1902 Encyclopedia > Silver

Silver




SILVER is widely diffused throughout the earth's crust, including the ocean, which contains a trace of the noble metal—minute, it is true, in a relative sense, but in absolute amount approaching 10,000 million tons. Of the varieties of silver ores, the following chiefly are metallurgically important:—(1) Reguline Silver, generally alloyed with mercury or gold, and if with the latter including sometimes a trace of platinum ; (2) Horn Silver, native chloride, AgCl; (3) Silver Glance, native sulphide, Ag2S ; (4) Silver-Copper Glance, (Ag,Cu)2S; (5) Pyrargyr-ite (" Rothgiiltigerz "), Ag8SbS3; (6) Stephanite, Ag5SbS4; (7) Polybasite, 9(Ag2,Cu2)S + (Sb2,As2)S3. Silver is also frequently met with in base-metallic ores, e.g., in lead ores and many kinds of pyrites. Unmixed silver minerals nowhere present themselves in large continuous masses. What we call " silver ores " are all more or less complex mixtures in which the non-argentiferous components are usually decidedly in the majority. Their metallurgic treat-ment depends chiefly on the nature of these admixtures, the state of combination of the silver being as a rule irre-levant in the choice of a process, because some at least of the noble metal is always present as sulphide, and our modes of treatment for it include all other native forms.

Amalgamation.—If a given ore is relatively free of base " metals " (metallurgically speaking), some process of " amalgamation " may be, and often is, resorted to.
In the Freiberg 'process the first step is to roast the (ground) ore with common salt, which converts the sulphide of silver into chloride (Ag„S + 2NaCl + 40 from the air = 2AgCl + Na2S04). The mass, along with certain proportions of water, scrap-iron, and mercury, is placed in barrels, which are then made to rotate about their axes so that the several ingredients are forced into constantly varying contact with o^e another. The salt solution takes up a small proportion of chloride, which in this (dissolved) form is quickly reduced by the iron to the metallic state (2AgCl + Fe = FeCl2 + 2Ag), so that there is, so to say, room made in the brine for another instalment of chloride of silver, which is reduced in its turn, and so on to the end,—the metal formed uniting with the mercury into a semi-fluid amalgam. Of this the bulk at least readily unites into larger continuous masses, which, on account of their high specific gravity, are easily separated from the dross mechanically. The amalgam is pressed in linen bags to eliminate a quantity of relatively silver-free liquid mercury (this of course is utilized as such in subsequent operations), and the remaining solid amalgam is subjected to distillation from iron re-torts, whereby its mercury is recovered as a distillate while a more or less impure silver remains in the retort. This process, after having been long wrought in Freiberg with great success, is now superseded there by the Augustin method (see below), but it survives in some other places, as, for example, the Washoe or Comstock district in the Sierra Nevada (United States). It is not used in Chili, Peru, and Mexico because of the scarcity of fuel.

The Mexican process, though far less perfect than that of Freiberg, evades this difficulty. It was tried for the first time, if not actually invented, by Bartolomeo de Medina in 1557. It was adopted in Mexico in 1566 and in Peru in 1574, and is in use in both countries and in Chili to this day. The stamped ore is ground into a fine paste with water ; this paste, after having been allowed to dry up a little in air, is placed on a stone floor along with a quantity of salt, and the two are trodden together by mules. On the following day there are added certain proportions of "magistral" (a kind of crude sulphate of copper made by-roasting copper pyrites) and of mercury, and the mules are kept going until the silver is as far as possible converted into amalgam, which takes from fifteen to forty-five days. The rationale of the process is not quite understood. According to Boussingault, the cupric chloride (formed by the salt from the sulphate) chlorinates part of the sulphide of silver, thus—

2CuCl2 + Ag2S = 2 AgCl + S + Cu2Cl2, and the cuprous chloride formed acts upon another portion of sulphide of silver, thus—

Cu2Cl2 + Ag2S = 2AgCl + Cu2S, and in this way all the sulphide of silver is gradually converted into chloride. The chloride is reduced to the metallic state by the mercury (AgCl + Hg = HgCl + Ag) with formation of calomel, the metallic silver uniting with the surplus mercury into amalgam. The calomel is allowed to go to waste.

