No article of food is used more constantly or in larger quantities than is water. At the same time, no food material is more dangerous as a carrier of disease. This is shown by the serious epidemics of typhoid fever and other maladies which infected water has caused. The dangers in this direction, at least for the inhabitants of large villages and cities, are augmented by the fact that their water supply comes in most instances from extensive watersheds, or from rivers or lakes, subject to pollution, and in any case, from sources not under the immediate control of the user and about the condition of which he generally has very little or no accurate knowledge.

316. Pure Water

Pure water, chemically speaking, consists entirely of the compound H20, without the presence of any other compound or foreign bodies. In this sense, there are no natural waters that are pure. Even the first rain water that falls brings down with it dust and gases from the air. The only waters that approximate to chemical purity are carefully distilled water and rain water, after the rain has been falling for a few hours. The term " pure," as popularly applied to water, as for instance to the water of springs, signifies that the water is free from undesirable compounds or bodies, that is, that it is of good sanitary quality. Natural waters from uninhabited regions and from carefully guarded sources in inhabited places are of this kind. Springs, streams, and lakes along which or near which there are no inhabitants, and the vicinity of which is not frequented by any number of people, or streams and storage reservoirs that are carefully policed, furnish the safest water supply, excepting, possibly, deeply driven wells where the water entering them is subject to effective ground filtration.

317. The Impurities Of Water

The various sources to water of compounds of bodies foreign to itself, some dangerous and some not, are as follows:

1. Substances washed out of the air by falling rain. These consist mainly of nitrogen compounds, nitrates and nitrites of ammonia, carbon dioxid, and particles of dust. The latter contains germ life which is dangerous only in exceptional cases. Rain water, at least after ground filtration, and certainly that which falls after the first hour or so, must be regarded as of excelleent sanitary quality.

2. Water takes into solution substances from the soil. When the soil has not been polluted with animal matter, these impurities do not convey disease. They consist mostly of inorganic salts, carbonates, nitrates, sulfates, and chlorides, chiefly of the bases potassium, sodium, calcium, magnesium, and iron. Occasionally some of the less common metals are found, such as lithium. Ordinarily, these salts are not present in such quantities in natural water as to render them unfit for domestic use, although some springs are so charged with certain compounds, of which magnesium sulfate is an example, that their water has a pronounced medicinal effect.

Organic matter exists only in those natural waters that ooze out of or filter through, deposits of vegetable matter such as leaf mold and peat. While such material in solution may give water a dark color and make it look very impure, there is no evidence that the health of the user would thereby be affected.

3. The waters that menace human health are generally those that are contaminated by human excreta. Both private and public water supplies may become unfit for use, in this way. Numerous instances are on record where wells have become infected from nearby cases of typhoid fever, thereby causing a spread of this disease, or where public water systems polluted with infectious material have caused serious epidemics of various maladies.

318. Certain Precautions Are Necessary To Insure Sanitary Water For Domestic Use

(1) No water should be used from a well which can, by any possibility, be contaminated from a nearby cesspool, or that is so built as to allow the direct entrance of surface wash. Farm wells remote from cesspools, with proper covers, and into which water can flow only after it filters through several feet of soil, are a fairly safe source of water, provided the excreta of diseased persons on the farm are properly disposed of. Even if a well is distant from a cesspool, care should be taken that the soil strata do not incline from the cesspool toward the well, especially where a sandy or gravelly surface stratum is underlaid by a stratum of dense clay. In villages and cities, wells should always be regarded with suspicion, because they are liable to contamination in ways that are not easily prevented.

(2) Public water supplies are safe only when the water is conveyed in pipes from an indisputably pure source, or where, if contamination is possible, the best modern devices are adopted to render the water sanitary. Water from a lake or stream into which sewage enters, or the borders of which are thickly populated, should always be regarded as dangerous to health,and the citizens of any village or city should protest against such a supply unless it is rendered sanitary by certain modern devices. The defense of the family against suspected water is thorough boiling, to kill any disease germs which it may contain.