CHAPTER XI.

IMPURITIES OF WATER.

=Properties of Water.=—When pure, water =is colourless=, or bluish when seen in large quantity. It should be quite =inodorous=. If, after keeping it for some time in a perfectly clean vessel, or if on heating it a smell is developed, the water is bad. Its =taste= should be pleasant and sparkling from the atmospheric gases dissolved in it. Bitterness generally indicates the presence of sulphate of magnesium (Epsom salts). Saltness is always a suspicious property, except in water obtained in the neighbourhood of salt mines or brine springs, or near the sea. It should be soft to the =touch=, and should dissolve soap easily. It should be bright and =clear=, and contain no suspended matters. Clear water is not necessarily pure, but turbid water is always to be rejected; the only exception being the brownish-tinged water from moors, which is not hurtful. In all other cases, printed matter should be legible through at least 18 inches of water in a clear glass cylinder. Thoroughly dissolved organic matter is less dangerous than suspended; the turbidity of water is therefore of great importance. But water may be bright and sparkling and apparently perfectly clear, and yet highly dangerous. The most important of the physical properties of water in regard to health are the absence of smell and turbidity, and these can be ascertained by even the most inexperienced. The chemical tests for the more important impurities are given (pages 85 to 87).

The impurities of water may be classed under four heads—gaseous, mineral, vegetable, and animal.

The gases ordinarily present in water cannot properly be regarded as impurities, inasmuch as they are always present, and greatly increase its palatableness. The dissolved nitrogen and oxygen bear to each other the proportion 1·42 to 1; where sewage contamination occurs, the oxygen will be diminished or disappear, owing to oxidation of the organic matter.

The amount of carbonic acid gas in water varies greatly. It may be considerable in chalk waters, and in contaminated well-water.

=Mineral Impurities.=—Mineral impurities are dissolved by water in its course through the soil, and so will vary with the character of the latter. 1. The water obtained from _granitic_ formations contains very little mineral matter, often not more than two to six grains per gallon. _Clay slate water_ is also generally very pure, as is the water from hard _trap rocks_. 2. The water from _millstone grit_ and _hard oolite_ is very pure, often containing only four to eight grains per gallon, chiefly calcium and magnesium sulphate and carbonate. 3. _Soft sand-rock waters_ usually contain thirty to eighty grains per gallon of sodium salts, with a little lime and magnesia. 4. _Loose sand and gravel waters_ vary greatly. They may be almost free from mineral matter, or the solids may be more than seventy grains per gallon, including much organic matter. 5. Waters from the _lias clays_ vary somewhat, but commonly contain a large quantity of calcium and magnesium sulphates. 6. _Chalk waters_ generally contain from seven to twenty grains of calcium carbonate, with smaller quantities of other salts. 7. _Limestone and magnesian limestone waters_ differ from the last, in containing more calcium sulphate and less calcium carbonate, as well as much magnesium sulphate and carbonate in the dolomite districts. 8. _Selenitic waters_ contain calcium sulphate in considerable quantities. 9. _Clay waters_ usually possess the characters of water from surface wells, and are objectionable. 10. _Alluvial waters_ generally contain a large amount of various salts, including the various calcium, magnesium, and sodium salts. 11. _Artesian well water_ varies greatly in composition. It may contain a large amount of sodium and potassium salts, or a small quantity of iron, or calcium salts.

The commonest and most important mineral constituent of water is calcium carbonate, next to this calcium sulphate. These two salts are the chief causes of =hardness of water=. For practical purposes as regards use in domestic matters and in manufactures, the most important classification of waters is into _hard_ and _soft_. The degree of hardness varies within wide limits—from rain-water, which has no hardness at all, to the water from new red sandstone rocks which sometimes possesses a hardness of 90 degrees; or wells in the gravel, in which it may be as much as 152 degrees.

The following _classification of waters_, according to the degree of hardness, beginning with the least hard and gradually increasing in hardness, is from the sixth report of the Rivers Pollution Commissioners:—1. Rain-water. 2. Upland surface. 3. Surface from cultivated land. 4. River. 5. Spring. 6. Deep-well. 7. Shallow-well water.

Calcium carbonate is the most common cause of hardness, and the hardness produced by it is remediable by boiling or chemical means. Calcium carbonate (chalk) is rendered soluble in water, by the carbonic acid contained in the latter, a double bicarbonate being thus formed. The air contained in the interstices of the soil through which water passes, often contains 250 times as much carbonic acid as ordinary air. The water, in percolating through the soil, dissolves this carbonic acid, and thus is able to take up a considerable amount of chalk.