The Augustin process of silver extraction is only a peculiar mode of metallifying and collecting the silver of an ore after it has been by some preliminary operation converted into chloride or sulphate. Either salt is brought into solution—the chloride by means of hot brine, the sulphate by means of hot water, acidified with oil of vitriol; the solution is separated from the insolubles, and made to filter through a bed of precipitated copper. The copper reduces the silver to metal, which remains on the bed as a spongy mass, while an equivalent quantity of copper chloride (or sulphate) passes through as a solution. The silver sponge is col-lected, freed from adhering copper by muriatic acid in contact with air, and then sent to the furnace. From the copper liquor that metal is precipitated in its original form by means of iron.

The silver furnished by any of these methods is never pure, even in the commercial sense. A general method for its purification is to fuse it up with lead and subject the alloy to cupellation (see LEAD, vol. xiv. p. 376). Cupel-silver is apt to contain small quantities of lead (chiefly), bismuth, antimony, copper, and more or less of gold, of which metals, however, only the first three are reckoned "contaminations" by the metallurgist. They can be removed by a supplementary cupellation, without added lead, at a high temperature. Addition of lead would remove the copper likewise, but it is usually allowed to remain and the alloy sent out as cupriferous silver, to be alloyed with more copoer and thus con-verted into some kind of commercial " silver " (see below). If gold is present to the extent of O'l per cent, or more, it is recovered by treatment of the metal with nitric acid or boiling vitriol. The gold in either case remains as such ; the silver becomes nitrate or sulphate, and from the solution of either salt is recovered by precipitation with metallic copper. Although nitric acid is the more expensive of the two parting agents, it is often now preferred because photography has created a la,rge demand for nitrate of silver. Compare GOLD, vol. x. p. 749.

For the " incidental " extraction of silver from essentially base-metallic ores the method in the case of all lead ores is simply to proceed as if only lead were present, and from the argentiferous lead produced to extract the noble metal by one of the processes described under LEAD (vol. xiv. p. 376-7), while for the treatment of sulphureous copper ores one method is so to smelt the ore (with, if neces-sary, an addition of galena or some form of oxide of lead) as to produce a regulus of lead and a " mat" of sulphide of copper, (Cu2S), which latter should contain as little lead as possible. The silver follows chiefly the lead, and is extracted from it by cupellation; but some silver remains in general even with a lead-free mat. Compare account of the Lautenbach process under LEAD.

A modern mode of extracting the silver from a copper mat is to roast it at a very low temperature, so as to produce a relatively large proportion of metallic sulphate, and then to destroy the bulk of the sulphate of copper by a judiciously-regulated higher tem-perature. The silver all remains as sulphate, which is extracted by hot dilute sulphuric acid and wrought by the Augustin method.

Very interesting is the process which was patented by Claudet for the remunerative extraction of the few hundredths of a per cent, of silver contained in that kind of cupriferous iron pyrites wdiich is now used, almost exclusively, for the making of vitriol. The "cinders," as returned by the vitriol maker, are habitually worked up for copper by roasting them with salt and lixiviating the roasted mass with water, when the copper dissolves as chloride, Cu2Cl2 and CuCl2. The silver goes with it, but for its precipita-tion no method was known until Field found that silver dissolved as AgCl in a chloride solution can be precipitated exhaustively by addition of the calculated proportion of a soluble iodide, as Agl. Claudet's process is only an adaptation of Field's discovery. After having diluted the copper liquor with a certain proportion of water he adds the weight of iodine, calculated from the assay, as solution of iodide of zinc, which produces a very impure precipitate of iodide of silver. From it he re-extracts the iodine, by treatment with zinc and dilute sulphuric acid, as iodide of zinc, which is used over again. The "silver precipitate," which now contains its silver as metal mixed with a large quantity of (chiefly) sulphate of lead, goes to the metal-refiner, who treats it as a lead ore.

Chemically Pure Silver.-—Even the best " fine " silver of commerce contains a few thousandth-parts of copper or other base metal. To produce perfectly pure metal the most popular method is to first prepare pure chloride (by apply-ing the method given below under " Chloride " to a nitric solution of any kind of ordinary "silver"), and then to reduce the chloride to metal, which can be done in a great variety of ways. One way is to mix the dry chloride intimately with one-fifth of its weight of pure quicklime or one-third of its weight of dry carbonate of soda, and to fuse down the mixture in a fire-clay crucible at a bright red heat. In either case we obtain a regulus of silver lying under a fused slag of chloride—2AgCl + (CaO or Na2C03) = 2Ag-r-(CaCl2-i-0 or 2NaCl + C02 + O). The fused metal is best granulated by pouring it from a suffi-cient height, and as a thin stream, into a mass of cold water. A convenient wet-way method for small quantities is to boil the recently precipitated chloride (which must have been produced and washed in the cold) with caustic soda-ley and just enough of sugar to take away the oxygen of the Ag20 transitorily produced. The silver in this case is obtained as a yellowish-grey heavy powder, which is easily washed by decantation; but it tends to retain unreduced chloride, which can be removed only by fusion with carbonate of soda.

