NITRATE AND PHOSPHATES OF SILVER.

807

presence of oxide of copper being unimportant. It crystallizes in square, colourless, anhydrous tables, which require an equal weight of cold water for solution. Boiling alcohol dissolves about a fourth of its weight of the salt, but deposits most of it on cooling. The nitrate fuses at 426° when heated, and if then cast into cylindrical moulds, it forms the sticks of lunar caustic (from luna, the alchemical name for silver) employed by surgeons as an escharotic. By a more elevated temperature it is decomposed, nitrite of silver is produced, and at a still higher temperature metallic silver is left.

Nitrate of silver, when pure, undergoes no change by the action of light; but it is readily decomposed by the combined action of light and organic matter, which it usually stains black. The stain thus produced cannot be removed by washing with soap and water; from this property it has been employed as the basis of an ink for marking linen, which may be prepared as follows:Dissolve 2 drachms of nitrate of silver and I drachm of gum arabic in 7 drachms of water, and colour the liquid with Indian ink (Brande). It is requisite to prepare the cloth first, by moistening the spot to be marked, with a solution of carbonate of sodium, which is allowed to become dry. This preparatory solution may consist of 2 ounces of crystallized carbonate of sodium, and 2 drachms of gum, dissolved in 4 ounces of water.* The black stains of nitrate of silver may be removed from the hands or from linen by the employment of a strong solution of iodide of potassium; cyanide of potassium is still more effectual. Dry nitrate of silver absorbs 3 atoms of ammonia, and if ammoniacal gas be passed into a concentrated solution of the salt, crystals having the composition (2 H,N,AgNO3) are deposited.

When metallic silver in fine powder is digested in a solution of nitrate of silver, it is dissolved, and a yellow solution is formed analogous to that obtained when lead is similarly treated (905).

(949) TRIPHOSPHATE OF SILVER (AgP→4, or 3 AgO,PO¿=419; sp. gr. 7321) is of a yellow colour, which is speedily changed by the action of light. The salt is very soluble in excess both of nitric acid and of ammonia. It is easily procured by precipitating a solution of the ordinary phosphate of sodium by one of nitrate of silver; it fuses if heated above redness. The pyrophosphate (Ag1P,,, or 2 AgO,PO;; sp. gr. 5'306) is obtained in like manner

*A solution of coal-tar in naphtha forms a cheap indelible marking ink, which resists the action of chlorine, and is used by bleachers to mark their goods.

808

CHARACTERS OF THE SALTS OF SILVER.

by precipitating the nitrate of silver by pyrophosphate of sodium; it is a white precipitate slowly darkened by light, and is easily fusible. The metaphosphate (AgPе ̧, or AgO,PO) is obtained by precipitation from the nitrate of silver by the metaphosphate of sodium; it forms a gelatinous mass, which softens even at a heat of 212°, and is soluble in excess of nitrate of silver. If boiling water be poured upon this precipitate, it fuses; acid is removed, and a submetaphosphate is left, consisting of (AgP0139 or 3 AgO, 2 PO; Graham).

(950) CHARACTERS OF THE SALTS OF SILVER.-The soluble salts of this metal are colourless, and nearly all are anhydrous; they do not redden litmus; they have a powerfully acrid, metallic, astringent taste, and act as irritant poisons. Before the blowpipe they are all readily reduced on charcoal to the metallic state, especially when mixed with carbonate of sodium. They give a yellowish bead with microcosmic salt in the oxidating flame. In solution the salts of silver present the following

reactions: :

The hydrates of the fixed alkalies give a brown hydrated oxide, insoluble in excess of the precipitant; ammonia, a brown precipitate, readily soluble in excess of ammonia; carbonates of potassium and sodium, a white carbonate of silver insoluble in excess, but soluble in carbonate of ammonium. Sulphuretted hydrogen and sulphide of ammonium give a black precipitate of sulphide of silver, not soluble in ammonia or in the sulphides of the alkaline metals. But the most characteristic test is the action of hydrochloric acid, or of a soluble chloride, which produces a white curdy precipitate of chloride of silver, insoluble in nitric acid, but readily soluble in ammonia; it is also soluble in hyposulphite of sodium, with which it forms an intensely sweet solution; cyanide of potassium also dissolves it: chloride of silver speedily assumes a violet tinge when exposed to light; this change is impeded by the presence of free chlorine as well as by that of free nitric acid, and is prevented by the admixture of a small proportion of chloride of mercury. lodide or bromide of potassium gives a yellowish-white precipitate of iodide or of bromide of silver, sparingly soluble in ammonia. Hydrocyanic acid and cyanide of potassium give a white curdy precipitate of cyanide of silver, which is soluble in excess of cyanide of potassium, easily soluble in ammonia, insoluble in diluted nitric acid, but soluble in boiling nitric acid if concentrated. Phosphoric, chromic, oxalic, tartaric, and citric acids all form insoluble precipitates with salts of silver. Indeed, silver

ESTIMATION OF SILVER.

809

furnishes a greater number of insoluble salts than any other metal; they are almost all neutral in composition, and generally of a dazzling white colour. Most of them, however, become black when exposed to the action of light. Nearly all of them are soluble in ammonia, and many of them also in nitric acid. Many metals reduce solutions of the salts of silver, and throw down the silver from them in a metallic state, as is beautifully shown by the action of mercury, which produces a crystalline deposit consisting of an amalgam of silver, forming what has been termed the arbor Dianæ.* Copper and zinc also precipitate silver from its solutions. Phosphorus becomes coated with metallic silver if placed in a solution of any of its salts. A solution of ferrous sulphate also precipitates silver in the metallic form from its solutions, if they do not contain free nitric acid. If a solution of ammonia-nitrate of silver be added to one of ferrous sulphate, an intensely black precipitate (Ag,O, 2 FeO,Fc,,; H. Rose) is produced. This reaction is extremely sensitive for very small quan

tities of iron.

