Chlorine reacts with “hypo” (sodium thiosulfate) with the formation of sodium chloride. Hypo is able to transform a large amount of chlorine, so that it proved a very satisfactory impregnating agent for the early cloth masks. Water reacts with chlorine under certain conditions to form hypochlorous acid, HOCl. In the presence of ethylene, this forms ethylene chlorhydrin, which was the basis for the first method of preparing mustard gas. In the later method, in which sulfur chloride was used, chlorine was used in the manufacture of the chloride. Chlorine reacts with carbon monoxide, in the sunlight, or in the presence of a catalyst, to form phosgene, which is one of the most valuable of the toxic gases. Chlorine and acetone react to form chloroacetone, one of the early lachrymators. The reaction of chlorine with toluene forms benzyl chloride, an intermediate in the preparation of bromobenzylcyanide. In a similar way, it is found that the greater number of toxic gases use chlorine in one phase or another of their preparation. One author has estimated that 95 per cent of all the gases used may be made directly or indirectly by the use of chlorine. Chlorine has been used in connection with ammonia and water vapor for the production of smoke clouds. The ammonium chloride cloud thus produced is one of the best for screening purposes. In combination with silicon or titanium as the tetrachloride it has also been used extensively for the same purpose. On the other hand one may feel that, whatever bad reputation chlorine may have incurred as a poison gas, it has made up for it through the beneficial applications to which it has lent itself. Among these we may mention the sterilization of water and of wounds. In war, where stationary conditions prevail only in a small number of cases, the use of liquid chlorine for sterilization of water is impractical. To meet this condition, an ampoule filled with chlorine water of medium concentration has been developed, which furnishes a good portable form of chlorine as a sterilizing agent for relatively small quantities of water. Chlorine has also been applied, in the form of hypochlorite, to the sterilization of infected wounds. The preparation of the solution and the technique of the operation were worked out by Dakin and Carrel. This innovation in war surgery has decreased enormously the percentage of deaths from infected wounds. CHAPTER VI PHOSGENE