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attributes similar power to certain fishes of the Nile, which he called choerus (xoipos), and which are supposed by some naturalists to belong to the modern genus Synodontis. The genus Doras is divided into two sections on account of the structure of the mouth. In some it is situated at the end of a depressed muzzle, and is provided with two broad bands of delicate teeth, both in the upper and lower jaws. In others the opening of the mouth is situated on the under side of a conical muzzle, and the opening is of a circular form-here the teeth are either wanting or are hardly visible; the maxillary barbules are sometimes furnished with small lateral branches. To the first of these sections belongs the Silurus costatus of Linnæus, a species found in the rivers of Guiana.

A species of Doras, described by Dr. Hancock, in the fourth volume of the Zoological Journal, p. 241, under the name of D. costatus, is a native of Demerara, where it is called the Flat-head Hassar: it possesses the singular property, says Dr. Hancock, of deserting the water, and travelling over land. In these terrestrial excursions large droves of the species are frequently met with during very dry seasons, for it is only at such periods that they are compelled to this dangerous march, which exposes them as a prey to many and such various enemies. When the water is leaving the pool in which they commonly reside, the Yarrows (a species of Esox, Linn.), as well as the second species of Hassar, to which I shall presently refer, bury themselves in the mud, while all the other fishes perish for want of their natural element, or are picked up by rapacious birds, &c. The flathead Hassars, on the contrary, simultaneously quit the place, and march over land in search of water, travelling for a whole night, as is asserted by the Indians, in search of their object. I have ascertained by trial that they will live many hours out of water, even when exposed to the sun's rays. Their motion over land is described to be somewhat like that of the two-footed lizard. They project themselves forwards on their bony arms by the elastic spring of the tail | exerted sideways. Their progress is nearly as fast as a man will leisurely walk. The strong scuta or bands which envelope their body must greatly facilitate their march, in the manner of the plates under the belly in serpents, which are raised and depressed by a voluntary power, in some mea sure performing the office of feet. It is said that the other species, the round head (Callichthys littoralis, Hancock), has not been known to attempt such excursions, although it is capable of living a long time out of its element; but, as I before observed, it buries itself in the mud in the manner of the Yarrows, when the water is drying up.

The Indians say these fishes, carry water within them for a supply on their journey. There appears to be some truth in this statement; for I have observed that the bodies of the Hassars do not get dry, like those of other fishes, when taken out of the water; and if the moisture be absorbed, or they are wiped dry with a cloth, they have such a power of secretion that they become instantly moist again. Indeed it is scarcely possible to dry the surface while the fish is living.'

Both the species of Hassar here mentioned, it appears, make nests in which they lay their eggs in a flattened cluster, and cover them over most carefully. This care does not end here. They remain by the side of the nest till the spawn is hatched, with as much solicitude as a hen guards her eggs; both the male and female Hassar, for they are monogamous, steadily watching the spawn, and courageously attacking any assailant. Hence the negroes frequently take them by putting their hands into the water close to the nest; on agitating which, the male Hassar springs furiously at them, and is thus captured.

The round-head forms its nest of grass; the flat-head, of leaves; both at certain seasons burrow in the bank; they lay their eggs only in wet weather. I have been surprised to observe the sudden appearance of numerous nests in a morning after rain occurs, the spot being indicated by a bunch of froth, which appears on the surface of the water over the nest; below this are the eggs, placed on a bunch of fallen leaves or grass, if it be the littoral species, which they cut and collect together. By what means this is effected seems rather mysterious, as the species are destitute of cutting teeth. It may possibly be by the use of their serrated arms, which form the first ray of the pectoral fins.'

Genus Callichthys, Linn.-The species of this genus have the body almost entirely covered by large bony plates, these forming four longitudinal ranges, two on each side: P. C.. No. 1361.

the head is also protected by bony plates; the mouth is but slightly cleft, and provided with four long barbules; the second dorsal has a bony spine in front; the foremost ray of the pectoral fins is strong, but that of the anterior dorsal is comparatively feeble and short. The species of Callichthys appear to be confined to the tropical portions of South America. [CALLICHTHYS.]

