366

EQUIVALENT AND ATOM OF SILVER.

tion, till the washings are quite tasteless. It may either be reconverted into nitrate by dissolving in nitric acid (p. 364), or fused in an earthen crucible with a little borax.

From the fixing solutions containing hyposulphite of soda, the silver cannot be precipitated by salt, because the chloride of silver is soluble in the hyposulphite. A piece of sheet copper left in this for a day or two will precipitate the silver at once in the metallic state.

Subchloride of silver (Ag2CI) has been obtained by the action of perchloride of iron upon metallic silver (Ag2 + Fe,Cl, = Ag2Cl + 2FeCl). It is black and insoluble in nitric acid. Ammonia decomposes it, dissolving out chloride of silver, and leaving metallic silver.

Bromide of silver (AgBr) is a rare Chilian mineral. Associated with chloride of silver, it forms the mineral embolite. It much resembles the chloride, but is somewhat less easily dissolved by ammonia.

Iodide of silver (AgI) is also found in the mineral kingdom. It is worthy of remark that silver decomposes hydriodic acid much more easily than hydrochloric acid, forming iodide of silver, and evolving hydrogen. The iodide of silver dissolves in hot hydriodic acid, and is deposited in crystals on cooling. By adding nitrate of silver to iodide of potassium, the iodide of silver is obtained as a yellow precipitate which, unlike the chloride, does not dissolve in ammonia. Iodide of silver dissolves in a boiling saturated solution of nitrate of silver, and the solution, on cooling, deposits crystals having the composition AgI. AgO. NO, which are far more sensitive to the action of light than iodide of silver itself, a circumstance which is taken advantage of by photographers. The crystals are decomposed by water, with separation of iodide of silver.

Sulphide of silver (AgS) is found as silver glance, which may be regarded as the chief ore of silver; it has a metallic lustre, and is sometimes found in cubical or octahedral crystals. The minerals known as rosiclers or red silver ores contain sulphide of silver combined with the sulphides of arsenic and antimony. The black precipitate obtained by the action of hydrosulphuric acid upon a solution of silver is the sulphide of silver. It may also be formed by heating silver with sulphur in a covered crucible. Sulphide of silver is remarkable for being soft and malleable, so that medals may even be struck from it. It is not dissolved by diluted sulphuric or hydrochloric acid, but nitric acid readily dissolves it. Metallic silver dissolves sulphide of silver when fused with it, and becomes brittle even when containing only 1 per cent. of the sulphide.

265. Equivalent and atomic weights of silver.-When finely divided silver is heated in a current of chlorine gas, 108 parts by weight of silver combine with 35.5 parts (1 eq.) of chlorine; hence 108 is taken to represent the equivalent of silver. The specific heat of silver shows its atomic weight to be represented by the same number as its equivalent, so that it is a monatomic element, and the atomic formula of its principal compounds are written thus: Oxide of silver, Ag; chloride of silver, AgCl; sulphide of silver, Ag,S.

EXTRACTION OF MERCURY FROM CINNABAR.

MERCURY.

367

266. Mercury (quicksilver) is the only metal which is liquid at the ordinary temperature, and since it requires a temperature of 39° below zero F. to solidify it, this metal is particularly adapted for the construction of thermometers and barometers. Its high boiling point (662° F.) also recommends it for the former purpose, as does its high specific gravity (13.54) for the latter, a column of about 30 inches in height being able to counterpoise a column of atmospheric air having the same sectional area, and a height equal to that of the atmosphere above the level of the sea. The symbol for mercury (Hg) is derived from the Latin name for this element, hydrargyrum (voop, water, referring to its fluidity, apyvpov, silver). Mercury is not met with in this country, but is obtained from Idria (Austria), Almaden (Spain), China, and New Almaden (California). It occurs in these mines partly in the metallic state, diffused in minute globules or collected in cavities, but chiefly in the state of cinnabar, which is a sulphide of mercury (HgS).

