The subject of the purity of potable waters possesses the highest degree of importance in its sanitary relations, and, particular attention has been bestowed upon methods of analysis that would serve to indicate the character and significance of existing impurities. The earlier processes of examination, which chiefly consisted in the determination of the mineral constituents of water, while of use in furnishing an idea of the general nature of the water regarded as an inorganic solution, almost totally failed to reveal the presence of the more subtle and important organic contaminations which are now known to exert an active influence in the propagation of zymotic diseases. During the past few years, decided progress has been attained in the analytical methods employed. Little is known of the exact nature of the organic constituents present in water that has received sewage contamination. They may be either of vegetable or animal origin, and it appears to be very probable that they constitute organised germs. But, although we are still unable to determine the constitution of these deleterious ingredients, it is at present possible to approximately ascertain the hygienic character of drinking water, and to distinguish, with a fair degree of accuracy, between a good and a bad sample. In arriving at a conclusion regarding the sanitary quality of water, it is, however, also needful to take into consideration the origin and surrounding conditions which affect the chances of contamination. Most of the more recent methods of water analysis are based upon the fact, that the putrefactive decomposition of harmful organic matter is attended by the genesis of certain compounds (such as ammonia, nitrites, and nitrates), of which quantitative estimations can be made. For the purpose of ascertaining the character of a potable water, the following determinations are usually necessary:-- 1. Colour, odour, and taste. 2. Total solid matter and loss on ignition. 3. Organic matter in solution. 4. Chlorine. 5. Ammonia, free and albuminoid. 6. Nitrogen, as nitrites and nitrates. Certain precautions should be observed in the collection of samples of water intended for examination. It is indispensable for this purpose to employ scrupulously clean glass stoppered bottles, which are washed out several times with the water previous to being filled. If a well or stream is to be sampled, the bottle should be entirely immersed in the water some distance from the sides of the stream, and, if taken from a pump or pipe, the latter should be cleansed by first running a considerable quantity of the water before charging the bottle.