Genus Arges, Cuv. and Val.- The principal characters of this genus are-teeth bifid at the extremity, and with the points curved inwards; palate destitute of teeth; opening of the mouth large; maxillary barbules two in number; anterior dorsal fin small, and with the front ray feeble; adipose fin long; the other fins with the outer rays prolonged into a filament.

The species which forms the type of this genus (Arges sabalo, Cuv. and Val.) is a small fish about eight inches in length, which was brought by Mr. Pentland from Upper Peru, being found in the reighbourhood of the mission of Santa Anna, at a height of from 4500 to 4800 French metres above the level of the sea. The specimen was given to M. Valenciennes, who prized it much, since it threw a light on the affinities of a fish described by Humboldt, under the name Pimelodus Cyclopum, relating to which that author has given such an interesting account. The Pime lodus Cyclopum, which M. Valenciennes thinks most probably belongs to the present genus, is about four inches in length, and is found in lakes at the height of 3500 metres above the level of the sea. But the most remarkable circumstance relating to these fishes is that they are frequently ejected in the eruptions from the volcanoes of the kingdom of Quito, and in such quantities that the fetid odour arising from their putrefaction was perceived at a great distance, and the putrid fevers which prevailed in those districts were attributed to the miasmata they produce. These fishes sometimes issued from the crater of the volcano, and sometimes from lateral clefts, but constantly at an elevation of from 5000 to 5200 metres above the level of the sea. In a few hours millions are seen to descend from Cotopaxi, with great masses of cold and fresh water. The genus Brontes, Cuv. and Val., is founded upon a fish possessing all the characters of the preceding genus (and which, it appears, like the Pimelodus, is thrown out from the volcanoes of Cotopaxi), but which differs in having no adipose fin.

Genus Astroblepus, Cuv. and Val., consists of but one species (the Astroblepus Grixalvii of Humboldt). This fish possesses all the characters of the genus Brontes, having, like it, the head depressed, the eyes directed upwards, a single dorsal fin, the external rays of the fins prolonged into a filament, and four branchiostegous rays, but it possesses no ventral fins. This fish is found at Rio de Palace, near Papayana, where it is known by the name pescado negro; it attains about fifteen inches in length.

Genus Heterobranchus, Geoff.-Here the head is furnished with a rough bony shield, which is flat and broader than in the other Silurians, on account of the lateral lamina furnished by the frontals and parietals, which cover the orbital and temporal bones. The operculum is still smaller than in the preceding fishes, and what chiefly distinguishes these fishes from others of the family is, that, besides the ordinary branchiæ, they have an apparatus ramifying like the branches of a tree adhering to the upper branch of the third and fourth branchial rays; the branchiostegous rays vary from eight or nine to fourteen or fifteen in number. The pectoral spine is strong and denticulated, but there is no bony spine to the dorsal fin. The body is elongated and naked, and the dorsal and anal fins are greatly extended. the longitudinal direction. The barbules are eight in number. The species inhabit the rivers of Africa, and some of those of Asia.

In some species the long dorsal fin is supported throughout by rays; these constitute the subgenus Clarias, Val.; and in others there is a dorsal fin supported by rays, and a second behind this, which is adipose. To them the term Heterobranchus is restricted in the Histoire Naturelle des Poissons.

Genus Saccobranchus, Cuv. and Val.-This genus is founded upon the Silurus Singio of Hamilton's Fishes of the Ganges,' which possesses some interesting peculiarities in its internal organization, pointed out by Mr. Wyllie, in the 'Proceedings of the Zoological Society, for May, 1840.

Genus Plotosus, Lacépède, is distinguished by the elongated form of the body and the possession of two dorsal fins,

VOL. XXII.-D

the hindermost being supported by rays as well as the other. The head is protected by a bony plate, the lips are fleshy and pendent, the jaws are furnished with strong and conical teeth, and the vomer with rounded teeth. The species inhabit India.