Stas recommends the following process as yielding a metal which comes nearer ideal purity. Slightly cupriferous silver is made into dry nitrate and the latter fused to reduce any platinum nitrate that may be present to metal. The fused mass is taken up in dilute ammonia and diluted to about fifty times the weight of the silver it contains. The filtered (blue) solution is now mixed with an ex-cess of solution of sulphite of ammonia, S03(NH4)2, and allowed to stand. After twenty-four hours about one-half of the silver has separated out in crystals ; from the mother-liquor the rest comes down promptly on application of a water-bath heat. The rationale of the process is that the sulphite hardly acts upon the dissolved oxide of silver, but it reduces some of the oxide of copper, 2CuO, to Cu20, with formation of sulphate S04(NH4)2. This Cu20 deoxi-dizes its equivalent of Ag20, forming Ag + Cu202, which latter is reduced by the stock of sulphite and reconverted into OuaO which now acts upon a fresh equivalent of Ag20 ; and so on to the end.

Pure silver (ingot) has a beautiful white colour and lustre; it is almost as plastic as pure gold, and, like it, very soft. It does not tarnish in natural air; but in air contaminated with ever so little sulphuretted hydrogen it gradually draws a black film of sulphide. The specific gravity of the frozen metal is 10'42 to 10-51, rising to 10'57 after compression under a die. It is the best con-ductor of heat and electricity. The expansion of unit length from 0° to 100° C. is 0-001936 (Fizeau). The specific heat is 0-0570 (Regnault), 0-0559 (Bunsen). It fuses at 954° C. (Violle)—i.e., far below the fusing point of copper or gold—without oxidation, unless it be in con-tact with a surface of silicate (porcelain glaze, &c), when a trace of silicate of Ag20 is produced. It volatilizes appreciably at a full red heat; in the oxyhydrogen flame it boils, with formation of a blue vapour. The fused metal readily absorbs oxygen gas (under fused nitre as much as twenty times its volume—Gay-Lussac). When the oxygenated metal freezes the absorbed gas goes off suddenly at the temperature of solidification, and, by forcing its way through the solid crust produces volcanic eruptions of metal which are sometimes very beautiful. The presence of even very little base metal in the silver prevents this "spitting," the base metal combining with the oxygen faster than it can be reabsorbed. Pure silver retains a trace of the absorbed oxygen permanently, and Dumas in an experiment on one kilogramme of metal extracted from it 82 milligrammes of oxygen in an ab-solute vacuum at 400°-500° C. Water, and ordinary non-oxidizing aqueous acids generally, do not attack silver in the least, hydrochloric acid excepted,—which, in the presence of air, dissolves the metal very slowly as chloride. A solution of common salt acts similarly, the liberated sodium becoming NaOH. Aqueous hydriodic acid, even in the absence of air, dissolves silver perceptibly, with evolution of hydrogen (Deville). Aqueous nitric acid dis-solves the metal readily as nitrate; hot vitriol converts it into a magma of crystalline sulphate, with evolution of sulphurous acid. Silver is absolutely proof against the action of caustic alkali leys, and almost so against that of fused caustic alkalies even in the presence of air. It ranks in this respect next to gold, and is much used to make vessels for chemical operations involving the use of fused caustic potash or soda. The ordinary " fine " metal is good enough for this purpose.





