ctionable bacterial bodies; and (3) injurious bacteria. The most
common forms of impurities are excess of organic matter and bacterial
contamination. The sanitary condition of water is greatly influenced by
the character of the soil through which it flows and the extent to which
it has been polluted by surface drainage.[88]

[Illustration: FIG. 61.--DIRT AND IMPURITIES IN A SURFACE WELL WATER.]

265. Mineral Impurities.--- The mineral impurities of water are mainly
soluble alkaline and similar compounds dissolved by the water in passing
through various layers of soil and rock. When water contains a large
amount of sodium chloride, sodium sulphate or carbonate, or other
alkaline salts, it is termed an "alkali water." Where water passes
through soil that has been largely formed from the decay of rocks
containing alkaline minerals, the water dissolves some of these minerals
and becomes alkaline. The kind of alkali determines the character of the
water; in some cases it is sodium carbonate, which is particularly
objectionable. The continued use of strong alkali water causes digestion
disorders, because of the irritating action upon the digestive tract.
Hard waters are due to the presence of lime compounds. In regions where
limestone predominates, the carbon dioxid in water acts as a solvent,
producing hard waters. Waters that are hard on account of the presence
of calcium carbonate give a deposit when boiled, due to liberation of
the carbon dioxid which is the material that renders the lime soluble.
Calcium sulphate, or gypsum, on the other hand, imparts permanent
hardness. There is no deposit when such waters are boiled. A large
number of minerals are found in various waters, often sufficient in
amount to impart physiological properties. Water that is highly charged
with mineral matter is difficult to improve sufficiently for household
purposes. About the only way is by distillation.[89]

266. Organic Impurities.--Water that flows over the surface of the
ground comes in contact with animal and vegetable material in various
stages of decay, and as a result some is dissolved and some is
mechanically carried along by the water. After becoming soluble, the
organic matter undergoes further chemical changes, as oxidation and
nitrification caused by bacteria. If the organic matter contain a large
amount of nitrogenous material, particularly of proteid origin, a series
of chemical changes induced by bacterial action takes place,