英文科學(xué)讀本 第五冊·Lesson 48 Heat, the Cause of Motion in the Air

Lesson 48 Heat, the Cause of Motion in the Air

We are going to begin this morning with an experiment, boys, said Mr. Wilson. "I have here a large glass jar. I have divided it into two parts, from the top to within an inch or so of the bottom, by fitting into it a cardboard partition. On one side of this partition I will lower into the jar a piece of lighted candle. I want one of you to come to the front and hold over the other side a piece of smouldering brown paper. Watch what takes place. The smoke from the smouldering paper passes down one side of the glass, then under the partition, and up the other side in which the candle is burning.

What is the meaning of this?

I suppose the heat of the candle flame has made the air on that side of the partition hot, sir, said Fred. "Yes, Fred, you are right; the heat of the burning candle has raised the temperature of the air all round it. But what follows from that?"

Air expands when it is heated, sir, replied Fred. "The heated, expanded air is of course lighter than it was, because the same quantity has to fill a greater space than it did at first."

That's very good, Fred, said Mr. Wilson. "The expanded air is lighter than it was—lighter than the rest of the air near it; lighter than the air below it. It is therefore forced upwards by the buoyancy of that heavier air around and below it, or, as we often say roughly, it ascends.

But as the warm light air rises and passes out at the top of one side, the colder, denser, heavier air from the other side rushes under the partition to take its place, and so prevent a vacuum. We should not have been able to see the moving current of air in the usual way, so we held the smoking paper over the mouth of the jar, and that showed the direction of the movement.

I remember, sir, you showed us this movement of the heated air by another contrivance in one of our lessons on the coal-mine, said Fred.

So I did, my lad. Suppose you tell us all about it now.

You had a small wooden box with two round holes cut in the bottom. The lid of the box was taken off, and the box was stood upside down on the table. You fitted two lamp-glasses into the round holes, and stood a lighted candle under one of them. I remember holding the piece of smouldering brown paper over the top of the other one for you. Our experiment just now brought it all back to my mind. The smoke passed down that glass and up the other, under which the candle was burning. It was the heat of the burning candle that caused the current of moving air.

Thank you, Fred, said Mr. Wilson. "I am very pleased to know that you try to remember what you are taught. Now let us see where all this leads us.

Think of the gas or the lamp burning in a room. Here the action is exactly the same. The heat of the flame raises the temperature of the air around, and this heated, expanded, lighter air rises towards the ceiling, the colder, denser air constantly moving in from all sides to take its place and force it upwards. The hottest air in the room is always near the ceiling, the coldest near the floor. The light hot air in the upper part of the room will do its best to escape, and float away higher and higher outside, if it can find a crack or an opening anywhere. When a room is very warm, pull the window-sash down about half an inch from the top, and hold a lighted candle near the opening. The flame will pass outwards through the crack, showing that there is a rush of air in that direction.

If next you open the door a little way, or hold the candle-flame near the crack at the bottom, the flame will be seen to blow into the room, owing to the inward rush of air there. The fire in the grate acts in the same way, although the heated air passes up the chimney with the smoke and not into the room. There is, however, a constant movement of cooler air through the room towards the fire to take the place of that which has floated away up the chimney."