SILVER ALLOYS.—Pure silver is too soft to make durable coins or vessels combining lightness with stability of form. This defect can be cured by alloying it with a little copper. All ordinary "silver" articles consist of such alloys. The proportion of silver in these (their "fineness") is habitually stated in parts of real silver per 1000 parts of alloy. In Great Britain all silver coins are made of "standard silver," the fineness of which, by legal definition, is 925. The toleration is 4 units (of pure silver in 1000 of alloy), i.e., a specimen passes as long as its fineness lies between 925 and 921 (compare MINT, vol. xvi. p. 483). As regards silver-plate the " Hall" in London refuses to stamp any poorer alloy. In Germany and in the United States all silver coins, in France and Austria the major silver coins, are of the fineness 900, with a toleration of 3 units. The minor coins of Austria are of the fine-ness 375 to 520; in France all silver coins under one franc contain 835 of silver, 93 of copper, and 72 of zinc in 1000 parts. The fineness prescribed by law or custom for "silver" articles is 950 or 800 (±5) in France, 750 in North Germany, 812-5 in South Ger-many, and 820 in Austria. All these alloys at least are liable to " liquation," which means that, although they are perfectly homo-geneous in the crucible, they freeze into layers of not absolutely the same composition. According to Leval, passing from the skin to the core of an ingot of 900 per mille silver the difference may amount to 3 units. Of all the alloys tried by that chemist only that composed according to the formula Ag3Cus, corresponding to 719 per mille of silver, remained perfectly homogeneous on freezing. He therefore recommends this alloy for coinage; unfor-tunately, however, any silver-copper alloy which contains less than about 750 per mille of noble metal tarnishes very perceptibly in the air. British standard silver is quite free of this defect, but it is inconveniently soft, far softer than the " 900 " alloy.

The extent to which the properties of silver are modified by addition of copper depends on the fineness of the alloy produced. The addition of even three parts of copper to one of silver does not quite obliterate the whiteness of the noble metal. According to Kamarsch the relative abrasion suffered by silver coins of the degrees of fineness named is as follows :—
Fineness 312 750 900 993
Abrasion 1 2-3 3'9 9 "5
The same observer established the following relation between fine-ness p and specific gravity in coins containing from 375 to 875 of silver per 1000 :—sp. gr. = 0-001647^ + 8-833.
The fusing points of all copper-silver alloys lies below that of pure copper; that of British standard silver is lower than even that of pure silver. For the alloys of silver with other metals than copper, see GOLD, PLATINUM, and NICKEL. The present writer has introduced an alloy of 91 of silver, 7 of gold, and 2 of nickel as a material far superior, on account of its higher rigidity, to fine silver for the making of alkali-proof vessels.

"Oxidized" silver is ordinary cupriferous silver superficially modified by immersion into sulphide of sodium solution (which produces a dark film of sulphide), or otherwise.

Silvering.—For the production of a silver coating on a base-metallic object we have chiefly two methods. One of these is to dissolve silver in mercury and to apply this amalgam to the (care-fully cleaned) surface of the object by means of a brush. The mercury then is driven away by heat, wdien a coherent film of silver remains, which adheres very firmly, is quite continuous, and needs not be thick to stand polishing and other surface treatment. This very old method is to this day the best for producing a strong coating, but it is dangerous to the health of the workmen, expen-sive, and troublesome, and has been almost superseded by the modern process of electroplating (see ELECTRO-METALLURGY, vol. viii. p. 116). Objects made of iron or steel must first be coated over with copper, and then treated as if they consisted of that metal. For Glass-Silvering, see MIRROR, vol. xvi. p. 500. Inscriptions on linen, consisting of black metallic silver and consequently proof against all ordinary processes of washing, can be produced by using suitably-contrived silver solutions as inks. A mere solution of nitrate of silver (1 to 8 of water) will do, if the surface to which it is applied has been prepared by impregna-tion with a solution of 6 parts of soda crystals and 17 of gum arabie in 30 of water, and subsequent ironing. The ink must be applied with a quill or gold pen (compare vol. xiii. p. 81).

SILVER COMPOUNDS.—(1) Nitrate of Silver (AgN03) is made by
dissolving fine silver in a moderate excess of nitric acid of 1 '2 sp.
gr., applying heat at the end. The solution on cooling deposits
crystals—very readily if somewhat strongly acid. Even a slightly
cupriferous solution deposits pure or almost pure crystals. Any
admixture of copper in these can be removed by fusing the dry
crystals, when the copper salt only is reduced to black oxide of
copper insoluble in water and thus removable, or by boiling the
solution with a little pure oxide of silver (Ag20), which precipitates
the CuO and takes its place. Nitrate of silver forms colourless
transparent sonorous plates, which, if free of organic matter, remain
unchanged in the light,—which agent readily produces black me-
tallic silver if organic matter be in contact with the salt or its
solution. One hundred parts of water dissolve, of nitrate of silver—
at 0° 11° 19°-5 110° C.
121-9 127-7 227 1111 parts.