Genus Aspredo, Linn.-The fishes of this genus, says Cuvier, present very singular characters, particularly in the flattening of the head and in the dilatation of the anterior portion of the trunk, which chiefly arises from that of the bones of the shoulder; in the proportionate length of the tail; in the small size of their eyes, which are placed in the upper surface of the head. The intermaxillaries are situated under the ethmoid, directed backwards, and are only furnished with teeth in their hinder margin. But the most striking character consists in there being no power of motion in the operculum, a character which distinguishes the present genus from all other osseous fishes. The branchial opening consists of a simple slit in the skin under the external edge of the head, and the branchiostegous membrane is provided with five rays; the dorsal fin is of moderate size; the anal is long; the tail moderate, and the adipose fin is wanting: the whole of the body is smooth and without bony plates. The species are found in the tropical parts of South America.

Genus Chaca, Cuv. and Val., which is the next in succession in the Histoire des Poissons, is founded upon the Platystacus Chaca of Buchanan Hamilton. It inhabits the rivers of India.

The genus Sisor is also founded upon a single species described (under the name Sisor rhabdophorus) by the author just mentioned, in his Fishes of the Ganges.

Genus Loricaria.-Linnæus gave this name to a group of Siluride distinguished by the head and body being covered throughout by large angular bony plates; they differ moreover from certain other Siluri which have the body protected by plates (such as Cullichthys and Doras), in having the opening of the mouth on the under side of the muzzle, in this respect approaching the genus Synodontis. The intermaxillaries are small and suspended beneath the muzzle, and the mandibles are transverse and not united; they are furnished with long and slender teeth, and these are flexible and terminate in a hook. The mouth is encircled by a large, circular, membranous veil; the pharyngeal bones are furnished with numerous teeth rounded like paving-stones. The true opercula are fixed as in Aspredo, but two small external plates, which are movable, appear to take their place. The branchiostegous rays are four in number. The first ray of the dorsal, pectoral, and anal fins is in the form of a strong spine.

This genus is subdivided into two subgenera. In the one (Hypostomus, Lacép.), there are two dorsal fins; the hinder one is small and provided with but one ray. The labial veil is covered with papillæ, and provided with a small barbule on each side. The belly is not protected by plates. The species are found in the rivers of South America. In the second subgenus, to which Lacépède restricts the term Loricaria, there is but one dorsal fin; the labial veil is furnished with several barbules, and sometimes beset with villosities; the belly is protected by plates. The species of this section are also found in South America.

then holding his court, who received him with every mark of distinction, but would not hear his message until he had himself returned to Ispahán, where he directed Silva to wait till his arrival. Accordingly, after a stay of two months at Kazwin, the Spanish envoy returned to Ispahán, where Shah Abbás arrived shortly after, in July, 1619. He granted Silva an audience; but though he manifested a wish to conclude a commercial treaty, and to be upon friendly terms with Spain, the Shah refused to subscribe to two conditions stipulated by the ambassador of Philip III., namely, that he should restore some fortresses belonging to Ormuz, which he had lately seized; and that he should exclude all other European nations from trading with his dominions. The negotiations for the treaty being thus suspended, Silva left Ispahán on the 25th of August, 1619, and returned by the same route to Goa, where he landed in November, 1620. From Goa he sailed to Spain, where he died in 1628.

·

During his residence in Persia Silva wrote an itinerary of his travels, with an account of such events as came within his observation; and a sketch of the manners and customs of the inhabitants of that empire. This work was never printed in the original Spanish, though a French translation appeared in 1667, under the title of L'Ambassade de Don Garcias de Silva Figueroa en Perse, contenant la Politique de ce Grand Empire, les Mœurs du Roi Shah Abbas, et une relation exacte de tous les Lieux de la Perse et des Indes où cet Ambassadeur a été l'espace de huit anneés qu'il y a demeuré,' par M. Wicqfort, Paris, 1667, 4to. It is one of the best accounts of Persia that we possess, and is much commended by Chardin. During his residence in Goa Silva also made an abridgment of Spanish history, which appeared at Lisbon soon after his death: Breviarium Historia Hispanica,' Lisbon, 1628, 4to. A Latin letter of his, dated Ispahan, 1619, and addressed to the Marquis of Bedmar, in which he gave a short account of his travels, was also published at Antwerp: Garcia Silva Figueroa, Philippi III. Hispaniarum Indiarumque Regis, ad Persarum Regem Legati, de Rebus Persarum Epistola,' Antw., 1620, 8vo.

