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 shine; 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.
[Illustration: 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 space in the night and loses still more heat into
space 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 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.
If a lamp glows for a long time, however, the glass really does become
hot. That is partly because there is not a perfect vacuum within it
(there is a little gas inside that carries the heat to the glass by
convection), and it is partly because the glass does not let quite
all of the radiant heat and light go through it, but absorbs some and
changes it to the regular conducted heat.
One practical use that is made of a knowledge of the difference
between radiant and conducted heat is in the manufacture of thermos
bottles.
EXPERIMENT 43. Take a thermos bottle apart. Examine it
carefully. If it is the standard thermos bottle, with the name
"thermos" on it, you will find that it is made of two layers
of glass with a vacuum between them. The vacuum keeps any
_conducted_ heat from getting out of the bottle or into it.
But, as you know, _radiant_ heat can flash right through a
vacuum. So to keep it from doing this the glass is silvered,
making a mirror out of it. Just as a mirror sends light back
to where it comes from, it sends practically all radiant heat
back to where it comes from. Heat, therefore, cannot get
into the thermos bottle or out of it either by radiation or
conduction. And that is why thermos bottles will keep things
very hot or ice-cold for such a long time.
Fill the thermos bottle with boiling water, stopper it, and
put it aside till the next day. See whether the water is still
hot.
[Illustration: FIG. 61. How a thermos bottle is made. Notice the double
layer of glass in the broken one.]
If we could make the vacuum perfect, and surround all parts of the
bottle, even the mouth, with the perfect vacuum, and if the mirror
were perfect, things put into a thermos bottle would stay boiling hot
or icy cold forever and ever.
WHY IT IS COOL AT NIGHT AND COLD IN WINTER. It is the radiation of
heat from the earth into space that makes the earth cooler at night
and cold in winter. Much of the heat that the earth absorbs from
the sun in the daytime radiates away at night. And since it keeps on
radiating away until the sun brings us more heat the next day, it is
colder just before dawn than at midnight, more heat having radiated
into space.
For the same reason it is colder in January and February than in
December. It is in December that the days are shortest and the sun
shines on us at the greatest slant, so that we get the least heat from
it; but we still have left some of the heat that was absorbed in the
summer. And we keep losing this heat by radiation faster than we get
heat from the sun, until almost spring.
_APPLICATION 33._ Distinguish between radiant and conducted
heat in each of the following examples:
(a) The sun warms a room through the window. (b) A room is
cooler with the shades down than up, when the sun shines on
the window. (c) But even with the shades down a room on the
sunny side of the house is warmer than a room on the shady
side. (d) When a mirror is facing the sun, the back gets hot.
(e) If you put your hand in front of a mirror held in the sun,
the mirror reflects heat to your hand. (f) If you put a plate
on a steam radiator, the top of the plate gradually becomes
hot. (g) If anything very hot or cold touches a gold or
amalgam filling of a sensitive tooth, you feel it decidedly.
(h) The handle of your soup spoon becomes hot when the bowl of
it is in the hot soup. (i) The moon is now very cold, although
it probably was once very hot.
INFERENCE EXERCISE
Explain the following:
181. Trees bend in the wind, then straighten up again. Why do
they straighten up?
182. A cloth saturated with kerosene and placed in the bottom
of a clock will oil the clockworks above it.
183. In cold weather the doorknob _inside_ the front door is
cold.
184. It is cool in the shade.
185. Clothes get hot when you iron them.
186. Potatoes fried in deep fat cook more quickly than those
boiled in water.
187. If you hold your hand near a vacuum electric lamp globe
that is glowing, some of the heat will go out to your hand at
once.
188. Rubbing silver with fine powder polishes it.
189. A mosquito can suck your blood.
190. A hot-water tank becomes hot at the top first, then
gradually heats downward. When you light the gas under an
ordinary hot-water heater, the hot water circulates to the
top of the boiler, while the cold water from the boiler pushes
into the bottom part of the heater, as shown in Figure 59.
What causes this circulation?