The solution is neutral to litmus. The salt dissolves in 4 parts of cold alcohol. Nitrate of silver fuses at 198° C. into a thin colour-less liquid, which stands even higher temperatures without decom-position. At a red heat it is reduced to metal. The fused salt, cast into the form of quill-sized sticks, is used in surgery as a cauterizing agent ('' lapis infernalis," or lunar caustic). The sticks gain in firmness if alloyed with a little nitrate of potash.
(2) Sulphate of Silver (AgoS04) forms white crystals soluble in 200 parts of cold or 68 of boiling water, but more soluble in dilute sulphuric acid. It stands a red heat without decomposition.

(3) Oxide of Silver (Ag20) appears as a dark-brown precipitate when a solution of the nitrate is mixed with excess of caustic potash or—preferably for preparative purposes—baryta water. It is slightly soluble in water, forming a very decidedly alkaline (to litmus) solution, behaving as if it contained the (unknown) AgOH. It seems to suffer reduction in the light. In hydrogen it loses its oxygen at 100° C. (Wonler), in air from about 250° C. upwards. Solutions of numerous organic substances and other agents reduce oxide of silver, more or less readily, to metal. Patter produced what he took to be a peroxide of silver by decom-posing a solution of the nitrate galvanically, in the form of black metallically-lustrous crystals, wdiich gathered at the positive pole. At 110° C. these decompose almost explosively, with evolution of the 12'77 per cent, of oxygen demanded by Ag202; yet according to Berthelot the crystals are 4Ag203. AgN03 + 2fi,0. But a hydrate of Ag403 is got by the action of peroxide of hydrogen on Ag20.

(4) salts Chloride of Silver (AgCl) comes down as a precipitate when solutions of silver are mixed with solutions of chlorides (for preparative purposes AgS"03 with HC1, which is preferable to NaCl). The mixture at first has the appearance of a milk, but on being violently shaken it divides into a curdy, heavy, easily settling precipitate and a clear solution,—more readily if the co-reagents are exactly balanced or the silver is in excess than when the precipitant predominates. Chloride of silver is as good as insoluble in water, but hydrochloric acid, and chloride solu-tions generally, dissolve it perceptibly. In dilute sulphuric and nitric acids it is as insoluble as in plain water. Even boiling oil of vitriol attacks it only very slowly. It is readily soluble in ammonia solution and reprecipitated therefrom on acidification. It dissolves in aqueous thiosulphate of soda, Na2S203, forming the very stable salt NaAg. S203, and in cyanide of potassium solution, forming KAg. (NC)2. From either solution the silver is conveniently recoverable only by sulphuretted hydrogen or sulphide of ammonium as an Ag2S precipitate. Chloride of silver fuses at 260° C. into a yellowish liquid, freezing into a transparent, almost colourless, glass of horn-like consistence (hence the name "horn-silver "). The specific gravity of frozen AgCl is 5 '45 (Karsten). It remains undecomposed, but volatilizes appreciably at a red heat. Hydrogen at a dull red heat reduces it to metal. A similar reduc-tion is effected in even the compact chloride by contact with zinc, water, and a little dilute sulphuric acid ; the reduction, however, proceeds rather slowly and is rarely quite complete. Unfused chloride of silver, when exposed to sunlight, becomes at first violet, then darker and darker, and at last black, through progressive de-chlorination. Yet even the black final product, according to Bibra, yields up no silver to hot nitric acid.

(5) Bromide of Silver (AgBr) closely resembles the chloride. The reduction on insolation is prevented by the presence of a trace of free bromine and promoted by that of nitrate of silver. Chlorine converts the hot fused salt into chloride.





(6) Iodide of Silver (Agl), while similar on the whole to the other two haloids, presents marked peculiarities. As formed by precipitation it is distinctly yellow ; it is insoluble in, but decol-orized by, ammonia ; it is less soluble in water and dilute nitric acid or other nitrate solutions than even the bromide, this latter exceeding in this sense the chloride. But boiling oil of vitriol decomposes it slowly, with elimination of iodine vapours and forma-tion of sulphate. Hydrogen at a red heat does not act upon it; nor is it at all easily decomposed by zinc and dilute acid. Pre-cipitated iodide of silver is characteristically soluble in solutions sf alkaline iodides and in those of nitrate of silver, with forma-tion of double salts, which, however, are all decomposed, more or less completely, by addition of much water. Pure iodide of silver, even if recently precipitated, is not changed by sun-light, but if contaminated with nitrate of silver it readily blackens. For action of light on silver haloids, see PHOTOGRAPHY.

