The hardness due to calcium sulphate is not removable by boiling. It is, therefore, called permanent hardness, to distinguish it from the temporary hardness of chalk waters, which is removable by boiling. It may, however, be partially removed by the addition of washing soda to the water, as well as the nitrate and chloride of calcium which are also present. The magnesium81 salts are not removable by boiling or soda. This is shown by the fact that the “fur” inside kettles does not usually contain magnesium salts.

The amount of hardness varies greatly in different waters. In the 경산오피 deep wells in magnesium limestone, it varies from 14°-57°; in the deep wells from chalk beds, it varies from 13° to 27° and may be higher. In the water from Bala Lake, Wales, the temporary hardness is 0°·1, the permanent hardness 0°·3; in the Loch Katrine water there is no temporary hardness, 0°·9 permanent hardness; in the water from the new red sandstone (Nottingham), the temporary hardness is 9°·6, permanent 10°·2; in a chalk spring at Ryde, temporary hardness 16°·7, permanent 3°·9 (Wanklyn). The total hardness in the metropolitan water supplies from the rivers Thames and Lea, varies from 13°·2 (Southwark Company) to 14°·6 (New River Company); in the Kent Deep Wells 20°·1; in deep wells from the chalk at Brighton it varies from 12° to 13°. In all these, the hardness is chiefly temporary.

The amount of permanent hardness is always great in water from clays, as the London, Oxford, Kimmeridge, and Lower Lias clays; or in places where there are large deposits of calcium sulphate, as at Montmartre, near Paris (hence the name Plaster of Paris, given to desiccated calcium sulphate). Water from fissures in the clay often contains, also, a large amount of organic matter.

Chlorides are always present in small quantities in water. As a rule the presence of more than 1 grain per gallon, i.e. ·7 parts per 100,000 of water, indicates contamination with some animal refuse, unless the water is derived from new red sandstone, or brine springs, or from the neighbourhood of the sea. This rule does not, however, hold universally good. The absence or the presence of only a minute quantity of chlorides indicates the probable absence of animal contamination; but in exceptional cases waters of the highest organic purity may contain more chlorides than the same bulk of sewage.

To determine the amount of Chlorine take 70 c.c. of the water, add a few drops of solution of potassium monochromate (KCrO₄). From a burette run in gradually a standard solution of silver nitrate (of such a strength that 1 c.c. of the solution is equivalent to 1 milligramme of chlorine). The silver solution forms milky chloride of silver by combination with the chlorine of the chlorides in the water. When all the chlorine is thus combined, the next drop of the silver solution forms a deep red tint with the chromate. The number of c.c.’s of the silver solution required to produce this effect, equals the number of milligrammes of chlorine in 70 c.c. of water, or the number of grains of chlorine in a gallon of water. To convert this into parts per 100,000, divide by 7 and multiply by 10.

To express the amount of chlorine in terms of common salt , multiply the parts per 100,000 of chlorine by 1·65.

Nitrates in any water are suspicious; but their import varies with the circumstances under which they occur. A minute quantity of ammonium nitrate is present in nearly all waters; and the water of deep wells, especially of wells in the chalk, which, as a rule is perfectly free from sewage, may be highly charged with nitrates.82 Nitrates, when derived from sewage, represent a completely oxidised condition of its nitrogenous matter. Crude sewage generally contains no nitrates. Nitrites as a rule indicate more recent contamination, and therefore greater danger than nitrates. The presence of more than a trace of phosphates is a strong indication of contamination with sewage matter.

To determine the amount of Nitrites and Nitrates the best known methods are by the indigo, the phenol-sulphuric, the aluminium, or the zinc-copper couple tests. For nitrites the metaphenylene-diamine test is employed (page 85). The following qualitative tests will suffice for elementary work.

Nitrates. An equal amount of a solution of brucine is added to the suspected water in a test-tube, then a little pure sulphuric acid is poured down the side of the tube. A pink zone is produced if nitrates are present in considerable amount.

Nitrites. A few drops of each of diluted sulphuric acid and of metaphenylene-diamine solution give a red colour with water after standing for a few minutes, if nitrites are present.

