A great variety of filters have been invented. The principle is the same in most of them; namely, to pass the water through some finely granular material, such as gravel, manganic iron, or charcoal, with the intention of arresting the passage of suspended particles. None of these filters which are in ordinary use give complete security; and in order that they should give even comparative and temporarily good results, it is necessary that the filtering material be frequently renewed. It is now known that to confer safety, water should be bacterio-logically pure, and free from the bacilli of disease. The only type of filter known that fulfils these conditions is the Pasteur-Chamberland model, and partially some of its imitations. In this filter the water is made to pass through a solid cylinder of porcelain base, specially tested to produce complete sterilisation of the water. This filter is now used extensively in laboratories and wherever it is important that water should be entirely free from dangerous organisms.