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SILVER, a metal which has been well known and highly valued from the remotest period-circumstances which are readily explained by the facts of its occurring frequently native, and possessing great lustre and fitness for immediate use without being subjected to any metallurgic process.

Ores of Silver.

Native Silver.-This occurs crystallized, arborescent, or dendritic, capillary, reticulated, granular, and massive. The primary form of the crystal is a cube. It has no cleavage Fracture hackly. Colour white, but externally often blackish, owing probably to the presence of a little sulphur. Hardness 25 to 3. Lustre metallic. Colour pure white, except when tarnished. Streak shining. Opaque. Specific gravity 1047. Malleable, but commonly less so than pure silver, probably owing to an admixture of other metals. Soluble in nitric acid, and the solution colourless when pure, but blue if copper be present; and if antimony, a white substance, and if gold, a black one remains undissolved. Fuses into malleable globules before the blowpipe.

Native silver is met with in most parts of the world. in the British Isles, Germany, Hungary, in the north of Europe, but especially, and in largest quantity, in Mexico and South America. Silver occurs in mixture or combination with other metals, as already hinted at. The first compound of this nature we shall describe is

Antimonial Silver. Stibiuret of Silver.-This occurs in

SILVA Y FIGUERO'A, GARCIA DE, was born of illustrious parents at Badajoz, in 1574. At the age of fifteen his father sent him to court, where he entered the household of Philip II. as page. He then joined the Spanish army in Flanders, where he greatly distinguished himself, and obtained the command of a company. Having subsequently shown some talent for diplomacy, he was de-crystals, in grains, and massive. Spatched by Philip III. on an embassy to Shah Abbás, king of Persia, who was willing to conclude a treaty of commerce with Spain. Silva embarked for Goa, where he arrived in 1614; but the governor of that place, who was a Portuguese, fearing lest Silva's mission should lead to an inquiry into the administration of the Spanish possessions in India, threw every impediment in his way, and refused to provide hum with a vessel and money to prosecute his journey, as he was ordered to do. Impatient at the delay, Silva embarked on board a native vessel and sailed for Ormuz, which port he entered on the 12th of October, 1617. Thence he sailed to Bandel (Bender Abassi) in the dominions of the Shah, when he was well received. He reached Ispahán on the 18th of April, 1618, by the then usual route of Lar and Shiráz. After a short residence in the latter place, Silva started for Kazwin, or Casbin, where Shah Abbás was

Primary form of the crystal a right rhombic prism. Cleavage parallel to the terminal plane and short diagonal of the prism. Fracture uneven. Colour silver white, or, when tarnished, yellowish white. Streak silver white. Lustre metallic. Opaque. Slightly malleable. Easily frangible. Hardness 3.5. Specific gravity 9 44 to 9.8.

Before the blow-pipe on charcoal readily melts, with the formation of white antimonial vapour, into a greyish globule, which is not malleable, but eventually pure silver is obtained. It is not totally soluble in nitric acid, oxide 'of antimony remaining undissolved.

The Massive Varieties are amorphous, and have a granular or foliated structure.

Antimonial silver is found in clay-slate at Andreasberg in the Harz; in Baden; near Guadalcanal in Spain; at Salzburg; and at Allemont in France,

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100 Telluric Silver occurs in coarse-grained masses. Colour grey. Lustre metallic. Soft. Somewhat malleable. Specific gravity about 85. It is dissolved by nitric acid, and when heated, and before the blow-pipe, or charcoal, gives a fused blackish mass, containing specks of metallic silver.

