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 furnaces and condensers and to avoid the salivation of the workmen. (6) The condenser should be easily and quickly cleaned during the operation of the furnace. (7) Both furnaces and condensers should have inclined iron plates in their foundations to prevent the infiltration of mercury. (8) There is a great need of some substance for the construction of quicksilver condensers which shall be strong enough to be made thin, be a good conductor of heat, and resistant to abrasion and the alternate action of heat and cold. It should also resist the action of mercury and warm dilute sulphuric acid, and be not too expensive.

Quicksilver is best removed from the “soot,” not by pressure, but by the opposite treatment. A machine in use for this purpose at New Almaden, devised by Colonel von Leicht, consists of an iron bowl, perforated at the bottom, in which revolves a vertical shaft carrying a propeller blade which tosses the soot (mixed with wood ashes and a little coal oil) into the air, so that the entangled mercury is free to run out through the bottom of the bowl. The residue from which no more mercury can be extracted mechanically is returned to the roasting furnace.

The losses of treatment are: (1) Furnace loss, which is easily reduced to nothing, and (2) condenser loss, which can never be zero. The latter consists of mercury lost as vapour and as mist, and its minimum amount is determined not by the richness of the ore but by the volume of escaping gases, their velocity and temperature. The percentage of loss will be higher with a poor than a rich ore. On a 3% ore the losses need not exceed 3 or 4% ore content. On a 1% ore they will run from 5 to 10%. But in poorly arranged plants under bad management they may easily be doubled or even trebled. The Huettner and Scott fine-ore furnace costs with condensers in California about $30,000, and roasts from 30 to 45 tons of ore (from 2 in. to dust) in 24 hours at a cost of from $1 to $0·62 per ton.

Purification.—Commercial mercury, as a rule, only needs to be forced through chamois leather or allowed to run though a very fine hole to become fit for all ordinary applications; but the metal, having the power of dissolving most other metals, is very liable to get contaminated, and requires then to be purified. For this purpose many chemical methods have been proposed; the commonest consist in allowing the metal to fall in a very fine stream through a column of a mixture of nitric acid and mercurous nitrate, or of sulphuric acid, or of potassium bichromate and sulphuric acid; the metal being subsequently dried and filtered through a perforated paper filter. The only really exhaustive method is distillation in a vacuum out of a glass apparatus. Many forms of apparatus have been devised to effect this. Recent researches have shown, however, that the metal so obtained is not chemically pure, there being found in the distillate traces of other metals. Absolutely pure mercury does not at all adhere to any surface which does not consist of a metal soluble in mercury. Hence the least quantity of it when placed on a sheet of paper, forms a neatly rounded-off globule, which retains its form on being rolled about, and, when subdivided, breaks up into a number of equally perfect globules, which tend to coalesce when sufficiently near to each other. The presence in it of the minutest trace of lead or tin causes it to “draw tails.” A very impure metal may adhere even to glass, and in a glass vessel, instead of the normal convex, form an irregular flat meniscus.

Properties.—Pure mercury is a freely flowing liquid, which does not wet objects placed in it, and has a silvery white colour and perfect metallic lustre; in very thin layers it transmits a bluish-violet light. It freezes at about –39° C. (Mallet gives –38·85°; Hutchins, –39·44°) with contraction, and the formation of a white, very ductile and malleable mass, easily cut with a knife, and exhibiting crystals belonging to the cubic system. When heated the metal expands very uniformly, and vaporizes at about 360°; the volatility is generally increased by the presence of impurities; its high expansion and the wide range of temperature over which it is fluid render it especially valuable as a thermometric fluid (see ). The vapour is colourless, and its density points to the conclusion that the molecules are monatomic. Its specific gravity at 0° is 13·5959, i.e. it 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 ); its electrical conductivity is involved in the definition of the ohm (see ); 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 and Scheele. The halogen elements 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 has no 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 consequently in aqua regia.

Mercury readily dissolves many metals to form a class of compounds termed amalgams, which have considerable applications in the arts.

Compounds of Mercury.

