. Preventive medicine and hygiene. >- o o o M X CI X -r o ^,2 C^o Q>«4-( t£pJ^ IKTEEPEETATION OF SANITARY WATER ANALYSIS 877 the nitrites diminish. It is evident that storage and filtration have littleeffect upon the ehlorin content of the water. The total residue diminishesas the result of storage^, sedimentation, and filtration. It will benoted, however, that this diminution is more marked with the fixedresidue than with the volatile residue. The number of bacteria decreases as the result of storage, but themost marked decrease occurs as the result of filtration. It should berememlDcre
. Preventive medicine and hygiene. >- o o o M X CI X -r o ^,2 C^o Q>«4-( t£pJ^ IKTEEPEETATION OF SANITARY WATER ANALYSIS 877 the nitrites diminish. It is evident that storage and filtration have littleeffect upon the ehlorin content of the water. The total residue diminishesas the result of storage^, sedimentation, and filtration. It will benoted, however, that this diminution is more marked with the fixedresidue than with the volatile residue. The number of bacteria decreases as the result of storage, but themost marked decrease occurs as the result of filtration. It should berememlDcred that all the bacteria in the filtered water do not representthose which have passed the filter. The effect of the few days storageupon this water does not very materially affect the number of B. coli,but there is a marked diminution in their number as the result offiltration. The analyses of surface waters, shown in the table on page 876, withdiagram showing the locations from which samples were obtained, will CULTIVATEDFIELDS. UPLAND Fig. 108.—Diagram Showing Location or Samples. repay careful study. This table and diagram were furnished throughthe kindness of Professor Whipple. The diagram above (Fig. 108) shows the location of the samplesemployed in Analysis No. 13. CHAPTEE VTHE PUEinCATION OF WATEK The ?ways in Avhich water may be purified for practical purposes arenot many. It is worth noting that most of the advances in water puri-fication come from the development of old empiric processes. It is onlyat long intervals that a new method or principle of treatment is dis-covered that is important enough to find a permanent place in the art ofwater purification. The principal methods at present serviceable for the purification ofwater upon a large scale are: (1) storage, (2) filtration, (3) chemicals,such as ozone, hypochlorite of lime, sulphate of aluminium or iron, (4)ultra violet rays. jSTo method of purifying water can be considered to approach asatisfactory hygieni
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