When next the boys assembled for their lesson, Mr. Wilson began by calling upon them to describe the mode in which heat passes through solids and liquids respectively.
"I want you to think once more," said he, "of the poker in the fire, and tell me exactly by what means the handle becomes heated, although it is some distance from the fire.""The heat travels through the poker, and through all solid bodies, by conduction, sir," said Fred. "One molecule of matter receives the heat, and passes it on to the next.""Right. Now tell me what takes place when a liquid is heated?""In liquids the heat is carried, or conveyed, by the upward flow of the heated molecules themselves, which are not fixed and stationary as they are in solids. We call this convection.""Exactly," replied Mr. Wilson, "and now that we are sure of our ground as to solids and liquids, we will turn our attention to the passage of heat through the air, which, you know, is a gas.
"One of you shall come to the front, and stand before the fire. You at once feel a sensation of warmth. The heat, of course, comes from the fire. We must find out how it travels through the air to your body.
"First, does it travel through the air by conduction? We will see. While you stand in the same position, I will hold this drawing-board between you and the fire. What do you observe?""I no longer feel any heat from the fire, sir.""No, you get no heat now. But if the fire sent out its heat through the air by conduction, all the particles of the air would be heated in succession, and you would still feel the heat, in spite of the board.
"If we stood in the open air exposed to the most brilliant summer sun, we should experience exactly the same thing. Immediately the screen was put between us and the sun, the sensation of heat would disappear.
"Hence we learn that, although heat does pass through the air from one body to another, it does not travel by conduction, nor does it heat, to any extent, the air through which it passes. Air is a non-conductor of heat.
"Heat, as you know, passes from the sun to the earth,but balloonists experience severe cold in the higher regions of the atmosphere, although they are then so much nearer the sun. The sun"s heat travels through the atmosphere without raising its temperature to any extent. "Lower down, at the surface of the earth, the air ismore or less warmed by the heat which is given out from the warm earth. We shall inquire further into this presently; we are now concerned only with the passage of heat through the air.
"Suppose we have an experiment. I have got something in the fire; it is a large iron ball. I daresay it is red-hot by this time. I want a dozen of you to come to the front, and stand in a circle in the body of the room. Now I willtake the ball out of the fire, and hang it by its chain in the middle of the circle. Of course, each of you will tell me that he feels the glow of heat from the red-hot ball. Now sit on the floor below the ball, and you will still feel the heat, and so you would if I could place you above it.
"You may take your places again; I have something else to show you now. Here is a large ball of wool, and you can see that I have stuck it full of pins, all pointing towards its center.
"I want you to imagine that, from every part of the surface of the red-hot ball, straight lines of heat are sent out through the air, much in the sameway as the pins appear in the surface of the ball of wool. This is the universal way in which heat travels through the air from one body to another. We callthese straight lines rays of heat. The drawing- board screen proved that the rays travel only in straight lines, for when it was placed in front of the heated body, it intercepted the rays, and no heat was felt. Heat, then, passes through the air in straight lines, which we call raysof heat, and we say that it travels by radiation. The body itself which sends out the heat we call a radiator.
"Just one more thought before we leave the subject. You stood in front of the fire just now and became warm; we naturally stand before the fire to warm ourselves. We are warmed by taking in the heat which is radiated from the fire. We might stand there too long, or get too close to the fire, and we should be glad to move away. We should be getting too hot; we should burn ourselves if we remained there.
"Other bodies placed in front of the fire would be warmed too by taking in this radiated heat. We say they absorb the heat, and we call the bodies themselves absorbers of heat.
"So then we see that one body radiates heat, andanother absorbs it, and the double process would go on till both bodies were at the same temperature.
"I have a pretty experiment to prove this, but as it would take up too much time during the lesson, I will show it to you after school. I shall simply hang up our red-hot ball again, and place half a dozen thermometers in a circle round it. You will have nothing to do but watch whathappens. You will see that, as the ball cools by radiating its heat, the mercury in each of the thermometers will rise by absorbing it, and after a time the ball and the thermometers will show the same temperature."