ANALYSIS.—In a solution of salts derived from purely oxygenated acids the least trace of silver can be detected by hydrochloric acid, which precipitates the silver as chloride (see above). The precipitate, when produced in a possibly complex solution, may include the chlorides of lead (PbCl) and mercurosum (Hg2Cl2). Repeated treatment of the (washed) precipitate with boiling water extracts the lead chloride; then by pouring ammonia on the precipitate we convert the Hg2Cl2 into an insoluble black body, while the chloride of silver dissolves and, from the filtrate, can be precipitated by acidification. For the quantitative determination of silver, the ordinary laboratory method is to bring the metal into solution as nitrate and then to throw it down as pure chloride. The chloride is washed, collected, dehydrated by fusion, and weighed. According to Stas, if 0 = 16, Ag = 107'93 and Cl = 35'454; hence the chloride contains 0'75273 of its weight of metal.

The assaying of silver ores is done preferably in the 1' dry way "; in fact relatively poor ores cannot be assayed satisfactorily in any other. The general method with sulphureous ores is to mix them, as powders, with (silver-free) oxide of lead and tartar, and fuse in a clay or graphite crucible. The regulus includes all the silver. The fuse is poured into a conical mould of cast-iron, when the metal goes to the bottom of the mould ; the ingot, after cooling, is easily separated from the adhering slag. The slag-free regulus is then placed in a little cupel made out of com-pressed bone-ash, and is heated in a muffle to redness and kept at this temperature in the current of air which pervades the muffle in virtue of its disposition in the furnace until all the lead and base metals generally have been sucked up by the porous cupel. The remaining "button" of metal is weighed, which gives the conjoint w-eight of the silver and gold, which latter metal is rarely absent. For its determination the button is rolled out into a piece of thin sheet, which is "parted" with nitric acid (see GOLD). The gold remains and goes to the balance; the weight of the silver is found by difference. Similarly, to determine the fineness of silver alloys, a known weight of the alloy—customarily 0'5 gramme—is "cupelled," with addition of a proportion of pure lead depending on the weight of base metal to be removed, as shown by the following table, which, however, holds strictly only for copper-silver alloys :—
Fineness 1000-900 80 units of lead per unit of copper.
900-850 64
800-750 53
,, below 750 50-40 ,,

In a well-appointed laboratory two operators who work into each other's hands can easily make several dozen of such assays in a day. Cupelling, indeed, is the promptest of all methods of ana-lysis, only the results are not quite as exact as is desirable in the ease of precious metal, part of the silver being lost by volatilization, and part by being sucked into the cupel. The error attains its maximum in the case of alloys of about 700 per mille, and with these comes to about TlTth of the weight of the silver to be determined. It of course can be, and always is being, corrected to some extent by "blank" assays made with known weights of pure silver and pure copper; but such corrections are not quite safe. Hence cupellation nowadays, in the mints at least, is used only for a first approximation, and the exact fineness determined by the " wet-way " process, invented by Gay-Lussac. See ASSAYING, vol. ii. p. 727.

A most excellent method for the quick determination of a not
approximately known weight of dissolved silver has been invented
by Volhard. This method rests on the fact that solutions of
sulphocyanates (including that intensely red salt Fe(NCS)3 which
is produced when, for instance, NCS.H is mixed with ferric sul-
phate) precipitate silver completely from even strongly acid solu-
tions, as NCS.Ag. A convenient reagent for the method is pro-
duced by dissolving NCS. NH4 grammes of (chlorine-free)
sulphoeyanate of ammonium in water to 1000 e.c. to produce a
solution of which 1 c. c. precipitates about -fa Ag = 10 '8 milligrammes
of silver. To determine the exact "titre," we dissolve, say, 540 milli-
grammes of pure silver in 1 '2 nitric acid, and next boil away every
trace of N203. We then dilute to say 50 c.c., add 5 c.c. of saturated
solution of iron alum (not less), and, lastly, run in sulphoeyanate
from the burette, until the red colour of ferric sulphoeyanate which
appears locally from the first, by addition of the last drop of NCS
solution, has become permanent on stirring. Supposing 49'3 c.c.
of solution to have been required to reach this point, every 1 c.c.
of reagent precipitates milligrammes of silver, and it, of course,
always does so, even, let us add, in the presence of (say) 70 per cent,
of copper beside 30 of silver in the alloy under operation. Volhard's
method is more exact, and, with a small number of samples, takes
even less time, than cupellation. (W. D.)

