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.
Section 17. Freezing and melting.
When water freezes in a pipe, why does the pipe burst?
What is liquid air?
Why does not the wire in an electric lamp melt when it is red hot?
Suppose we looked at a piece of ice through the imaginary microscope that shows us the molecules. The ice molecules would be different from the iron molecules in size, but they would be vibrating back and forth in exactly the same way, only with less motion. It is because they have less motion that we say the ice is colder than the iron. Then let us suppose that the sun was shining on the ice while we watched the ice molecules.
First we should see movements of the ice molecules become gradually more rapid, just as the iron molecules did when the iron was warmed. Then, as they moved faster and faster, they would begin to bump into each other and go around every which way, each molecule bumping first into one neighbor, then into another, and bouncing back in a new direction after each collision. This is what causes the ice to melt. When its molecules no longer go back and forth in the same path all the time, the ice no longer keeps its shape, and we call it water—a liquid.
Almost all solid substances will melt when they are heated. Or, to put it the other way around, every liquid will freeze solid if it gets cold enough. Even liquid air (which is ordinary air cooled and compressed until it runs like water) can be frozen into a solid chunk. Some things will melt while they are still very cold; solid air, for instance, melts at a temperature that would freeze you into an icicle before you could count ten. Other things, such as stones, are melted only by terrific heat.