英文科學(xué)讀本 第五冊(cè)·Lesson 40 Radiators and Absorbers

Lesson 40 Radiators and Absorbers

I think, said Mr. Wilson, "you clearly understand what is meant by a radiator."

Yes, sir, replied Will. "A radiator is a heated body, which cools by giving off its heat in rays."

That's right, my lad. I have a great deal more to say on this subject, and our best course will be to commence the new lesson with a simple experiment. I have here a couple of meat tins; one, you see, is bright inside and out, the other I have coated with lamp-black on the outside. Some few minutes back I filled both tins with boiling water from the kettle. We are now going to examine them. I will plunge a thermometer into each. Now, in the first place, I filled both vessels, as I told you, with boiling water from the same kettle, and therefore the water in both was then at the same temperature. Let us see what the thermometers tell us now. The water in the black-coated vessel is much cooler than that in the bright one. It has, in fact, cooled nearly twice as fast."

How have these vessels cooled?

They have both cooled by radiation, sir, answered Fred. "The heat has passed through the air in rays, and I suppose the radiation has been going on more rapidly in the blackened vessel than in the bright one."

Yes, Fred, you are right, replied Mr. Wilson, "and the same thing would have happened if, instead of coating the vessel with lamp-black, I had covered it with dark-colored linen, or brown paper. Dark-colored bodies, and those with rough surfaces, are the best radiators; white and polished surfaces are bad radiators.

Before the lesson began I stood two other tins, similar to these, in front of the fire. Of course all this time the fire has been radiating heat, and the two vessels have been absorbing it. We will now take them up and examine them. We find that the black one is hotter than the bright one; find so it would have been if, instead of the black coating, it had been covered with brown paper, or some dark-colored substance, such as the linen we just mentioned. Hence we learn that dark-colored bodies and those with rough surfaces absorb heat more quickly than white and polished surfaces, like that of the tin. So, then, the best radiators are the best absorbers of heat, and bad radiators are bad absorbers.

But we have still to learn, continued Mr. Wilson. "what has become of the heat in the one case, for they both received the same amount of heat from the fire, and yet the bright tin is not so hot now as the black one.

The black vessel absorbed the rays of heat; the polished one would not absorb them, but sent them back. We say the rays were reflected. That is to say, bad absorbers send back, or reflect, the rays of heat, instead of taking them in. The knowledge of scientific facts is of little use, unless it is turned to account in some way. You will readily see now why all metal utensils, such as kettles, urns, tea and coffee pots, as well as metal dish-covers for the table, should be kept bright; and also why the engineer is careful to keep the steam-pipes of his engines well polished."

Yes, sir, said Fred, "the bright polished surfaces are bad radiators; they will not allow the heat to pass away."

Have you ever noticed, asked Mr. Wilson again, "that the polished fire-irons in front of the fire are often scarcely warm when the black fender is very hot? Water will boil much sooner in a kettle covered with soot than in a brightly polished one. All cooking vessels should, therefore, be black and rough on the outside in order that they may absorb heat rapidly.

Light-colored clothing is most suitable for summer wear. In tropical countries people almost universally dress in white garments. These, being bad absorbers, throw back, or reflect, the sun's rays, which dark clothing would rapidly absorb. On the same principle the slated roofs of buildings may be white-washed in summer to keep the place cool.

The gardener trains his best grape-vines and gets his earliest ripe grapes on rough dark walls. Can you think out the reason for this, Fred?

I suppose, sir, replied Fred, "the wall, being a good absorber, and also a good radiator of heat, takes in the sun's rays rapidly, and as rapidly radiates them back on the fruit, and so ripens it quickly."

That's a very thoughtful answer, said Mr. Wilson.

Now let us, lastly, think of the snow and winter-time. Snow neither absorbs nor radiates heat. It protects the plants in the ground by preventing the heat from passing away by radiation, while it melts very slowly in a thaw owing to its low absorbing power.