All water used for drinking purposes is impure in the chemical sense, since it contains some inorganic matters or salts, and in most cases organic matter also. It is difficult to define precisely what should be considered an impure water in a sanitary sense, and the best we can do is to indicate probabilities in the absence of positive evidence of the production of disease by the suspected water. So far as inorganic impurities are concerned, the most important, from the sanitary point of view, are the salts of lead, magnesia, and lime, but in this country these are so rarely the cause of disease that they hardly require special notice. The physician should, however, bear in mind possibilities of lead-poisoning in some obscure cases which he will meet. The diseases due to impure water are certain specific fevers, diarrhoeal diseases, and some affections due to parasites which find entrance to the body through this medium. The water-supply is to be suspected in case of prevalence of diarrhoeal disease in a community, and especially if the outbreak be sudden and affect a number of persons and families. Sudden outbreaks of cholera, typhoid fever, or malarial fever, confined to a limited locality, should lead to careful examination of the water-supply. The impurity in water which causes these diseases is supposed to be either organic or the product of organic life, and at present the prevailing opinion is that the really dangerous impurities consist of minute living organisms or {183} germs. It is usual to estimate the impurity of water by the amount of organic matter present, but it is evident that this alone can give no positive information, since by this standard milk and soup would be very dangerous. Much depends upon the character of the organic matter, whether it is derived from the animal or vegetable kingdom--whether it is in a state of fermentation or putrefaction, etc. etc.; but the presence of specific germs in it is the most important part of all, and at the same time the most difficult to ascertain. Nitrogenous organic matter in a state of decomposition is dangerous, yet it does not always produce disease, even when ingested in comparatively large quantity, as in case of "high" game or tainted meat; and it is easy to find instances where water strongly polluted with sewage has been used for a considerable period without producing marked ill effects. It is, however, so extremely probable as to be for practical purposes certain, that water contaminated with the discharges from persons suffering from certain diseases will produce similar diseases in those who drink it, and there is also enough evidence that water containing filth of various kinds either produces or promotes disease to warrant much more attention to this subject than has heretofore been bestowed upon it.