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.