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. 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?