gold solution value chloride coins metal acid water nitrate ratio
SILVER COMPOUNDS. - (1) .fl'itrafe of Silver (AgNO3) 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 CeO 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.
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 colourless liquid, which stands even higher temperatures without decomposition. 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.
Sulphate of Silver (Ag2SO4) 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.
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) Ag0H. It seems to suffer reduction in the light. In hydrosgen it loses its oxygen at 100° C. (Wohler), 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. Hitter produced what he took to be a peroxide of silver by decomposing a solution of the nitrate galvanically, in the form of black metallically-lustrous crystals, which 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.AgNO3+ 2H20. But a hydrate of Ag403 is got by the action of peroxide of hydrogen on Ag20.
Chloride of Silver (AO) comes down as a precipitate when solutions of silver salts are mixed with solutions of chlorides (for preparative purposes AgNO3 with HCI, which is preferable to Neel). 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 solutions '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 reprempitated therefrom on acidification. It dissolves in aqueous thiosnlphate of soda, Na2S203, forming the very stable salt NaAg.S503, and in cyathje of potassium solution, forming KAg.(NC)2. From either solution the silver is conveniently recoverable only by sulphnretted 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 AgCI 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 reduction 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. Unfiesed 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.
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.
Iodide of Silver (AgI), 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 decolorized 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 formation of sulphate. Hydrogen at a red heat does not act upon it ; nor is it at all easily decomposed by zinc and dilute acid-. Precipitated iodide of silver is characteristically soluble in solutions of alkaline iodides and in those of nitrate of silver, with formation 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 sunlight, but if contaminated with nitrate of silver it readily blackens. For action of light on silver lialoids, 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 Hg2C12 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-10713 and CI =35154 ; hence the chloride contains 0/5273 of its weight of metal.
The assaying of silver ores is done preferably in the "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 compressed 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 weight 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 Gomm). flee 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 II „ 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 analysis, 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 raffle, and with these conies to about 11-,,th 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 eupellation 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 snlphocyanates (including that intensely red. salt Fe(NCS)3 which is produced when, for instance, NCS.H is mixed with ferric sulphate) precipitate silver completely from even strongly acid solutions, as NCS. Ag. A convenient reagent for the method is produced by dissolving /1-3 NCS.NH4 grammes of (chlorine-free) sulphocyanate of ammonium in water to 1000 c.c. to produce a solution of which 1 c. c. precipitates about I1-0- Ag - 10'8 milligrammes of silver. To determine the exact "titre," we dissolve, say, 540 milli-grammes of pure silver in 11 nitric acid, and next boil away every trace of N503. We then dilute to say 50 c.c., add 5 c.c. of saturated solution of iron alum (not less), and, lastly, run in sulphocyanate from the burette, until the red colour of ferric sulphocyanate 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 ft milligmmmes of silver, and it, of course, always does so, oven, 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 quantities. 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 Reef 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 civilization.
Cost of Production. - In nearly all silver ores there is duced on the average at a loss. Such 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.2 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 Sabicans was at one time one gold to two silver ; and so late as the 17th century silver and gold were valued equally in Japan.3 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 Pannonia, 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 61 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 71 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 10,Z. 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 131, and, as Spain then monopolized 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 141, adopted the Spanish ratio of 151 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 fabricate 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 IL (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 phenomenon appears as a "fall of silver," by which term it is commonly known. This alleged fall, its causes, consequences, 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 Commission 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,' 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.)