Many means have been adopted of removing impurities from natural water, in order to render it potahle. Muddy water is easily rendered clear and bright by processes of filtration on a large scale. In places where the only available water is muddy, the purification is effected in what are termed " filtering tanks." These consist of large, water-tight basins, on the bottom of which is placed a layer of small stones ; above these is placed a second layer of coarse sand or g,ravel ; over this again a layer of fine sand, and at the top a layer of river sand. The muddy water is introduced from above and filters through the several layers, collecting in the bottom one. From thence it passes into reservoirs, or shafts built vertically in the basin, and having their walls so perforated at the lower extremity that nothing but filtered water can pass through them ; this water is puftiped up from the shafts when required. Iron tubes perforated below are sometimes used instead of the laickwork reservoirs. The greater portion of the suspended impurities con tained in the water is retained in the uppermost layer of sand, which has, consequently, to be renewed from time to time.
In order to ren3ove decaying organic matters or impure gases held in solution, powdered charcoal is frequently used as the filtering medium. In this way, not only are all suspended matters elimi nated, but water which is coloured brown and possesses an offensive taste and smell, from the presence of the above matters, may be rendered clear, tasteless, and inodorous. The cost of the charcoal, however, which soon becomes impure and useless, prevents its spplication to this purpose on a large scale. And it is possible that charcoal which has become saturated with organic impuri ties at a low temperature may give up a portiou of the absorbed substance when the water to be filtered has a higher temperature. Carbon filtAars are frequently used on a small scale with great advantage for the filtration of impure waters. (See Filtration.) Drinking water should be clear and eolourlms, that is, absolutely free from suspended im purities, such as clay, organic matter, &c. It should contain small quantities of dissolved carbonate of lime, chloride of sodium, oxygen, and carbonic acid gases. It should not contain any salts of lime and magnesia, except the carbonates, nor the smallest trace of any nitrates from which the presence of ammonia or nitrogenous organic matter may be inferred. When drinking water is 'boiled to dryness, it should leave a residue of frona 10 to 30 grains of solid matter for every 100,000 grains of water, and of this quantity about one-half should he carbonate of lime. Water containing less than 10 parts of solid constituents in every 100,000 is soft and insipid, and less fitted for chinking purposes than that which contains a higher proportion. Of the free gases held in solution hy the water, it should always contain 0-8 per cent. by volume of oxygen, 0-7 per cent. of nitrogeu, and a considerable quantity of free carbonic acid. Suspended and organic impurities should invariably he removed, if present, by passing the water through a carbon filter. Indeed, no water should ever be used for drinking purpo.,es, especially in large towns, which has not been subjected to careful filtration.
Water that is to be used Lai brewing ales and porter should contain a considerable quantity of saline constituents, and principally of carbonate and sulphate of lime ; that used in brewiog the best Burton ales contains from 10 to 20 grains per gallon of each. Common salt is also a valuable constituent. There should be no organic matter. Analyses of some of the beet waters for brewing will bo found in the article on Beer. For wine-making, tbe water employed ahould contain a
smaller proportion of mineral constituents than that required for beer. The very beat would be rain water, to which the necesaary proportion of the various salts has been added. Thia, however, would be impossible in practice, and it ia found most convenient to use river or stream water, the composition of which is known and may be conatantly relied on. The water of aprings, or of atagnant ponde and marehes, which might contain putrid organic matter, either in suspension or solution, must not on any account be employed. The same conditiona apply to water which is to be need in preparing whisky, or other spirit ; it should contain a mall proportion of mineral salts ; it should be free from organic matters ; and it should always be clear and bright. For making infusiona of tea and coffee, the moat suitable ia a soft water, or one containing no mita of lime, or very little. Rain water, carefully filtered, is the beet for this purpose.
Below are given some typical analyses of watere obtained from the differcut sourcea described, namely, lakes, rivers, and deep wella The tbree following analyses, made by Professor Wanklyn, are of samples taken from the deep walla at Croydon, in Surrey. No. 1 is from the well in Waterworks Yard ; No. 2 from that in Mint Walk ; and No. 3 from the Old Well ; they are expressed in grains per gallon :— The three following analyses, made by Professor Wanklyn, show the composition of the water supplied to London by the West Middlesex, New River, and Kent companies respectively. The first is obtained from the Thames at Hampton, the second from the Lea and other rivers, and the third entirely from wells in the chalk ; the analyses are expressed in grains per gallon:— The growing importance of deep wells as sources of drinking water renders it necessary that the geological character of the strata in which they are situate should be subjected to careful investigation, and a good deal of trustworthy and interesting information has been gathered together on this head during the last few years. The influence exerted upou the water by the different strata through which it passes is very great, and hence the subject is oue of considerable importance. In its desceut through the different beds, the water passes downwards with greater or less rapidity, according to the porosity of the strata, until at length it reaohes one which is impermeable : this stratum forms a kind of floor upon which the water is stored, the quantity depending upon the thickness and extent of the strata above, and their degrees of porosity. These impermeable, or dry, strata have no influence upon the water which lies upon them further than assisting to store it. When such an impermeable bed lies upon the surface, the rain falling. upon it, not being able to penetrate, runs away and collects in brooks and streams; hence, iu districts where the upper beds are of this natnre, there iB an abundance of stream water. In its downward course, the water is completely filtered from all suspended organic and other matters, being thus rendered beautifully clear and bright ; it also dissolves a portion of the soluble salts, generally of lime, with which it comes in contact in the various strata, and becomes, in consequence, more er less " hard." The seven most important permeable or water-bearing strata in this country are : (1) Chalk and upper Greensand ; (2) Lower Greensand ; (3) Purbeck and Portland beds; (4) Coral Reg and Grit ; (5) °elites and upper Lias sands ; (6) Middle Li0.8 ; and (7) New Red Sandstone.