over the land and has thus accumulated the waste of pasture and
stable and, it may be, of dumping ground.

[Illustration: FIG. 37.--As water flows over the land, it gathers
filth and disease germs.]

It is not necessary that water should be absolutely free from all
foreign substances in order to be safe for daily use in drinking; a
limited amount of mineral matter is not injurious and may sometimes be
really beneficial. It is the presence of animal and vegetable matter
that causes real danger, and it is known that typhoid fever is due
largely to such impurities present in the drinking water.

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 reënter 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