The metal is extracted from the sulphide at Idria by roasting the ore in a kiln (fig. 252), which is connected with an extensive series of con

densing chambers built of brick-work. The sulphur is converted, by the air in the kiln, into sulphurous acid gas, whilst the mercury passes off in vapour and condenses in the chambers.

At Almaden the extraction is conducted upon the same principle, but the condensation of the mercury is effected in earthen receivers (called aludels) opening into each other, and delivering the mercury into a gutter which conveys it to the receptacles.

The cinnabar is placed upon the arch (A, fig. 253) of brick-work, in which there are several openings for the passage of the flame of the wood fire kindled at B; this flame ignites the sulphide of mercury, which burns in the air passing up from below, forming sulphurous acid gas and vapour of mercury (HgS+ O2 = Hg + SO2), which escape through the flue (F) into the aludels (C), where the chief part of the mercury

Fig. 253.

condenses, and runs down into the gutter (G). The sulphurous acid gas

escapes through the flue (H), and any mercury which may have escaped condensation is collected in the trough (D), the gas finally passing out through the chimney (E), which provides for the requisite draught.

In the Palatinate, the cinnabar is distilled in cast-iron retorts with lime, when the sulphur is left in the residue as sulphide of calcium, and sulphate of lime, whilst the mercury distils over

4HgS 4CaO = 3CaS+ CaO. SO, + Hg, .

The mercury found in commerce is never perfectly pure, as may be shown by scattering a little upon a clean glass plate, when it tails or leaves a track upon the glass, which is not the case with pure mercury. Its chief impurity is lead, which may be removed by exposing it in a thin layer to the action of nitric acid diluted with two measures of water, which should cover its surface, and be allowed to remain in contact with it for a day or two, with occasional stirring. The lead is far more easily oxidised and dissolved than the mercury, though a little of this also passes into solution. The mercury is afterwards well washed with water and dried, first with blotting-paper, and then by a gentle heat. Mercury is easily freed from mechanical impurities by filtering it through a cone of paper, round the apex of which a few pin-holes have been made.

267. Although mercury in its ordinary condition is not oxidised by air at the ordinary temperature, it appears to undergo a partial oxidation when reduced to a fine state of division, as in those medicinal preparations of the metal which are made by triturating it with various substances which have no chemical action upon it, until globules of the metal are no longer visible. Blue pill and grey powder, or hydrargyrum cum cretâ, afford examples of this, and probably owe much of their medicinal activity to the presence of one of the oxides of mercury.

268. Uses of mercury.—One of the chief uses to which mercury is devoted is the silvering of looking-glasses, which is effected by means of an amalgam of tin in the following manner: a sheet of tin-foil of the same size as the glass to be silvered is laid perfectly level upon a table, and rubbed over with metallic mercury, a thin layer of which is afterwards poured upon it. The glass is then carefully slid on to the table, so that its edge may carry before it part of the superfluous mercury with the impurities upon its surface; heavy weights are laid upon the glass so as to squeeze out the excess of mercury, and in a few days the combination of tin and mercury is found to have adhered firmly to the glass; this coating usually contains about 1 part of mercury and 4 parts of tin. In this and all other arts in which mercury is used (such as barometer-making) much suffering is experienced by the operatives, from the poisonous action of the mercury.

The readiness with which mercury unites with most other metals to form amalgams is one of its most striking properties, and is turned to account for the extraction of silver and gold from their ores. The attraction of the latter metal for mercury is seen in the readiness with which it becomes coated with a silvery layer of mercury, whenever it is brought in contact with that metal, and if a piece of gold leaf be suspended at a little distance above the surface of mercury, it will be found, after a time, silvered by the vapour of the metal which rises slowly even at the ordinary temperature. From the surface of rings which have been accidentally whitened by mercury, it may be removed by a moderate heat, or by warm dilute nitric acid, but the gold will afterwards require burnishing.

