METALLURGY OF MERCURY. § 314. Mercury is sometimes found native in small globules disseminated throughout certain bituminous strata in the neighborhood of cinnabar. Cinnabar, the sulphide of mercury, has been previously mentioned as the chief ore of this metal. Subchloride of mercury is sometimes found in the mineral kingdom; it is commonly termed horn-mercury, and is occasionally crystallized in four-sided rectangular prisms. An iodide of mercury has been found in Mexico. EXTRACTION OF MERCURY.-The extraction of mercury is exceedingly simple. in consequence of the great volatility of the metal. At Idria and Almaden, where the metal is chiefly extracted, the cinnabar is roasted, with free access of air, in reverberatory furnaces of a particular construction; the sulphur is oxidized and converted into sulphurous acid, while the mercury passes off in vapor, and is condensed, either by passing through a long series of earthenware adapters, or in brick chambers of considerable size; if any of the sulphide passes over unchanged, it is moulded into bricks with clay, or placed in saucers, and again roasted with the next charge of ore. Mercury is also sometimes extracted by distilling the ore with iron or lime; in the latter case, sulphide of calcium and sulphate of lime are formed : 4HgS+4Ca0=3CaS+CaO.SO,+Hg, If the ore itself contain a sufficient amount of limestone, as is sometimes the case, it is only requisite to moisten it with a little water, and to distil it. In any other case, the ore is mixed with a certain amount of slaked lime. The distillation is effected in iron retorts.1 ASSAY OF THE ORES OF MERCURY.-The determination of mercury in a specimen of cinnabar is effected by mixing 100 grs. with 4 or 5 parts of dry carbonate of soda and about 10 parts of quicklime, heating the mixture in a hard glass tube placed in a combustion-furnace, and collecting the mercury which distils over; this may be washed with water by decantation, to remove any particles of lime, dried in the water-bath, and weighed. It will be found advantageous to place a little bicarbonate of soda at the closed end of the tube, so that by heating it at the conclusion of the operation, carbonic acid may be evolved, to sweep away all mercury-vapors out of the tube. PHARMACEUTICAL PREPARATIONS OF MERCURY.-Mercury in various forms of combination is frequently used in medicine. The red or nitric oxide, as it is termed, is sometimes applied externally. The suboxide has also been used. Calomel is the principal form in which mercury is administered medicinally, and since it is termed by some chemists the chloride, and by others the subchloride of mercury, it would be well if its common name (calomel) were always retained in prescriptions, since a mistake between the two chlorides would be almost inevitably fatal. Violette recommends the application of high-pressure steam to the purification of mercury; he allows the steam to pass first through an iron worm, in which its temperature is raised to about 700° F., and then conducts it into an iron retort, in which is inclosed the vessel containing the mercury; the delivery-pipe, from which the steam issues, dips into the mercury. The metal is thus raised to the proper temperature for distillation, and the resulting vapor rapidly removed from the retort by the steam, passing into an ordinary condensingapparatus. This process appears to present many advantages over the ordinary process of distillation. Corrosive sublimate (bichloride of mercury), in small doses, is an important remedial agent. White precipitate, or hydrargyri ammonio chloridum, is used as an external application. Turbith or turpeth mineral, hydrargyri oxydum sulphuricum, see p. 458, is seldom employed. The sulphide and the two iodides of mercury are found in the preparations of the Pharmacopoeia. Several preparations of mercury are employed medicinally, which are obtained by triturating metallic mercury with various substances which have no chemical action upon it, until globules of metal are no longer visible; examples of these are seen in blue pill, blue ointment, hydrargyrum cum cretâ, &c. It is yet undecided in what form the mercury exists in these preparations, some chemists asserting that they contain the metal itself, and others maintaining that it is present in the form of suboxide; the former opinion, namely, that the metal is present in a very finely divided state, seems the most feasible, for it is difficult to believe that a substance which has so feeble an affinity for oxygen at the ordinary temperature should be oxidized during the short period occupied by the trituration; perhaps only a part of the mercury is present in the form of suboxide. Sul-alembroth is a mixture of corrosive sublimate with an equal weight of chloride of ammonium, which dissolves more readily in water than pure corrosive sublimate, in consequence of the formation of a double-salt. LEAD. Sym. Pb. Eq. 103.7. Sp. Gr. 11.445. § 315. The metal now before us is of great importance, because both itself and its compounds are, and have been from remote antiquity, applied to many useful purposes. The metallurgy of lead will, as usual, be discussed at the end of the section. Preparation of Pure Lead.-In order to obtain this metal in a state of purity, nitrate of lead is calcined to expel the nitric acid, and the residual oxide of lead fused in a Hessian crucible lined with charcoal, when a button of pure metal is obtained. Or precipitated sulphate of lead may be reduced by charcoal or nascent hydrogen (p. 172). Properties.