. By this method pure water is obtained, but this
method of purification cannot be used conveniently in the home
(Section 25).

(_c_) _Filtration_. In filtration, the water is forced through
porcelain or other porous substances which allow the passage of water,
but which hold back the minute foreign particles suspended in the
water. (See Laboratory Manual.) The filters used in ordinary dwellings
are of stone, asbestos, or charcoal. They are often valueless, because
they soon become choked and cannot be properly cleaned.

The filtration plants owned and operated by large cities are usually
safe; there is careful supervision of the filters, and frequent and
effective cleanings are made. In many cities the filtration system is
so good that private care of the water supply is unnecessary.

71. The Source of Water. In the beginning, the earth was stored with
water just as it was with metal, rock, etc. Some of the water
gradually took the form of rivers, lakes, streams, and wells, as now,
and it is this original supply of water which furnishes us all that we
have to-day. We quarry to obtain stone and marble for building, and we
fashion the earth's treasures into forms of our own, but we cannot
create these things. We bore into the ground and drill wells in order
to obtain water from hidden sources; we utilize rapidly flowing
streams to drive the wheels of commerce, but the total amount of water
remains practically unchanged.

The water which flows on the earth is constantly changing its form;
the heat of the sun causes it to evaporate, or to become vapor, and to
mingle with the atmosphere. In time, the vapor cools, condenses, and
falls as snow or rain; the water which is thus returned to the earth
feeds our rivers, lakes, springs, and wells, and these in turn supply
water to man. When water falls upon a field, it soaks into the ground,
or collects in puddles which slowly evaporate, or it runs off and
drains into small streams or into rivers. That which soaks into the
ground is the most valuable because it remains on the earth longest
and is the purest.

[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,