According to the earlier of these patents, a halogen, more particularly iodine, is adsorbed on the carbon, whereas according tothe later of the two patents a gas containing mercury and chlorine is passed over activated carbon, as a result of which the mercury content of the gas can be reduced to 0.01 mg/m.sup.3.Moreover, from Soviet Chemical Industry 1972 (No. 8) p. 535-536, and from Tsvet.