let in cold air that pushed the smoke up the chimney of this room at once.
Why opening a door stops the smoking of a fire-place.