-Lead has a bluish-gray color, and when freshly cut, considerable lustre. It is exceedingly soft, may be easily scratched with the nail, or cut with a knife, and leaves a dark trace upon paper. Lead is very malleable, and may be beaten into thin leaves, but these are easily split, from the imperfect tenacity of the metal; for the same reason it cannot be drawn out into very fine wire; a wire of inch in diameter will not support more than 20 lbs. Exposed to air, lead is soon tarnished, probably becoming covered with a film of suboxide. If lead be prepared, in a very finely divided state, it will be pyrophoric. The tartrate of lead is prepared by mixing solution of tartaric acid with a slight excess of ammonia, evaporating to neutrality, and precipitating with acetate of lead; the precipitate is washed with cold water. tube (closed at one end, and constricted near the open end, so as to be readily sealed) as long as any fumes are evolved; the tube is then sealed with the blowpipe flame. If the extremity of this tube be broken off, and the mixture of finely divided lead, with carbon, be scattered into the air, it burns with a red flash. Heated in close vessels, lead fuses at about 635° F. (335° C.), and emits perceptible vapors at a red heat; however, it is not sufficiently volatile to be distilled. If fused lead be allowed to cool slowly, it crystallizes in octohedra. Lead is often deposited in crystals, when slowly precipitated from its solutions by various metals; the so-called Saturn's-tree is obtained by suspending a bundle of zinc-turnings in a solution of acetate of lead, acidified with acetic acid, when the crystals of lead are deposited upon the zinc, giving it a pretty arborescent appearance. When fused in air, lead oxidizes rapidly, becoming covered first with an iridescent pellicle, and subsequently with a yellow powder, which is litharge, or (prot-) oxide of lead; at a red heat the litharge fuses, and must be removed from the surface, if a continuous oxidation be required. Lead is capable of dissolving a certain amount of oxide of lead, which renders it harder, hence the alteration in physical properties which the metal suffers when long fused in contact with air. Pure lead is not affected by perfectly pure water (free from air), at the ordinary temperature; but if air be present, the metal is oxidized at its expense, and the oxide thus formed combines with the carbonic acid accompanying the air, to form a basic carbonate of lead, which is deposited as a coating of minute crystals upon the metal. The water will then be found to contain lead; but if saline matters (especially such as may yield insoluble lead-compounds, above all, the sulphates) be present in considerable quantity, no lead will be found in solution, for a film of an insoluble lead-salt (e. g. the sulphate) will be formed upon the surface of the metal, and will protect it from further oxidation. Lead decomposes steam slowly, at a white heat, litharge being formed. It does not decompose water in presence of acids. Lead is scarcely attacked by hydrochloric or dilute sulphuric acid; concentrated sulphuric acid dissolves it, with the aid of heat, as sulphate of lead, sulphurous acid being evolved: Pb+2(HO.SO,)=PbO.SO,+SO,+2HO. Concentrated nitric acid acts but slowly upon lead, a film of nitrate of lead, insoluble in that acid, being formed; dilute nitric acid, especially if heated, dissolves the metal rapidly, as nitrate. § 316. This oxide is supposed by some chemists to compose the dark film which forms upon the surface of lead when exposed to air. The suboxide may be prepared by heating oxalate of lead (PbO.CO,), in an oil-bath, to about 572° F. (300° C.), as long as any gas (carbonic acid and carbonic oxide) is disengaged: 2(PbO.C,O,)=Pb20+3CO,+CO. Suboxide of lead is a black powder; when heated in close vessels to a temperature exceeding 700° F. (370° C.), it is decomposed into metallic lead and oxide of lead. If heated in air, it burns, and is converted into the oxide. Suboxide of lead is insoluble in water; acids and alkalies decompose it into oxide of lead, which dissolves, and metallic lead. OXIDE, OR PROTOXIDE, OF LEAD, PbO. Eq. 111.7. MASSICOT (the oxide which has not been fused). LITHARGE (crystalline oxide obtained by fusion). Preparation. Some details relating to the preparation of this compound upon a large scale will necessarily be given when we describe the reduction of lead from its ores; suffice it for the present to say that it is produced by the oxida tion of lead in air, under the influence of a high temperature. Pure oxide of lead may be obtained by strongly heating the nitrate. By dissolving oxide of lead in caustic soda, and slowly evaporating the solution, white dodecahedral crystals are obtained. When hydrated oxide of lead is boiled with a quantity of potassa insufficient to dissolve it, it is converted into a brownish-yellow, crystalline anhydrous oxide, which becomes of a pale yellow color when heated. If a solution of caustic soda be boiled with an excess of litharge, the clear liquid deposits, on cooling, rose-colored nearly cubical crystals of oxide of lead; if these be heated to dull redness, and allowed to cool slowly, they remain red, but if suddenly cooled, become yellow. Properties. From what we have said