_Domestic Purification of Water._ Something may here be said, from a bacteriological point of view, relative to what is called _domestic purification_. There is but one perfectly reliable method of sterilising water for household use, viz., _boiling_. As we have seen, moist heat at the boiling point maintained for five minutes will kill all bacteria and their spores. The only disadvantages to this process are the labour entailed and the "flat" taste of the water. Nevertheless in epidemics due to bad water it is desirable to revert to this simple and effectual purification. There are a large number of filters on the market with, in many cases, but little modification from each other. The materials out of which they are made are chiefly the following: carbon and charcoal, iron (spongy iron or magnetic oxide), asbestos, porcelain and other clays, natural porous stone, and compressed siliceous and diatomaceous earths. From an extended research in 1894 by Dr. Sims Woodhead and Dr. Cartwright Wood our knowledge of the quality of these substances as protectives against bacteria has been largely increased. They concluded that a filter failed to act in one of two ways. It was either pervious to micro-organisms, or its power of filtering became modified owing to (_a_) structural alteration of its composition, or to (_b_) the growing through of the micro-organisms. The conditions which chiefly influence the growth of bacteria through a filter appear to be the temperature, the intermittent use of the filter, and the species of bacteria. The higher the temperature and the longer the organisms are retained in the filter the more likely is it that they will grow through, and in the next usage of the filter appear in the filtrate. As to the species, those multiplying rapidly and possessing the power of free motility will naturally appear earlier in a filtrate than others. Woodhead and Wood, from their searching and most able investigation, concluded that the Pasteur-Chamberland candle filters (composed of porcelain formed by a mixture of kaolin and other clays) were the only filters out of the substances named above which were reliable and protective against bacteria. They tested over three dozen of the Pasteur filters, and "in every case these gave a sterile filtrate." Pure cholera bacillus in suspension (5000 bacilli to every cc.) and typhoid bacillus in suspension (8000 per cc.) were passed through these filters, and not a single bacillus was detectable in the filtrate. The Berkefeld filter (siliceous earth) came second on the list as an effective filter, and had but the fault of not being a "continuous" steriliser. A certain Parisian filter ("Porcelaine d'Amiante"), made of unglazed porcelain, rendered water absolutely free from bacteria. Its action was, however, very slow. Setting aside these three efficient filters, we are face to face with the fact that most filters do not produce germ-free filtrates, even though they are nominally guaranteed to do