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65. Destructive Action of Water. The action of water in stream and sea, in springs and wells, is evident to all; but the activity of ground water—that is, rain water which sinks into the soil and remains there—is little known in general. The real activity of ground water is due to its great solvent power; every time we put sugar into tea or soap into water we are using water as a solvent. When rain falls, it dissolves substances floating in the atmosphere, and when it sinks into the ground and becomes ground water, it dissolves material out of the rock which it encounters (Fig. 30). We know that water contains some mineral matter, because kettles in which water is boiled acquire in a short time a crust or coating on the inside. This crust is due to the accumulation in the kettle of mineral matter which was in solution in the water, but which was left behind when the water evaporated. (See Section 25.)

FIG. 30.—Showing how caves and holes are formed by the solvent action of water.

The amount of dissolved mineral matter present in some wells and springs is surprisingly great; the famous springs of Bath, England, contain so much mineral matter in solution, that a column 9 feet in diameter and 140 feet high could be built out of the mineral matter contained in the water consumed yearly by the townspeople.

Rocks and minerals are not all equally soluble in water; some are so little soluble that it is years before any change becomes apparent, and the substances are said to be insoluble, yet in reality they are slowly dissolving. Other rocks, like limestone, are so readily soluble in water that from the small pores and cavities eaten out by the water, there may develop in long centuries, caves and caverns (Fig. 30). Most rock, like granite, contains several substances, some of which are readily soluble and others of which are not readily soluble; in such rocks a peculiar appearance is presented, due to the rapid disappearance of the soluble substance, and the persistence of the more resistant substance (Fig. 31).

FIG. 31.—The work of water as a solvent.

We see that the solvent power of water is constantly causing changes, dissolving some mineral substances, and leaving others practically untouched; eating out crevices of various shapes and sizes, and by gradual solution through unnumbered years enlarging these crevices into wonderful caves, such as the Mammoth Cave of Kentucky.

66. Constructive Action of Water. Water does not always act as a destructive agent; what it breaks down in one place it builds up in another. It does this by means of precipitation. Water dissolves salt, and also dissolves lead nitrate, but if a salt solution is mixed with a lead nitrate solution, a solid white substance is formed in the water (Fig. 32). This formation of a solid substance from the mingling of two liquids is called precipitation; such a process occurs daily in the rocks beneath the surface of the earth. (See Laboratory Manual.)

FIG. 32.—From the mingling of two liquids a solid is sometimes formed.

Suppose water from different sources enters a crack in a rock, bringing different substances in solution; then the mingling of the waters may cause precipitation, and the solid thus formed will be deposited in the crack and fill it up. Hence, while ground water tends to make rock porous and weak by dissolving out of it large quantities of mineral matter, it also tends under other conditions to make it more compact because it deposits in cracks, crevices, and pores the mineral matter precipitated from solution.

FIG. 33.—Mineral matter precipitated from solution is deposited in crevices and forms veins.

These two forces are constantly at work; in some places the destructive action is more prominent, in other places the constructive action; but always the result is to change the character of the original substance. When the mineral matter precipitated from the solutions is deposited in cracks, veins are formed (Fig. 33), which may consist of the ore of different metals, such as gold, silver, copper, lead, etc. Man is almost entirely dependent upon these veins for the supply of metal needed in the various industries, because in the original condition of the rocks, the metallic substances are so scattered that they cannot be profitably extracted.

Naturally, the veins themselves are not composed of one substance alone, because several different precipitates may be formed. But there is a decided grouping of valuable metals, and these can then be readily separated by means of electricity.

67. Streams. Streams usually carry mud and sand along with them; this is particularly well seen after a storm when rivers and brooks are muddy. The puddles which collect at the foot of a hill after a storm are muddy because of the particles of soil gathered by the water as it runs down the hill. The particles are not dissolved in the water, but are held there in suspension, as we call it technically. The river made muddy after a storm by suspended particles usually becomes clear and transparent after it has traveled onward for miles, because, as it travels, the particles drop to the bottom and are deposited there. Hence, materials suspended in the water are borne along and deposited at various places (Fig. 34). The amount of deposition by large rivers is so great that in some places channels fill up and must be dredged annually, and vessels are sometimes caught in the deposit and have to be towed away.

FIG. 34.—Deposit left by running water.

Running water in the form of streams and rivers, by carrying sand particles, stones, and rocks from high slopes and depositing them at lower levels, wears away land at one place and builds it up at another, and never ceases in its work of changing the nature of the earth's surface (Fig. 35).

FIG. 35.—Water by its action constantly changes the character of the land.

68. Relation of Water to Human Life. Water is one of the most essential of food materials, and whether we drink much or little water, we nevertheless get a great deal of it. The larger part of many of our foods is composed of water; more than half of the weight of the meat we eat is made up of water; and vegetables are often more than nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes have over 90 per cent. of water, and most fruits are more than three fourths water; even bread, which contains as little water as any of our common foods, is about one third water (Fig. 36).

FIG. 36.—Diagram of the composition of a loaf of bread and of a potato: 1. ash; 2, food; 3, water.

Without water, solid food material, although present in the body, would not be in a condition suitable for bodily use. An abundant supply of water enables the food to be dissolved or suspended in it, and in solution the food material is easily distributed to all parts of the body.

Further, water assists in the removal of the daily bodily wastes, and thus rids the system of foul and poisonous substances.

The human body itself consists largely of water; indeed, about two thirds of our own weight is water. The constant replenishing of this large quantity is necessary to life, and a considerable amount of the necessary supply is furnished by foods, particularly the fruits and vegetables.

But while the supply furnished by the daily food is considerable, it is by no means sufficient, and should be supplemented by good drinking water.

69. Water and its Dangers. Our drinking water comes from far and near, and as it moves from place to place, it carries with it in solution or suspension anything which it can find, whether it be animal, vegetable, or mineral matter. The power of water to gather up matter is so great that the average drinking water contains 20 to 90 grains of solid matter per gallon; that is, if a gallon of ordinary drinking water is left to evaporate, a residue of 20 to 90 grains will be left. (See Laboratory Manual.) As water runs down a hill slope (Fig. 37), it carries with it the filth gathered from acres of land; carries with it the refuse of stable, barn, and kitchen; and too often this impure surface water joins the streams which supply our cities. Lakes and rivers which furnish drinking water should be carefully protected from surface draining; that is, from water which has flowed over the land and has thus accumulated the waste of pasture and stable and, it may be, of dumping ground.