Zinc plates are amalgamated, as it is termed, for use in the galvanic battery, by rubbing the liquid metal over them under the surface of dilute sulphuric acid, which removes the coating of oxide from the surface of the zinc. The amalgam of zinc is not acted on by the diluted sulphuric acid used

MERCUROUS AND MERCURIC OXIDES.

369

in the battery (see p. 20) until the circuit is completed, so that no zinc is wasted when the battery is not in use. A combination of 5 parts of mercury and 2 parts of zinc is also used to promote the action of electrical machines. The addition of a little amalgam of sodium to metallic mercury gives it the power of adhering much more readily to other metals, even to iron. Such an addition has been recommended in all cases where metallic surfaces have to be amalgamated, and especially in the extraction of silver and gold from their ores by means of mercury.

Iron and platinum are the only metals in ordinary use which can be employed in contact with mercury without being corroded by it. Mercury, however, adheres to platinum.

The following definite compounds of mercury with other metals have been obtained by combining them with excess of mercury, and squeezing out the fluid metal by hydraulic pressure, amounting to 60 tons upon the inch:

The amalgam of silver (AgHg) has been found in nature, in dodecahedral crystals. A very beautiful crystallisation of the amalgam of silver (Arbor Diana) may be obtained in long prisms having the composition AgHg,, by dissolving 400 grains of nitrate of silver in 40 measured ounces of water, adding 160 minims of concentrated nitric acid, and 1840 grains of mercury; in the course of a day or two crystals of 2 or 3 inches in length will be deposited.

269. Oxides of mercury.-Two oxides of mercury are known, the suboxide Hg,O, and the oxide HgO; both combine with acids to form salts.

Suboxide of mercury, black oxide or mercurous oxide (Hg,O), is obtained by decomposing calomel with solution of potash, and washing with water (HgCl + KO= Hg,O+ KCl). It is very easily decomposed by exposure to light or to a gentle heat, into oxide of mercury and metallic mercury.

Red oxide of mercury, or mercuric oxide (HgO), is formed upon the surface of mercury, when heated for some time to its boiling point in contact with air. The oxide is black while hot, but becomes red on cooling. It is used under the name of red precipitate in ointments, and is prepared for this purpose by dissolving mercury in nitric acid, evaporating the solution to dryness, and gently calcining the nitrate of mercury (HgO. NO) until the nitric acid is expelled. The name nitric oxide of mercury refers to this process. It is thus obtained, after cooling, as a brilliant red crystalline powder, which becomes nearly black when heated, and is resolved into its elements at a red heat. It dissolves slightly in water, and the solution has a very feeble alkaline reaction. A bright yellow modification. of the oxide is precipitated when a solution of corrosive sublimate is decomposed by potash (HgCl + KO= HgO + KCl); the yellow variety is chemically more active than the red.

When oxide of mercury is acted on by strong ammonia, it becomes converted into a yellowish white powder which possesses the properties of a strong base, absorbing carbonic acid eagerly from the air, and combining readily with other acids. It is easily decomposed by exposure to light, and, if rubbed in a mortar when dry, is decomposed with slight detonations, a property in which it feebly resembles fulminating silver (p. 364). The composition of this substance is represented by the formula 4HgO. NH,. 2HO, and it is sometimes called ammoniated oxide of mercury. When exposed in vacuo over oil of vitriol, it loses 2HO, becoming 4HgO. NH3, but if heated to about 260° F., it evolves another equivalent of water and becomes brown.* * It has been stated that by heating it for some time in a current of dry ammonia, the whole of the hydrogen may be expelled as water, leaving the oxide of tetra-mercurammonium, NHg,O, which is very explosive, and combines with water to form a hydrate which produces salts with the acids.

370

MERCURIC CHLORIDE OR CORROSIVE SUBLIMATE.

