The empty shell, after inspection, are loaded on trucks, together with the appropriate number of “boosters,” which screw into the top of the shell and thereby close them. The trucks are run by an electric storage battery locomotive to the filling unit. The shell are transferred by hand to a conveyor, which carries the shell slowly through a cold room. During this passage of about 30 minutes, the shell are cooled to about 0° F. The cooled shell are transferred to shell trucks, each truck carrying 6 shell. These trucks are drawn through the filling tunnel by means of a chain haul operated by an air motor to the filling machine. Here the liquid phosgene is run into the shell by automatic machines, so arranged that the 6 shell are at the same time automatically filled to a constant void. The truck then carries the filled shell forward a few feet to a small window, at which point the boosters are inserted into the nose of the shell by hand. The final closing of the shell is then effected by motors operated by compressed air. The filling and closing machines are all operated by workmen on the outside of the filling tunnel. [Illustration: FIG. 25.—Filling Livens’ Drums with Phosgene.] The filled shell are conveyed to the shell dump, where they are stored for 24 hours, nose down on skids, in order to test for leaks. TACTICAL USE Phosgene was first used in cloud attacks in December, 1915. These attacks continued for about nine months and were then gradually replaced, to a large extent, by gas shell attacks. Phosgene was first found in German projectiles in November, 1916. These shell were known as the D-shell. Besides pure phosgene, mixtures of phosgene and chloropicrin, phosgene and superpalite, and phosgene and diphenylchloroarsine have been found. [Illustration: FIG. 26.—Interior of a Shell Dump.] The English introduced the use of projectors in the Spring of 1917. They have a decided advantage over shell in that they hold a larger volume of gas and readily lend themselves to surprise attacks. As the Germans say, “the projector combines the advantages of gas clouds and gas shell. The density is equal to that of gas clouds and the surprise effect of shell fire is also obtained.” Toward the close of the war, the Germans made use of a mixture of phosgene and pumice stone. A captured projector contained about 13 pounds of phosgene and 5½ pounds of pumice. There seems to be some question as to the value of such a procedure. Lower initial concentrations are secured; this is due, in part of course, to the smaller volume of phosgene in the shell containing pumice. Pumice does seem to keep the booster from scattering the phosgene so high into the air, and at the same time does not prevent the phosgene from being liberated in a gaseous condition. This would indicate that pumice gives a more even and uniform dispersion and a more economical use of the gas actually used.