The two chlorides of mercury are mercuric chloride (HgCl2) and mercurous chloride (HgCl or Hg2Cl2). The first is often called corrosive sublimate, and the second calomel. To prepare mercurous chloride, dry mercuric chloride, mercury, and a common salt solution are rubbed in a mortar to a uniform mixture, which is sublimed, that is, evaparated by heat, and the calomel is condensed in steam or air. Another method is by placing solution of mercurous nitrate in a dilute solution of common salt; the mercuric chloride in the white precipitate resulting. It must be thoroughly washed in water, and dried. Mercuric chloride is often produced by the evaporation of a mixture of dry common salt and mercuric sulphate in equal parts. Or metallic mercury may be heated in chlorine gas; or hot hydrochloric acid may be used to dissolve mercuric oxide (red precipitate), when the required material crystallises out on cooling.