The amount of hardness in any given water is expressed in degrees, one degree being equivalent to a grain of calcium carbonate in a gallon of water. =Clarke’s soap test= is employed to detect the amount of hardness. It consists of a solution of soap of a known strength. Soft water will form a lather at once with this; hard water will only form a lather after all the calcium salt is neutralised. The amount of Clarke’s solution required before a lather is produced, will give an estimate of the amount of hardness.

=To Determine the Total Hardness= take 70 c.c. of the water and place in a stoppered bottle. From a burette run in a sufficient quantity of the standard soap solution (of which 1 c.c. equals 1° of hardness), to produce a lather on shaking the water, which remains unbroken after standing five minutes. Thus, if 7·5 c.c. of the soap solution were required, the hardness is 6°·5, as 1 c.c. of the solution is required to produce a lather in soft water. The 6°·5 means 6·5 milligrammes of calcium carbonate in 70 c.c. or 6·5 grains in a gallon of the water.

=To Determine the Permanent Hardness= boil 70 c.c. of the water in a flask for half-an-hour; allow the precipitated carbonates of calcium and magnesium to settle. Some of the latter will be re-dissolved. Carefully decant, and make up the liquid to the original 70 c.c. with distilled water. Filter through fine filter paper and estimate hardness as above.

The amount of soap wasted in consequence of the hardness of water is very great. Thus, in the case of water of one degree of hardness, as every gallon contains one grain of chalk, 7,000 gallons would contain 7,000 grains—that is, a pound. But every grain of chalk wastes 8 or 9 grains of soap; therefore, a pound of chalk, contained in 7,000 gallons, would waste about 8½ pounds of soap. But nearly all waters are harder than this, and they not uncommonly possess a hardness of 20° or more. If the hardness be 20°, the waste would be 170 pounds of soap. This quantity would be easily used annually in a family of seven or eight persons, if we include the washing of clothes. The amount of money thus wasted can be easily estimated.

Not only does soft water require less soap, but it is much more suitable for making tea and soup, and for boiling meat and vegetables—both time and fuel being saved. The reason why better tea is made when a little carbonate of soda is added to the water is that the chalk is by this means precipitated.

Carbonate of calcium is precipitated from water by boiling it; carbonic acid being driven off, the neutral salt falls to the bottom of the vessel. This is the origin of the “fur” inside kettles, which lessens their conductivity to heat, and renders necessary a greater consumption of fuel.

The chalk may also be removed by adding to the water, while still in the reservoir, some milk of lime—that is, quicklime made into a milky solution with water. This is done on a large scale at various waterworks. The reaction may be expressed thus:—

Calcium bicarbonate + calcium oxide = calcium carbonate + calcium carbonate.

The calcium carbonate, as it is precipitated, carries down with it organic and other matters, thus clearing and purifying the water.

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 magnesium 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 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 (AgCl) 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 (NaCl), 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. 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 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 thus 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 1∕2 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 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 ammonia 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.

METHODS OF WATER ANALYSIS.

The following scheme of =qualitative examination= may be followed, when an immediate opinion is required as to a water. It can only be trusted when the examination shows pollution. The following results will be obtained, for instance, when a minute quantity of urine is added to a gallon of water.

(1) The water has a faint odour.

(2) Its colour is greenish yellow in bulk.

(3) On adding a few drops of Nessler’s solution, a deep yellow colour appears.

(4) A few drops of an acid solution of permanganate of potassium become yellow when added to it.

(5) Acidify some of the water in a test-tube with nitric acid, then add silver nitrate solution. Distinct cloudiness is produced, much greater than with pure tap water.

(6) Addition of hydrochloric acid and barium chloride solution shows a much greater quantity of sulphates than the same quantity of tap water.

(7) A quantity of the water evaporated in a porcelain dish over a Bunsen’s flame gives a white residue, which speedily turns brown, with a urinous odour.

(8) Ignite the ash and add some nitric acid to oxidise it more completely. Then dissolve in distilled water, and add acid molybdate solution. A yellow colour, followed by a precipitate, indicates high phosphates and sewage pollution.

