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?
Wet clothes get dry when they are hung on the clothes-line. The water in them _evaporates_. It turns to invisible vapor and disappears into the air. Water and all liquids evaporate when they are long exposed to the air. If they didn"t--well, let us imagine what the world would be like if all evaporation should suddenly stop:
You find that your face is perspiring and your hands as well. You wipe them on your handkerchief, but soon they are moist again, no matter how cool the weather. After wiping them a few more times your handkerchief becomes soaking wet, and you hang it up to dry. There may be a good breeze stirring, yet your handkerchief does not get dry.
By this time the perspiration is running off your face and hands, and your underclothes are getting drenched with perspiration.
[Ill.u.s.tration: FIG. 48. An evaporating dish.]
You hurry into the house, change your clothes, bathe and wipe yourself dry with a towel. When you find that your wet things are not drying, and that your dry ones are rapidly becoming moist, you hastily build a fire and hang your clothes beside it. No use, your clothes remain as wet as ever. If you get them very hot the moisture in them will boil and turn to steam, of course, but the steam will all turn back to water as soon as it cools a little and the drops will cling to your clothes and to everything around the room. You will have to get used to living in wet clothes. You won"t catch cold, though, since there is no evaporation to use up your heat.
But the water problem outside is not one of mere inconvenience.
It never rains. How can it when the water from the oceans cannot evaporate to form clouds? Little by little the rivers begin to run dry--there is no rain to feed them. No fog blows in from the sea; no clouds cool the sun"s glare; no dew moistens the gra.s.s at night; no frost shows the coming of cold weather; no snow comes to cover the mountains. In time there is no water left in the rivers; every lake with an outlet runs dry. There are no springs, and, after a while, no wells. People have to live on juicy plants. The crops fortunately require very little moisture, since none evaporates from them or from the ground in which they grow. And the people do not need nearly as much water to drink.
Little by little, however, the water all soaks too deep into the ground for the plants to get it. Gradually the continents become great deserts, and all life perishes from the land.
All these things would really happen, and many more changes besides, if water did not evaporate. Yet the evaporation of water is a very simple occurrence. As the molecules of any liquid bounce around, some get hit harder than others. These are shot off from the rest up into the air, and get too far away to be drawn back by the pull of the molecules behind. This shooting away of some of the molecules is evaporation. And since it takes heat to send these molecules flying off, the liquid that is left behind is colder because of the evaporation. That is why you are always cold after you leave the bathtub until you are dry. The water that evaporates from your body uses up a good deal of your heat.
[Ill.u.s.tration: FIG. 49. Diagram ill.u.s.trating how in the evaporation of water some of the molecules shoot off into the air.]
Gasoline evaporates more quickly than water. That is why your hands become so cold when you get them wet with gasoline.
Since heat is required to evaporate a liquid, the quickest way to dry anything is to warm it. That is why you hang clothes in the sun or by the stove to dry.
Try these experiments:
EXPERIMENT 33. Read a thermometer that has been exposed to the room air. Now dip it in water that is warmer than the air, taking it out again at once. Watch the mercury. Does the thermometer register a higher or a lower temperature than it did at the beginning? What is taking up the heat from the mercury?
EXPERIMENT 34. Put a few drops of water in each of two evaporating dishes. Leave one cold; warm the other over the burner, but do not heat it to boiling. Which evaporates more quickly?
WHY THE SEA IS SALT. You remember various fairy stories about why the sea is salt. For a long time the saltness of the sea puzzled people.
But the explanation is simple. As the water from the rains seeps through the soil and rocks, it dissolves the salt in them and continually carries some of it into the rivers. So the waters of the rivers always carry a very little salt with them out to sea. The water in the ocean evaporates and leaves the salt behind. For millions of years this has been going on. So the rivers and lakes, which have only a little salt in them, keep adding their small amounts to the sea, and once in the sea the salt never can get out. The oceans never get any fuller of water, because water only flows into the ocean as fast as it evaporates from the ocean. Yet more salt goes into the ocean all the time, washed down by thousands of streams and rivers. So little by little the ocean has been growing more and more salty since the world began.
[Ill.u.s.tration: FIG. 50. A view of the Dead Sea.]
Great Salt Lake and the Dead Sea, unlike most lakes, have no rivers flowing out of them to carry the salt and water away, but rivers flow into them and bring along small amounts of salt all the time. Then the water evaporates from Great Salt Lake and the Dead Sea, leaving the salt behind; and that is why they are so very salty.
When people want to get the salt out of sea water, they put the sea water in shallow open tanks and let the water evaporate. The salt is left behind.
EXPERIMENT 35. Dissolve some salt in warm water until no more will dissolve. Pour the clear liquid off into an evaporating dish, being careful not to let any solid particles of the salt go over. Either set the dish aside uncovered, for several days, or heat it almost to boiling and let it evaporate to dryness. What is left in the dish?
_APPLICATION 28._ Some girls were heating water for tea, and were in a hurry. They had only an open stew pan to heat the water in.
"Cover the pan with something; you"ll let all the heat out!"
Helen said.
"No, you want as much heat to go through the water as possible. Leave the lid off so that the heat can flow through easily," said Rose.
"The water will evaporate too fast if the lid is off, and all the heat will be used up in making it evaporate; it will take it much longer to get hot without the lid," Louise argued.
"That"s not right," Rose answered. "Boiling water evaporates fastest of all. We want this to boil, so let it evaporate; leave the lid off."
What should they have done?
_APPLICATION 29._ Two men were about to cross a desert. They had their supply of water in canvas water bags that leaked just enough to keep the outside of the bags wet. Naturally they wanted to keep the water as cold as possible.
"I"m going to wrap my rubber poncho around my water bag and keep the hot desert air away from the water," said one.
"I"m not. I"m going to leave mine open to the air," the other said.
Which man was right? Why?
INFERENCE EXERCISE
Explain the following:
151. When you go up high in an elevator, you feel the pressure of the air in your ears.
152. Water is always flowing into Great Salt Lake; it has no outlet; yet it is getting more nearly empty all the time.
153. A nail sinks while a cork floats in water.
154. Steep hillsides are paved with cobblestones instead of asphalt.
155. If you place one wet gla.s.s tumbler inside another you can pull them apart only with difficulty, and frequently you break the outer one in the attempt.
156. Sausages often break their skins when they are being cooked.
157. A drop of water splashed against a hot lamp chimney cracks it.
158. When you shoot an air gun, the air is compressed at first; then when it is released it springs out to its original volume and throws the bullet ahead of it.
159. Leather soles get wet through in rainy weather, while rubbers remain perfectly dry on the inside.
160. When you want to clean a wooden floor, you scrub it with a brush.
SECTION 19. _Boiling and condensing._