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A wrought diamond of 3 carats is worth
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£72 126 200

800 3,200 7,200 12,800 20,000 28,000 80,000

carats.

Beyond this weight such a method of calculation is not, however, applicable, in consequence of the difficulty of finding purchasers for the more valuable gems.

Of the numerous diamonds exhibited, by far the largest and most valuable is the Koh-i noor, formerly the property of Runjeet Singh. This jewel, which is the property of her Majesty, is one of the largest in the world, and is valued at £2,000,000 sterling. Besides this mag. nificent diamond the Exhibition contains a vast collection of jewels of inferior weight and value, among which may be mentioned a unique blue diamond weighing 177 grains, the property of Mr. Hope.

Of the other large diamonds in the world, the following are the most remarkable: That mentioned by Tavernier as belonging to the Emperor of Mogul, a now extinct kingdom, weighing, in the rough state, 900

It was found in the Golconda nine in the year 1550, and is the size of a hen's egg divided through the middle in the direction of its smallest diameter. Among the crown jewels of Russia is a diamond weighing 195 carats. It is the size of a pigeon's egg, and was formerly the eye of the idol Sheringham. Thence it was stolen by a French soldier who deserted in the Malabar service, and who found the means of purloining the gem. He escaped with it to Madras, where he disposed of it for £2,100 to a captain of a ship, who afterwards sold it to a Jew for six times that amount. The Jew subsequently disposed of it to a Greek merchant, who afterwards sold it to the Empress Catharine for £90,000 in ready money and an annuity of £4,000. The most perfect and beautiful diamond hitherto found is probably that brought from India by an English gentleman of the name of Pitt, who sold it to the Duke of Orleans, by whom it was placed among the crown jewels of France. This jewel weighs rather more than 136 carats, and was sold for the sum of £100,000.

A model of a portion of the Nizam diamond--the remainder being unfortunately chipped off-is shown in the Indian department. The manner in which this diamond was found, about 20 years since, in the Nizam's territories, is rather interesting. It was first seen in the hands of a native child, who was playing with it in ignorance of its value. The sum of eight "aunas” being offered for it, excited the suspicion of the parents of the child, and led ultimately to the discovery that the bright stone was a real diamond. The diamond, after passing through many hands, was purchased by a native banker for 70,000 rupees, and it is now in the possession of his highness the Nizam. The stone is of an irregular, oval shape. Its length is 2.43, its greatest breadth 1.85, and its average thickness 0.92 inch. The actual weight of the Nizam diamond is 1,108 grains, being equal to 277 carats of weight for the rough diamond; and as the rough stones are usually taken to give but one half their weight when cut and polished, we should have 108, carats; or a weight between the Pitt or Regent diamond (1864 carats) and that of the Grand Duke of Tuscany, (139 carats,) and the weight of the Nizam diamond. Had the diamond remained entire, its weight, when cut and polished, would have been 1558 carats, which would have placed it between the Tuscan and the great Russian diamond of 195 carats.

From the circumstances of the Nizam diamond not being polished, it is not known whether it is lik-ly to prove one of the first water; but there is every probability that it is so, as the natives of India are too good judges of diamonds to mistake a topaz for one. And an additional proof of its value may be learned from the fact that a native gave, for the broken fragment, a sum not less than 75,000 rupees.

The diamonds coming from Brazil are usually smaller than those procured from India. But the mines of the former country annually furnish from 10 pounds to 13 pounds weight of this precious mineral, of which from 800 to 900 carats only are fit for jewelry; the remainder, under the name of “port,” being used for other purposes such as the cutting of glass and the grinding or polishing of precious stones.

Among the other minerals much prized by the jeweller may be mentioned the sapphire, which, when perfectly transparent, and of a good color, is as highly esteemed as the diamond. This gem is almost entirely composed of alumina, the various colors of different individual specimens being occasioned by extremely minute admixtures of the metallic oxides. T'hose having a blue color are know u as oriental sapphires, whilst others not having the same oxides in combination are differently colored, and consequently receive various distinct names When red, they are called oriental rubies; when yellow, oriental topazes; when violet, oriental amethysts; aud when they are hair brown, adamantine spar.

The finest blue specimens of this gem have been procured from Ceylon. The most esteemed red varieties come from the Capellan mountains, in the kingdom of Ava; and smaller stones of the saine kind are occasionally met with in Saxony, Bohemia, and Auvergne. Amethysts are principally brought from the Carnatic or the Malabar coast, and elsewhere in the East ludies. The adamantine spar is chiefly obtained from the Malabar coast, but is less used than the other varieties for ornamental purposes. Of these several kinds the red is by far the most valuable à ruby of 3} carats, and perfect in form and color, having been valued at the same price as brilliants having an equal weight.

