Mercury combines directly with chlorine, and the first product of combination is _calomel_ or _mercurous chloride_, Hg_{2}Cl_{2}. This is obtained, as above stated, in the form of a white precipitate by mixing solutions of mercurous salts with hydrochloric acid or with metallic chlorides. A precipitate of calomel is also obtained by reducing a boiling aqueous solution of corrosive sublimate, HgCl_{2}, with sulphurous anhydride. It is likewise produced by heating corrosive sublimate with mercury.[22 bis] Calomel may be distilled (although in so doing it decomposes and recombines on cooling from a state of vapour); its vapour density equals 118 compared with hydrogen (= 1) (_see_ Note 23). In the solid state its specific gravity is 7·0; it crystallises in rhombic prisms, is colourless, but has a yellowish tint, turns brown from the action of light, and when boiled with hydrochloric acid decomposes into mercury and corrosive sublimate. It is used as a strong purgative. _Corrosive sublimate or mercuric chloride_, HgCl_{2}, can be obtained from or converted into calomel by many methods.[23] An excess of chlorine (for instance, _aqua regia_) converts calomel and also mercury into corrosive sublimate. It owes its name corrosive sublimate to its volatility, and, in medicine up to the present day, it is termed _Mercurius sublimatus seu corrosivus_. The vapour density, compared with hydrogen (= 1) is 135; therefore its molecule contains HgCl_{2}. It forms colourless prismatic crystals of the rhombic system, boils at 307°, and is soluble in alcohol. It is usually prepared by subliming a mixture of mercuric sulphate with common salt, HgSO_{4} + 2NaCl = Na_{2}SO_{4} + HgCl_{2}. Corrosive sublimate combines with mercuric oxide, forming an oxychloride or basic salt,[23 bis] of the composition HgCl_{2},2HgO (magnesium and zinc form similar compounds). This compound is obtained by mixing a solution of corrosive sublimate with mercuric oxide or with a solution of sodium bicarbonate. In general, with both mercurous and mercuric salts, there is a marked tendency to form basic salts.[24] [22 bis] Calomel (in Japanese 'Keyfun') has been prepared in Japan (and China) for many centuries, by heating mercury in clay crucibles with sea salt, which contains MgCl_{2} and gives HCl. The vapour of the mercury reacts with this HCl and the oxygen of the air and forms calomel: 2Hg + 2HCl + O = Hg_{2}Cl_{2} + H_{2}O. The calomel collects on the lid of the crucible in the form of a sublimate (Divers, 1894). [23] HgCl_{2} is partially converted into calomel even in the act of dissolving in ordinary water, especially under the action of light.