Mode of Occurrence.—Silver is rarely found in the native state, and then only in comparatively small quanti-ties. Most of the ores of silver are difficult to reduce, and it is therefore deemed safe to regard this as the last of the three great coining metals which came into use. Silver is originally as widespread as gold, occurring in nearly all the volcanic rocks and some of the Primary ones. In the Silver Eeef district of Utah it is found in sedimentary sandstone, though this appears to have undergone some change from volcanic action. But gold remains unaltered by the action of the elements, and is often carried away long distances from its original place of occurrence by the breaking down of the rocks which contain it and their formation anew elsewhere, either as other rocks or as " placers " of gravel or sand, containing gold easily washed out by hand or with rude appliances. Silver, on the contrary, is only to be found in the rocks where it originally occurs. When these are broken down or worn away, the silver is either driven into new mineral combinations, or, more commonly, dissipated and lost. Hence silver is only to be obtained by subterranean mining, and demands the aid of capital and associated labour. The greater rapidity with which gold can be obtained has often influenced the legal relation of value between these two metals, and its bearing upon prices, commerce, and civilization.
Cost of Production.—In nearly all silver ores there is some gold, and in nearly all gold ores some silver. In the £70,000,000 worth.of metal produced from the Comstock lode of Nevada nearly one-half in value consisted of gold. For this and other reasons, it is impossible to determine the general average cost of producing gold and silver from all the mines during any reasonably long period of time. If recent statistics are to be trusted, both metals are produced on the average at a loss. Sneh is alleged to have been the case in California, Australia, and Nevada, countries whose combined product has equalled in value nearly £600,000,000.

Value.—In some ancient states the value of silver appears to have been superior to that of gold. Agatharchides informs us that such was the case in ancient Arabia; and Tacitus says the same of ancient Germany. Strabo alleges that the ratio of value in a country bordering that of the Sabseans was at one time one gold to two silver; and so late as the 17th century silver and gold were valued equally in Japan. Going back to a remote antiquity, silver appears to have been everywhere equal in value to gold until the silver mines showed signs of exhaustion, when, as the principal coins were of copper and silver, and prices were commonly expressed in these coins, the threatened decrease of money was probably averted and a profit secured for the state by raising the legal value of gold coins. In Greece, in the time of Herodotus (cf. iii. 95), gold was 13 times the value of silver, at which ratio it appears to have stood for a long period.

"When the Romans acquired the placer mines of Pan-nonia, Dacia, Spain, Gaul, &c, they made their principal coins of gold; and at a later period, when the supplies of this metal fell off, they raised the legal value of silver coins to one-tenth that of gold ones of like weight and fineness. This ratio was afterwards changed to 11, and still later to 12 silver for 1 gold. In the Arabian states of the 7th century the ratio was about 6J for 1; yet in France at the same time it was 10 for 1; in England during the 12th century it was 9 for 1; in France during the 14th century certain silver and gold coins of like weight bore the same value, hence the ratio was 1 for 1; in Castile and Leon in 1454-74 it was 7| for 1. Speaking broadly, between the rise of Mohammedanism and the opening of the silver mines of America the value of silver compared with gold gradually rose. It is evident that there were two lines of ratios, the one having an Indo-Arabic, the other a Romano-Germanic origin, and that the conflict of ratios—which only ceased when America was discovered and a great coinage of the precious metals occurred in Spain—gave rise to many of those otherwise inexplicable lowerings of coins, of one or the other metal, which characterize this period.

In Spain, by the edict of Medina (1497), the ratio was lOf. When America was plundered the first fruits were gold, not silver; whereupon Spain, in 1546, and before the wealth of the silver mines of Potosi was known, raised the legal value of gold to 13^, and, as Spain then mono-polized the supplies of the precious metals, the rest of the world was obliged to acquiesce in her valuation. During the following century Portugal obtained such immense quantities of gold from the East Indies, Japan, and Brazil that the value of her imports of this metal exceeded ¿£3,000,000 a year, whilst those of Spain had dwindled to £500,000 in gold, and had only increased to £2,500,000 in silver. Portugal now governed the ratio, and in 1688 raised the value of gold to 16 times that of silver. Except during a brief period of forty years, this ratio has ever since been maintained in Spanish and British America and the United States. A century later the spoils of the Orient were exhausted, the Brazilian placers began to decline, and Portugal lost her importance. Spain thus again got control of the ratio, and, as her colonial produce was chiefly silver, she raised its value in 1775 from one-sixteenth to one-fifteenth and a half that of gold for the Peninsula, permitting it to remain at one-sixteenth in the colonies. France, whose previous ratio (that of 1726) was 14J, adopted the Spanish ratio of 15| in 1785, and has adhered to it ever since. These three historical ratios, and the bearing of each upon the others, have influenced all legislation on the subject, and, where there was no legislation, have governed the bullion markets for more than two centuries.

