ammonia. Now, when the pressure is removed, the ammonia boils vigorously and in so doing takes up a great deal of heat; for just as much heat is needed to turn the liquid ammonia back into the ammonia gas, as was given off by the gas when it was compressed to a liquid. So the pipes become very cold, and when they are run through a salt brine, they cool it to perhaps -15° C. This cold salt brine is used for cooling purposes, just as hot water is used for heating. If we put it around molds, or tanks of water, heat is taken from the water until it is frozen (Fig. 81). Cold brine is also used in cold-storage

Expansion Coils and Ice Molds

FIG. 81. - When liquid ammonia is allowed to boil rapidly, it takes up heat from the salt brine and thus cools it below the freezing point of water.

warehouses and in refrigerator cars to produce a low temperature. In this way butter, eggs, meat, and fruits are kept cold to prevent their spoiling (Fig. 82).

127. How Is Ice Cream Frozen? You know how ice cream is made. The cream is placed in a pail surrounded by a mixture of salt and ice. As the freezer is turned, the cream comes in contact with the cold walls of the pail and is frozen. But why does a mixture of ice and salt give a colder temperature than ice alone?

When we draw water from a faucet and put ice into it, the ice melts. Where does the heat come from that is needed to melt the ice? It comes from the water; therefore the water is cooled, if there is enough ice, to 0° C., which is the temperature of melting ice or freezing water.

FIG. 82.

Rooms in which meats and other perishable goods are kept are cooled by pipes carrying cold brine.

What happens when we put salt upon ice? The ice begins to melt, forming water, and the salt dissolves in the water, producing a salt solution, or salt brine. The ice is really melting in salt brine. The heat needed to keep up the melting comes from the brine. But as salt brine does not freeze until it is cooled to about -20° C., the melting of the ice will continue until this temperature is reached. Now, while a temperature of 0° C. is not low enough to freeze cream, one of -20° C. is.

How is the making of artificial ice different from the

freezing of cream? In the making of ice we produce a cold brine by the rapid boiling of liquid ammonia; in freezing ice cream we cool the brine by the rapid melting of ice in the brine.

128. How Do Bodies of Water Affect Climate ? Have you ever wondered why peaches are grown so successfully in the "peach belt" on the eastern shore of Lake Michigan, while they cannot be grown successfully at many places that are much farther south, but away from the water? Can you tell why grapes are grown in large quantities on the eastern and southeastern shores of Lake Erie, in New York, Pennsylvania, and Ohio? The answer is found in a peculiar quality, or property, of water.

But

If you try to heat a pound of water and a pound of iron over two burners giving the same amount of heat, the iron reaches 100° C. long before the water does. when you stop heating them, the iron cools much more rapidly than the water. If you have a pound of iron and one of hot water, both at 100°, and put each separately into a pan of ice, the heat in the water will melt about 9 times as much ice as the heat in the iron will. We say that water has a greater heat capacity than the iron has. Now the land, like iron, has a small heat capacity. Because of this it is heated up more rapidly by the sun than water is, but it also cools more rapidly. So a place that is inland, or away from the water, has great extremes of heat and cold, while a place near a large body of water is kept warmer in winter and cooler in summer by the presence of the water.

We can now answer the question regarding the peach orchards on the eastern shore of Lake Michigan and the grape vineyards on the southeastern shore of Lake Erie. The prevailing cold winds of the winter in the region of the Great Lakes are from the northwest, and the winds are warmed as they pass over these large bodies of water. As a result the weather of winter is less severe on the eastern shores than on the western shores of these lakes.

Why is the eastern shore of Lake Michigan dotted with summer resorts? Why do people go to the shore of the ocean in hot weather?

This effect of water upon the land is much greater if currents of cold or of warm water flow along the shore. Thus the shores of Alaska, British Columbia, and the Pacific Northwest have a mild winter climate, owing to the Japan current which passes along the coast. The Gulf Stream, containing warm water, carries heat from the tropics up to the western coast of Europe and keeps open water in the most northern Norwegian ports.

129. How Does Water Change the Earth's Surface?Did you ever wonder what becomes of all the rain? About 35,000 cubic miles of it fall upon the land every year. This is nearly seven times as much water as is stored in our Great Lakes. The rain that falls upon the earth and washes away the soil, the crystals of frost that freeze within the cracks of the rocks and burst the rocks apart, the great sheets of snow and ice that have moved over large areas of the earth these have chiseled the land into its present forms. Water is thus not only the storehouse of the sun's heat, but also the great sculptor of the earth's surface.

You have heard of the Grand Cañon of the Colorado River and of other deep cañons, or gorges, like it (Fig. 83). The Grand Cañon is 217 miles long, 8 to 15 miles wide, and a mile deep. Do you think that the river just ran down into a deep crack in the mountains and took it for its bed? No; the river itself dug its own bed by wearing away the rock little by little, century after century. The Ohio has carried the wearing-away process farther and flows through a peaceful valley bordered by rounded hills; the Mississippi, Nile, and Hoang-Ho have worn down their valleys to low, fertile flood plains, but little above sea level. What force causes water to fall and to run downhill to the sea? What lifted the water to the mountain tops?

Why is it that the seashore is so interesting to you? Because there are long stretches of sandy beaches, great waves, and ships on long voyages. The shoreline is a place of great interest to the scientist also. There the land and sea meet and carry on their agelong struggle for the possession of the dry ground. The rain wears away the land, the rivers carry their sediments to the sea, and the sea, not satisfied with the work of its allies, the rain and the rivers, pounds away at the shore itself, slowly cutting it away, so as to extend the empire of water. Which will win? The average height of the continents above sea level is about 2300 feet; but the average depth of the sea is about six times as great, or 13,000 feet. So, if the sea could get the complete mastery, its waters could cover all the land with a layer of water about two miles deep.

130. Exercises. - 1. Explain why water is believed to be a compound rather than an element.

2. Is a cubic foot of water heavier, or lighter, than a cubic foot of ice? How do you know?