802

PEROXIDE AND SULPHIDE OF SILVER.

of hydrate of potash. It is necessary to be aware of these facts, as it is a most dangerous substance, and might be produced unintentionally. Friction or pressure, even when under water, occasions it to explode: and when dry, its detonation often occurs without any assignable cause. Acids immediately decompose it into an ammoniacal salt, and the corresponding salt of silver. The composition of this body, owing to its dangerous character, has not been accurately determined, but it is generally supposed to be a nitride, similar to that which is obtainable from mercury. Peroxide of silver (Ag2, or AgO,?); Sp. Gr. 5'474.—This compound is procured in dark grey acicular crystals, when a dilute solution of nitrate of silver is decomposed by means of the voltaic current. The peroxide of silver accumulates upon the positive plate, but it always retains a certain quantity of undecomposed nitrate of silver. It is a conductor of the voltaic current. Acids decompose it, forming a salt of argentic oxide, whilst oxygen gas escapes. It is also decomposed by ammonia, with effervescence, owing to escape of nitrogen.

It

(943) SULPHIDE OF SILVER (Ag2S=248, or AgS=124); Sp. Gr. 72: Comp. in 100 parts, Ag, 87∙1; §, 12′9.—This compound is the principal ore of silver. It is found native, sometimes crystallized in cubes or octohedra, at other times massive. has a leaden-grey metallic lustre, from which it derives its mineralogical name of silver glance. Sulphide of silver is isomorphous with subsulphide of copper, and sometimes displaces it in certain minerals, such, for example, as polybasite, and fahlerz or grey copper ore (879).

Silver has a very powerful attraction for sulphur. The metal becomes tarnished, owing to the formation of a film of sulphide, if it be exposed to the action of sulphuretted hydrogen in the gaseous state, even though largely diluted with air; and a black spot is immediately produced upon its surface by contact with a solution of a sulphide of one of the metals of the alkalies or alkaline earths. Sulphide of silver may be prepared by transmitting a current of sulphuretted hydrogen through solutions of the salts of silver, in which it forms a black precipitate; or it may be obtained by heating silver with an excess of sulphur in a covered crucible. The sulphide of silver fuses, and forms a dark grey crystalline mass as it cools, and the excess of sulphur is volatilized.

Sulphide of silver is soft enough to allow of its being cut with a knife; it also possesses sufficient malleability to receive impressions from a die. It is not a conductor of the voltaic current when cold, but if heated it readily transmits the current without

SULPHIDE, AND CHLORIDES OF SILver.

803

undergoing decomposition. It is easily fusible, and if heated in closed vessels may be melted without becoming decomposed; but if roasted in the air, the sulphur is gradually converted into sulphurous anhydride, and metallic silver is left: during this operation a portion of it is usually converted into sulphate of silver, which afterwards requires an elevated temperature for its decomposition.

Sulphide of silver is decomposed when boiled with concentrated sulphuric acid, sulphurous anhydride and sulphate of silver being formed. Strong nitric acid also dissolves it by the aid of heat.* Boiling hydrochloric acid converts it into chloride of silver, with evolution of sulphuretted hydrogen. Chloride of copper converts it into chloride of silver, with the formation of cupreous chloride and sulphide of copper: this change is much facilitated by the presence of chloride of sodium in a moist state, as by its means both the chloride of silver and the cupreous chloride are dissolved at the moment of their formation. These reactions become important in the extraction of silver from its ores (931). Sulphide of silver is also decomposed when heated with the alkalies, and a similar effect is produced by igniting it with iron, copper, lead, and many other metals.

Sulphide of silver is not soluble in solutions of the sulphides of the alkaline metals; but it may be made to unite with many other metallic sulphides when fused with them. A native compound of this description is found in red silver ore, which is a double sulphide of silver and antimony, 3 Ag,S,Sb,S,. In this mineral a portion of sulphide of antimony is often displaced by sulphide of arsenic.

(944) CHLORIDES OF SILVER.-There are two chlorides of silver, the subchloride, Ag,Cl, and the protochloride, AgCl.

