We get our heat from the sun; then why is it so cold up on the
mountain tops?
What is coldness?
Here is an interesting and rather strange thing about heat and
expansion. Although heat expands things, yet expansion does not heat
them. On the contrary, if a thing expands without being heated from an
outside source, it actually gets cold! You see, in order to expand, it
has to push the air or something else aside, and it actually uses up
the energy of its own heat to do this. You will understand this better
after you do the next experiment.
EXPERIMENT 31. Wet the inside of a test tube. Hold the mouth
of the test tube against the opening of a carbon dioxid
tank. Open the valve of the tank with the wrench and let the
compressed gas rush out into the test tube until the mouth
of the test tube is white. Shut off the valve. Feel your test
tube.
What has happened is this: The gas was tightly compressed in the tank.
It was not cold; that is, it had some heat in it, as everything has.
When you let it loose, it used up much of its heat in pushing the air
in the test tube and all around it out of the way. In this way it lost
its heat, and then it became cold. _Cold means absence of heat_, as
dark means absence of light. So when the compressed gas used up its
heat in pushing the air out of its way, it became so cold that it
froze the water in your test tube.
[Illustration: FIG. 46. The expansion of the compressed gas freezes
the moisture on the tube.]
ONE REASON WHY IT IS ALWAYS COLD HIGH UP IN THE AIR. Even on hot
summer days aviators who fly high suffer from the cold. You might
think that they would get warmer as they went up nearer the sun; one
reason that they get colder instead is this:
As you saw in the last experiment, a gas that expands gets very cold.
Air is a kind of gas. And whenever air rises to where there is not so
much air crowding down on it from above, it expands. So the air that
rises high and expands gets very cold. Consequently mountains which
reach up into this high, cold air are snow covered all the year round;
and aviators who fly high suffer keenly from the cold. There are
several reasons for this coldness of the high air. This is just _one_
of them.
_APPLICATION 26._ Explain why air usually cools when it rises;
why high mountain tops are always covered with snow.
INFERENCE EXERCISE
Explain the following:
131. You should not fill a teakettle brim full of cold water
when you are going to put it on the stove.
132. It is harder to erase an ink mark than a pencil mark.
133. Bearings of good watches, where there is constant rubbing
on the parts, are made of very hard jewels.
134. You feel lighter for an instant when you are in an
elevator which starts down suddenly.
135. When men lay cement sidewalks, they almost always make
cracks across them every few feet.
136. To cool hot coffee one sometimes blows on it.
137. It is much easier to turn the latch of a door with the
knob than with the spindle when the knob is off.
138. Patent-leather shoes do not soil as easily as plain
leather shoes.
139. We use rubber bands to hold things together tightly.
140. As air goes up it usually cools.