=The Complete Systematic Examination= is (_a_) physical, (_b_) bacteriological, and (_c_) chemical. Of the =physical tests=, _colour_, which should never be yellow or brown except for peaty water, is important. _Taste_ is a somewhat uncertain guide, but any badly-tasting water should be rejected. The _odour_ on heating to 80° F. in a closed flask may indicate pollution. The degree of _hardness_ can be roughly tested by rubbing between the hands. The absence of _turbidity_ is most important, as suspended impurities are more dangerous than all others. Printed matter should be legible through a column of 18 inches of water.

=Microscopally= the suspended matter in water which has been allowed to settle should be examined. Particles of vegetable matter, _e.g._ fibres of cotton, linen, cells of potato, or spiral cells of cabbage, are important as indicating domestic impurities. Bits of wool, hair, wings and legs of insects and epithelium may be discovered. The presence of algæ, diatoms and desmids, or of water-fleas, cannot be held to indicate pollution, as these are found in all running streams and in many wells. The eggs and embryos of worms are much more serious.

=Bacteria= are almost invariably present in water. The majority of these micro-organisms are harmless. But as they may number among them the germs producing diseases like enteric fever and cholera, the estimation of their number and particularly of any deviation from the number usually present in a given water, and if possible the detection of special disease-producing bacteria, are very important. This method has been made more practicable since Koch’s method of “plate cultivation” of bacteria was discovered. A small quantity of the water to be examined (kept surrounded by ice until this test is applied, to prevent multiplication of bacteria in the bottle), is mixed with sterilised gelatine which has been melted over a water bath. Then the mixture is spread in a thin layer on a glass plate and allowed to solidify, having been covered to prevent atmospheric germs from settling on the gelatine. The bacteria in the water thus become fixed, each growing and forming “colonies” dotted over the plate. These colonies can be recognised by their size and appearance, and by sub-culturing according to recognised methods. The number of such colonies, and the number of bacteria, from which, presumably, such colonies sprang in 1 c.c. of filtered Thames water is usually much below 100; in the water before filtration many thousands are present. It has been suggested that no water should be regarded as wholesome which contains more than 100 bacteria in each c.c.

This standard is, however, obviously arbitrary. Chalk water ought to have a smaller number than this; river waters may have more, and yet be wholesome. Everything depends on the character of the bacteria found. The detection of the _Bacillus coli communis_, which is present in sewage, and normally in the human intestine, is very suggestive of contamination by sewage. The bacteriological method of examination of water is still in its infancy.

CHEMICAL ANALYSIS.

(1) The =total solids= are ascertained by evaporating a given quantity of the water to dryness, and weighing.

(2) =Determination of Chlorine= (see page 81).

(3) =Determination of Hardness= (see page 80).

(4) The =Determination of Nitrites= is based on the reddish-brown colouration produced when an acid solution of metaphenylene diamine is brought into contact with a weak solution of nitrous acid. 100 c.c. of the water under examination are placed in a clean glass cylinder. Add 1 c.c. of H₂SO₄ solution (1 in 3), then 2 c.c. of metaphenylene diamine solution (5 grains in 1 litre of water with a little H₂SO₄ added). Stir well with a glass rod. If a colouration is produced at once, a smaller quantity of water must be taken, and made up to 100 c.c. with pure distilled water. The quantity of nitrous acid present is measured by introducing different fractions of a c.c. of the standard sodium nitrate solution[3] into similar glass cylinders. Each is then made up to 100 c.c. with distilled water, and the metaphenylene diamine solution and acid added as before. The colour develops slowly; time must, therefore, be allowed in matching.

(5) The =Determination of Nitrates= can be conveniently made by the following method. When phenyl-hydrogen sulphate solution is poured upon a nitrate, and sulphuric acid is formed, picric acid is formed:—

(C₆H₅)HSO₄ + 3 HNO₃ = C₆H₂(NO₂)₃OH + H₂SO₄ + 2 H₂O.

The addition of free ammonia in excess forms yellow ammonium picrate, the intensity of the colour of which is an index of the picrate, and of the nitrate from which it was produced. (_a_) Evaporate 25 c.c. of the water under examination, and (_b_) 5 c.c. of standard KNO₃ solution (containing 1 part N in 100,000) to dryness in two porcelain dishes over the water bath. Add 1 c.c. of phenyl-sulphate solution to each of these as soon as cool, stir well with a glass rod, then add 1 c.c. distilled water to each dish and 3 drops of strong H₂SO₄. Next add 25 c.c. of water to each dish, and after heating for five minutes over the water bath, add solution of ammonia to each dish in excess. A yellow colour is produced in proportion to the amount of nitrate present. Transfer the liquids to glass cylinders, and dilute each to 100 c.c. Take 50 c.c. of the solution showing the least colour, and dilute the other with distilled water, until it has the same tint.