Subchloride of silver (Ag,Cl) does not appear to have been obtained in a perfectly pure form. It is usually directed to be procured by digesting leaves of pure silver in a solution of chloride of copper or of perchloride of iron; it forms black scales which are not acted upon by nitric acid, but are resolved by ammonia into chloride of silver and metallic silver.

Chloride of silver (AgCl=143'5); Sp. Gr. 5'552: Comp. in 100 parts, Ag, 75°27; Cl, 24*73.-This compound is found native, either crystallized in cubes, or as a compact semi-transparent

*Nitrate of silver forms with the sulphide a yellow compound, Ag,S,AgNO,, insoluble in cold nitric acid, but it is decomposed when washed with boiling water. It is left in the form of a yellow powder when silver containing sulphide is dissolved in warm nitric acid of sp. gr. about 1.2.

804

CHLORIDE OF SILVER.

mass, known by the name of horn silver. It is procured as a dense white flocculent precipitate on adding hydrochloric acid or the solution of any chloride to a soluble salt of silver: when moist it quickly assumes a violet colour by exposure to the sun's light; a similar change is produced gradually by diffused daylight. The subchloride appears to be formed under these circumstances, and chlorine is set free. If the chloride be moistened with a solution of nitrate of silver and exposed to the sun in a thin layer, a strong odour of hypochlorous acid is immediately developed.

Chloride of silver is insoluble in pure water, and in all the diluted acids. A solution of silver containing not more than 1 part of the metal in 200,000 of water is immediately rendered opalescent by the addition of hydrochloric acid. Chloride of silver is however taken up by boiling hydrochloric acid and by strong solutions of the chlorides of metals of the alkalies and alkaline earths, with which it forms crystallizable double salts; they are decomposed if their solutions are diluted; advantage is taken of this circumstance in the extraction of silver (note, p. 785). Chloride of silver is decomposed by digestion with a solution of bromide or of iodide of potassium, bromide or iodide of silver being produced, while chloride of potassium is obtained in solution. Field, by whom this result was observed (Q. J. Chem. Soc. x. 236), has proposed to employ it for determining the proportions of chlorine, bromine, and iodine in the analysis of a mixture in which they occur together.

Chloride of silver melts at a temperature of about 500°, and when strongly heated it is partially volatilized; on cooling it forms a horny, semi-transparent, scctile mass. It is not decomposed when heated with carbon; but it is easily reduced by hydrogen if it be heated in a current of the gas, hydrochloric acid being formed, whilst metallic silver is set free; zinc, and iron, and many of the easily oxidizable metals, also reduce moist chloride of silver. On the large scale this process is turned to account in the refining of silver (940). It is not necessary that the chloride of silver be freshly precipitated, though, if it be, the operation is more rapid; if a cake of the fused chloride be laid upon zinc or on iron and covered with acidulated water, it will after some days be completely reduced to a spongy mass of metallic silver.

Weak alkaline leys do not act upon chloride of silver, but if a concentrated solution of potash be boiled upon it, chloride of potassium is formed, and a dense black oxide of silver is produced; the addition of sugar to this mixture reduces the oxide rapidly to the state of metallic silver. A solution of ammonia dissolves the chloride freely, and deposits it again, by evaporation at ordinary

BROMIDE AND IODIDE OF SILVER.

805

temperatures, in transparent colourless crystals; if the solution be boiled with potash, fulminating silver is deposited. The solid chloride absorbs ammoniacal gas rapidly, and leaves it unaltered when heat is applied (369). When chloride of silver is ignited with the carbonates of the alkali-metals, chlorides of their basyls are formed and pure silver is left: this reaction furnishes a means of procuring large quantities of silver in a state of purity; 4 AgCl + 2 Na ̧¤¤ ̧= · 4 NaCl + 2 ЄO2+2+2 Ag2. Chloride of silver is soluble in solutions of the hyposulphites, forming compounds of an intensely sweet taste: by evaporating these solutions crystalline double hyposulphites may be procured (419). Cyanide of potassium likewise dissolves chloride of silver, forming chloride of potassium and a double cyanide of silver and potassium. The soluble sulphites also dissolve chloride of silver.

