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.)

[Illustration:  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.

[Illustration:  FIG. 31.—­The work of water as a solvent.]

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).

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.)