Regarding hypochlorite solutions a phenomenon of more scientific interest than of practical importance has been noted by Breteau[12] who found that alkaline solutions of sodium hypochlorite containing 0.94 per cent of available chlorine lost 3.6 per cent of their titer on dilution with 80 volumes of water; also that this loss was increased by the addition of small quantities of salt (sodium chloride) and more so by carbonates and bicarbonates. The author has noted similar losses on diluting bleach solutions and that the loss increased on standing. The loss can be explained by the decomposition of hypochlorous acid, in the presence of light, into hydrochloric acid and oxygen. 2HClO = 2HCl + O_{2} CHLORINE WATER. When a solution of chlorine in water is used as a germicide the chemical reactions that occur differ materially from those of hypochlorite solutions. On solution in water, hydration or solvation probably takes place with the production of heat. Cl_{2}·Aq. = 2,600 calories. Chlorine water is comparatively stable but decomposes under the influence of light in accordance with the equation Cl_{2} + H_{2}O = 2HCl + O; a similar reaction occurs in the presence of organic matter or any substance capable of oxidation. Chlorine water contains only minute traces of hypochlorous acid and there is no evidence that the endothermic reaction Cl_{2}·Aq + H_{2}O = HClO·Aq + HCl·Aq -2600 - 68,460 = -29,930 - 39,315 - 1815 occurs in a measurable degree. From thermochemical considerations hypochlorous acid and chlorine water should be about equally active as oxidising agents. 2HClO·Aq = 2HCl + O_{2} + 18,770 calories 2Cl_{2}·Aq + 2H_{2}O = 2HCl + O_{2} + 15,340 calories 2Cl_{2}· + Aq + 2H_{2}O = 2HCl + O_{2} + 20,540 calories When a solution of chlorine or hypochlorite is added to water as a germicidal agent, a variety of reactions occur the character of which is determined by the nature of the mineral and organic matter in the water and the type of chlorine compound added. The general reactions are of three types (1) oxidation of the organic matter, (2) direct chlorination of the organic matter, and (3) a bactericidal action. In the treatment of waters that contain appreciable amounts of organic matter almost all the chlorine is consumed in reaction (1) and even with filter effluents it is probably true that oxidation accounts for the greater portion of the chlorine consumed. The author has found that a dosage of 0.02 part per million of available chlorine was more effective in destroying _B. coli_ in distilled water than 0.40 p.p.m. in a water absorbing 9.5 p.p.m. of oxygen (30 mins. at 100° C.). Reaction (1) can be adequately explained by the nascent oxygen hypothesis and it is this reaction that determines the dosage required for effective sterilisation. (See Chap. III.)