The tar which remains behind flows through a tube to the cistern. From the tar separator the gas goes through _scrubbers_ (fig. 12), where the gas is washed free of ammonia and part of the sulphuretted hydrogen and carbon dioxide with water. The scrubbers are tower-like vessels filled with coke or charcoal through which the gas passes from below upwards, encountering a spray of water. Several scrubbers in series are used, so that the water constantly becomes richer in ammonia. Mechanical scrubbers are much used, so-called standard washers; they are rotating, horizontal cylinders having several chambers filled with staves of wood half dipping in water. In them the same principle of making the gas meet an opposing stream of water is employed, so that the last traces of ammonia are removed from the gas. The various purifying apparatus through which the gas has to pass cause considerable resistance to its flow. Escape in various ways would occur had the gas to overcome it by its own pressure, and too long contact of the gas with the hot walls of the retorts would be detrimental. Hence an exhauster is applied to the system which keeps the pressure to the right proportion in the retorts and drives on the gas. [Illustration: FIG. 12.—Washer or Scrubber] After purification in the scrubbers _dry purification_ follows, having for its object especially removal of compounds of sulphur and cyanogen and carbon dioxide. To effect this several shallow receptacles are used, each having a false bottom upon which the purifying material is spread out. The boxes are so arranged that the gas first passes through purifying material which is almost saturated and finally through fresh material, so that the material becomes richer in sulphur and cyanogen compounds. The _gas purifying material_ formerly used was slaked lime, and it is still frequently used, but more generally bog iron ore or artificially prepared mixtures are used consisting mostly of oxide of iron. The saturated purifying material is regenerated by oxidation on spreading it out in the air and turning it frequently. After having been thus treated some ten times the mass contains 50 per cent. sulphur, and 13 to 14 per cent. ferrocyanide. [Illustration: FIG. 13.—Manufacture of Illuminating Gas. Diagrammatic view (_after Lueger_) A Retort setting and hydraulic main; B Condensers and coolers; C Exhauster; D Well; E Water tank; F Tar extractor; G Scrubber; H Purifier; I Station meter; K Gas holder; L Pressure regulator.] The _naphthalene_ in illuminating gas does not separate in the condenser, and therefore is generally treated in special apparatus by washing the gas with heavy coal tar.