It is found at the silver-mines of Savdinski, in the Altai Mountains, Siberia.

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Iodide of Silver. Herreralite.-Occurs massive in thin plates, which are silver or greyish white, and which become bluish by exposure to the air. Transparent. Translucent Lustre resinous to adamantine; in thin laminæ flexible and malleable. Melts on charcoal before the blow-pipe, vapour of iodine being evolved, and globules of silver remaining. Found at Abarradon near Mazapil, in the state of Zacatecas, Mexico, in serpentine.

Sulphuret of Silver. Vitreous Silver. Silver Glance. Henkelite.-Occurs crystallized and massive. Primary form a cube. Fracture fine-grained and uneven; sometimes small and flat conchoidal. Colour lead-grey; blackish when tarnished. Lustre metallic. Opaque. Hardness 20 to 25. Malleable. Sectile. Specific gravity about 7.2. When heated by the blow-pipe, sulphur is expelled and silver remains. It occurs in Saxony, Bohemia, and in great abundance in Mexico. It has been occasionally found in Cornwall, and in most silver-mines.

Analysis, (1) by Klaproth, of a specimen from Freiberg, (2) by Berzelius:

Boessingault.

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100⚫

100 100°

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100

98 100⚫

in them.

100

Streak shin

Arsenical Antimonial Silver, or rather Arsenio ferru- Derived from the decomposition of the last mentioned. Black Sulphuret of Silver. Earthy Silver Glance.ginous Antimonial Silver.-This substance occurs mammillated or in small globular and reniform masses, and dark lead-grey, inclining to black. Devoid of lustre, or Occurs massive and pulverulent. Fracture uneven. Colour sometimes investing other substances. When untarnished it is nearly silver white, but is commonly tarnished yellowish only feebly glimmering. Somewhat sectile. or blackish; its lustre is metallic. It is harder than anti-ing, metallic. It is found in Norway, Siberia, Hungary, &c., monial silver, but is sectile and brittle. Specific gravity 94. usually investing other silver-ores or filling up cavities Before the blow-pipe antimony and arsenic are volatilized with the alliaceous smell, and a globule of impure silver Sulphuret of Silver and Arsenic. Light Red Silver. Colour cochineal remains. Its localities are nearly the same as those of Proustite.-Primary form a rhomboid. antimonial silver. Klaproth obtained from a specimen from to aurora red; streak lighter. Lustre adamantine. TransAndreasberglucent to transparent. Specific gravity 5.5 to 5'6. Silver Antimony Iron Arsenic

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12.75

4.00

44.25
35*

96

The native compounds of silver next to be described are those in which it occurs in combination with the nonmetallic elements. It is not found simply combined with oxygen, nor at all with azote, hydrogen, or fluorine.

Chloride of Silver. Horn Silver. Muriate of Silver. Laxmannite.-This ore occurs crystallized and massive. Primary form of the crystal a cube. No cleavage. Fracture uneven. Hardness 10 to 15. Yields to the pressure of the nail. Streak shining. Specific gravity 475 to 5.55. Translucent. Opaque. Lustre resinous. Colour grey, yellowish, greenish, and blue of various shades. Malleable and sectile. Fusible in the flame of a candle. Heated with potash by the blow-pipe, yields a globule of metallic silver. Insoluble in nitric acid, but dissolved by ammonia. When rubbed with a piece of moistened zinc, the surface becomes covered with metallic silver.

This ore occurs in various parts of Europe and America, along with others of the same metal. The largest masses, which are of a greenish colour, are brought from Mexico and Peru. It is found in veins, chiefly in primitive rocks. Two specimens from Peru (1) and from Saxony (2), analyzed by Klaproth, gave

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Sulphuret of Silver and Antimony. Ruby Silver. Dar Red Silver. Braardite.-Occurs crystallized and massive. Primary form a rhomboid. Cleavage parallel to the primary planes, usually indistinct. Fracture conchoidal. Colour, by reflected light, from lead-grey to iron-black; by transmitted light, from brilliant to dark red. Lustre adamantine. Translucent. Opaque. Hardness 2.0 to 2.5. Extremely brittle. Streak red. Specific gravity 5'8 to 5.9.

