英文科学读本 第五册·Lesson 12 Boiling

Lesson 12 Boiling

You explained in one of our lessons, sir, said Fred, "the manner in which heat is carried through certain bodies by conduction, and you told us that this mode of conveying heat is confined to solids, liquids being very bad conductors of heat. But liquids get hot as well as solids. Will you explain to us how the heat travels in these bodies?"

Yes, boys, said Mr. Wilson, "I will try to make this quite clear to you. But in the first place I want you to think of the one great distinction between solid bodies and liquids."

In solids the molecules or particles have a certain fixed position with respect to each other, sir, and that position never alters, but in liquids the molecules are free to move. Cohesion is strong in solids, but weak in liquids.

Quite right, said Mr. Wilson, "and this will help you to remember that, when a solid body is heated by conduction, the heat affects it, molecule by molecule, each one transmitting to the next the heat which it has itself received, and so on. The great thing to keep in mind is that the molecules themselves do not move, or change their position in any way. After this little retrospect into our past work, we shall now be prepared to inquire into the mode by which liquids become heated.

Suppose we begin with a little experiment. We will place a piece of ice in the bottom of this large test-tube, with something forced down upon it to keep it there. This coil of wire will do as well as anything. As soon as I fill the tube with water, you will see the necessity for the coil of wire. The ice is lighter than the water, and would rise to the surface if it were not held down. I am now going to fix the tube obliquely over the flame of the Bunsen burner, in such a position as to heat only the upper part of it. In a short time the water will begin to rise in temperature. The thermometer will prove this, if we place it in the liquid. This rise in temperature continues until the water actually boils. We can see that the water in the upper part of the tube is in a state of ebullition, and steam-vapor is rising from it. If we test it now with the thermometer, that instrument will register 212°F.

But what do we see in the bottom of the tube? The ice remains in it unmelted. That part of the tube, and the water in it, are as cold as they were at first. It would, in fact, take hours before the heat could make any sensible difference in the water below the flame. The heat has readily passed upwards, but it cannot descend. Let us now remove the tube, and place over the flame, in the same oblique position, this strip of metal. As before, we will allow the flame to play only on the upper part of the metal. Your sense of touch will be sufficient to tell you that the lower end is gradually being heated, as well as the upper part, and in a short time even that end would be too hot for you to handle.

How has the heat been carried downwards through the metal?

The heat has been carried by conduction, sir, said Will, "and, now I remember, it does not matter whether the poker slants upwards, or downwards, or rests horizontally in the fire. The heat always travels to the other end."

That's a very thoughtful answer, my lad, said Mr. Wilson. "Conduction acts downwards as well as upwards, horizontally or obliquely as well as vertically. It is quite unaffected by the position of the body through which it acts.

This proves, therefore, that the heat did not travel through the water by conduction. Had the heat been conducted through the water, the lower part of the liquid would have become as hot as the upper—the ice would have quickly melted. We will now replace the tube in the stand, but direct the flame this time on the lower part of it.

Now notice the result. The water is rapidly heated, the ice melts and disappears, and in a few minutes the whole of the liquid in the tube is seen to boil. The water has been uniformly heated, because the heat has travelled upwards from bottom to top. We have boiled the water. We will take an early opportunity of investigating more closely the mode by which heat travels in liquids. We have at present been merely leading up to this interesting subject."