70.  Methods of Purification.  Water is improved by any of the following methods:—­

(a) Boiling.  The heat of boiling destroys animal and vegetable germs.  Hence water that has been boiled a few minutes is safe to use.  This is the most practical method of purification in the home, and is very efficient.  The boiled water should be kept in clean, corked bottles; otherwise foreign substances from the atmosphere reenter the water, and the advantage gained from boiling is lost.

(b) Distillation.  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.]