Water is composed of hydrogen and oxygen, its chemical formula being H20. Water is very widely distributed in nature; it covers three-fourths of the earth's surface, and is present in the soil and rocks and in all animal and vegetable organisms. The human body is composed of about 70 per cent water. The body obtains its water supply from foods and beverages, and a small amount of water is formed chemically in the body by the oxidation of the hydrogen of foods.

Water is a very important solvent, dissolving many sub-stances. It also holds many organic substances in suspen-sion. Water which is chemically pure contains no foreign substance of any kind. Distilled water or condensed steam is the only chemically pure water. Water that is hygieni-cally pure contains no substance which is injurious to the health of those drinking it.

Harmful or disease-producing bacteria and decaying ani-mal and vegetable matter are the chief sources of water con-tamination, although the presence of many chemicals makes it unfit for drinking. The disease-producing bacteria most commonly found in drinking water are the typhoid bacilli. Many cases of typhoid fever may be traced to the water supply or to milk which has been diluted with impure water. The common house fly has been found to be a carrier of the typhoid bacillus and should never be allowed to settle on food or on utensils which hold it.