[Illustration:  FIG. 35.—­Water by its action constantly changes the character of the land.]

68.  Relation of Water to Human Life.  Water is one of the most essential of food materials, and whether we drink much or little water, we nevertheless get a great deal of it.  The larger part of many of our foods is composed of water; more than half of the weight of the meat we eat is made up of water; and vegetables are often more than nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes have over 90 per cent. of water, and most fruits are more than three fourths water; even bread, which contains as little water as any of our common foods, is about one third water (Fig. 36).

[Illustration:  FIG. 36.—­Diagram of the composition of a loaf of bread and of a potato:  1. ash; 2, food; 3, water.]

Without water, solid food material, although present in the body, would not be in a condition suitable for bodily use.  An abundant supply of water enables the food to be dissolved or suspended in it, and in solution the food material is easily distributed to all parts of the body.

Further, water assists in the removal of the daily bodily wastes, and thus rids the system of 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.