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the aluminum collects at the bottom in a melted form, while the chlorine is removed by combination with the sodium.

4.-Rose obtains aluminum from cryolite (3 NaF,AIF) by fusing it with sodium. For this purpose Wöhler recommends 7 parts of chloride of sodium to be melted with 9 of chloride of potassium, and the mass thus furnished to be finely powdered and intimately mixed with its own weight of cryolite in fine powder. This powder is to be introduced with a fifth or a sixth of its weight of sodium (arranged in alternate layers of the powder and the metal), into a dry earthen crucible, which is to be heated rapidly in a wind furnace. An intense reaction occurs, and a portion of the sodium burns off. The mixture is then heated for about a quarter of an hour until it is in liquid fusion, and is then allowed to cool. The aluminum generally collects at the bottom into a well-formed button, which is frequently crystalline on its surface. In some experiments the quantity of reduced metal amounted to one-third of the proportion present in the mineral employed.

(661) Properties.-As prepared by Deville's process, aluminum is a white malleable metal, nearly resembling zinc in colour and hardness it may be rolled into very thin foil, and admits of being drawn into fine wire; after it has been rolled, it becomes much harder and more elastic. It conducts electricity with about one-third the power of silver. Aluminum is remarkably sonorous, and emits a clear, musical sound when struck with a hard body. Fused aluminum crystallizes readily as it cools, apparently in regular octohedra; its point of fusion is below that of silver. It may be heated intensely in a current of air in a muffle without undergoing more than a superficial oxidation, and it is but slowly oxidized when heated to full redness in an atmosphere of steam. When heated in the form of foil with a splinter of wood in a current of oxygen it burns with a brilliant bluish-white light.

Nitric acid, whether concentrated or diluted, is without action upon aluminum at the ordinary temperature, and dissolves it very slowly even when boiled upon the metal. Hydrochloric acid, on the contrary, both when concentrated and when diluted, attacks it rapidly, forming chloride of aluminum, whilst hydrogen is disengaged. Solutions of the alkalies, especially when aided by heat, also attack aluminum with energy, producing alumina, which is dissolved by the alkaline solution, whilst hydrogen gas is liberated. From its lightness and inalterability in the air, aluminum has been applied to the preparation of small weights: but some difficulty is experienced in working the metal for

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want of a suitable solder. articles.

ALUMINA.

It is chiefly used for ornamental

Aluminum readily forms alloys with copper, silver, and iron, but it may be melted with lead without any combination between the two metals taking place. Its alloys with copper are very hard, and susceptible of a high polish; they vary in colour from white to golden yellow, according to the proportion of the two metals one of these, a beautiful alloy of a golden yellow colour, containing about 10 per cent. aluminum, is manufactured by Messrs. Bell under the name of aluminum bronze. Copper of high purity is needed. Aluminum also combines readily with carbon and silicon, forming greyish, granular, brittle, and crystalline compounds, which present a considerable analogy to cast iron. It does not combine with mercury.

Finely divided aluminum burns brilliantly in the vapour of sulphur, and forms a black sulphide (Al,S), of semi-metallic appearance, which is rapidly decomposed by water, with formation of hydrate of alumina and sulphuretted hydrogen.

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(662) ALUMINA (Al,0,= 103, or Al,O =515); Sp. Gr. of ruby, 3'95; Composition in 100 parts, Al, 53'39; →, 46·61.—This is the only known oxide of aluminum: from its isomorphism with the sesquioxide of iron, and its general resemblance to it in properties, it is regarded as a sesquioxide. It forms one of the materials that enter most largely into the composition of the superficial strata of the earth. It is the basis of all the varieties of clay, and is present in greater or less quantity in almost every soil. Alumina occurs nearly pure, and crystallized in six-sided prisms, in corundum, in which mineral it has a specific gravity of 3'95, and is hard enough to cut glass. The sapphire and the ruby are also composed of this earth, tinged with a small quantity of oxide of chromium. They are only inferior to the diamond in hardness. Emery, which from its hardness is so largely used in grinding and polishing, after it has been powdered and levigated, is another form of alumina, coloured with oxides of iron and manganese.

