Phosgene was first prepared by John Davy in 1812, by exposing a mixture of equal volumes of carbon monoxide and chlorine to sunlight; Davy coined the name “phosgene” from the part played by light in the reaction. While phosgene may be prepared in the laboratory by a number of other reactions, it was quite apparent that the first mentioned reaction is the most economical of these for large scale production. The reaction is a delicate one, however, and its application required extended investigation. The United States was fortunate in that, for some months previous to the war, the Oldbury Electrochemical Company had been working on the utilization of their waste carbon monoxide in making phosgene. The results of these investigations were given to the government and aided considerably in the early work on phosgene at the Edgewood plant. [Illustration: FIG. 23.—Furnace for Generating Carbon Monoxide.] Of the raw materials necessary for the manufacture of phosgene, the chlorine was provided, at first by purchase from private plants, but later through the Edgewood chlorine plant. After a sufficient supply of chlorine was assured the next question was how to obtain an adequate supply of carbon monoxide. A method for this gas had not been developed on a large scale because it had never been necessary to make any considerable quantity of it. The French and English passed oxygen up through a gas producer filled with coke; the oxygen combines with the carbon, giving carbon monoxide. The oxygen was obtained from liquid air, for which a Claude liquid air machine may be used. The difficulty with this method of preparing carbon monoxide was that the amount of heat generated was so great that the life of the generators was short. Our engineers conceived the idea of using a mixture of carbon dioxide and oxygen. The union of carbon dioxide with carbon to form carbon monoxide is a reaction in which heat is absorbed. Therefore by using the mixture of the two gases, the heat of the one reaction was absorbed by the second reaction. In this way a very definite temperature could be maintained, and the production of carbon monoxide was greatly increased. [Illustration: FIG. 24.—Catalyzer Boxes Used in the Manufacture of Phosgene.] Carbon dioxide was prepared by the combustion of coke. The gas was washed and then passed into a solution of potassium carbonate. Upon heating, this evolved carbon dioxide. Phosgene was then prepared by passing the mixture of carbon monoxide and chlorine into catalyzer boxes (8 feet long, 2 feet 9 inches deep and 11 inches wide), which are made of iron, lined with graphite and filled with a porous form of carbon. Two sets of these boxes were used. In the first the reaction proceeds at room temperature, and is about 80 per cent complete. The second set of boxes were kept immersed in tanks filled with hot water, and there the reaction is completed.