Smoke is not drawn up a chimney, but is pushed up.
You hear people sometimes say of a chimney that itdrawswell, 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-placein 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 thewindow 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.
You can now understand why it is that we open a door or window to stop the smoking of a fire-place. It is because we want the help of some more cold air to push the smoke up. In some fire-places we can never make a fire without its smoking, unless we have a door or a window open a little while at first. The reason that the fire is not apt to smoke after it has been going some time is that the chimney has become well heated, and so makes the air very thin and light as it goes up; and the lighter the air is, you know, the more easily it is pushed up, just as you can raise a bag of feathers more easily than you can raise a block of wood.
Experiments with a light.
One thing more I must tell you about the cold air coming into a room where there is a fire. Suppose that you open a door into a cold entry. Now, if you hold a light near the floor by the open door, the flame will be blown inward; but if you hold it up at the top of the door, it will be blown outward toward the entry. Why is this? It is because the cold air of the entry comes in at the lower part of the opening, while some of the warm air of the room goes out at the upper part to take the place of the cold air that comes in. The warm air is above the cold air, because it is lighter. It is the cold air coming in that blows the light when you hold it low down, and it is the warm air going out that blows it when you hold it up high. The warm air that goes out is less in quantity than the cold air that comes in. The reason is that there is cold air coming into the entry all the time from outdoors, by every crevice and hole, and this, in part, supplies theplace of the air that goes in from the entry to the room. The flame, therefore, is not blown as strongly when you hold the light above as when you hold it below.
I told you in Chapter I. that nothing will burn without air. The air that presses toward a fire feeds it, as it is expressed. It does not all go up the chimney as heated air. Some of it is used in the burning of the wood and coal; and what goes up the chimney is, as I have told you in the first part of this chapter, partly heated air and partly gas.
A free supply of air necessary to make a fire burn well.
Anecdote.
Now a fire will not burn well unless it has a free supply of air. Fresh air must keep coming to it to feed it. But this can not be unless there is a good upward current from the fire. Firemen very well understand this in putting out fires. If the fire be inside of a building, the more shut up it can be kept the less rapidly will the fire spread, and the more easily can it be put out. If all the doors should be opened, and the windows broken out, the fire would rage, because the air would come in freely at the doors and lower windows, and go out freely at the upper windows. The fire would then have the same upward current that it has in a chimney. I will relate to you an anecdote, which will show how much can be saved by understanding such things. A fire was discovered early one morning by a flickering light shining through the windows in the upper room of a shop. An acquaintance of mine was among the first to get there, and he found a man about to beat the door in with an axe, so as to get at the fire. He kept him from doing this, and would not let him touch the door till they had got a good supply of water on hand. After he was satisfiedthat there was enough water to put out the fire, he then let the man use the axe, and they rushed up and easily put out the fire. If he had let him break open the door at first, it would have let in the air to feed the fire, and the fire would have got well agoing before the water was brought; and, as it was in a block of wooden buildings, we should have, had a great fire.
Tall chimneys of factories.
Lamp chimneys.
The brisker the upward current of a fire is, the more briskly does the fire burn. This is the reason that foundries and other factories, where they want a very hot fire, have such tall chimneys. The air and gas in such a chimney are kept hot for some time, instead of being cooled by spreading out in the open air. The current, therefore, up the chimney is very rapid, and so fresh air comes rapidly to the fire, and makes it burn very briskly. For the same reason, a very brilliant light is given by those lamps that have tall glass chimneys. The wick is thus made to burn briskly.
Questions.—Why does smoke go up a chimney? What is smoke? What is there in smoke that you can see? What is soot? Tell how it is that the smoke is pushed up the chimney. What is said about the air in a room where there is a fire? What will happen to a light if you hold it near the fire-place? What if you hold it near a crack in the wall of the room? Tell about the rooms with folding-doors between them. Why do we open a door or a window to stop the smoking of a fire-place? Why is a fire-place not apt to smoke when the fire has been going for some time? Tell about holding a light at the lower part and at the upper part of a door that opens out into a cold entry. How is some of the air that presses toward a fire used? What is necessary to have a fire burn well? What is said about a building that is on fire inside? Tell the anecdote about the fire in a shop. Why do some factories have tall chimneys? What is said about the chimneys of some lamps?
Questions.—Why does smoke go up a chimney? What is smoke? What is there in smoke that you can see? What is soot? Tell how it is that the smoke is pushed up the chimney. What is said about the air in a room where there is a fire? What will happen to a light if you hold it near the fire-place? What if you hold it near a crack in the wall of the room? Tell about the rooms with folding-doors between them. Why do we open a door or a window to stop the smoking of a fire-place? Why is a fire-place not apt to smoke when the fire has been going for some time? Tell about holding a light at the lower part and at the upper part of a door that opens out into a cold entry. How is some of the air that presses toward a fire used? What is necessary to have a fire burn well? What is said about a building that is on fire inside? Tell the anecdote about the fire in a shop. Why do some factories have tall chimneys? What is said about the chimneys of some lamps?