Supposing the 100 c.c. of the sample required to be diluted to 150 c.c.—

Then the amount of N will be 150∕100 × 5∕25 = ·3 parts per 100,000.

If the two solutions (_a_) and (_b_) when diluted have the same tint, then the

Amount of N in the sample = 5∕25 = ·2 parts per 100,000.

(6) =Determination of Organic Matter.= Frankland’s =combustion process= involves the use of delicate and costly apparatus, and is seldom employed. In this process the organic carbon is evolved as carbonic acid, and the nitrogen as such.

=Wanklyn’s ammonia process= is based on the reduction of organic matter to ammonia. Part of this ammonia, =free or saline ammonia=, is simply combined with carbonic, nitric, or other acids, or is easily derived from the urea of urine, CH₄N₂O + 2H₂O = 2(NH₄)₂CO₃. Another part is only set free when the water is boiled with a strongly alkaline solution of permanganate of potassium. This is called the =albuminoid ammonia=.

In carrying out this method, a retort is taken, and after having been washed out, first with a little sulphuric acid, and then with some of the water to be analysed, 500 c.c. of the latter is put in, and the retort is connected with a condenser, and distillation begun; 50 c.c. of the distilled water is collected in a cylindrical glass tube called a Nessler glass. To this 1½ c.c. of Nessler’s reagent (mercuric iodide dissolved in a solution of potassic iodide and made alkaline by potass) are added. A rich brown colour is produced, if any ammonia is present in the distillate. The amount of ammonia in the distillate is determined by exactly imitating its colour by adding a known quantity of a standard solution of ammonium-chloride to 50 c.c. of ammonia-free distilled water, and then Nesslerising as before. Each c.c. of the dilute standard ammonium chloride solution is equivalent to ·00001 gramme of ammonia (NH₃).

If the first 50 c.c. of water distilled over gives only a slight colouration with the Nessler solution, no more water needs to be distilled over for free ammonia. If more is present, two more 50 c.c.’s must be distilled over, and the amounts of the standard solution required for imitating the test in each Nesslerised 50 c.c. added together. Thus, if 2 c.c. were needed. This

= ·00002 grm. NH₃, which is contained in 500 c.c. of the water

= ·00002 × 200 = ·004 parts saline NH₃ in 100,000 of water.

The free ammonia having been distilled over, 50 c.c. of an alkaline permanganate solution (containing 8 grammes KMnO₄ and 200 grammes of NaOH in 1100 c.c. of distilled water, boiled until the bulk is reduced to 1,000 c.c.) is poured into the retort, and distillation is begun again. Three successive 50 c.c.’s of water are collected, and then the distillation stopped. Each of these is Nesslerised, and the tint imitated as before with standard ammonia solution. The three amounts of ammonia thus found to be present are added together; and when multiplied by 200, we obtain the amount of albuminoid ammonia in 100,000 parts of water. This test is universally employed by water analysts along with the next test.

The amount of =Oxygen Absorbed= from permanganate of potassium is regarded as an approximate test of the amount of organic matter in water. Qualitatively this forms a favourite method of testing the purity of water. Two glass cylinders are taken, one filled with distilled water, one with the water to be tested. To each is added a given small amount of an acid solution of permanganate of potassium. The distilled water to which permanganate has been added will retain its pink colour; while, if the water being tested is very impure, it will speedily become decolourised. The rapidity and degree of decolourisation are a rough test of the amount of impurity. A rapid decolourisation proves the presence of organic matter having an animal origin, or of sulphuretted hydrogen, iron, or nitrites. Sulphuretted hydrogen is rarely present, and can be easily recognised by its smell; iron or nitrites are readily distinguished by their appropriate tests. In the absence of these, the rapid discolouration is an indication of animal contamination.