Massive Varieties.-Structure granular, compact, lamellar, dendritic, amorphous.

It is found in many parts of Europe and America, as Germany, Norway, Mexico and Peru, and also in Cornwall.

According to Bonsdorff, a specimen from Andreasberg yielded by analysis

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Sulphuret of Silver and Antimony. Miargyrite.-Occurs crystallized. Primary form an oblique rhombic prism. Cleavage imperfect. Fracture uneven. Colour iron-black in mass; but in thin fragments deep red by transmitted light. Nearly opaque. Lustre bright metallic. Hardness 2:0 to 2.5. Very sectile. Streak dark red. Surfaces of the crystals usually striated. Specific gravity 52 to 5'4.

It is found with argentiferous arsenical pyrites at Braunsdorft, near Freiberg, Saxony.

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Sulphuret of Silver and a little Iron. Biegsamer Silbergianz.-Occurs crystalline and massive. Crystals small and tabular. Cleavage parallel to the terminal planes. Colour nearly black. Lustre metallic. Very soft. Readily separable into thin flexible laminæ.

Found only in Hungary and at Freiberg.

According to Wollaston, this mineral (which is extremely rare) consists of sulphuret of silver with a little iron.

Sulphuret of Silver and Iron. Sternbergite. Flexible Sulphuret of Silver.-Occurs crystallized. Primary form a right rhombic prism. Cleavage parallel to the terminal plane, distinct. Laminae very flexible. Colour dark-brown, often with a blue tarnish. Streak black. Lustre metallic. Hardness 10 to 15. Specific gravity 4.2 to 4:25.

It is found at Johanngeorgenstadt, Schneeberg, and Joachimstahl in Bohemia, with other silver-ores.

A specimen from the last-mentioned locality yielded, according to the analysis of Zippe

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Sulphuret of Silver and Copper. Silberkupferglanz.Occurs massive. Compact. Fracture brilliant, granular, flat conchoidal. Colour dark lead-grey. Streak shining. Lustre metallic. Opaque. Soft. Specific gravity 6·25. Found at Schlangenberg, near Colivan in Siberia. Analysis by Stromeyer:

Sulphur
Silver
Copper
Iron

15.96

52.87

30.83

00:34

100

Bismuthic Silver.-Occurs in acicular crystals and massive. Fracture uneven. Colour, when first broken, lead-grey, but liable to tarnish.

Massive Varieties disseminated, amorphous. Fracture fine-grained, uneven. Lustre metallic. Opaque. Soft. Sectile and brittle.

It is found accompanying pyrites and galena at Schappach in the valley of Kinzig, Baden. Analysis by Klaproth :

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Seleniuret of lead, with a little iron 6.79

96:40

Seleniuret of Silver and Copper. Eukairite.-Occurs massive. Structure granular. Colour grey. Lustre shining. Disposed in films on calcareous spar. Sweden.

Found in a copper-mine at Skrickerum in Smaland,

Analysis by Berzelius :

Selenium
Silver
Copper

Earthy matter

Carbonic acid and loss

26*

38.93

23.05

8.90

312

100.

uneven. Colour

too Carbonate of Silver and Antimony. Selbite.-Occurs
massive and disseminated. Fracture
greyish-black. Structure fine granular.
Opaque. Soft. Brittle. Heavy.

Found at Altwolfach in the Black Forest.
Analysis by Selb:-

Carbonic acid.

Silver

Oxide of antimony and a trace of

copper.

Lustre metallic.

12

72.6

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15.5

100.1

This analysis cannot however be correct, if the ore contain carbonate of silver.

