Since the very earliest days when the process of bleaching was investigated it was considered to be a process of oxidation and it is not surprising that Lavoisier and his pupils, who had noted the strong decolourising action of the gas discovered previously by Scheele, should regard it as a compound that contained oxygen. They were confirmed in this view by the fact that an aqueous solution of the gas slowly evolved oxygen when placed in bright sunlight, and lost its bleaching properties. Watt disproved this and showed that the evolution of oxygen was due to the action of the chlorine on water. Cl_{2} + H_{2}O = 2HCl + O. The bleaching action was not due to the chlorine "per se" but to the nascent oxygen produced in the presence of moisture. Later, when bleach and other chlorine compounds came into use as deodourisers, their action was attributed to the oxygen produced and when their germicidal properties became known it was natural to assume that the destruction of bacteria was due to the same cause. Some of the earlier experimental work supported this view. Fischer and Proskauer[1] found that humidity played an important part in chlorine disinfection, probably because it favoured oxidation. In air saturated with moisture micro-organisms were killed by 0.3 per cent of chlorine in three hours but when the air was dry practically no action occurred. They concluded that chlorine was not directly toxic. Warouzoff, Winogradoff, and Kolessnikoff[2] were unable to confirm the results of Fischer and Proskauer and found that a mixture of chlorine gas and air killed tetanus spores in one minute. The nascent oxygen hypothesis was clearly and succinctly expressed by Prof. Leal during the hearing of the Boonton, N. J., case and the following abstracts have been taken from his evidence: "... That on the addition of bleach to water the loosely formed combination forming the bleach splits up into chloride of calcium and hypochlorite of calcium. The chloride of calcium being inert, the hypochlorite acted upon by the carbonic acid in the water either free or half bound, splits up into carbonate of calcium and hypochlorous acid. The hypochlorous acid in the presence of oxidisable matter gives off its oxygen; hydrochloric acid being left. The hydrochloric acid then drives off the weaker carbonic acid and unites with the calcium forming chloride of calcium. "That the process was wholly an oxidising one, the work being done entirely by the oxygen set free from the hypochlorous acids in the presence of oxidizable matter.... "We have used during our investigations, the term 'potential oxygen' as expressing its factor of power. When set free, it is really nascent or atomic oxygen and is, in its most active state, entirely different from the oxygen normally in water...." The reactions suggested are expressed in the following equations: (i). 2CaOCl_{2} = CaCl_{2} + Ca(OCl)_{2} (ii). Ca(OCl)_{2} + CO_{2} + H_{2}O = CaCO_{3} + 2HClO