“The remaining solution percolates from the cathode compartment through the asbestos diaphragm into the anode compartment and flows from the end of the cell, containing from 8 to 12 per cent caustic soda, admixed with 14 to 16 per cent salt, into an open trough and into a pipe in the trench and through this pipe by gravity to the weak caustic storage tanks located near the caustic evaporator building. [Illustration: FIG. 22.—Nelson Electrolytic Cell, showing the Interior Arrangement of the Cell.] “The gas piping from the individual cell units to and including the drying equipment is of chemical stoneware. The piping is so designed that the gas can be drawn from the cells through the drying equipment at as near atmospheric pressure as possible in order that the gas can be kept nearly free of air. When operating, the suction at the pump was kept at ¹/₂₀ inch or less. The quality of the gas was maintained at a purity of 98.5 to 99 per cent. The coolers used were very effective, the gas being cooled to within one degree of the temperature of the cooling water, no refrigeration being necessary. The drying apparatus consisted of a stoneware tower of special design containing a large number of plates, and thus giving a very large acid exposure. There was practically no loss of vacuum through the drying tower and cooler. The gas pumping equipment consisted of two hydroturbine pumps using sulfuric acid as the compressing medium. The acid was cooled by circulation through a double pipe cooler similar to those used in refrigerating work. The gas was delivered under about five pounds pressure into large receiving tanks located just outside the pump rooms, and from these tanks into steel pipe mains which conducted the gas to the chemical plant.” The purity of the gas was such that it was not found necessary to liquefy it for the preparation of phosgene. PROPERTIES Chlorine, at ordinary atmospheric pressure and temperature, is a greenish yellow gas (giving rise to its name), which has a very irritating effect upon the membranes of the nose and throat. As mentioned above, at a pressure of 16.5 atmospheres at 18° C., chlorine is condensed to a liquid. If the gas is first cooled to 0°, the pressure required for condensation is decreased to 3.7 atmospheres. This yellow liquid has a boiling point of -33.6° C. at the ordinary pressure. If very strongly cooled, chlorine will form a pale yellow solid (at -102° C.). Chlorine is 2.5 times as heavy as air, one liter weighing 3.22 grams. 215 volumes of chlorine gas will dissolve in 100 volumes of water at 20°. It is very slightly soluble in hot water or in a concentrated solution of salt. Chlorine is a very reactive substance and is found in combination in a large number of compounds. Among the many reactions which have proved important from the standpoint of chemical warfare, the following may be mentioned: