Service-pipes are usually made of lead, and, after moderate use, become coated on their internal surface with insoluble compounds (sulphate of lead), which prevent contamination of the water by them. When the water is not very hard, however, a slight amount of lead may be dissolved by it. It is said that Cochituate water (Boston) always contains traces of lead, but that no well-authenticated case of poisoning from this source has ever been reported. Croton water (New York), which has stood overnight in the pipes, is said to contain one tenth of a grain of lead per gallon—sufficient to produce poisoning in some instances. One case of this sort has been known. If drinking-water is drawn from tanks, they should never be lined with lead, but should be made of iron, or of wood lined with tinned and planished copper. (See “Plumbing Regulations,” 44.) Water passing through galvanized-iron pipes always contains zinc salts—not, however, in injurious amount. Such pipes soon rust. Dirt and other suspended matters should be removed by means of a =filter=. A good household filter must be made of a material which can not communicate any injurious or offensive quality to the water that passes through it; it must remove all suspended particles, so as to render the water bright and clear; it must be easy to clean, or so arranged that the filtering material can be readily renewed. The action of a filter is either mechanical or chemical; in the latter case, the organic matters contained in the water are oxidized in the filter. There are innumerable patent filters in the market, to be attached to the faucet, but these can only act as strainers. There is no material known which can be introduced into the small space of a tap-filter and accomplish any real =purification= of the water which passes through at the ordinary rate of flow. (Nichols.) The only points to be looked to, therefore, in purchasing a tap-filter, are its efficiency as a strainer, and the facility of cleansing or renewing the filtering material. Where the pressure is not too great, a closely woven cotton-flannel bag, fastened to the tap, makes as good a filter as any. For large filters (in cisterns, etc.), those which contain animal charcoal are the most efficient. In places where the drinking-water is drawn from wells, it is sometimes polluted by leakage from cesspools, privy-vaults, stables, and refuse matters lying on the surface of the ground in their vicinity. It has been demonstrated beyond a doubt that epidemics of typhoid fever have often originated and spread in this way. Even when no specific disease is caused, water polluted from such sources often causes diarrhœal disorders and various forms of indigestion in those who drink it.