=To Determine the Amount of Oxygen Absorbed=, two glass-stoppered bottles, each holding about 350 c.c. are required. Into one, 250 c.c. distilled water, and into the other the same amount of the water under examination are placed. To each are then added 10 c.c. of standard permanganate of potassium solution[4] and 10 c.c. of a standard pure 25 per cent sulphuric acid solution. The two bottles, after being shaken, are placed in a water-bath at 27°C for four hours. At the end of this time add a few drops of potassium iodide solution to each bottle. The pink is now replaced by a yellow colour.[5] A standard thiosulphate solution (Na₂S₂O₃, 5H₂O)[6] is placed in a burette. From this the thiosulphate solution is run into the control bottle until the yellow colour almost disappears. Now a few drops of starch solution are added, and a blue colour is produced. The thiosulphate is then added cautiously until all the blue colour disappears. The amount of thiosulphate necessary for this is read off on the burette. The same process is repeated with the bottle containing the sample of water. The starch acts as an indicator. The amount of iodine liberated is an index of the amount of permanganate in the water, which has not been used up by its impurities. The amount of iodine liberated is measured by the amount of thiosulphate required to decolourise the solution. Thus—

2 Na₂S₂O₃ + I₂ = 2 NaI + Na₂S₄O₆.

Suppose that 20 c.c. of thiosulphate solution were required to decolourise the iodine liberated in 250 c.c. of a sample of water, while the distilled water required 25 c.c. Then 25 c.c. thiosulphate represents 10 c.c. of the permanganate solution = ·001 grains of available oxygen.

25-20 = 5

As 25 c.c. = ·001 grm. O, 5 c.c. = 5∕25 of ·001 = ·0002 grm.

This is the amount of O absorbed by 250 c.c. of the sample.

Therefore „ „ „ 100,000 „ = ·08 grm.

It is usual to make a similar determination of the amount of oxygen absorbed in fifteen minutes.

The =Interpretation of Results= of analysis is more difficult than the analysis. A single analysis may be misleading, unless the source of the water is known. Constancy in composition or analysis is almost as important a criterion of purity as the actual character of the constituents. A knowledge of the source is essential in interpreting results of analysis, as the chemical composition of water varies with its source. The following rules are only approximately correct, and are subject to the above general considerations. The _total dissolved solids_ in river-water are usually 10 to 30 parts in 100,000. Shallow well-water may contain from 30 to 200 parts or even more, and deep well-water from 20 to 70 parts.

_Saline Ammonia_ in water is commonly of animal origin, ammonia (NH₃) being one of the first products of decomposition of nitrogenous animal refuse. Upland surface water usually contains about ·002 parts per 100,000, but it may reach ·008 or more if the land over which the water passes has been manured. Shallow well-water may be free from ammonia, or this may be very excessive in amount. Deep well-water may contain no ammonia or any amount up to ·1 per 100,000. Its presence is suspicious if the albuminoid ammonia is above a trace, or if the oxygen absorbed is appreciable in amount. Generally water is suspicious if saline ammonia is up to ·01 per 100,000. _Albuminoid Ammonia_ indicates the amount of organic nitrogenous matter present in the water. It should not exceed ·005 parts per 100,000, while at the same time the saline ammonia should not usually exceed ·01 per 100,000. For _Oxygen consumed_ the following table of the weight of oxygen required for 100,000 parts of water is given by Clowes and Coleman:—

┌────────────────────┬─────────────────────┬─────────────────────────┐ │ │UPLAND SURFACE WATER.│WATER FROM OTHER SOURCES.│ │ _Water of_ ├─────────────────────┼─────────────────────────┤ │ _Great purity_ │Not exceeding ·1 │Not exceeding ·05 │ │ _Medium purity_ │From ·1 to ·3 │From ·05 to ·15 │ │ _Doubtful purity_│From ·3 to ·4 │From ·15 to ·20 │ │ _Impure_ │Exceeding ·4 │Exceeding ·20 │ └────────────────────┴─────────────────────┴─────────────────────────┘

The presence of more than 1 and still more so of 2 grains of _Chlorine_ per 100,000 of water is most suspicious, except in saline districts. _Nitrites_ if present in an appreciable quantity indicate comparatively recent contamination by sewage. In deep well-water they may be produced by deoxidation of nitrates. _Nitrates_ in upland surface waters should not be equivalent to more than ·03 of N. per 100,000; in shallow well-waters the amount varies greatly; in deep well-waters it may be excessive. As a rule it ought not to be equivalent to more than 5 parts of N. per 100,000 of water; but the significance of nitrates depends greatly on the source of the water and on the amount of the other constituents present.

Chemical analysis alone cannot ascertain the safety of a given drinking water. A minute amount of impurity inappreciable to analysis may be competent to produce disease; while another water may be drunk with impunity, which contains considerable organic matter. Chemical analysis “can tell us of impurity and hazard, but not of purity or safety” (Buchanan). An accurate opinion as to the character of a drinking water can only be expressed when one knows the amount of each chief constituent (as above), and whether these amounts deviate from the same water at other times or from other waters in the vicinity.