=176. Molecular Motion in Solids.=--Evidence of molecular motion in liquids is given by expansion on heating, evaporation, and diffusion. Do any of these lines of evidence apply to solids? It is a fact of common experience that solids do become larger on heating. Spaces are left between the ends of rails on railroads so that when they expand in summer they will not distort the track. Iron tires are placed on wheels by heating them until they slip on easily. Then on cooling, the iron shrinks and presses the wheel tightly. Many common demonstrations of expansion are found in lecture rooms. The fact of the evaporation of a solid is often detected by noticing the odor of a substance. The odor of moth balls is one example. Camphor also evaporates. Heated tin has a characteristic odor noted by many. Ice and snow disappear in winter even though the temperature is below freezing. Wet clothes, "freeze dry," that is, dry after freezing, by evaporation. A few crystals of iodine placed in a test-tube and gently heated form a vapor easily seen, even though none of the iodine melts. Where the vapor strikes the side of the tube, it condenses back to dark gray crystals of iodine. This change from solid directly to gas and back again without becoming liquid is called _sublimation_. A number of solids are purified by this process. Important Topics 1. Cooling effect of evaporation, rate of evaporation affected by six conditions. 2. Effects of molecular motion in solids: (a) Expansion, (b) Evaporation, (c) Sublimation. Exercises 1. Does sprinkling the streets or sidewalks cool the air? Why? 2. Give an illustration for each of the factors affecting evaporation. 3. Give an illustration for each of the three evidences of molecular motions in solids. 4. Since three-quarters of the earth's surface is covered with water, why is not the air constantly saturated? 5. If the air has the temperature of the body, will fanning the perfectly dry face cool one? Explain. Will the effect be the same if the face is moist? Explain. 6. What is the cause of "Cloud Capped" mountains? 7. Why does the exhaust steam from an engine appear to have so much greater volume on a cold day in winter than on a warm one in summer? 8. What causes an unfrozen pond or lake to "steam" on a very cold day in winter, or on a very cool morning in summer? 9. As the air on a mountain top settles down the sides to places of greater pressure, how will its temperature be affected? its relative humidity? Explain. 10. On our Pacific coast, moist winds blow from the west over the mountains. Where will it rain? Where be dry? Explain. CHAPTER VIII HEAT AND WORK (1) HEAT MEASUREMENT AND SPECIFIC HEAT