Lead is an occasional contamination of slightly acid waters. The purest and most oxygenated waters act most readily on lead; as also those containing organic matter, nitrates or nitrites. Waters containing chlorides also act 경산오피 on lead, the chloride of lead being sufficiently soluble to produce poisonous symptoms. Upland surface waters derived from moorlands in certain districts, e.g. around Sheffield, have been found to be capable of dissolving considerable lead from lead service-pipes. The water taken first from the tap in the early morning is the most heavily charged with lead. Such waters are very soft; but other moorland soft waters do not dissolve lead. It is the water having a slightly acid reaction which possesses this property. The source of this acid, whether sulphuric acid from the products of combustion in a neighbouring town, or an organic acid, is uncertain. The plumbo-solvent action of such water is greatest in autumn, when the amount of acid is at its maximum. The property of dissolving lead is removed by passing the water on a large scale over filters of sand, spongy iron, chalk, or limestone. The addition of a small quantity of carbonate of soda has the same effect. In such districts the use of tin-lined iron pipes for domestic services has been recommended, but these are liable to fracture when bent. Pipes consisting of an outer case of lead and an inner pipe of tin with a layers of asbestos between have also been placed on the market. (See also page 68.)

Hard waters have the least action on lead; a coating of insoluble carbonate of lead being formed on the interior of the pipe, which prevents any further action. Thus the use of lead pipes for water containing carbonates or sulphate, or calcium phosphate, is comparatively safe. Hard water containing carbonic acid gas under pressure will dissolve a small amount of carbonate of lead; this explains the cases of lead poisoning from soda water which was formerly supplied in syphon bottles with lead tubes.

Lead is dissolved much more easily by water if other metals are in contact with it, as iron, zinc, or tin, galvanic action being thus83 set up. Zinc pipes containing some lead are very dangerous, especially with the distilled water used on board ships.

To determine the presence of lead in water, place a given quantity, say 100 c.c. in a white dish, and stir with a rod dipped in a solution of ammonium sulphide; if the water becomes coloured, this is generally due to the presence of iron or lead. If the colour remains after adding a drop or two of hydrochloric acid, lead is present.

To determine the amount of lead, a standard solution of lead acetate containing 1 ∕ 10 milligrammes of lead in 1 c.c., is made by dissolving ·183 gramme of crystallised lead acetate in a litre of distilled water. Place 100 c.c. of the water to be examined in a Nessler glass, acidify by a few drops of acetic acid; now add ½ c.c. of a saturated solution of ammonium sulphide. A brownish-black discoloration is produced if lead is present. To a second Nessler glass, containing 100 c.c. of distilled water, the same amounts of acetic acid and ammonium sulphide are added, and then a sufficient quantity of the standard lead solution is added, until the tints of the contents of the two Nessler glasses are identical. The amount of the standard solution added being known, we know the amount of lead in 100 c.c., and the amount per litre (1,000 c.c.) will be tenfold. Thus if 2 c.c. of the solution were required for matching colours, there were ·2 parts of lead per 100,000 of water, or ·14 grains per gallon.

Traces of Iron are sometimes present in water, giving it an astringent taste. Such water is apt to turn brown; and tea made from it is very dark.

Organic Impurities.—Organic impurities may be either vegetable or animal, the latter being by far the most dangerous. The water from moorlands is often brown, but this is not noxious. Growing plants, again, may be beneficial to water, by absorbing dissolved organic matter, and aiding its oxidation. Decaying vegetable matter is objectionable in water, and may set up diarrhœa.

The most important organic impurity of water has an animal origin—from sewage; the liquid or solid excreta (i.e. the urine or fæces) gaining accidental access to the water. Besides sewage, the eggs of various intestinal worms 경산오피 have been swallowed with water; and in a few cases, even leeches. But whatever the source of the organic matter contained in water, it contains nitrogen as an essential constituent; and tends under the influence of warmth, and therefore especially in summer, to undergo putrefactive changes, owing to the action of bacteria. These split up the more complex molecules of organic matter into simpler matter; ammoniacal compounds and salts, of which the most important are nitrites and nitrates, being final products of their activity. The detection of nitrates, and still more of nitrites, is important, as they may indicate the occurrence of previous sewage contamination. These products are quite harmless in water, except as an indication that the water has been polluted, and that possibly a certain proportion of the nitrogenous matter in the form of the complex organic matter forming the germs of such diseases as enteric fever, may still be present. Organic matter may be suspended or dissolved, the former being most dangerous to health. The germs or microbes causing disease consist of suspended, i.e. particulate matter. The amount of organic matter is determined by the amount of free ammonia84 and albuminoid ammonia which are present (Wanklyn’s process), by Frankland’s combustion process, or by Forschammer’s oxygen process; all of which give indications, rather than an exact estimate of its amount.