In order to obtain alumina, it is sufficient to ignite pure ammonia alum (H ̧NAI”” 2 SO4, 12 H2O) intensely for some time; the water, ammonia, and sulphuric acid are expelled, and anhydrous alumina is left, in the proportion of 1134 parts of alumina for 100 of the crystallized salt. It is, however, nearly impossible to drive off the last portions of sulphuric acid, as the salt swells up enormously, and forms a white, porous, infusible mass, which is an extremely bad conductor of heat. Alumina may also be

procured from alum quite free from iron; the salt should be dissolved in water and precipitated by carbonate of potassium in slight excess: the liquid should be warmed, and the precipitate well washed; but since traces of potash always adhere to it obstinately, it must be redissolved in hydrochloric acid, and then thrown down by ammonia or carbonate of ammonium; in which case it falls as a white, semitransparent, bulky, gelatinous hydrate, which must be again thoroughly washed. In this form alumina is completely soluble in a solution of potash, and is readily taken up by acids. On drying, it contracts very much, and forms a yellowish translucent mass, like gum, retaining 3 H. Diaspore is a natural hydrate (Al,,, H,O), which decrepitates strongly when heated, and falls to powder. Alumina may also be obtained from aluminate of sodium (663) by adding hydrochloric acid in quantity just sufficient to form chloride of sodium.

The hydrate of alumina when ignited loses its water, and at a certain temperature presents an appearance of sudden incandescence; it contracts greatly at the moment that this effect is produced, and is afterwards nearly insoluble in acids. Hydrate of alumina is strongly hygroscopic, and adheres to the tongue when applied to it.

Alumina fuses before the oxyhydrogen blowpipe, and yields a colourless, transparent mass, resembling corundum. Gaudin states that artificial crystals, having the form and hardness of the ruby, may be obtained by calcining equal parts of sulphate of potassium and alum, and introducing the mixture in fine powder into a crucible lined with lampblack. The cover is then to be luted on, and the crucible exposed to the highest heat of a forge for a quarter of an hour. In this operation the sulphuric acid of the sulphate of aluminum is expelled, the sulphate of potassium is reduced to sulphide of potassium, and this compound dissolves a portion of the liberated alumina, depositing it in minute prismatic colourless crystals, during the slow cooling of the mass, These crystals may be cleansed from adhering impurities by digestion in dilute aqua regia. Similar crystals have also been obtained by Deville, who has succeeded in imitating the hue both of the ruby and the sapphire. Alumina forms salts with the more powerful acids, but these salts are readily decomposed : they all have an acid reaction; and indeed alumina possesses properties which approach somewhat to those of an acid, for it has a strong tendency to unite with basic oxides. The spinelle ruby, for example, is a native aluminate of magnesium (MgOAl,0), and gahnite is an aluminate of zinc (Zn✪Al ̧Ð ̧). Fremy has

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ALUMINA-ALUMINATE OF SODIUM.

also obtained a white granular compound of alumina with potash, to which he assigns a composition corresponding with the formula (K2OAI,O). When the solution of alumina in potash is exposed to the air it absorbs carbonic acid, and a terhydrate of alumina is deposited in regular crystals.

Alumina when combined with silica forms clay, which is the basis of porcelain and of earthenware. To the dyer and the calico-printer the compounds of alumina are of high value: the hydrate of alumina has the property of combining intimately with certain kinds of organic matter, and when salts of aluminum are mingled with coloured vegetable or animal solutions, and precipitated by the addition of an alkali, the alumina carries down the greater portion of the colouring matter, forming a species of pigments termed lakes. By soaking the cloth with a preparation of aluminum, the earth attaches itself to the fibre; and if cloth thus prepared be plunged into a bath of the colouring matter, it becomes permanently dyed. Most colouring matters would be removed by washing, were it not for the intervention of some mordant, or substance which thus adheres to the fibre as well as to the colouring matter. Binoxide of tin and the sesquioxides of iron and chromium resemble alumina in this respect, and are largely used as mordants in dyeing calicoes and woollens.