above, it will be seen that the physical properties of the oxide of lead differ much according to the method of preparation; thus it may vary in color from white to red or yellow; all these varieties occur in the litharge of commerce, which often contains traces of silver, and occasionally of copper. The different varieties of the oxide give, when powdered, a product having the ash-gray color of litharge. Oxide of lead, as obtained by the calcination of the nitrate, has a yellow color, which becomes darker, and ultimately brownish-red on heating; it fuses at an intense red heat, and is somewhat volatile at a very high temperature. On cooling from the fused state, it crystallizes in shining plates (litharge). The true primitive form of oxide of lead appears to be the octohedron with a rhombic base. When exposed to air, it slowly absorbs carbonic acid. Heated in air to about 572° F. (300° C.) it absorbs oxygen, and is converted into minium, a compound of oxide with binoxide of lead. Oxide of lead is slightly soluble in pure water; about 7000 parts of the latter take up 1 part of oxide of lead; the solution has an alkaline reaction. The presence of a small quantity of saline matter hinders the solution of the oxide. Water holding sugar in solution is capable of dissolving a considerable quantity. Oxide of lead is a powerful base, forming numerous well-defined salts. The soluble neutral salts have an acid reaction. It is also capable of dissolving in alkaline solutions, forming compounds, some of which are crystallizable, and have received the name of plumbites, though the powerful basic properties of the oxide of lead appear to forbid the supposition that it should ever be capable of playing the part of an acid; these are more probably double compounds similar to those which we occasionally see formed between powerful bases. Oxide of lead is very easily reduced by hydrogen or carbon, with the aid of heat. Silicic acid, at a high temperature, readily combines with oxide of lead; hence litharge should not be fused in earthen crucibles, since it corrodes them rapidly. Oxide of lead is isomorphous with baryta and lime, which, indeed, in many of its chemical relations, it much resembles. Uses.-Litharge is employed in preparing white-lead, in the manufacture of glass, and in glazing some kinds of earthenware. A compound of litharge with lime is sometimes used for dyeing the hair of a purplish-black color; the color is due to the production of sulphide of lead from the sulphur existing in hair. It is also an important reagent in assaying. Hydrated oxide of lead, PbO.HO, is obtained as a white crystalline precipitate when a solution of a lead salt is decomposed by a caustic alkali; it is easily dehydrated by heat, and dissolves in the alkalies. NITRITE OF LEAD, PbO.NO,.-This salt is prepared by passing carbonic acid through an aqueous solution of basic nitrite of lead, filtering off the carbonate of lead which is precipitated, and evaporating the solution, when yellow prismatic crystals of the nitrite are deposited. Nitrite of lead is employed for preparing other nitrites by double decomposition. Basic nitrite of lead, 4PbO.NO,,HO, is obtained by boiling a solution of nitrate of lead for a considerable period with an excess of metallic lead. It is a crystalline salt of a pink color; its solution has a very alkaline reaction. Another basic nitrite, 2PbO.NO,,HO, is deposited in yellow needles when the salt 2PbO.NO, is boiled with metallic lead. NITRATE OF LEAD, PbO.NO. Preparation. In order to prepare this salt, metallic lead, the oxide, or its carbonate is dissolved in nitric acid, and the solution allowed to crystallize. Properties.-Nitrate of lead crystallizes in hard anhydrous octohedra, which are sometimes transparent and sometimes opaque. They are unalterable by exposure to air. When heated, nitrate of lead decrepitates, fuses, and is decomposed into oxide of lead, which remains behind, whilst peroxide of nitrogen and oxygen are evolved : PbO.NO, PbO+NO,+0. Nitrate of lead is somewhat sparingly soluble in water, 1 part of the salt requiring about 7 parts of cold water; it is more soluble in hot water. This salt is almost insoluble in nitric acid; hence, when alloys containing lead are treated with that solvent, a considerable quantity of water should be added after the oxidation is completed, in order to insure the solution of the nitrate of lead. Nitrate of lead is used in the laboratory for the preparation of peroxide of nitrogen; it is also occasionally employed as a reagent. Three basic nitrates of lead have been obtained. PbO.NO,,PbO.HO is obtained in colorless crystals when a solution of nitrate of lead is boiled with oxide or carbonate of lead, and the filtered liquid allowed to cool. PbO.NO,,3(PbO.HO) is produced when the neutral nitrate is treated with a slight excess of ammonia; if digested with a large excess of ammonia, the hydrated oxide of lead is obtained. The other nitrate contains 6 eqs. PbO, and is prepared by partially decomposing the neutral nitrate with ammonia. Two compounds of nitrate with nitrite of lead are known; they are prepared by dissolving lead in a solution of the nitrate. If a very dilute solution of nitrate of lead be digested at about 158° F. (70° C.) with a quantity of metallic lead in the proportion of 1 eq. for each equivalent of nitrate, the metal is gradually dissolved, forming a yellow solution, which, |