It now contains Hg,O,NH2, and may be regarded as a compound of oxide of mercury with ammonia in which one equivalent of hydrogen is displaced by mercury (NH,Hg, 3HgO), a view which would explain, in a simple manner, the evolution of ammonia when the substance is fused with hydrate of potash

NH2Hg, 3HgO + KO. HO = NH3 + 4HgO + ко.

This substance is sometimes called mercuramine; it forms salts with the acids; the sulphate of mercuramine has the composition (NH,Hg, 3HgO)SO3.

By passing ammonia gas over the yellow oxide of mercury as long as it is absorbed, and heating the compound to about 260° F. in a current of ammonia as long as any water is evolved, a brown explosive powder is obtained, which is believed to be a nitride of mercury, NHg, representing ammonia in which the hydrogen has been displaced by mercury. It yields salts of ammonia when decomposed by hydrated acids.

270. The salts formed by the oxides of mercury with the oxygen-acids are not of great practical importance. Protonitrate of mercury or mercurous nitrate is obtained when mercury is dissolved in nitric acid diluted with five volumes of water; it may be procured in crystals having the composition Hg2O. NO, 2Aq. The prismatic crystals which are sometimes sold as protonitrate of mercury consist of a basic nitrate, 3(Hg2O. NO), Hg2O. HO, prepared by acting with diluted nitric acid upon mercury in excess. When this salt is powdered in a mortar with a little common salt, it becomes black from the separation of suboxide of mercury

3(Hg20. NO5), Hg,O. HO + 3NaCl = 3Hg,Cl + 3(NaO. NO2) + Hg,O + HO; but the neutral nitrate is not blackened (Hg,O. NO, + NaCl = Hg,Cl + NaO. NO,). These nitrates cannot be dissolved in water without partial decomposition and precipitation of yellow basic nitrates.

Nitrate of mercury or mercuric nitrate is formed when mercury is dissolved with an excess of strong nitric acid, and the solution boiled. It is better to prepare it by saturating strong nitric acid, diluted with an equal measure of water, with oxide of mercury. It may be obtained in crystals of the formula 2(HgO. NO,), Aq. Water decomposes it, precipitating a yellow basic nitrate, which leaves oxide of mercury when long washed with water.

Sulphate of suboxide of mercury or mercurous sulphate (Hg,O. SO,) is precipitated as a white crystalline powder when dilute sulphuric acid is added to a solution of protonitrate of mercury.

Sulphate of mercury or mercuric sulphate (HgO. SO,) is obtained by heating 2 parts by weight of mercury with 3 parts of oil of vitriol, and evaporating to dryness. Mercurous sulphate is first produced, and is oxidised by the excess of sulphuric acid. It forms a white crystalline powder, which is decomposed by water into a soluble acid sulphate, and an insoluble yellow basic sulphate of mercury, HgO. SO,. 2HgO, known as turbith or turpeth mineral, said to have been so named from its resembling in its medicinal effects the root of the Convolvulus turpethum.

271. CHLORIDES OF MERCURY.-The chlorides are the most important of the compounds of mercury, the subchloride being calomel (Hg.Ĉl) and the chloride, corrosive sublimate (HgCl). Vapour of mercury burns in chlorine gas, corrosive sublimate being produced.

Corrosive sublimate, chloride of mercury, bichloride or perchloride of mercury, or mercuric chloride, is manufactured by heating 2 parts by weight of mercury with 3 parts of strong sulphuric acid, and evaporating to dryness, to obtain mercuric sulphate (Hg + 2(HO.SO) = HgO. SO, + 2HO + SO2), which is mixed with 1 part of common salt and heated in glass vessels (HgO. SO, + NaCl = NaO. SO2 + HgCl), when sulphate of soda is left, and the corrosive sublimate is converted into vapour, condensing on the cooler part of the vessel in lustrous colourless masses, which are very heavy (sp. gr. 5'4), and have a crystalline fracture. It fuses very easily (at 509° F.) to a perfectly colourless liquid, which boils at 563° F., emitting an extremely acrid vapour, which destroys the sense of smell for some time. This vapour condenses