foul and poisonous substances.

The human body itself consists largely of water; indeed, about two
thirds of our own weight is water. The constant replenishing of this
large quantity is necessary to life, and a considerable amount of the
necessary supply is furnished by foods, particularly the fruits and
vegetables.

But while the supply furnished by the daily food is considerable, it
is by no means sufficient, and should be supplemented by good drinking
water.

69. Water and its Dangers. Our drinking water comes from far and
near, and as it moves from place to place, it carries with it in
solution or suspension anything which it can find, whether it be
animal, vegetable, or mineral matter. The power of water to gather up
matter is so great that the average drinking water contains 20 to 90
grains of solid matter per gallon; that is, if a gallon of ordinary
drinking water is left to evaporate, a residue of 20 to 90 grains will
be left. (See Laboratory Manual.) As water runs down a hill slope
(Fig. 37), it carries with it the filth gathered from acres of land;
carries with it the refuse of stable, barn, and kitchen; and too often
this impure surface water joins the streams which supply our cities.
Lakes and rivers which furnish drinking water should be carefully
protected from surface draining; that is, from water which has flowed
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 reenter the
water, and the advantage gained from boiling is lost.

(_b_) _Distillation_