Mr. Crum (Q. J. Chem. Soc. vi. 216) has described a remarkable modification of hydrate of alumina, which, in the presence of a very small proportion of acetic acid, is largely soluble in water, and is coagulated and rendered insoluble by a minute trace of sulphuric acid. It appears to be probable from the experiments of Péan de St. Gilles (Chem. Gaz. 1855, p. 165) that sesquioxide of iron admits of a similar modification: these compounds will be further alluded to when the salts of acetic acid are described.

(663) Aluminate of Sodium (Na,AlO3).—This compound now forms an article of commerce. It is obtained by heating Bauxite, a hydrated aluminous peroxide of iron, which contains from 60 to 75 per cent. of alumina, and only from 1 to 3 per cent. of silica. It is mixed in fine powder with carbonate of sodium or soda ash, and heated to bright redness, until no effervescence occurs on the addition of an acid. On lixiviation, the aluminate is dissolved out and separated by filtration into a vessel, from which, to accelerate the operation, the air is exhausted. The filtrate when evaporated to dryness, gives a whitish, infusible, but freely soluble compound, which furnishes a valuable material in the preparation of lakes for pigments, as well as for the purposes

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of a mordant to the calico-printer, which will probably to a large extent supersede the use of the different forms of alum. The silica remains behind in the form of an insoluble aluminosilicate of sodium.

If a solution of aluminate of sodium is exposed to the action of a current of carbonic acid, carbonate of sodium is produced, and hydrate of alumina precipitated contaminated with soda. If hydrochloric acid in quantity sufficient to neutralize the soda be added to a solution of the aluminate, the alumina. is precipitated in a form in which it may be washed; but the precipitate is simply dried when it is to be used in the preparation of alumina, for which it is chiefly required; the presence of chloride of sodium being advantageous in the subsequent operations. A curious reaction occurs when solutions of aluminate of sodium and chloride of aluminum are mixed in equivalent proportions; chloride of sodium is formed, and the alumina from both compounds is precipitated in the form of hydrate; 2 (NaAlO3) +Al¿CI=2 Al ̧Ð ̧+6 Na¤l.

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(664) CHLORIDE OF ALUMINUM (AlCl = 268); Sp. Gr. of Vapour, 9'34; Mol. Vol. ; or (Al,Cl=134).—The anhydrous chloride of aluminum cannot be formed directly by dissolving alumina in hydrochloric acid, and evaporating to dryness; since during the expulsion of the water, a great part of the acid is also driven off. It may be procured as a yellow, anhydrous, volatile sublimate, by a process devised by Oersted:-alumina, mixed with charcoal powder, is made up into paste with starch or oil, and subdivided into pellets: these pellets are charred in a covered crucible, and then exposed to ignition in a current of dry chlorine. In this operation, carbon, in a very finely divided state, is mixed with the alumina; when the mass is heated with chlorine, the carbon unites with the oxygen of the alumina, and the chlorine seizes the liberated aluminum; Al,0, +3+3 Cl2 = Al,Cl ̧ +3 €0. The chloride of aluminum condenses in the cool part of the tube in a crystalline, somewhat translucent mass, or as an amorphous powder.

In preparing the chloride of aluminum in the laboratory, an apparatus similar to that shown in fig. 338 may be used: b is a vessel containing a mixture of black oxide of manganese and hydrochloric acid, for generating chlorine; a is a water-jacket, for applying a moderate heat; c is a wash-bottle containing water; d contains sulphuric acid: e is a bent tube filled with pumice-stone soaked with oil of vitriol, to remove the last traces of moisture; g is an earthen retort filled with the mixture of charcoal and alumina, heated by a charcoal fire. The chlorine