[Illustration: FIG. 38.--How springs are formed. _A_, porous layer;
_B_, non-porous layer; _C_, spring.]

Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock
through which it cannot pass, and which effectually hinders its
downward passage. In such regions, there is an accumulation of water,
and a well dug there would have an abundant supply of water. The
non-porous layer is rarely level, and hence the water whose vertical
path is obstructed does not "back up" on the soil, but flows down hill
parallel with the obstructing non-porous layer, and in some distant
region makes an outlet for itself, forming a spring (Fig. 38). The
streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams
and rivers evaporation occurs, the water once more becomes vapor and
passes into the atmosphere, where it is condensed and again falls to
the earth.

Water which has filtered through many feet of earth is far purer and
safer than that which fell directly into the rivers, or which ran off
from the land and joined the surface streams without passing through
the soil.

72. The Composition of Water. Water was long thought to be a simple
substance, but toward the end of the eighteenth century it was found
to consist of two quite different substances, oxygen (O) and hydrogen
(H.)

[Illustration: FIG. 39.--The decomposition of water.]