Arseniate of Silver and Iron. Gansekothig-erz; Goosedung Silver-ore.-Occurs massive. Mammillated. Fracture conchoidal; sometimes earthy, and mixed with cobaltore. Colour yellow or pale green. Streak white. Lustre resinous. Found chiefly in the mines of Clausthal in the Harz; Sulphuret of Silver, Antimony, and Copper. Romelite. and also in Cornwall, and at Allemont in France. Mine d'Argent grise Antimoniale.-Occurs crystallized. It does not appear to have been accurately analyzed. Primary form a right rhombic prism. Cleavage parallel to Having now mentioned the principal minerals which conthe lateral planes. Colour nearly silver-white. Lustre tain silver, it is to be observed that few of them are largely shining, metallic. Opaque. Hardness 2 to 25. Ex-worked as ores: the principal are native silver, chloride of tremely brittle. Specific gravity 5.5 to 5'6.

It consists principally of sulphur and the metals above named, but in proportions not yet determined. Sulphuret of Silver, Arsenic, Antimony, and Copper. Polybasite. Brittle Silver.-Occurs crystallized. Primary form a right rhombic prism. Cleavage imperfect. Fracture uneven. Colour iron-black. Lustre metallic. Translucent. Opaque. Hardness 2.0 to 2.5. Specific gravity 6.269. Occurs in Bohemia, Saxony, and other parts of Europe; and in Mexico and Peru.

Analysis (1) of a specimen from Mexico by Rose, and (2) from Freiberg by Brandes:

silver, and sulphuret of silver. The first, when the quantity is considerable, is separable by mere fusion; the chloride and the sulphuret are obtained by amalgamation with mercury; the sulphuret being first converted into a chloride by treatment with common salt, &c. A considerable quan tity of silver is also procured from the lead-ore of this country by cupellation.

Properties of Silver.

The properties of silver are, that it has a purer white colour than any other metal; it has great brilliancy, and is susceptible of a very high polish. Its specific gravity is about 104 when cast, and 10.5 to 10'6 when stamped or rolled. It is sufficiently soft to be cut with a knife. It is very malleable and ductile, so that it may be beaten into leaves about 1-10,000th of an inch in thickness, and drawn into wire much finer than a human hair. It does not rust or oxidize by exposure to the air, but when the air contains sulphureous vapours it tarnishes, becoming first yellowish and afterwards black. Three metals only, viz. iron, copper, Sulphuret of Silver, Iron, Copper, Bismuth, and Lead. and platinum, exceed silver in tenacity; a wire 0-787 of

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line in diameter supports rather more than 187 pounds without breaking. When exposed to a bright red heat silver melts, which, according to Daniell, is equivalent to 1873 of Fahrenheit; on fusion its appearance is extremely brilliant, and during this it absorbs oxygen from the air to the amount of about 22 times its volume, and this it gives out either by cooling or by being poured into water. When leafsilver or fine silver-wire is heated by voltaic electricity, it burns with a fine green flame; if intensely heated in the open fire, it boils, and a portion is vaporized.

Oxygen and Silver combine to form three compounds, viz. suboxide, protoxide, and peroxide.

Protoxide of Silver is prepared by oxidizing and dissolving the metal in dilute nitric acid; when lime or barytes water, or solution of potash or soda, is added to the solution of nitrate of silver, a precipitate is formed, which is the protoxide of silver, composed of

One equivalent of oxgyen One equivalent of silver

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Equivalent.

8

108

116

The properties of this oxide are, that it is of a brownish colour, inodorous, tastless, very slightly if at all solubie in water; it is decomposed by the action of light, being reduced to metallic silver and oxygen gas, and the same effect is produced by heat. It is insoluble in the alkalis or alkaline earths in general, but is rapidly and largely dissolved by ammonia. Nitric, acetic, sulphuric, and some other acids combine with it readily, but it is decomposed by hydrochloric acid, the results being chloride of silver and water. It gives a yellow colour to glass and porcelain. This is the oxide which is the basis of all the common salts of silver.

Suboxide of Silver was first procured by Faraday, by the partial decomposition of the protoxide; when the ammoniacal solution of this is exposed to the air, its surface becomes covered with a pellicle or dark film, which is the suboxide in question; it is probably owing to the decomposition of a portion of the ammonia, which in this case yields hydrogen to a part of the oxygen of the protoxide of silver.