(945) BROMIDE OF SILVER (AgBr=188); Sp. Gr. 6'353: Comp. in 100 parts, Ag, 57'44; Br, 42:56.-This constitutes a rare mineral which has been found in Chili; but it occurs in combination with chloride of silver in variable proportions in tolerable abundance at the mine of Chañarcillo, in Atacama. The bromide may be formed artificially by adding a solution of bromide of potassium to one of nitrate of silver. It is of a yellowish colour, is insoluble in water, and is much less soluble in ammonia than the chloride. Acids do not dissolve it, but chlorine disengages vapours of bromine from it, and chloride of silver is produced. Bromide of silver fuses below a red heat. It is soluble in a concentrated solution of bromide of potassium, and in other bromides, with which it forms double salts, which are decomposed by dilution with water. Both the bromide and the iodide of silver are soluble in a solution of hyposulphite of sodium.

(946) IODIDE OF SILVER (AgI=235); Sp. Gr. 5'5: Comp. in 100 parts, Ag, 45'96; I, 54'04.—This compound is found in Mexico, mixed with carbonate of calcium, native silver, and sulphide of lead. It may be procured artificially by precipitating a solution of the nitrate of silver by one of iodide of potassium, when a pale yellow, flocculent deposit occurs, which is but slowly acted on by light, is insoluble in acids, and almost so in ammonia. It may also be obtained by acting upon metallic silver with hydriodic acid, which dissolves the metal with evolution of hydrogen, and gradually deposits six-sided prisms of the iodide. It fuses easily into a mass which becomes yellow and opaque on cooling. It is decomposed by zinc in the presence of moisture. Chlorine displaces the iodine from the salt. Iodide of silver is soluble in a hot solution of hydriodic acid, which on cooling deposits flaky

806

FLUORIDE AND SULPHATE OF SILVER.

crystals of a compound of the acid with iodide of silver (AgI,HI). The iodide is likewise soluble in concentrated solutions of iodide of potassium.

Fluoride of silver (AgF=127) is freely soluble in water; it is obtained by dissolving the oxide or the carbonate of silver in diluted hydrofluoric acid, but it is partially decomposed on evaporating its solution.

(947) Sulphate of SILVER (Ag2SO=312, or AgO,SO ̧=156); Sp. Gr. 5322: Comp. in 100 parts, Ag¿Ð, 74'36; SO3, 25'64.— When silver is boiled with sulphuric acid, a portion of the acid is decomposed and gives oxygen to the silver, which is converted into a sulphate, while sulphurous anhydride escapes: the sulphate is dissolved by the excess of acid, but is deposited in great part on the addition of water, of which it requires 90 times its weight for solution. It may be obtained in small rhombic prisms, which are isomorphous with those of sulphate of sodium. They fuse readily; for their decomposition they require a temperature higher than is needed to decompose the sulphates of iron or copper. (See note, p. 785.) Small quantities of gold are separated from silver on the large scale, by boiling 1 part of the alloy, finely granulated, in cast-iron vessels with 2 parts of oil of vitriol; the gold is left behind as a fine powder. The solution of silver is afterwards diluted till of a specific gravity of 1.200, introduced into leaden vessels, and the silver precipitated in the metallic form from the solution by bars of metallic copper. This process has been economically applied to the extraction of the gold contained in old silver coin, even where the proportion of gold did not exceed 1 part in 2000. It cannot be advantageously practised upon alloys containing more than about 200 parts of gold per 1000. If copper be present, its proportion should not exceed 4 per cent. of the mass; otherwise the sulphate of copper, owing to its sparing solubility in the acid, impedes the operation. Crystallized sulphate of silver absorbs 1 equivalent of ammonia with rapidity. A hot solution of ammonia dissolves the salt freely, and on cooling deposits crystals composed of (4 H ̧N‚Ag2SO1).

(948) NITRATE OF SILVER (AgNO,, or AgO,NO;=170); Sp. Gr. 4336: Comp. in 100 parts, Ag2Ð, 68·23; N1⁄2Ð ̧, 3177, or Ag, 63 51.-This salt is readily formed by dissolving silver in moderately strong nitric acid. If standard silver be employed in its preparation, the oxide of copper is easily separated from the solution by boiling it upon freshly precipitated oxide of silver, which may be obtained by precipitating a portion of the same solution by caustic potash, and washing the precipitate, the