The emerald is a precious stone, of a beautifully green color, valued next to the diamond, and in the same rank as the oriental ruby and sapphire. It occurs crystallized in regular six-sided prisms, and has a specific gravity of 2.70. In composition this gem may be considered as a double silicate of alumina and glucina, mixed with variable small portions of iron and a little lime. The most beautiful emeralds are obtained froin Peru, where they occur as a kind of grey schist, mixed with greater or less quantities of carbonate of lime. A good stone of this kind, weighing four grains, is valued at from £4 to £5; and one of 24 grains realized, at the sale of M. De Dree's cabinet, 2,4001., or nearly £100.

The garnet is a vitreous mineral, belonging to the cubic system, and of which the predominating form is the rhomboidal dodechaedrun. Its constituents are silica, alumina, lirpe, and tide me. esua..y found discrminated in the primitive borras 3. så fregendy occurs in goess and clay slate. Gamets are atency Eet wah in many parts of Eur pe, particularly in Germacy: tur use of Pers are the most estremned.

The crysolite, called the “peridot" by Hang and the French mineral. ogists, is, probably, the trpaz of ihe ancients. It is the setest of the precious stones, being scratched by a file or a frazment of quartz. Quartz, in a crystalline form, is also frequentis cut scramental purproses; and when limpid and epurely free from flas. is a very beautiful su ne. When exisung in the form of calcedony, and rancusly colored by metallic oxides, the substance receives the same of cats ere, plasma, chryscopase, onyx, sardonyx, &c. It has a riueous fusire, a conchoidal fracture, and a specific gravity of 2.69.

Opal, or uncleavable quartz, has a conchoidal fracture, with a resinous or vitreous Justre, accompanied by a strong play of colors. It occurs in kidney-shaped or stalactitic concretions, and has a specific gravity of 2.091. Hungary was long the only locality of precious opal, where it occurs in connexion with common opal, in a sort of phephyritis formation. Lately, however, some very fine specimens of this substance have been discovered in the Faroe islands; and most beautiful ones, sometimes quite transparent, are obtained near Gracias a Dios, in the province of Honduras, in America.

Electro-Metallurgy.

It is scarcely eleven years since electricity began to be applied success. fully to the arts. About the year 1840, electro medals, brittle and friable, but still successful repetitions of the originals, began to pass into circu. lation. Since that period, the laws regulating the deposition of the metal have been determined, the most appropriate solution for every metal has been learned, and the operation of electrotyping has been dig. nified into an art. In all electro decompositions the metal is deposited atom by atom, so that however minute ihe object is, however fine the workmanship, the difficulty of its application is in no way enhanced. Everything is faithfully copied, and our countryman, Professor Silliman, has actually been enabled to multiply the iridiscent colors which gives jis unequalled beauty to mother-of-pearl. In fact, there seems to be no bounds to the power of delicately representing objects by this wonderful process. Bunches of grapes, leaves of the fern, the tiniest spires shooting from a blade of grass, the stamens and pistils of flowers, wings of insects, down of feathers, and even the eyes of the common house-fly, are produced in model without difficulty, and in cxquisite perfection.

The mode in which these results are obtained is generally that which is called the battery process; troughs and batteries of such as were em. ployed in the usual modus operandi being exhibited at various stands in the Exhibition. In this trough gold and silver have latterly been reduced by the motion of the electric force. During this reduction of metals, various processes are simultaneously conducted. At the negative pole, when the metal is being deposited, the sanie amount of metal is produced from the solution. But at the same time that the metal is being reduced from the fluid, a similar amount of metal is dissolving at the positive pole, and thus the precipitating trough is a manufactory for the generation of the metallic salt, and a decomposing apparatus for the reduction of the metal. With regard to gold and silver, which are generally precipitated from cyanides, more metal is apt to be reduced than that which is dissolved; and hence experimenters have thought that electricity has made gold.

In the rear of the Exhibition was placed one most interesting example of electro-metallurgic deposition. It was a complete model of the Britannia bridge, which has rendered the Menai straits so great an object of attraction. Every part of the model was made to scale, every rivet was represented, every smallest portion of iron, steel, casting, or wrought work, was depicted; and the tubes, exact in their reduced proportions, were placed as they were on the day when science achieved their successful fixture over the deep abyss.