The compounds of silver exhibit a less strongly marked tendency to form double salts than is the case with the other noble metals.

(951) Estimation of Silver.-Silver may be estimated either in the metallic state, as in the process of cupellation, or in the form of chloride, 100 parts of which, after fusion, correspond to 75'27 of the metal. This precipitation is best effected by acidulating the liquid with nitric acid, and adding hydrochloric acid in slight excess. After the precipitate has been collected and dried, it should be detached from the filter, and fused in a porcelain capsule; on burning the filter, the portions of chloride retained by it are reduced partially to the metallic state by the hydrogen of the paper; the ash must therefore be moistened, first with nitric, and then with hydrochloric acid, to reconvert it into chloride: the excess of acid must afterwards be expelled by heat.

(952) Separation of Silver from other Metals.-This is readily effected by means of hydrochloric acid. If lead be present, the solution must be diluted largely: should mercury be in solution, it must be converted into a salt of the red oxide by boiling the liquid with nitric acid, after which the silver may be precipitated in the form of chloride.

* On one occasion I found the long prismatic thin crystals to have the composition Ag,Hg,, containing 26 45 per cent. of metallic silver.

§ III. GOLD: Au=1966. Sp. Gr. 19:34; Fusing-pt. 2016°.

(953) This valuable metal has been prized from the earliest ages of the world. It is found in small quantities in numerous localities, and always occurs in the native state, either crystallized in cubes, octohedra, or tetrahedra,-or in plates, in ramified masses, or in nodules or nuggets, which sometimes weigh many pounds.* Native gold is always alloyed with silver; small quantities of osmium and iridium, copper, antimony, and, in some rare instances, tellurium, are found accompanying it. No regular veins of gold are met with; it commonly occurs either in primitive or volcanic rocks, or in the alluvial deposits of certain rivers. Its most celebrated mines are those of California and Australia; and those of Mexico, Chili, Brazil, and Peru. In California the gold is chiefly found upon the Sacramento and its tributary streams, in deposits formed by the disintegration of quartz and granite. In Australia the gold is also associated with quartz, and occurs in slate rocks equivalent to the Cambrian formations of England and Wales, in the detritus of which the most productive gold-fields occur, in the deep gullies at the base of the rocky ranges of clay-slate, mica schist, red and yellow sandstone. In the alluvial portion the gold is usually found at a depth of from 10 to 40 feet, resting upon a "bottom" of pipeclay. A good deal of gold is also obtained from the Ural Mountains; gold has also been obtained in Wales, in the Cader Idris district. Many of the rivers of Africa likewise contain it among their sands, as do those of Hungary, Transyl vania, and Piedmont: in these countries it is principally extracted from the river sands by gipsies.

Extraction. The operations for obtaining gold from its deposits differ from those required by almost every other metal, in being for the most part purely mechanical. According to Mr. Wathen (The Golden Colony, p. 71), the Australian digger formerly used a cradle for washing the ore, but this is not adapted to the stiff clays of the Australian gold-fields. The miner now, after having raised the "washing stuff" from the pit, introduces it into the "puddling tub," which is merely one-half of a porter-cask. The tub is half filled with the washing stuff, water is baled in from the creek, and the whole worked about with the spade until the clay has become diffused through the water; this

* A specimen of native gold, nearly free from earthy impurities, from the Kingower diggings, Australia, weighing 1743 oz., was exhibited in England in the early part of 1858, and still larger masses have been found subsequently.

EXTRACTION OF GOLD.

811

turbid water is poured off and fresh water added, until, by repetition of the washing, "nothing but clean gravel, sand, and gold remains. The gold is now readily separated from the gravel by means of a cradle, or simply by a tin dish. In the latter case, the dish is held half-immersed obliquely in water, and the gravel gradually washed away from the gold by the dexterous handling of the dish."

The Californian "Long Tom" consists of a trough about 16 inches wide, and 10 or 12 feet long, inclined so as to cause the water to run rapidly down: an iron grating, perforated with holes as large as a sixpence, forms the lower end, and is tilted in an opposite direction to the trough. Through the trough a current of water is kept constantly flowing. The auriferous earth is thrown in at the head, and as it is washed down by the stream, it is worked about with the spade; the earth and clay are quickly washed away; when the clean gravel reaches the lower end, it is arrested by the iron grating, and removed with a shovel, while the gold and sand fall through into a box placed beneath. tents of the box are again washed to extract the gold.

The con

Auriferous quartz is first crushed, then stamped and ground to powder, and from the powder the gold is subsequently extracted by amalgamation.

Much of the gold in circulation before the discovery of the deposits in Australia and California was obtained from auriferous pyrites. This mineral is coarsely pulverized, either before or after roasting, and washed: the heavier particles of gold subside, and are extracted from this concentrated portion by amalgamation, the excess of mercury being separated by distillation. Various methods are adopted for washing the auriferous material: in Mexico this operation is usually performed by negresses, who having pulverized the ore under flat stones, agitate it in wide, shallow, wooden dishes, separating the lighter portions with much dexterity. In Europe, the pyrites is ground and amalgamated, in mills constructed for the purpose. Those who wash the river sands usually select some spot at the bend of the stream, where the mud appears to be black or reddish; as it is here, if anywhere, that the gold is found. The most favourable time is when the waters are subsiding after storms or heavy rains; the sand is concentrated either by washing it in shallow vessels, or else by allow• ing it to pass through a succession of troughs. Amalgamation is afterwards resorted to, and the product is distilled, as in the analogous process for obtaining silver.

(954) Properties.-Gold is of a rich yellow colour and high