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rapid evaporating power, without receiving fresh accessions of water.

Alum.

This is a well-known earthy salt, found native only in small quantities, but very largely manufactured from certain argillaceous strata, generally distinguished as alum-clays and shales. It is composed of alumina, potash, and sulphuric acid, has a sweet and astringent taste, and is a powerful styptic. It is much used in dyeing and in calico-printing, in consequence of the attraction its base has for colouring matter; it is also used in lake colours, in leather-dressing, in the preparation of paper pastes, in clarifying liquors, and by candlemakers to harden and whiten the tallow. The shales from which it is prepared are calcined, exposed to air, lixiviated, and the solution so obtained mixed with sulphate of potash, and crystallised. The most extensive alum works in Britain are those at Hurtlett and Campsie, near Glasgow, where it is prepared from certain of the coal shales; and at Whitby, in Yorkshire, from an inexhaustible stratum of alum slate belonging to the lias formation. The best foreign alums are the roch alum, imported from Smyrna, and the Roman alum, prepared at La Tólfa, near Rome–either of which brings fully double the price of the British manufacture, the annual value of which is estimated at £22,000. Alum is also extensively produced in China, whence India obtains her main supply,

Nitrate of Potash.

This is the saltpetre of ordinary language--a salt composed of nitric acid and potash. It is of very varied utility, being used in the manufacture of gunpowder, signal-lights, nitric and sulphuric acids, and in dyeing, metallurgy, curing of meat, and in medicine. The sal-prunella of the shops is the ordinary saltpetre purified and moulded into cakes and little balls. Our main supply of saltpetre is derived from Bengal, where it exists in the soil, and from which the rough nitre or crude saltpetre of commerce is obtained by washing, evaporation, and crystallisation. From 10,000 to 15,000 tons of this salt are annually imported into Britain. In France, Germany, and other continental countries, the salt is produced artificially on what are called nitrebeds.

Nitrate of Soda, This salt, sometimes known by the name of cubic nitre, possesses properties similar to those of saltpetre, differing chiefly in being more pungent in taste, more soluble in cold water, and more inclined to attract moisture from the atmosphere. It differs also in the form of its crystals these being of a rhomboid form, while those of saltpetre are six-sided prisms. It is obtained almost wholly from South America, where it occurs in immense

deposits in the high districts of Atacama and Tarapaca in Peru. Indeed, according to Darwin, a great proportion of the surface of the southern regions of South America consists of salinas, or salt plains, from which common salt, and the sulphates and nitrates of soda, might be procured in any quantities—these occurring sometimes as an efforescence, sometimes in crystallised strata, but oftener mingled with clay, sand, and other earthy impurities. One deposit which he visited in 1835 was full 3300 feet above the Pacific, and consisted of a hard stratum, between two and three feet thick, of the nitrate mingled with the sulphate of soda, and a good deal of common salt. It lay close beneath the surface, and followed, for a length of 150 miles, the margin of a grand basin or plain, which, from its outline, must once have been a lake, or more probably an inland arm of the sea, as iodic salts were abundant in the stratum. This salt was first imported from Iquique in 1830, and so rapidly has its commercial value increased, that, ten years after, about 150,000 hundredweights were shipped for Great Britain alone. In 1835, Mr Darwin found the selling price at Iquique 14s. per 100 pounds--the main part of the expense being its transport from the mountains on mules and asses. It is principally used as a manure, and as a top-dressing for pasture, its advantages being very perceptible on all but wet plashy soils; it is also used in the preparation of nitric acid, and for many of the purposes to which saltpetre is applied; but, owing to its deliquescent properties, it is not adapted for the manufacture of gunpowder.

Natron,

Natron or trona is a native sesquicarbonate of soda, found as an efflorescence or as deposit in sandy soils in Egypt, Mexico, and other countries. It has many of the properties of the two preceding salts, and, according to Herodotus, was employed by the Egyptians in the process of embalming.