Mercury forms two well-defined series of salts—the mercurous salts derived from the oxide Hg2O, and the mercuric salts from the oxide HgO; the existence of these salts can hardly be inseparably connected with a variable valency, i.e. that mercury is monovalent in mercurous, and divalent in mercuric compounds, for according to Baker mercurous chloride or (q.v.) has the formula Hg2Cl2.

Mercurous Oxide, Hg2O, is an unstable dark-brown powder formed when caustic potash acts on calomel; it is decomposed by light or on trituration into mercury and mercuric oxide Mercuric oxide, HgO, occurs in two forms: it is obtained as a bright-red crystalline powder (also known as “red precipitate,” or as mercurius praecipitatus per se) by heating the metal in air, or by calcining the nitrate, and as an orange-yellow powder by precipitating a solution of a mercuric salt with potash; the difference is probably one of subdivision. The yellow form is the most reactive and is transformed into the red when heated to 400°. If the red oxide be heated it becomes black, regaining its colour on cooling, and on further heating to 630° it decomposes into, mercury and oxygen. It is slightly soluble in water, to which it imparts an alkaline reaction and strongly metallic taste. A peroxide is obtained as a brown solid from mercury and slightly acid 30% hydrogen peroxide at low temperatures.

Mercurous and mercuric chlorides, known respectively as (q.v.) and (q.v.), are two of the most important salts of mercury. Mercurous bromide, Hg2Br2, is a yellowish-white powder, insoluble in water. Mercuric bromide, HgBr2, forms white crystals, sparingly soluble in cold water, readily in hot, and prepared by the direct union of its components. Mercurous iodide, Hg2I2, is a yellowish-green powder obtained by heating its components to about 250°, or by triturating them with a little alcohol; it is also obtained by precipitating a solution of mercurous nitrate with potassium iodide. It is blackened by exposure to light. Mercuric iodide, HgI2, exists in two crystalline forms. By mixing solutions of mercuric chloride and potassium iodide under a microscope, yellow rhombic plates are seen to be formed which are transformed very quickly into scarlet quadratic octahedra. On heating to about 126° the red form is transformed into the yellow modification; on cooling the reverse gradually occurs, and immediately if the yellow iodide be touched. Mercuric iodide is insoluble in water, but soluble in absolute alcohol; and also in potassium iodide solution, with the formation of K2HgI4, which may be obtained in lemon-yellow crystals. A strongly alkaline solution of this salt is known as Nessler's reagent, and is specially used for determining traces of ammonia (see below). Mercuric iodide dissolves in other iodide solutions to form similar compounds; these solutions are characterized by their exceptionally high specific gravity, and hence are employed in density determinations (see ). It also forms many other double salts. Oxidation with strong nitric acid gives the iodate, Hg(IO3)2. An iodide, Hg2I3, intermediate between mercurous and mercuric iodides, is obtained as a yellow insoluble powder by precipitating mercurous nitrate with a solution of iodine in potassium iodide. Mercurous fluoride, Hg2F3, and mercuric fluoride, HgF2, are unstable substances obtained from the corresponding oxide and hydrofluoric acid.

Mercurous Nitrate, Hg2(NO3)2.2H2O, is obtained as a white crystalline salt soluble in water by dissolving the metal in cold dilute nitric acid; if the metal be in excess a basic salt Hg2(NO3)2.2HG2O.Hg2.3H2O is obtained. Several other basic salts are known. By adding ammonia to a solution of mercurous nitrate a black precipitate of variable composition, known in pharmacy as mercurius solubilis Hahnemanni, is obtained.

Mercuric Nitrate.—By dissolving mercuric oxide in strong nitric acid there is obtained a thick liquid which will not crystallize, and which gives on the addition of strong nitric acid a white precipitate of 2Hg(NO3)2.H2O. Water decomposes it to give basic salts of variable composition. By dissolving the oxide in dilute nitric acid, the basic salt Hg(NO3)2.HgO.H2O, crystallizing in needles, is obtained.

Mercurous Sulphide, Hg2S, is an unstable black powder obtained by acting with sulphuretted hydrogen, diluted with carbon dioxide, on calomel at −10°. It decomposes into mercuric sulphide and mercury at 0°. Mercuric sulphide, HgS, is one of the most important