4. When you put a cold lamp chimney upon a lighted kerosene lamp, the inside of the chimney becomes covered with mist. What substance is the mist? Where does it come from? Why does the bottom of a teakettle filled with cold water become wet when you put it over a gas flame?

5. Name several uses of salt.

6. How has civilization been helped by the invention of matches?

CHAPTER XIV

WATER

123. Where Is Water Found? How much there is to know about the common substance "water"! You think of it as a liquid that flows from hydrants or is pumped from wells, or that flows in streams, from the little springs and brooks to the great rivers of the earth; you think, too, of ponds and lakes and the mighty oceans. You must also think of it as rain, as clouds, as ice and snow, as the invisible water vapor that rises from teakettles and from the sea, to become a part of our atmosphere, until it condenses and returns once more to the earth's surface. Water is not only on the surface and in the atmosphere, but in the rocks as well. No matter where we dig, we find it, even in the desert. Water is a large part of all plants and animals as well as of our food. The following table shows how much of it there is in our bodies and in some of our foods:

The farmer looks for water anxiously in the growing season, because he says, "No one ever starved in a wet

"There is no life without

We already know some

year"; the scientist says: water."

Our

124. What is Water Like? thing of what water is like. In a small amount it is without color, but a large amount of it looks blue. drinking water has a taste, because of substances dissolved in it, but water that has nothing in it is tasteless and flat, much like boiled water. We have learned that the boiling point of water is 100° C. and is used as a standard mark on the thermometer, as is also its freezing point, 0° C. A cubic foot of water weighs about 62.5 pounds and a cubic centimeter of it at 4° C. weighs one gram. If you put warm water in a flask with a very small neck and cool the flask, the water shrinks in volume until its temperature is 4° C. (39° F.); then it expands until its temperature is 0° C. Water is therefore most dense, or heavy, at 4° C. and not at 0° C. Because of this fact the water at the bottom of lakes is rarely cooled lower than 4° C.; for if it becomes colder, it expands, and rises instead of sinking. When water freezes, it expands, so that 100 cubic feet of water become 109 cubic feet of ice. So when we have ice at the top, we have water near 0° C. just under the ice, and water near 4° C. at the bottom. Because this is so, water animals and plants can live through the winter and in the deep waters of the frigid zone. Ice, like water, is blue in large

masses.

You know how readily salt, sugar, soda, and many other substances disappear in water; we say they dissolve, and that water is the solvent. Other liquids, such

as alcohol, are used to dissolve substances which water cannot, but water is our most common solvent.

Hold a lump of sugar so that its lower side just touches the surface of some water in a clear glass and watch the appearance of the liquid. The oily appearance is caused by the sinking of the sugar solution before it has mixed with the rest of the water. Do the same with a lump of copper sulphate (bluestone or blue vitriol). Are the solutions of sugar and blue vitriol heavier or lighter than the water used? What are the colors of the solutions?

125. How Does Water Boil? Watch the water in a pan as it nears boiling. At first there are tiny bubbles that escape; these are chiefly air that was dissolved in the water. Then, as the temperature rises, there are larger bubbles of steam that start from the bottom, but do not rise to the top. These condense and cause the singing" of the water, but the water is not yet boiling. Finally the bubbles rise to the surface and burst; as they do so, they have force enough to push out the air. The bubbles jump up and down when there is real boiling. Then and then only, will a thermometer register the boiling point of water.

We must remember that the temperature at which water boils depends on how heavily the atmosphere presses down upon the water. As we ascend a mountain and leave more and more of the atmosphere below us, water does not need to be heated so hot to make it boil. The steam bubbles do not need to have as much force to push away from the water as they did when the air was pressing harder upon them. The boiling point is lowered about 1° C. for every 960 feet we ascend above sea level. So water boils at 92.3° C. at Mexico City, 7500 feet above the sea. At Denver, 5500 feet high, it boils at about 95° C. At about what temperature will it boil on Pikes Peak, 14,108 feet high?

126. How Is Ice Made? Years ago ice was carried to tropical countries in ships, but modern methods of living require so much ice that a great deal of our ice is manufactured, even in temperate zones. How can we make ice in summer? We have already learned that the reason why perspiration cools us is that the heat needed to evaporate it comes from our bodies (cf. § 71). Water evaporated in a porous jug cools the water inside.

ETHER
Drops of

water

Substances that evaporate rapidly can be made to take away so much heat that very low temperatures can be obtained. Pour out a little ether into a shallow pan, such as the cover of a baking-powder can (Fig. 80), set it on a few drops

FIG. 80. By causing the ether to evaporate rapidly you can freeze of water placed on some folded the water under the pan. Use a writing paper, and by means of long tube and perform the experiment near a window with an out- a tube and bellows (your cheeks will serve as bellows) force air The dish can be frozen to

going draft.

rapidly through the ether. the paper! The principle of rapid evaporation is used to make artificial ice. Think what a blessing it is that hot countries can have ice for keeping food and water cool.

The liquid that is used to freeze water is liquid ammonia. This is not ammonia water, but the gas ammonia which has been strongly compressed until it turns to the liquid state.

Usually the liquid ammonia is made in strong pipes (Fig. 81). When the gas is compressed into the liquid form, it becomes hot; so men cool it by spraying running water over the pipes which hold the