here 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