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140. Can a City Filter Its Water? How do you suppose a city filters its water? City filter systems are made of beds of sand; these are often acres in extent. Sand is loose and contains much air. The oxygen of the air (cf. § 34) can thus penetrate far into it and destroy the bacteria of disease. After soaking through the sand, the water enters reservoirs from which it is pumped through the water "mains into the houses. But the large filters, like the small ones, must be emptied sometimes and allowed to lie idle, so that the sand may be purified by sunlight and air.

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141. May Water be Purified by Chemicals? — The bacteria of disease that are present in water may be destroyed by means of certain chemicals. One of these is chloride of lime, or "bleaching powder." This is a white solid that can be bought in metal cans. A certain amount of it when put into the city's water supply will make the water fit to drink, but it may give the water a slight taste. You can make a purifying solution for your own water supply, if you wish, in the following way:

Rub an even teaspoonful of chloride of lime with a little water until all the lumps are broken up and you get a smooth paste. Then mix the paste with four cupfuls of water. Put this solution into a bottle, stopper it tightly, and let it settle. To purify two gallons of water, add a teaspoonful of the clear chloride of lime solution, stir up the water, and let it stand for ten minutes.

142. What is Hard Water? We often hear water spoken of as "hard," or as "soft," water. If you live in a village or in the country, you have probably noticed that water from a cistern is often used for washing pur

poses, while water from the faucet or well is used for drinking purposes. Cistern water is very likely to be impure, in that it contains many bacteria, and it may, therefore, be unfit to drink. Why is it better, then, for washing? When soap is used in the soft rain water of the cistern, a lather, or suds, is quickly and easily formed, but the soap lathers with great difficulty in the hard water. Rain water is always soft. Since well and spring waters were once rain water, they must have become hard in their passage through the ground. In seeping through rock and ground layers, the water dissolves some substances which make it hard. Some of these substances are made insoluble when the water is boiled "hardness" of water is deand come out as "scale" (Fig. 89); others must be forced out by means of some chemical, such as ammonia. what substances besides ammonia are used to make water soft. Does soap soften hard water?

Boiler Scale:

FIG. 89.

How the

posited on the inside of a teakettle.

Ask your mother

143. Exercises. 1. Is your city water hard or soft? Does the inside of your teakettle have a scaly deposit? Where does it come from? What is the source of your water supply? Put a clean pebble into your teakettle and leave it there for some weeks. Examine it from time to time to see if it increases in size.

2. What is the danger in drinking surface water?

3. What are the dangers in camping? How may they be overcome?

4. What are the ways of purifying water?

5. Why is it necessary to clean a filter frequently?

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6. What is the value of a trap"?

7. Make a definition for distillation.

8. Find out what industries in your city, or the city nearest you, need a large supply of water and why.

9. Find out from the water department of your city how many gallons of water the city uses in a year. What is the population of your city? How much is its per capita consumption of water?

CHAPTER XVI

ROCKS AND SOIL

144. What is the Earth's Crust?

What do you

Atmosphere
Water
Crust

suppose the inside of the earth would look like, if we could cut it through its center into two great halves? This is something men have often wondered about. In such a cutting, or cross section (Fig. 90), there would be at least four different layers to think of. First, there is the atmosphere, or gas layer; then there is the water layer that covers about of the earth; the third is the land layer, or crust, upon which we live; the fourth is the inside, or core.

Core

Crust
Water
Atmosphere

FIG. 90. This gives us an idea how a cross section of the earth would look. Of course, the outer layers are not nearly so thick as the figure shows them.

In this chapter we consider the third layer the crust. You may have heard people say: "Now we are at the bed rock of the argument." What did they mean by this? They meant that they were through with all the little, outside reasons and had reached the most important of all. In the earth's crust there are two layers: the mantle rock and the bed rock. The mantle rock is the loose, outer covering of soil, sand, clay, gravel, and

pebbles. This may usually be worked without much difficulty. But the bed rock underneath is firm, hard, and difficult to cut through. A railroad cut or a road cut is sometimes deep enough to cut through the mantle rock into the bed rock. Mantle rock is much thinner at some places than others; in fact the bed rock sometimes comes to the surface and we have an outcrop. Have you ever seen a river which has cut down to bed rock?

Limestone

Shale

Limestone

Surface of

Shale

Sandstone

Shale

Sandstone

Shale

a

145. What are the Classes of Rocks? Examine Water piece of soft coal and note that a cross section of the coal is marked by a multitude of parallel lines, because the coal is made of thin layers. Limestone looks much the same. Such rocks are called stratified, or " made in layers.' One layer of material seems to have been

FIG. 91. A cross section of the rock layers under the Niagara Falls. Note how the layers of hard rock are being undermined by the wearing away

of the soft rocks. (After Gilbert.)

spread evenly over another, just as one leaf of a book is laid upon another.

The rocks into which Niagara Falls (Fig. 91) has cut are good examples of stratified rocks. The rock under the Niagara River is a hard limestone. This covers a soft shale layer and farther down we find some sandstone layers. Limestone, sandstone, and shale are stratified rocks.

Some rocks are not composed of layers. They are formed of crystallized masses which are interlaced and

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