The following conditions also affect the solvent power in direct proportion: the duration of contact; the temperature; the pressure; and the purity of the lead. In general one may say that turbid and hard waters have the least plumbo-solvent power.
In the new England States lead poisoning from this source is a problem to be constantly borne in mind and is usually overcome by using brass pipes instead of lead pipes.
4. Source of Water is practically derived from three sources: (a) Rain water, which is the ultimate source of all water. Practically it is of domestic importance only, in the United States. If care and attention are directed to the following points a satisfactory supply will be secured: (1) The material and care of the collecting surface; (2) Wastage of the first flow, and (3) Location and construction of the storage cistern.
(b) Surface water, including the water of streams, lakes, and impounding reservoirs. Such water is greatly exposed to con tamination and its dissolved as well as suspended matter is very variable in amount. Surface water is widely utilized as a source for municipal supplies because of the vast amount available.
(c) Ground water, including the water from wells and springs. In its underground passage this water is usually subjected to a varying degree of natural filtration and is less subject to contamination. It is of importance as both a domestic and municipal source of supply.
5. The Hygienic Examination of informa tion concerning the past history of water from a given source of supply, and hence of its hygienic value, can be secured by a physical, chemical and bacterial examination. Of still greater value than such an examination however, is a thorough inspec tion of the source of the supply by one competent to judge pos sible sources of contamination. Laboratory examinations should be supplemental to such surveys. The following fea tures are usually considered in a laboratory examination of water because of their direct or indirect bearing upon health: (a) Odors.—These are in themselves harmless, but because of their offensive character result in complaints, and a lessened consumption of water. They are due to volatile essential oils liberated from the bodies of microscopic green plants (algae), and are usually most pronounced in stagnant water containing very little or no dissolved oxygen.
(b) Taste is derived from matter in solution, either mineral or gaseous. It may render a supply unpalatable.
(c) Color is of vegetable origin and is due to material in solution.
(d) Turbidity is due to material in suspension, the water is said to be muddy. It may, if excessive, result in a lessened consumption of the water.
(e) Reaction.—The reaction of water is normally alkaline, due to the carbonates and bicarbonates of calcium and mag nesium in solution. Acid waters are rare, and usually owe their acidity to either trade or mine wastes, though swamp waters may be normally acid.
(I) Total Solids.—The amount of dissolved matter in water varies widely. Quantities in excess of soo parts per million are commonly considered to render the water unfit for use.
(g) Hardness is either temporary or permanent. Temporary hardness is due to the carbonates or bicarbonates of calcium or magnesium, while permanent hardness is due to the chlorides or sulphates of calcium or magnesium. The importance of hardness is economic, inasmuch as hard waters require a much greater amount of soap to make them suitable for wash ing purposes than do soft waters.
(h) Organic matter is of itself of doubtful importance, al though some outbreaks of diarrhea have been ascribed to the presence of toxic substances of putrefactive origin. Ordinarily the chemical examination ascertains the scope of the changes which the nitrogenous compounds have undergone, as indicating something of the changes which have taken place in dissolved organic matter. The determination of nitrates and nitrites is considered of greatest importance. The nitrites sug gest recent organic pollution, though if present in small quan tities, their significance is slight. The nitrates suggest past or distant pollution. Chlorine suggests remote urinary contamination, though in interpreting these results one must bear in mind the so-called normal chlorine, i.e., the chlorine normally present in water, which is derived from sea spray or salt deposits.