[Illustration: FIG. 39.—Diagram of Carbon Monoxide Canister, CMA3.] The metallic oxide mixture was the direct result of an observation that specially precipitated copper oxide with 1 per cent silver oxide was an efficient catalyst for the oxidation of arsine by oxygen. After a study of various oxide mixtures, it was found that a mixture of manganese dioxide and silver oxide, or a three component system containing cobaltic oxide, manganese dioxide and silver oxide in the proportion of 20:34:46 catalyzed the reaction of carbon monoxide at room temperature. The studies were extended and it was soon found that the best catalysts contained active manganese dioxide as the chief constituent. This was prepared by the reaction between potassium permanganate and anhydrous manganese sulfate in the presence of fairly concentrated sulfuric acid. It also developed that the minimum silver oxide content decreased progressively as the number of components increased from 2 to 4. The standard catalyst (Hopcalite) finally adopted for production consisted of 50 per cent manganese dioxide, 30 per cent copper oxide, 15 per cent cobaltic oxide and 5 per cent silver oxide. The mixture was prepared by precipitating and washing the first three oxides separately, and then precipitating the silver oxide in the mixed sludge. After washing, this sludge was run through a filter press, kneaded in a machine, the cake dried and ground to size. While it is not difficult to obtain a product which is catalytically active, it requires a vigorous control of all the conditions and operations to assure a product at once active, hard, dense and resistant as possible to the deleterious action of water vapor. [Illustration: FIG. 40.—Tanks and Press for Small Scale Manufacture of Carbon Monoxide Absorbent.] Hopcalite acts catalytically and therefore only a layer sufficiently deep to insure close contact of all the air with the catalyst is needed. One and a half inches (310 gm.) were found ample for this purpose. The normal activity of Hopcalite requires a dry gas mixture. This was secured by placing a three-inch layer of dry granular calcium chloride at the inlet side of the canister. Because of the evolution of heat, a cooling arrangement was also used in the Hopcalite canisters.