The English ordnance department at Southampton exhibited some capital samples of electrotyped plates. After the plates are engraved, the next process is to form a matrix, which is kept for the purpose of producing as many other duplicate plates as may be required. The plan thus carried out is as follows: Î'he battery trongh is an immense tank, sufficient to hold enough fluid to charge the batteries for twelve months. The zinc plate is arranged in the middle, and plates of the best Sheffield plated copper are used for the negative pole. The back and cipreous edge are thoroughly coated with varnish, and the silver surface is covered with finely divided platinum. This form of battery answers well in the hands of the highly disciplined and effective corps of sappers and miners, but most other manufacturers find that a płatinum silver plate, although much dearer, is still preferable to the platinized plated copper. The precipitating trough is placed upon a truck. The positive pole consists of a thick plate of copper, which is arranged at the bottom of the trough. The plate to be copied is placed at the top of the vessel, to prevent particles of dirt from falling upon it, and the proper diffusion of the newly made metallic salts is obtained by a mechanical arrangement which agitates the whole vessel. The plates exhibited were seven in number—the first three being an original, duplicate, and matrix, to illustrate the manner in which corrections, surveys, new lines of railroad, or new buildings, were inserted without injury or alteration to the original An impression on paper accompanies the original, showing its state before the matrix was taken.

In the department of the Austrian Imperial Printing Office was an electro deposit thirty feet long, to deinonstrate the extent to which these deposits may be made. This was, a short time since, described in the Philosophical Magazine, and was afterwards carried out in England. In another department of Austria an invention was shown called chemi. typy, said to have been discovered by Püll, of Copenhagen. A zinc plate is taken and covered with etching ground; it is then etched, and the surface covered with an easy fusible metal. The plate is then scraped, so as to leave the metal in the hollow parts produced by the etching. The surface is then again etched to revise the part o‘the zinc plate for the elevation of the design; it is then, like a wood cut, fit for printing.

In the Denmark department was an example of electro stylography. A cast is made in a black compound, which is silvered over. This is drawn upon by cutting through the silver. An electro rererse is then made, and an electro-plate from this reverse, which is ready for printing. This is called electro-stylography.

It is a singular fact that the first idea of electro gilding was given by Brugnatelli nearly fisty years ago; and he states in a letter to Van Mons, which was published at the same time in most of the countries of Europe, that he had gilded two vessels by making them the negative pole of a voltaic circuit. Notwithstanding this publication, the old mercury gilding was carried out till the first idea of electro metallurgy had been given. To Elkington is due the merit of employing the compound of metal with the cyanide of potassium, which has afforded to the electro. gilder and the electro-plater such facilities that these processes can be employed by any person. The gilding or silvering solutions can be easily made by boiling the acids of the metals in a solution of the cyanide of potassium, or by a process which is more used by some manufacturers—that of inaking a large plate of pure metal the positive pole of the solution. In comparing the results which have been obtained by different manufacturers, it appears that a very thin layer will give as good an appearance to the eye as a very thick coating. Hence in buying these articles the purchaser must rely entirely upon the honesty of the manufacturer.

Electro-plating is undoubtedly made to subserve the purposes of false coinage. We ourselves have seen electro-plated sixpences, shillings, and half crowns in England which rung as clear as silver. They may always be detected, however, by the bitter taste of the cyanide which is always left in small quantities on the coin; by their lightness, nearly one-third less in weight than the original metal; and by being much more easily bent.

Electro statues were well represented in the Exhibition. The electro deposition of the head of Baron Macketti's horse was, perhaps, the best example. The electro statue of the Duke of Gloucester, modelled by Westmacott for the English House of Lords, by Elkington, was a most excellent electro-cast, and was by many considered the best thing in its way in the Exhibition. The Death of a Welch prince, supported by a female figure, a bust of Jupiter, a side-board with electro-bronze panels, and figures of Ariadne and the Fawn, all exhibited by Elkington, were very fine. In the hands of this firm (Messrs. Elkington) the manufacture of electro-plated goods promises to form a very important branch of industry. They are employing 750 hands in the production of these goods, have two large factories at work in Birmingham, and another in the process of erection, and are bringing steam power to bear upon their extended processes.

Prussia exhibited one small electro statue, admirably well done. The process of making electro statues, however, is expensive. We must first have the cost of the copper, to which must be added the cost of the zinc dissolved in the battery, and to these the cost of the sulphuric acid to dissolve it. Further, the moulds are expensive, and the labor difficult. In spite of all these charges, electro-metallurgy is increasing, even for electro statues. Some of the zinc statues of Kiss, the great artist of the age, are electro-coppered; but the plan he adopts he has not made known.

Electro bas-reliefs are said to be less difficult. They are well represented now in the bazaars and toy-shops of our country. Electro copper

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