=Turbidity= is usually considered to exert an effect upon the dosage required but no definite evidence has been adduced in support of this hypothesis. Turbidity is generally caused by the presence of very finely divided suspended matter, usually silt or clay, which is inert to hypochlorites. The condition that produces turbidity, however, produces a concomitant increase in the pollution and some of the organisms are embedded in mineral or organic material that prevents access of the chlorine to the organisms which consequently survive treatment. A larger concentration is required to meet these conditions but it is not necessitated by the turbidity _per se_. =Effect of Light.= Light exerts a marked photo-chemical effect on the germicidal velocity of chlorine and hypochlorites. When chlorinated water is passed through closed conduits and basins the effect of light is of course nil but in open conduits and reservoirs this factor is appreciable and reduces the necessary contact period. The effect of light on laboratory experiments made with colourless glass bottles is so marked as to make it impossible to compare the results obtained on different days under different actinic conditions. The following figures illustrate the effect of sunlight: EFFECT OF SUNLIGHT -----------------+------------------------------------- | AVAILABLE CHLORINE 0.35 P.P.M. Contact Period. +-------------------+----------------- | Exposed to Bright | Stored in Dark | Sunlight (April) | Cupboard. -----------------+-------------------+----------------- Nil | 215 | 215 30 minutes | 130 | 145 1 hour | 122 | 136 2-1/2 hours | 61 | 130 3-1/2 hours | 0 | 32 -----------------+-------------------+----------------- =Determination of Dosage Required.= The dosage required for the treatment of a water can only be accurately determined by treating samples with various amounts of chlorine and estimating the number of bacteria and _B. coli_ after an interval of time equal to that available in practice. The temperature of the water during the experiment should be the same as that of the water at the time of sampling. In order to limit the range covered by the experiments the approximate dosage can be ascertained from Diagram V if the amount of oxygen absorbed by the water is known. This diagram is calculated on the amount of available chlorine, present as chlorine or hypochlorite, that will reduce the _B. coli_ content to the U. S. Treasury standard (2 _B. coli_ per 100 c.cms.) in two hours. If the oxygen absorbed values are determined by the four-hour test at 27° C. they should be multiplied by two.