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