Common Science Part 17

170. Candies like fudge and nougat become hard and dry when left standing several days open to the air.

SECTION 20. _Conduction of heat and convection._

Why does a feather comforter keep you so warm?

When you heat one end of a nail, how does the heat get through to the other end?

How does a stove make the whole room warm?

Here is a way to make heat run a race. See whether the heat that goes through an iron rod will beat the heat that goes through a gla.s.s rod, or the other way round:

[Ill.u.s.tration: FIG. 56. The metal b.a.l.l.s are fastened to the iron and gla.s.s rods with drops of wax.]

EXPERIMENT 41. Take a solid gla.s.s rod and a solid iron rod, each about a quarter inch in diameter and about 6 inches long.

With sealing wax or candle grease stick three ball bearings or pieces of lead, all the same size, to each rod, about an inch apart, beginning 2 inches from the end. Hold the rods side by side with their ends in a flame, and watch the b.a.l.l.s fall off as the heat comes along through the rods. The heat that first melts off the b.a.l.l.s beats.

[Ill.u.s.tration: FIG. 57. Does the heat travel faster through the iron or through the gla.s.s?]

What really happens down among the molecules when the heat travels along the rods is that the molecules near the flame are made to move more quickly; they joggle their neighbors and make them move faster; these joggle the ones next to them, and so on down the line. Heat that travels through things in this way is called _conducted_ heat.

Anything like iron, that lets the heat travel through it quickly, is called a _good conductor_ of heat. Anything like gla.s.s, that allows the heat to travel through it only with difficulty, is called a _poor conductor_ of heat, or an _insulator_ of heat.

A silver spoon used for stirring anything that is cooking gets so hot all the way up the handle that you can hardly hold it, while the handle of a wooden spoon never gets hot. Pancake turners usually have wooden handles. Metals are good conductors of heat; wood is a poor conductor.

An even more obvious example of the conducting of heat is seen in a stove lid; your fire is under it, yet the top gets so hot that you can cook on it.

When anything feels hot to the touch, it is because heat is being conducted to and through your skin to the sensitive little nerve ends just inside. But when anything feels cold, it is because heat is being conducted away from your skin into the cold object.

AIR CARRIES HEAT BY CONVECTION. One of the poorest conductors of heat is air; that is, one particle of air can hardly give any of its heat to the next particle. But particles of air move around very easily and carry their heat with them; and they can give the heat they carry with them to any solid thing they b.u.mp into. So when air can move around, the part that is next to the stove, for instance, becomes hot; this hot air is pushed up and away by cold air, and carries its heat with it. When it comes over to you in another part of the room, some of its heat is conducted to your body. When air currents--or water currents, which work the same way--carry heat from one place to another like this, we say that the heat has traveled by _convection_.

[Ill.u.s.tration: FIG. 58. Convection currents carrying the heat of the stove about the room.]

Since heat is so often carried to us by convection,--by warm winds, warm air from the stove, warm ocean currents, etc.,--it _seems_ as if air must be a good conductor of heat. But if you shut the air up into many tiny compartments, as a bird's feathers do, or as the hair on an animal's back does, so that it cannot circulate, the pa.s.sage of heat is almost completely stopped. When you use a towel or napkin to lift something hot, it is not so much the fibers of cotton which keep the heat from your hand; it is princ.i.p.ally the very small pockets of air between the threads and even between the fibers of the threads.

COLD THE ABSENCE OF HEAT. Cold is merely the absence of heat; so if you keep the heat from escaping from anything warm, it cannot become cold; while if you keep the heat from reaching a cold thing it cannot become warm. A blanket is just as good for keeping ice from melting, by shutting the heat out, as it is for keeping you warm, by holding heat in.

_APPLICATION 32._ Explain why ice is packed in straw or sawdust; why a sweater keeps you warm.

Select from the following list the good conductors of heat from the poor conductors (insulators): gla.s.s, silver, iron, wood, straw, excelsior, copper, asbestos, steel, nickel, cloth, leather.

[Ill.u.s.tration: FIG. 59. Diagram of a hot-water heater. What makes the water circulate?]

INFERENCE EXERCISE

Explain the following:

171. If the axle of a wheel is not greased, it swells until it sticks fast in the hub; this is a hot box.

172. When you have put liquid shoe polish on your shoes, your feet become cold as it dries.

173. The part of an ice-cream freezer which holds the cream is usually made of metal, while that which goes outside and contains the ice and salt is usually made of wood.

174. The steam in a steam radiator rises from a boiler in the bas.e.m.e.nt to the upper floors.

175. When you throw a ball, it keeps going for a while after it leaves your hand.

176. Clothes keep you warm, especially woolen clothes.

177. The Leaning Tower of Pisa does not fall over.

178. It is almost impossible to climb a greased pole.

179. Heat goes up a poker that is held in a fire.

180. A child can make a bicycle go rapidly without making his feet go any faster than if he were walking.

CHAPTER FIVE

RADIANT HEAT AND LIGHT

SECTION 21. _How heat gets here from the sun; why things glow when they become very hot._

If we were to go back to our imaginary switchboard we should find a switch, between the heat and the light switches, labeled RADIATION.

Suppose we turn it off:

Instantly the whole world becomes pitch dark; so does the sky. We cannot see the sun or a star; no electric lights s.h.i.+ne; and although we can "light" a match, it gives no light. The air above the burning match is hot, and we can burn our fingers in the invisible flame, but we can see nothing whatever.

Yet the world does not get cold. If we leave the switch off for years, while the earth remains in darkness and we all live like blind people, it never gets cold. Winter and summer are alike, day and night are just the same. Gradually, after many ages, the ice and snow in the north and in the far south begin to melt as the warmth from the rest of the world is conducted to the polar regions. And the heat from the interior of the earth makes all the parts of the earth's surface warmer. Winds almost stop blowing. Ocean currents stop flowing. The land receives less rainfall, until finally everything turns to a desert; almost the only rain is on the ocean. Animals die even before the rivers dry up, for the flesh eaters are not able to see their prey, and since, without light, all green things die, the animals that live on plants soon starve. Men have to learn to live on mushrooms, which grow in the dark. The world is plunged into an eternal warm, pitch-black night.

[Ill.u.s.tration: FIG. 60. It is by radiation that we get all our heat and light from the sun.]

Turning off the radiation would cause all these things to happen, because it is by radiation that we get all our heat from the sun and all our light from any source. And it is by radiation that the earth loses heat into s.p.a.ce in the night and loses still more heat into s.p.a.ce during the winter.

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 s.p.a.ce. 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 s.p.a.ce 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 s.p.a.ce, 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 s.p.a.ce, even the tiny s.p.a.ces between molecules. The fast moving particles of the sun joggle the ether up there, and make ripples that spread out swiftly all through s.p.a.ce. 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 pa.s.s through empty s.p.a.ce 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.