Mercury

MERCURY, in chemistry, a metallic element which is easily distinguished from all others by its being liquid at even the lowest temperatures occurring in moderate climates (symbol Hg, atomic number 8o, atomic weight, 200.6, numerous isotopes 197 to 204). This metal does not appear to have been known to the ancient Jews, nor is it mentioned by the earlier Greek writers. Theophrastus (about 30o B.c.) mentions it as prepared from cin nabar by treatment with copper and vinegar ; Dioscorides obtained it from the same mineral with the aid of iron, using at the same time a primitive distillation apparatus. With the alchemists it was an important substance. When, about the beginning of the i6th century, chemistry and scientific medicine came to merge into one, this same mysterious element of "mercury" played a great part in the theories of pathology; and the metal, in the free as in certain combined states, was looked upon as a powerful medicinal agent.

Occurrence.

Mercury occurs in nature chiefly in the form of a red sulphide, HgS, called cinnabar (q.v.), which, as a rule, is accompanied by more or less of the reguline metal—the latter being probably derived from the former by some secondary re action. The most important mercury mines in Europe are those of Almaden in Spain and of Carniola, Italy, and in America those of California and Texas. Deposits also occur in Russia, the Ba varian palatinate, in Hungary, Tuscany, Transylvania, Bohemia, Mexico, Peru and in some other countries.

Mercury occurs in formations of all ages from the Archean to the Quaternary, and it has been found in both sedimentary and eruptive rocks of the most varied character, e.g., conglomerates, sandstones, shales, limestones, quartzites, slates, serpentines, crystalline schists, and eruptive rocks from the most acid to the most basic.

In former times quicksilver deposits were supposed to be formed by sublimation, but from a careful study of the California occurrences S. B. Christy was convinced as early as 1875 that this was unlikely, and that deposition from hot alkaline sulphide solutions was more probable. By treating the black mercuric sul phide with such solutions, hot and under pressure, he succeeded in producing artificial cinnabar and metacinnabarite. He also showed that the mineral water at the New Almaden mines, when charged with sulphydric acid and heated under pressure, was capable of effecting the same change, and that this method of pro duction agreed better with all the facts than the sublimation theory. (See "Genesis of Cinnabar Deposits," Amer. Jour. Science, xvii., 453.) The investigations of Dr. G. F. Becker on the "Quick silver Deposits of the Pacific" (U.S. Geol. Survey, Mon. xiii.,

1888) established the correctness of these views beyond doubt.

Properties.

Pure mercury is a coherent, mobile liquid, which does not wet glass or objects placed in it; it is silvery white with a metallic lustre; in very thin layers it transmits a bluish-violet light. It freezes at about —39° C with contraction, and the forma tion of a white, very ductile and malleable mass (sp. gravity 14.193 at m.p.) easily cut with a knife, and exhibiting crystals belonging to the cubic system. When heated the metal expands very uniformly, and vaporizes at about 36o° (b.p. mm.) ; the volatility is generally increased by the presence of impurities; its high expansion and the wide range of temperature over which it is liquid render it especially valuable as a ther mometric fluid. (See THERMOMETRY.) The vapour is colourless, and its density points to the conclusion that the molecules are monatomic. The specific gravity of liquid mercury at o° is 13.5959 and 13.690 at its m.p. Mercury is about half as heavy again as copper volume for volume, a quarter as heavy again as lead, and nearly twice as heavy as zinc ; this property is turned to account in the construction of barometers and air-pumps. Its specific heat is about 0.0333 (see CALORIMETRY) its electrical conductivity is involved in the definition of the ohm (see CON DUCTIVITY) ; and its thermal conductivity is about two-thirds that of silver.

Pure mercury remains unchanged in dry air, oxygen, nitrous oxide, carbon dioxide, ammonia and some other gases at ordinary temperatures ; hence its application for collecting and measuring gases. In damp air it slowly becomes coated with a film of mercurous oxide; and when heated for some time in air or oxygen it becomes transformed into the red mercuric oxide, which decomposes into mercury and oxygen when heated to a higher temperature; this reaction is of great historical importance, since it led to the discovery of oxygen at the hands of Priestley. The halogens and sulphur combine directly with the metal. Mercury is unattacked by dilute sulphuric acid; the strong acid, however, dissolves it on heating with the formation of sulphur dioxide and mercurous or mercuric sulphate according as mercury is in excess or not. Hydrochloric acid when concentrated has only very slight action. Dilute nitric acid readily attacks it, mercurous nitrate being formed in the cold with excess of mercury, mercuric nitrate with excess of acid, or with strong acid, in the warm. The metal dissolves in solutions containing chlorine or bromine, and conse quently in aqua regia.

Mercury dissolves many metals to form compounds termed amalgams, of considerable importance to the arts.