In cities and towns which have a common water-supply, the water sometimes contains impurities dissolved from the pipes through which it runs, or dirt and vegetable débris stirred up from the bottom of the sources of supply, or brought down into them by heavy storms or melting snows.

Service-pipes are usually made of lead, and, after moderate use, become coated on their internal surface with insoluble compounds (sulphate of lead), which prevent contamination of the water by them. When the water is not very hard, however, a slight amount of lead may be dissolved by it. It is said that Cochituate water (Boston) always contains traces of lead, but that no well-authenticated case of poisoning from this source has ever been reported. Croton water (New York), which has stood overnight in the pipes, is said to contain one tenth of a grain of lead per gallon—sufficient to produce poisoning in some instances. One case of this sort has been known. If drinking-water is drawn from tanks, they should never be lined with lead, but should be made of iron, or of wood lined with tinned and planished copper. (See “Plumbing Regulations,” 44.)

Water passing through galvanized-iron pipes always contains zinc salts—not, however, in injurious amount. Such pipes soon rust.

Dirt and other suspended matters should be removed by means of a filter. A good household filter must be made of a material which can not communicate any injurious or offensive quality to the water that passes through it; it must remove all suspended particles, so as to render the water bright and clear; it must be easy to clean, or so arranged that the filtering material can be readily renewed. The action of a filter is either mechanical or chemical; in the latter case, the organic matters contained in the water are oxidized in the filter. There are innumerable patent filters in the market, to be attached to the faucet, but these can only act as strainers. There is no material known which can be introduced into the small space of a tap-filter and accomplish any real purification of the water which passes through at the ordinary rate of flow. (Nichols.) The only points to be looked to, therefore, in purchasing a tap-filter, are its efficiency as a strainer, and the facility of cleansing or renewing the filtering material. Where the pressure is not too great, a closely woven cotton-flannel bag, fastened to the tap, makes as good a filter as any. For large filters (in cisterns, etc.), those which contain animal charcoal are the most efficient.

In places where the drinking-water is drawn from wells, it is sometimes polluted by leakage from cesspools, privy-vaults, stables, and refuse matters lying on the surface of the ground in their vicinity. It has been demonstrated beyond a doubt that epidemics of typhoid fever have often originated and spread in this way. Even when no specific disease is caused, water polluted from such sources often causes diarrhœal disorders and various forms of indigestion in those who drink it.

The determination of such pollution is a matter of great delicacy and difficulty, and can only be trusted to an expert. Waters polluted by organic matters often contain an excess of gaseous constituents, and are clear, sparkling, and palatable, presenting to the uninstructed eye no indication of impurity. There are certain tests, however, which can be used by any person of intelligence, when, if positive results are obtained, an expert should be called in to determine the source and character of the contamination.

The pollution of water by decomposing animal matters is always to be suspected, if there are evidences of the presence of chlorine or nitrogen in the water, as these are invariable constituents of animal excreta. These substances are found in combination—the former in chloride of sodium, and the latter in the so-called nitrites and nitrates.[2] Their presence is determined as follows:

Chlorine.—Tests: Nitrate of silver (twenty-five cents a drachm), twenty-four grains to one ounce of distilled water, and dilute nitric acid (ten cents an ounce). Pour a few drops of each into the suspected water. If chlorine is present, there will be a cloudy-white precipitate of chloride of silver, which will gradually turn darker. One grain of chlorine to a gallon of water gives a haze; four grains a marked turbidity, and ten grains a considerable precipitate. In case chlorine is found, and any particular source is suspected, a pailful of salt (chloride of sodium) and water may be thrown into the place from which the leakage is supposed to come, and the water again examined, after a few hours, to see whether the amount of chlorine has increased.

Nitrates.—Tests: Pure sulphuric acid and a saturated solution of sulphate of iron (copperas). Add an equal bulk of the acid to any quantity of the water in a test-tube. The mixture will become very hot. Wait until it is cool, and then pour in the iron solution gently, so that it will float above the mixed acid and water. If nitrates are present, there will be an olive-colored layer where the fluids meet.

Nitrites.—Test-mixture: Iodide of potassium (fifty cents per ounce), one part; starch, twenty parts; water, five hundred parts. Make the starch-solution first, and filter when cold; then add the iodide of potash. Add to the suspected water this mixture, and then a little dilute sulphuric acid. If nitrites are present, there will be an immediate blue color.

Organic Matters in general.—Test: Eight grains of chemically pure permanganate of potash in one ounce of distilled water. In half a pint of the suspected water in a tumbler, put one drop of the solution. If the red color disappears in one half hour, add more. For every drop that loses color in the half-pint there will be found one and a half to two grains of putrid organic matter in a gallon of the water. If the action is rapid, the matter is probably animal; if slow, vegetable.

To purify such water, if it must be used, drop in the solution until a slight red tinge remains. The organic matter is then all oxidized and rendered harmless. It is better, however to boil such water before using it for drinking.

Precautions with regard to Drinking-Water.

Do not drink water that has been standing long in lead pipes, or lead cisterns or tanks.

Filter it before drinking.

See that the current of ground-water in the well from which you get your drinking-water is from the well toward any possible source of contamination (privy-vault, cesspool, etc.), and not vice versa.

If the use of a suspected water is unavoidable, boil it first. It can be rendered palatable by an infusion of tea or coffee.