The formula recommended for the preparation of tablets is halazone 4 per cent, sodium carbonate, 4 per cent (or dried borax 8 per cent), and sodium chloride (pure) 92 per cent. Halazone and halazone tablets, when tested in the author's laboratory on the coloured Ottawa River water seeded with _B. coli_, have given rather inferior results. With 1 tablet per quart, over six hours were required to reduce a _B. coli_ content of 100 per 10 c.cms. to less than 1 per 10 c.cms. Clear well waters gave excellent results and large numbers of _B. coli_ were reduced to less than 1 per 10 c.cms. in less than thirty minutes. McCrady[A] has also obtained excellent results with various strains of _B. coli_ seeded into the colourless St. Lawrence water. [A] Private communication. BIBLIOGRAPHY [1] Raschig. Chem. Zeit., 1907, =31=, 926. [2] Rideal. S. J. Roy. San. Inst., 1910, =31=, 33-45. [3] Race. J. Amer. Waterworks Assoc., 1918, =5=, 63. [4] Race. Eng. and Contr., 1917, =47=, 251. [5] Contract Record. Aug. 15, 1917, 696. [6] Muspratt and Smith. J. Soc. Chem. Ind., 1898, =17=, 529. [7] Dakin and Dunham. Brit. Med. Jour., 1917, No. 2943, 682. CHAPTER X RESULTS OBTAINED The object of adding chlorine or chlorine compounds to water is for the purpose of destroying any pathogenic organisms that may be present. In a few instances some collateral advantages are also obtained but, in general, no other object is aimed at or secured. Chlorination does not change the physical appearance of water; it does not reduce or increase the turbidity nor does it decrease the colour in an appreciable degree. The chemical composition is also practically unaltered. When bleach is used there is a proportionate increase in the hardness but the amount is usually trifling and is without significance. During 1916 when the Ottawa supply was entirely treated with bleach at the rate of 2.7 parts per million (0.92 p.p.m. of available chlorine) the average increase in the total hardness as determined by the soap method was 2.5 parts per million. When chlorine is added to prefiltered water, as an adjunct to filtration, an increase in the number of gallons filtered per run has been noted at some plants. This increase is not so great with rapid as with slow sand filters but in some instances it has led to appreciable economies. Walden and Powell[1] of Baltimore, found that the addition of a quantity of bleach equal to approximately 0.50 p.p.m. of available chlorine enabled the alum to be reduced from 0.87 to 0.58 grain per gallon. The percentage of water used in washing the filters was also reduced, from 4.1 per cent to 2.9 per cent, whilst the filter runs were increased on the average by one hour and ten minutes. The net saving in coagulant alone amounted to 30 cents per million gallons.