Sulphur. Though sulphur or brimstone be an elementary substance, sui generis, and, strictly speaking, does not come under the head of saline substances, yet it may, without much impropriety, be considered in this place, as often occurring in efflorescent salts or crystals. It is a yellow brittle mineral product, found in most parts of the world, but most abundantly in volcanic regions, and in the immediate neighbourhood of burning mountains, such as Ætna, Hecla, &c. It occurs either as an efflorescence on the surface, or in masses mingled with clay, ashes, and other volcanic products. Our chief supply is obtained from Sicily, whence it is imported, as dug from the mines, in square masses or blocks, called rough brimstone. Sulphur is also obtained artificially from the sulphurets of copper, iron, and other metals; but the facility with which native material can be secured, prevents its artificial

production from being followed to any great extent. Unlike most other materials of commerce, the formation of sulphur is still going forward wherever volcanic agency is in a state of activity. It appears to be sublimed by the subterranean heat through the crevices and fumeroles of the mountains; and this collects either as a slight efflorescent crust on the surface, or in crystals and in masses throughout the material of the ejected clays, ashes, &c. Speaking of the sulphur mountains of Iceland, Sir George Mackenzie says, " At the foot of an elevation, in a hollow formed by a bank of clay and sulphur, steam rushed with great force and noise from among the loose fragments of rock. Ascending still higher, we came to a ridge composed entirely of sulphur and clay, joining two summits of the mountain. Here we found a much greater quantity of sulphur than on any other part of the surface we had gone over.

It formed a smooth crust, from a quarter of an inch to several inches in thickness : the crust was beautifully crystallised. Immediately beneath it we found a quantity of loose granular sulphur, which appeared to be collecting and crystallising as it was sublimed along with the steam. Sometimes we met with clay of different colourswhite, red, and blue—under the crust; but we could not examine this place to any depth, as the moment the crust was removed, steam came forth, and proved extremely annoying. We found several pieces of wood, which were probably the remains of planks that had been formerly used in collecting the sulphur, small crystals of which partially covered them. There appears to be a constant sublimation of this substance, and were artificial chambers constructed for the reception and condensation of the vapours, much of it might probably be collected. As it is, there is a large quantity on the surface, and by digging, there is little doubt that great stores may be found.” Such is the usual origin of native sulphur-a substance of greater commercial value to a country like Britain than the most

of our readers may imagine. It is employed for making gunpowder, sulphuric acid which is indispensable to so many manufacturing processes-cinnabar, and for a variety of other purposes in the arts, as well as being used medicinally-requiring altogether an annual supply little short of 20,000 tons.

PRECIOUS STONES.

All our so-called “precious stones”-the diamond, ruby, emerald, amethyst, &c.—are but compounds of carbon, alumina, silica, lime, &c. and might therefore, so far as their mineralogical character is concerned, have been considered under the sections already presented. As none of them, however, occur in rocky masses, but rather as crystals, geodes, and concretions within other rocks, and as fashion has generally set a price upon them wholly disproportioned to their utility, it may be as well to treat them as an independent class. Our limits will only permit us to

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mention a few of the more esteemed; seeing that lapidaries, jewellers, and others have vastly increased the nomenclature of precious stones by giving individual names to specimens which are, in reality, but varieties of the same substance.

Diamond.

