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