Filters are now regarded as essential in every hospital, but it is quite possible for too much confidence to be placed in filters. The filter that will run fast is likely to be the favorite, whether it really filters or not. Elmer G. Horton, B. S., bacteriologist to the Ohio State Board of Health, in speaking of the weaknesses of domestic filters, says: “If the rate of filtration is not moderately fast, the consumer will not buy. The maker realizes this, and slights efficiency in order to gain a more rapid flow. The filtering material is made coarser or more porous, and, with the resistance decreased, the water passes through more readily, and so do the bacteria. “No filter will continuously yield a perfect effluent without being often cleaned. Whatever a filter takes out of the water remains in the filter until removed by cleansing; and if that cleansing be delayed the filter will dispose of the accumulated filth by way of the passing water. We must expect an unclean or foul filter to yield an effluent of like character, although that effluent may appear clear. “After all, the more harmful things in a water are not so much the easily discernible ones—mud, floating vegetation, iron, odors, etc., but the invisible living micro-organisms, and especially those forms capable of producing disease in man. It is true, and greatly to be regretted, that clearness is the criterion by which the average citizen measures the efficiency of his filter, but let it never be forgotten that the clearness of a water is no guarantee of its purity. This bottle of water which I pass around is clear enough, and no doubt looks good enough to drink, but it actually contains not thousands but millions of typhoid fever bacilli. “A sample of filtered water from the filter in the dining room of a large boarding-house was examined and found to contain five times as many bacteria as the unfiltered water. “Recently samples of filtered and unfiltered water were taken from a filter in one of the offices of the State House. The 2,200 bacteria per cc. of the untreated water had increased to 14,000 in passing through their “germ-proof” filter. Organic matter collects in the filter and serves as food, and there is developed a breeding nest for the bacteria.” Parkes in his manual of hygiene suggests this plan for the examination of water: First, a physical examination of water; second, a microscopical examination of suspended matters and sediment; third, a quantitative examination of dissolved solids; fourth, a biological examination. [Sidenote: Air]