Tests (Physical).—1. To observe colour, stand in tall colourless glass cylinder on white ground. If very turbid allow to settle, and examine sediment by microscope for evidence of sewage contamination (linen fibres, hairs, epithelium) and for moving organisms. Slight turbidity is best noted by filling a clean quart flask and holding it towards the light with some dark object as a window pane between. Taste and odour most marked when the water is made lukewarm. 2. For poisonous metals add one drop of strong colourless ammonium sulphide to about 1000 grains of water in glass cylinder, and observe if liquid darkens. If the coloration or precipitate disappears on adding acid, it is iron; if it remains, lead or copper is present, either of which condemns the water. 3. For chlorine add couple of drops of nitric acid to a little of the water and a crystal or drop of solution of nitrate of silver. If the water turns very milky it is a bad sign; make, if possible, a comparative experiment with water of known composition. 4. The residuum, if any, of evaporation is impurity; if it be organic matter, smoke and a peculiar odour will be evolved, as the residue becomes dry and charred. 5. Neither litmus, syrup of violets, nor turmeric are discoloured or affected when moistened with pure water; if the first two are reddened it indicates an acid; if the litmus is turned blue or the turmeric is turned brown, an alkali is present. 6. If a precipitate is formed or a fur or crust deposited on the vessel during ebullition it indicates the presence of carbonates of calcium, magnesium, or iron. 7. Calcium salts produce a white precipitate with oxalate of ammonium. 8. The liquid filtered off from 7, on standing with phosphate of sodium and ammonium (microcosmic salt), gives a white precipitate if magnesium be present. 10. Tincture or infusion of galls turns water containing iron black. When this takes place both before and after the water has been boiled, the metal is present under the form of sulphate; but if it only occurs before boiling, then ferrous carbonate may be suspected, and it will be precipitated as a reddish powder by
exposure to air and heat. 11. Ferrocyanide of potassium gives a dark blue precipitate in water containing a ferric salt; and a white one, turning blue by exposure to the air, in water containing a ferrous salt. 12. If sulphuric acid be run into water and allowed to cool, and a crystal of sulphate of iron dropped into the water, a dark brown cloud round the crystal indicates nitrates; the bleaching of indigo added to the hot mixture of equal parts water and pure oil of vitriol also indicates the presence of these salts. 13. Sulphuric acid or sulphates is indicated by a soluble salt of barium throwing down a white precipitate insoluble in nitric acid.
Water, Quantitative Analysis of.—The quantitative analysis of potable water is confined to the following: total residue, hardness temporary and permanent, chlorine, ammonia, nitrates and nitrites, and organic matter.
Of these, all but the first two are intended to throw light on the organic contamination of the water. Chlorine, ammonia, and nitrates and nitrites are in themselves innocuous substances, but are estimated because they supplement the somewhat imperfect information obtained from the organic matter itself. A sewage-polluted supply being an agent in propagating zymotic diseases, a knowledge of the source of the organic matter in a water is of the highest importance.
Before passing to the mode of estimating the above items it may be desirable to explain the object of each analysis and the interpretation which may be placed on the results.
Total solid residue includes all the substance, organic or mineral, dissolved in the water. Everything beyond the two gases which enter into the combination of the water being useless, the ‘residue’ of a water is sometimes called the ‘total solid impurity.’ The less residue left by a water on evaporation the better, but a water need not be objected to for drinking purposes till the residue reaches 40 grains per gallon. For raising steam a water should not contain more than 20 grains, and should be, if possible, much less.
The hardness, or soap-wasting power of a water, is chiefly determined on economic grounds. Unless the hardness is very excessive, the hardness or softness of the water does not appear to materially affect the health of the consumer. Hardness is caused by salts of lime and magnesia. If the property of hardness be caused by the presence of bicarbonates of the above substances, the water is said to be ‘temporarily’ hard, for by boiling or adding lime as above described, the hardness may be reduced without affecting the potability of the supply; but when the hardness is due to calcium or magnesium sulphates it is called ‘permanent’ hardness, for it is not then practicable to remove the hardening ingredients without adding some more objectionable substance. The average hardness of the
four classes of pure water is shown in the analyses given above. Thames water has a total hardness of 15°, Loch Katrine water, as supplied to Glasgow, 0·70, on Clarke’s scale.
Chlorine.—Except in places near the sea, or in salt-bearing strata, an unpolluted water does not contain more than the merest trace of chlorine. Sewage, however, contains a large quantity of chlorine as sodic chloride (common salt) derived from the salt used in cooking, &c. Hence a mixture of sewage with water becomes known by the quantity of chlorine present. It is not safe to drink a water containing such an excessive quantity of chlorine as 4 grains per gallon. The chlorine in Ullswater and Thames water is ·7 and 1·1 grains per gallon respectively. Sewage has about 8 grains on the average.
Ammonia.—This determination acquires significance because it is one of the early substances produced by the decomposition of animal matter. It therefore indicates, when present in large quantities, recent contamination by sewage. Rain always contains a small amount of ammonia, and deep wells occasionally show ammonia derived from the reduction of nitrates by the oxygen-seeking organic matter. The above inferences must, therefore, be applied with caution.