The point of application in American water-works practice varies considerably (Longley[11]). In 57 per cent of those cases in which it is employed as an adjunct to filtration, it is used in the final treatment; in 26 per cent it is used after coagulation or sedimentation and before filtration; in the remaining 17 per cent it is applied before coagulation and filtration. The report of the committee adds: "The data at hand do not give any reasons for the application before coagulation. In general, an effective disinfection may be secured with a smaller quantity of hypochlorite, if it is applied after rather than before filtration. It should be noted that the storage of chlorinated water in coagulating basins, and its passage through filters, tend to lessen tastes and odors contributed by the treatment and this fact may in some cases account for its use in this way." =Contact Period.= Other things being equal, the efficiency of the treatment will vary directly, within certain limits, with the contact period. When a chlorinated water has to be pumped to the distribution mains directly after treatment, the dosage must be high enough to secure the desired standard of purity within twenty to thirty minutes. The chlorine is sometimes not completely absorbed in this period and may cause complaints as to tastes and odours. The examples given above show that the lack of contact period can be largely compensated by ensuring proper admixture. Experience has amply demonstrated that there is no necessity to use heroic doses for water that is delivered for consumption almost immediately after treatment, and that, with proper supervision, complaints can be almost entirely prevented. The general effect of the effect of contact period is shown in Tables VIII and IX on page 37. Another example of a coloured water is given in Table XI, whilst Table XII shows the results obtained with a colourless water. TABLE XI.[A]--EFFECT OF CONTACT PERIOD -----------------+------------------------------- | CHLORINE, PARTS PER MILLION. Contact Period. +-------+-------+-------+------- | 0.30 | 0.40 | 0.55 | 1.21 -----------------+-------+-------+-------+------- Nil | 3,800 | ... | ... | ... 1 minute | 1,400 | 120 | 0 | 0 10 minutes | 720 | 5 | 0 | 0 20 minutes | 35 | 0 | 0 | 0 -----------------+-------+-------+-------+------- [A] Results are _B. coli_ per 10 c.cms. TABLE XII.--EFFECT OF CONTACT PERIOD AVAILABLE CHLORINE 0.27 PART PER MILLION