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 space between the top of the fruit and the cover. 149. Water pipes burst in the winter when it is very cold. 150. When people want to make iron castings, they first melt the iron, then pour it into molds. They leave it in the molds until cold. After that the iron holds the shape of the molds. Explain why the iron changes from a liquid to a solid. SECTION 18. _Evaporation._ Why is it that when ink is spilled it dries up, but when it is in the bottle it does not dry up? What put the salt into the ocean? Why do you feel cold when you get out of the bathtub?