英文科學讀本 第五冊·Lesson 30 Steam

Lesson 30 Steam

I want you to think once more about the water boiling in the flask over the Bunsen burner, and some of the things it taught us, said Mr. Wilson.

Water, at the ordinary pressure of the atmosphere, boils at 212°F. What do we mean by saying that the water boils?

Water is said to boil, sir, when it is being converted from the liquid into the gaseous state—when it passes off as steam, said Fred.

Right, said Mr. Wilson. "We know that the water itself at this point—the boiling-point—stands at 212°, but what is the temperature of the steam as it flies off?"

The steam too is 212°, sir. Neither the steam nor the water ever exceeds that temperature, although I can't yet understand what becomes of all the heat which the water continues to receive after it has begun to boil.

Ah, my lad, said Mr. Wilson, "I am glad to find you puzzled on this point. It is the most remarkable fact about the process of boiling, and I can see you have been thinking about it. This additional heat is not lost—it is not wasted. It is used up in the work of changing the water into steam. It is in the steam, although we cannot register its presence by the thermometer. It is hidden away, so to speak, in the steam.

We speak of it as latent heat; the word latent means hidden away. It is this latent heat which has overcome the natural force of cohesion in the water, and driven the molecules or particles so far apart that they form now, not a liquid, but a gas. If the steam were passed into a cold chamber, it would be robbed of this latent heat, and the molecules, with nothing to keep them asunder, would rush together by the force of cohesion, and form little round globules of liquid again. The steam would be condensed.

You have seen the steam rushing from the spout of the kettle, or from the funnel of a locomotive. Why should it rush in this way? I will tell you. At the moment of the change, so great is the force of this latent heat that the water expands suddenly to 1700 times its original bulk. That is to say, a cubic foot of water raised to the boiling-point would make 1700 cubic feet of steam, nearly enough to fill a room 12 feet long, 12 wide, and 12 high. It is this great expansive force of steam—or, as we sometimes call it, the elastic force of steam—which makes it so useful as a mode of motion.

Suppose we have a little experiment now. I will half fill this test-tube with water, cork it with a cork that fits it rather loosely, and place it over the flame of the Bunsen burner to boil. Presently, as the water boils, the cork flies out with a sudden pop, and steam may be seen issuing from the tube. Either the cork must be forced out, or the sudden expansion will shatter the tube. The steam will, by its expansive power, force its way out in some direction. There is practically no limit to its expansive force, except the strength of the vessel which holds it. You may form some idea of what this statement means when I tell you that sometimes the strongest iron boilers are shattered into pieces by the expansive force of the steam within them.

I think, continued Mr. Wilson, "you will now be able to grasp the meaning of another very interesting experiment.

I have here a flask with a long neck, the same size throughout; I will pot some water in it. This piston fits the tube, so as to be capable of moving freely up and down in it, but at the same time it is air-tight. We will fix the flask over the flame of the Bunsen burner, and leave the water to boil. Now watch the result. As the water boils, the expansive power of the steam will force the piston up the tube. The very moment this happens I shall remove the lamp with one hand, and with the other squeeze a sponge of cold water over the flask. Note what happens. Immediately the flask is cooled, the piston falls in the tube.

If we repeat this again and again, the result will be the same. The piston will rise as the water boils, and fall when the flask is cooled. Why is this?

The piston rises because the expansive force of the steam is sufficient to overcome the pressure of the atmosphere on its upper surface. But let us see what effect the cooling process has on the steam in the flask. The cooling condenses the steam into drops of water. The water does not occupy so great a space as the steam, and consequently a vacuum is formed.

The pressure of the air from the outside then reasserts itself to restore the balance and so forces the piston down once more.