These experiments merely indicate the dosage required for exceptional conditions such as it is inconceivable would ever occur in water-works practice. No information is available regarding the actual _B. typhosus_ content of waters that have caused epidemics of typhoid fever, but for the present purpose it may be assumed that the extreme condition would be a pollution by fresh sewage giving a _B. coli_ content of 1,000 per c.cm. or 200 times worse than the average condition that can be satisfactorily purified without overloading a filter plant (500 _B. coli_ per 100 c.cms.). Experiments made by the author indicate that a suspension of 1,000 _B. coli_ per c.cm. in water, in the absence of organic matter, can be reduced to a 2 _B. coli_ per 100 c.cms. standard (the U.S. Treasury Standard) by 0.1 p.p.m. of available chlorine in ten minutes at 65° F. This experiment indicates the amount of chlorine that is required for the bactericidal action only; such a dosage could never be used in practice to meet a pollution of this degree because of the accompanying organic matter. In actual practice the author has experienced the above condition but once, and on that occasion the _B. coli_ were derived from soil washings and not from fresh sewage. The amount of chlorine required for germicidal action is small, and the main factors that determine the dosage necessary to obtain this action are (1) the content of readily oxidisable organic matter, (2) the temperature of the water, (3) the method of application of the chlorine and (4) the contact period. =Oxidisable Matter.= The oxidisable matter may be divided into two classes (_a_) inorganic and (_b_) organic. The inorganic constituents naturally found in water, that are readily oxidisable, are ferrous salts (usually carbonates), nitrites, and sulphuretted hydrogen, and these react quantitatively with chlorine until fully oxidised. The oxygen value of chlorine is approximately one-quarter (actually 16: 71) the available chlorine content in accordance with the equation Cl_{2}/71 + H_{2}O = 2HCl + O/16. One part per million of available chlorine will oxidise 1.58 p.p.m. of ferrous iron; 0.197 p.p.m. of nitrous nitrogen; and 0.479 p.p.m. of sulphuretted hydrogen. TABLE VI.[A]--EFFECT OF COLOUR TEMPERATURE 63° F.