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.

[Ill.u.s.tration: 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 k.n.o.b than with the spindle when the k.n.o.b 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 b.u.mp into each other and go around every which way, each molecule b.u.mping 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.

When the little particles of water that make up the clouds become very cold, they freeze as they gather and so make snowflakes. When the little particles of water in the air, that usually make dew, freeze while they are gathering on a blade of gra.s.s, we call it frost.

When raindrops are carried up into colder, higher air while they are forming, they freeze and turn to hail. When snow or frost or hail or ice is heated, it melts and turns back to water.

[Ill.u.s.tration: FIG. 47. Why did the bottle break when the water in it turned to ice?]

But here is a strange fact: although heat usually expands things, water expands when it _freezes_. Like everything else, however, water also expands when it becomes hot, as you found when you made a kind of thermometer, using a flask of water and a gla.s.s tube. But if you should put that flask into a freezing mixture of ice and salt, you would find that when the water became very cold it would begin to expand a little immediately before it froze.

And it is very lucky for us that water does expand when it freezes, because if it did not, ice would be heavier than water is. But since the water expands as it freezes, ice weighs less than water and floats. And that is why lakes and oceans and rivers freeze over the top and do not freeze at the bottom. If they froze from the bottom up, as they would if the ice sank as it formed, every river and lake would be solid ice in the winter. All the harbors outside the tropics would probably be ice-bound all winter long. And the ice in the bottom of the lakes and rivers and in the ocean would probably never melt.

So in the case of freezing water, and in the case of a couple of metals, there is a point where coldness, not heat, makes things expand.

EXPERIMENT 32. Take a ketchup bottle with a screw cap and a cork that fits tightly. Fill it to the top with water; put a long pin beside the cork while you insert it, so that the water can be crowded out as the cork goes down; then when you have pushed the cork in tightly, pull out the pin. Screw the cap on the bottle so as to hold the cork fast. Put the bottle in a pail or box, and pack ice and salt around it. Within an hour you should be able to see what the freezing water does to the bottle.

_APPLICATION 27._ Explain why ice is lighter than water; why we have no snow in summer.

INFERENCE EXERCISE

Explain the following:

141. Sealing wax is held over a candle flame before it is applied to a letter.

142. Automobile tires tighten upon a sudden change from cold weather to hot.

143. When paper has been rolled, it tends to curl up again after being unrolled.

144. Seats running across a car are much more comfortable when a car starts and stops, than are seats running along the sides.

145. You cannot siphon water from a low place to a higher one.

146. Candles get soft in hot weather.

147. Meteorites fall to the earth from the sky.

148. When you preserve fruit and pour the hot fruit into the jars, you fill the jars brim full and screw on the cap air-tight; yet a few hours later the fruit does not fill the jars; there is some empty s.p.a.ce between the top of the fruit and the cover.

149. Water pipes burst in the winter when it is very cold.