According to Wöhler, it may be obtained also by subjecting citrate of silver to a temperature of 212°. Suboxide of silver appears to be a di-oxide, composed ofOne equivalent of oxygen Two equivalents of silver

Equivalent

8

216

224

It does not readily, if at all, form salts with acids. Peroxide of Silver has been stated to be obtained by electrizing a weak solution of silver. It separates at the positive pole in the state of minute acicular crystals.

Sulphuric and phosphoric acid decompose it with formation of respective salts of the protoxide, and by ammonia it is acted upon and decomposed with great energy. It appears to be a binoxide, composed of

Two equivalents of oxygen

One equivalent of silver

Equivalent

16

108

124

Chlorine and Silver readily combine, and the compound, as already mentioned, forms one of ore of silver.

It may be artificially formed in several ways, first by heating the metal in a finely divided state in the gas, or by adding any soluble chloride, as common salt, to nitrate or any soluble salt of silver, except the hyposulphite.

When recently precipitated, or if kept from the action of light, chloride of silver is perfectly white, but by exposure to daylight it becomes slowly bluish-white, and eventually almost black. The direct rays of the sun produce this effect almost instantaneously; on this property is founded its use in photogenic drawing: the exact nature of the change which takes place does not appear to have been satisfactorily determined. This chloride is quite insoluble in water, either cold or hot; the stronger acids take it up sparingly, and it is precipitated from them by dilution; it is dissolved however to some extent by hyposulphurous acid, and readily and largely by ammonia. It is decomposed by hydrosulphuric acids, and soluble sulphurets, which immediately blacken it by converting it into sulphuret of silver; it is also decomposed by hydrogen gas, and by iron and zinc when put into contact with it and water. By mere heat it undergoes no change except fusion, and when it has solidified on cooling, it has the appearance of horn; hence the name of horn silver for the native chloride.

[blocks in formation]

Equivalent

18

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Sulphur and Silver form sulphuret of silver; this compound has been already noticed as existing in nature and constituting the vitreous silver-ore. It may be prepared by direct action, as by heating alternate layers of silver and sulphur; thus obtained, it is a soft malleable dark-coloured compound; it may be procured also by decomposing solution of nitrate or of ammoniuret of silver by hydrosulphuric acid, hydrosulphates, or soluble sulphurets. It is insoluble in water, ammonia, or other alkalis or acids, except nitric acid, which decomposes and is decomposed by it with the formation of sulphate of silver. It is composed of—

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Iodine and Silver readily combine when hydriodic acid or iodide of potassium is added to a solution of nitrate of silver. The iodide of silver formed is precipitated of a greenish-yellow colour: it is insoluble in water or ammonia, and decomposed when heated with potash; when fused, it acquires a red colour, and is discoloured by light; in the invention of the Daguerreotype, a film of this compound, on the surface of a polished plate of silver, is the substance that receives the impressions of light. It is decomposed by concentrated nitric or sulphuric acid. It is composed of

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Ammoniuret of Silver.-Protoxide of silver dissolves with great readiness in ammonia, and by careful operation the substance discovered by Berthollet, and called fulminating silver, is obtained. It should be prepared only in very small quantity at a time, on account of the facility and violence with which it explodes; in exploding it forms water, sets free azotic gas, and metallic silver, remains; it is procured by adding a small quantity of solution of ammonia to oxide of silver; a portion is dissolved, and a black powder, which is the fulminating ammoniuret of silver remains; it may be also formed by adding solution of potash from the ammonionitrate of silver; a very gentle heat or slight friction causes. it to explode, sometimes even before it is dry. Its exact composition has not been determined.

We come now to the compounds of the oxacids and oxide of silver, or the oxisalts of silver; it is the protoxide only which enters into combination with acids; at least they are the only well known compounds. The first we shall mention is

Nitrate of Silver.-This is one of the most important

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