The most highly-prized of precious stones is the diamond, a crystalline mineral of unsurpassed lustre and hardness. It is the hardest known substance, and can be polished or cut only by its own dust or powder; hence the common saying of “diamond cut diamond.” When perfectly pure, it is as transparent as a drop of the purest water, in which state it is known as a diamond of the first water; and in proportion as it falls short of this perfection, it is said to be of the second, third, or fourth water, till it becomes a coloured one. Coloured diamonds are generally yellow, blue, green, or red, and the higher the colour, the more valuable they are, though still inferior to those absolutely transparent. Diamond, as has been proved by numerous experiments, consists solely of carbon, being, in fact, a crystallised charcoal. Diamonds were originally discovered in Bengal, but they have since been found in other parts of India, in the East India islands, in the Brazils, and recently in the Ural Mountains. They occur chiefly in alluvial deposits of gravel and sand, lying in detached octohedral crystals, sometimes with plain, but more frequently with rounded surfaces. The finest are cut for ornamental purposes into brilliants, having curvilineal faces both at top and bottom; or into rose diamonds ; that is, those having their tops or upper surfaces cut into a number of triangular facets, but quite flat beneath. The black, dirty, and flawy ones, and those unfit for being cut, are pulverised for the purpose of polishing others, besides being applied to various uses in the arts. Fractured portions, with good cutting edges, are usually set for glaziers' cutting pencils, in which state they are worth from twelve to twenty shillings.' It is the ornamental diamonds that bring the exorbitant prices so frequently mentioned in modern history, their value depending upon shape, colour, and purity, and being fixed at so much per carat of 35 troy grains. “The largest diamond ever known was brought to the king of Portugal from Brazil. It is uncut, weighs 1680 grains, and its value is often quoted at £5,644,800. Similar extravagant valuations are applied to the famous Russian one weighing 195 carats; to that in the possession of the Great Mogul, weighing, cut, 280 carats ; and to others; but it does not appear that any sum exceeding £150,000 has ever been given. The last great sale of jewels was in London in 1837, for the distribution of the Deccan booty, obtained by the army under the Marquis of Hastings. On that occasion the magnificent Nassau diamond, weighing 357} grains, of the purest water, brought only £7200.9 The Russian diamond, says another

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authority, is of the size of a pigeon's egg, and was purloined from a Brahminical idol by a French soldier; it passed through several hands, and was ultimately purchased by the Empress Catharine for the sum of £90,000, and an annuity of £1000, Perhaps the most perfect and beautiful diamond hitherto found is a brilliant brought from India by a gentleman of the name of Pitt, who sold it to the Regent, Duke of Orleans, for the sum of £100,000; its weight, 136 carats.

Sapphire-Ruby-Topaz-Garnet, &c. These may be conveniently grouped together as consisting essentially of crystallised alumina-traces of magnesia, silica, fluoric acid, chromic acid, &c. constituting the specific distinctions. The sapphire is of various colours—the blue being generally known among jewellers and lapidaries as the sapphire; the red, the Oriental ruby, and, next to the diamond, the most valuable of gems; and the yellow, the Oriental topaz. Corundum, or adamantine spar, is nearly allied to the sapphire, and, with the exception of the diamond, is the hardest substance known. It is almost a pure crystallised alumina, consisting of more than ninety per cent. of that substance, with a little silica and iron. It is found in India, China, and some parts of Europe ; and is used in the East for the same purposes to which diamond powder is applied in England. Emery, so called from Cape Emeri, in the island of Naxos, is but a variety of corundum, with an admixture of iron, which gives to it a bluish-gray colour. From its extreme hardness, its powder is largely employed in the polishing of glass and metals, and in the cutting of gems and other minerals--all of which are abraded by it, with the exception of the diamond. The ruby, found chiefly in the sand of rivers in Ceylon, Pegu, and Mysore, is also of various colours—the scarlet-coloured being distinguished as spinelle ruby; the pale or rose-red, balass ruby; and the yellowish-red, rubicelle. The topaz likewise presents various shades between yellow and wine-colour; but, from its large per centage of silica, is harder than either of the preceding The best varieties are known as the Brazilian, the Saxon, Siberian, and Scotch. The garnet, another well-known mineral, belongs to the same section, the varieties being essentially of alumina, with silica, magnesia, iron, &c. The most valuable is the precious garnet, almandine, or carbuncle, which is commonly a transparent, red, and beautiful mineral, either crystallised or in roundish grains. It is found in Ceylon, Pegu, and Greenland. The pyrope, a blood-red variety, found in Germany and Ceylon, is perfectly transparent, and, in roundish or angular grains, is perhaps next in value. The common garnet is not transparent like the preceding, and is most frequently of a dull-redor blackish-brown. It is found plentifully in Scotland, Sweden, and other countries where the primitive rocks abound; but comparatively few specimens are fit for the jeweller.

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