CHAPTER XXIV.
WHAT HEAT DOES.
Heat makes most things larger, or expands them, as it is commonly expressed. I will give you some examples of this.
I have already told you in Chapter XIII. how heat expands air. You remember the two experiments with the bladder before the fire. You remember also what I told you about the expansion of the air in apples and chestnuts by heat.
Experiment showing how air is expanded by heat.
Here is represented another experiment which shows that heat expands air. A glass tube, with a bulb on the end of it, is put with its open end into a tumbler of water. Of course the tube is full of air. Now, on putting the warm hand on the bulb, as represented, the air in it will be warmed. The air, therefore, swells, and there is not room for it all in the tube; and so some of it escapes in bubbles, as you see, through the water.
Snapping of burning wood.
The snapping wood, you know, often throws out sparks. These are parts of the wood partly burned that happen to be right on the spot where the confined heated air was that has broken loose. The more porous wood is, the more apt it is to snap. The solid walnut seldom snaps; but the chestnut, which is very porous, is always snapping. So, too, dry wood snaps more than green, because the sap has dried up, and air has taken its place in the pipes of the wood.
Air expanded by heat, as you have before seen, always rises. It is pushed up, as I have already told you, by the colder air, which is heavier. This keeps the air always moving. It is never still, for heat is always acting upon it. Even when it is so still that we say there is no wind, it is not perfectly still. There is all the time the going up of warm air and the coming down of that which is colder. You can see this in a room if you shut it up so as to make it quite dark, and let only a little light in by partly opening one shutter. Though the air seems to you to be perfectly still, you will see, where this light is let in, little motes flying up and down. This is because there are currents in the air, and these are made by heat.