In burning this type of smoke mixture in a cylinder, it is essential that free burning be allowed. It has been found that if combustion is at all smothered, and the smoke forced to escape through a comparatively small opening, it will be gray instead of dense black. III. Various attempts have been made to utilize the heat evolved when the Berger type smoke mixture reacts to volatilize or mechanically disperse various colored inorganic substances, and especially iodine. These were unsuccessful. Modifications, such as Strontium nitrate 1 part Powdered iron 2 parts Iodine 3 parts were also tried, but while such mixtures ignited easily, burned freely and evenly, and gave a continuous heavy purple cloud, they were very sensitive to moisture and capable of spontaneous ignition. The most satisfactory and successful colored smokes are those produced by the volatilization of organic dye materials. This practice seems to have originated with the British, who produced such smokes by volatilizing or vaporizing special dyes by igniting mixtures of the dye, lactose and potassium chlorate and smothering the combustion. In selecting dyestuffs for this purpose it was at once recognized that only those compounds can be used which are volatilized or vaporized without decomposition by the heat generated when the mixture is ignited and the combustion smothered. It was also found that the boiling point and melting or volatilization point of the colored compound must be close enough together so that there is never much liquid dye present. Since all colored organic compounds are destroyed if subjected to sufficient heat, the mixture must be so prepared and the ignition so arranged that the heat generated is not sufficient to cause this destruction. The oxidizing agents used in the combustion mixture may be either potassium or sodium chlorate. The nitrate is not satisfactory. Lactose has proven the best combustible. Powdered orange shellac is fairly satisfactory but offers no advantage over lactose. The following dyes have been found to give the best smokes: Red “Paratoner” Yellow Chrysoidine + Auramine Blue Indigo Purple Indulin (?) Green Auramine Yellow + Indigo At the beginning of the war, the only colored smoke used by the United States Army was a yellow smoke. The smoke mixture used in all signals, excepting the smoke torch, was the old arsenic sulfide mixture. The following smoke signals were adopted during the World War: Signal Parachute Rocket Yellow and Red V. B. Parachute Cartridge Yellow 25 mm. Very Parachute Cartridge Yellow 35 mm. Signal Cartridge Yellow 35 mm. Signal Cartridge Red 35 mm. Signal Pistol 25 mm. Very Signal Pistol V. B. Rifle Discharger Cut THE TACTICAL USE OF SIGNAL SMOKES