Filters, containing solid materials capable of absorbing free chlorine, have also been used for removing the excess of the germicidal reagent. Iron borings and aluminium were used experimentally by Thresh[11] but the process was not commercially developed. The "De Chlor" filter, in which carbon is the active substance, has been installed at several water works in England (Reading, Exeter, Aldershot) with apparently successful results. The Reading experimental installation, described by Walker,[12] consisted of a steel drum, 8 feet 3 inches in width, the top and bottom being domed. In the upper portion, 10 feet 9 inches in depth, provision was made for thorough admixture of the bleach solution and water and a subsequent storage of thirty minutes. The lower section of the filter was divided into three compartments, the first and last of which contained graded silica; the middle compartment was filled with a layer (20 inches deep) of specially prepared granulated charcoal or carbon. The filter was operated under pressure and passed an average of 192,000 Imp. gallons per day, the rate being 32,000 Imp. gallons per square yard per day. Water from the pre-filters (polarite and sand) was treated with bleach to give a concentration of 1 p.p.m. of available chlorine and passed through the De Chlor filter. The average bacteriological results obtained during the first six months operation were as follows: Bacteria Per c.cm. B. coli Index Gelatine 3 Days at 20° C. Per 100 c.cms. Raw river water 6,775 600 Water from pre-filters 579 119 Water from De Chlor filter 33 Nil Free chlorine could not be detected by chemical tests in the filtered water which was also free from abnormal tastes and odours. It is stated that the carbon has to be removed and revivified periodically. The filter was washed about once per week, the wash water being only one-tenth of one per cent. The experimental filter was operated for nearly two years before being removed to permit the erection of larger units having a total capacity of one million Imp. gallons per day. BIBLIOGRAPHY [1] Hooker. Chloride of Lime in Sanitation, New York, 1913. [2] Griffen and Hedallen. J. Soc. Chem. Ind., 1915, =34=, 530. [3] Hale. Proc. N. J. San. Assoc., 1914. [4] Adams. J. Amer. Pub. Health Assoc., 1916, =6=, 867. [5] Ellms and Hauser. J. Ind. and Eng. Chem., 1913, =5=, 915 and 1030; _ibid._, 1914, =6=, 553. [6] Wallis. Ind. Jour. Med. Res., 1917, =4=, 797. [7] Le Roy. Comptes rend., 1916, =163=, 226. [8] Winkler. Zeit. angew. Chem., 1915, =28=, 22. [9]: Rideal, E. K. and Evans. Analyst, 1913, =38=, 353. [10] J. Amer. Pub. Health Assoc. 1915, =5=, 921. [11] Thresh. Internat. Congress Appl. Chem., 1908. [12] Walker. Jour. Roy. Inst. Pub. Health, Jan., 1911. CHAPTER VII LIQUID CHLORINE