We do not get our heat from the sun by conduction; we cannot, because there is nothing between us and the sun to conduct it. The earth's air, in amounts thick enough to count, goes up only a hundred miles or so. It is really just a thin sort of blanket surrounding the earth. The sun is 93,000,000 miles away. Between us and the sun there is empty space. There are no molecules to speak of in that whole vast distance. So if heat traveled only by conduction,—that is, if radiation stopped,—we should be so completely shut off from the sun that we should not know there was such a thing.
But even if we filled the space between us and the sun with copper or silver, which are about the best conductors of heat in the world, it would take the heat from the sun years and years to be conducted down to us. Yet we know that the sun's heat really gets to us in a few minutes. This is because heat can travel in a very much quicker way than by conduction. It radiates through space, just as light does. And it can come the whole 93,000,000 miles from the sun in about 8 minutes. This is so fast that if it were going around the world instead of coming from the sun, it would go around 7-1/2 times before you could say "Jack Robinson,"—really, because it takes you at least one second to say "Jack Robinson."
We are not absolutely sure how heat gets here so fast. But what most scientists think nowadays is that there is a sort of invisible rigid stuff, not made of molecules or of anything but just itself, called ether. (This ether, if there really is such a thing, is not related at all to the ether that doctors use in putting people to sleep. It just happens to have the same name.) The ether is supposed to fill all space, even the tiny spaces between molecules. The fast moving particles of the sun joggle the ether up there, and make ripples that spread out swiftly all through space. When those ripples strike our earth, they make the molecules of earth joggle, and that is heat. The ripples that spread out from the sun are called ether waves.
But the important and practical fact to know is that there is a kind of heat, called radiant heat, that can pass through empty space with lightning-like quickness. And when this radiant heat strikes things, it is partly absorbed and changed to the usual kind of heat.
This radiant heat is closely related to light. As a matter of fact, light is only the special kind of ether waves that affect our eyes. Radiant heat is invisible. The ether waves that are visible we call light. In terms of ether waves, the only difference between light and radiant heat is that the ripples in light are shorter. So it is no wonder that when we get a piece of iron hot enough, it begins to give off light; and we say it is red hot. What happens to the ether is this: As the molecules of iron go faster and faster (that is, as the iron gets hotter and hotter), they make the ripples in the ether move more frequently until they get short enough to be light instead of radiant heat. Objects give off radiant heat without showing it at all; the warmth that you feel just below a hot flatiron is mainly radiant heat.
When anything becomes hot enough to glow, we say it is incandescent. That is why electric lamps are called incandescent lamps. The fine wires—called the filament—in the lamp get so hot when the electricity flows through them that they glow or become incandescent, throwing off light and radiant heat.
It is the absorbing of the radiant heat by your hand that makes you feel the heat the instant you turn an electric lamp on. Try this experiment:
Experiment 42. Turn on an incandescent lamp that is cold. Feel it with your hand a second, then turn it off at once. Is the glass hot? (The lamp you use should be an ordinary 25, 40, or 60 watt vacuum lamp.)
The radiant heat from the incandescent filament in the lamp passed right out through the vacuum of the lamp, and much of it went on through the glass to your hand. You already know what a poor conductor of heat glass is; yet it lets a great deal of radiant heat pass through it, just as it does light. As soon as the lamp stops glowing, the heat stops coming; the glass is not made hot and you no longer feel any heat. In one way the electric filament shining through a vacuum is exactly like the sun shining through empty space: the heat from both comes to us by radiation.