=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.