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 mi