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.]

If we send an electric current through water (acidulated to make it a
good conductor), as shown in Figure 39, we see bubbles of gas rising
from the end of the wire by which the current enters the water, and
other bubbles of gas rising from the end of the wire by which the
current leaves the water. These gases have evidently come from the
water and are the substances of which it is composed, because the
water begins to disappear as the gases are formed. If we place over
each end of the wire an inverted jar filled with water, the gases are
easily collected. The first thing we notice is that there is always
twice as much of one gas as of the other; that is, water is composed
of two substances, one of which is always present in twice as large
quantities as the other.

73. The Composition of Water. On testing the gases into which water
is broken up by an electric current, we find them to be quite
different. One proves to be oxygen, a substance with which we are
already familiar. The other gas, hydrogen, is new to us and is
interesting as being the lightest known substance, being even "lighter
than a feather."

An important fact about hydrogen is that in burning it gives as much
heat as five times its weight of coal. Its flame is blue and almost
invisible by daylight, but intensely hot. If fine platinum wire is
placed in an ordi