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.