But the greatest dangers in water are beyond the power of analysis to recognize: a single cholera or typhoid evacuation discharged into a reservoir may set up an epidemic, though from its extreme dilution all the room chemistry would fail to detect it. Public health records abound with such cases. It is not organic matter as such, nor matter in a state of putrescence; it is not even faecal matter in itself, that constitutes the real danger. Such pollution may or may not affect the digestion or induce diarrhoea; indeed it frequently happens that people have for years drunk with impunity a water containing palpable amounts of excrement. But the indication of such organic or faecal pollution is a danger-signal, for if healthy evacuations have gained access to the water, those of diseased persons may do so too, and the use of the water must be absolutely prohibited until the pollution has been stopped without possibility of renewal.

Some useful information is furnished by an ordinary microscopical examination of the sediment or suspended particles in water, whether inorganic (as sand. chalk, or clay) or organic (as vegetable debris, or the undigested fibres and cells contained in excrement), or living animal and vegetable organisms. Some of these organisms are constantly to be found in running waters and lakes of a high degee of purity, to the maintenance of which they may even contribute, but others are present only or chiefly in polluted waters, the organic matter in which provides their nourishment.

No report on a water can be considered complete or satisfactory without a bacteriological examination, which may be confined to an estimation of the number in a cubic centimetre of the water, proved to be living by their development under cultivation into separate "colonies"; or may be extended so as to include the endeavour to determine the presence of those known to be the cause of certain diseases, or to be constantly associated with them. In making the latter examination, any suspicious forms are transplanted, and what are called "pure cultures made, and subsecmently submitted to various staining processes by which they may be more positively identified.

But after the most exhaustive examination, chemical and biological, it must be remembered that the result applies only to the water at the moment when the sample was taken; that negative evidence as to bacteria means only that particular forms were absent from the few drops examined; and that pollution may occur at long and uncertain intervals. Hence the necessity for an inspection of the surrounding's, and a knowledge of the nature and structure of the soil, as well as of the direction of the flow and the fluctuations in the level of the ground-water. A rise of the ground-water, a single heavy fall of rain, and many an accidental occurrence, may lead to the fouling of a source that had maintained its purity for months or years. All such possibilities must be excluded before a supply can be pronounced absolutely and permanently safe.

The bottle used for taking a sample of water for analysis, should have a capacity of about half a gallon, and be perfectly clean; it should be either new. or one that has contained a mineral acid which can leave nothing adhering to the glass; such a bottle can be had of any chemist. It should be stoppered or closed with a new and clean cork. If the water to be examined be that of a well, the pump should be worked for a minute or two before the sample is taken; if from a public supply, the tap should be left running for at least three to five minutes, unless the question to be answered be its tendency to dissolve lend from the pipes; and if the water be that of an open or bucket well, or of a river or lake, the bottle should be plunged some distance below the surface. In every case it must be repeatedly rinsed oat with the water to be analysed before being finally filled, then sealed and forwarded without delay.

For bacteriological examination a much smaller quantity - a few ounces - will suffice, but the absolute cleanliness of the bottle is even more necessary; to ensure this it must be sterilized by being plunged open in boiling water for five minutes, and the stopper reinserted directly the bottle is taken out Prompt delivery of the water is imperative, for if many hours be allowed to elapse, especially in warm weather, the bacteria may undergo a material increase in number.