Meanwhile an economical school arose which, while conceding it to be necessary that the state should fabri-cate coins, denied it the right to limit the number of coins, or to exact payment (seigniorage) for coinage. This school found expression in the Act 18 Charles II. (1666), which permitted private persons to have coined for them an unlimited quantity of gold or silver, at the public mint, free of charge. Similar Acts were passed in Holland, France, and other countries. But the crown retained the right to regulate the nominal value of gold and silver coins, the exercise of which has had the greatest influence on the relative market value of those metals.

To check abuses of this prerogative the economical school next directed its efforts towards the adoption of one in place of two metals for full legal tender coins. The principal advocates of this change during the last century-were Dutot (1739) and Desrotours (1790), and during the present one Lord Liverpool (1808), De Quincey (1849), and Chevalier (1856). The policy thus advocated was practically adopted in Holland and England during the 18th century, and by the latter definitively in 1816. It was accepted by the Monetary Conference assembled at Paris June 20, 1867, and by the Commercial Convention at Berlin October 20, 1868. In 1871 it was practically, though not definitively, adopted by Germany, and since that date by several smaller states, including distant Japan. In France (1874) and the United States (1873-78) the policy pursued has been a waiting one. Full legal tender silver coins continue to be employed for money, but the state has ceased to coin silver on private account. Either Germany, France, or the United States may, by simple enactment, and without recoinage or change of coins, return to the " bimetallic " basis of money.

The closure of the mints of all important commercial countries to silver, while they have remained open to the free coinage of gold at a fixed valuation, has enhanced the purchasing power of gold, compared with either silver or other commodities, about one-fourth. The price of uncoined silver being usually quoted in gold, this pheno-menon appears as a "fall of silver," by which term it is commonly known. This alleged fall, its causes, conse-quences, and remedies, constitute the " Silver Question."

Production.—In the principal producing countries—the United States, Mexico, Chili, and Peru—mining is free, and there are no official returns of the production, which is therefore mere matter of conjecture. In the United States it is the custom to value silver bullion at one-sixteenth that of gold. This unduly swells the value of the conjectural product of that country more than one-fourth (see Report of the United States Monetary Com-mission of 1876, Appendix, pp. 1-66). From a careful consideration of the bullion movement, the total annual product of silver throughout the world at the present time is estimated at between 50 and 60 million ounces, at which figure it has remained steady upwards of ten years.

Consumption in the Arts.—Direct inquiries as to the quantity of silver used in the arts have met with little success, and the statistics so obtained are defective. But the total production of silver in the Western World, from the discovery of America to the present time, has been, in value, about 1400 million pounds sterling, of which about 300 million pounds remain in coins. Consequently 1100 millions, or nearly four-fifths, have been consumed in the arts, lost, &c, or exported to Asia. There are estimated to be about 50 or 60 million pounds
sterling worth of silver coins in India,1 and some trifling amounts each in China, Japan, Persia, &c. On the whole it appears quite safe to estimate the average annual consumption of silver in the arts and through wear, tear, and loss as fully equal to three-fourths of the production. Lowe in 1822 estimated it at two-thirds. Silver is principally used for plate and jewellery; it is also consumed in photography, and in numerous chemical preparations, such as lunar caustic, indelible ink, hair dyes, fulminating powder, &c. (A. DE.)


Footnotes

Compare CHEMISTRY, vol. v. p. 528-530 ; also MINING, MINT, and MONET.

Preferably blackened for visibility bv incorporation of some Chinese ink (carbon).

DelMar, Hist. Prec. Metals, chap. xxxi.
Boeckh, Political Economy of the Athenians, book i. chap. 6,
B Sir Edward J. Keed, Japan, chap, xviii.; DelMar, Money and Civilization, chap. xx.

11. B. Chapman, Financial Department of Government of India.



The above article was written by two authors:
-- Part 1: W. D.
-- Part 2: A. DE.





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