which leaves the solution colorless. This property is turned to advantage in calico-printing, where the compounds of alumina are largely used as mordants. The chief aluminous mordant is the acetate, which is decomposed by a boiling heat, with precipitation of a basic acetate, capable of combining with coloring matters; hence, if a pattern be printed in acetate of alumina, and the stuff be then steeped in a hot color-bath, the basic acetate will be precipitated in the fibres, and will fix the color there.

Aluminate of Potassa, KO.Al,O,.-This compound may be obtained in white granular crystals, by slowly evaporating a solution of alumina in potassa. The crystals have a sweet taste, and a strongly alkaline reaction.

A solution of alumina in potassa is sometimes used as a mordant.

The Aluminate of Magnesia, MgO.Al,O,, is found as a very hard mineral, termed spinelle; it crystallizes in octohedra.

Nitrate of Alumina (AlО.зNO) is prepared by dissolving alumina in nitric acid; it may be obtained from an acid solution in colorless oblique rhombic prisms, containing eighteen eqs. of water. The crystals are very deliquescent and soluble.

SULPHATE OF ALUMINA, Al,O,.3SO.

§ 210. This salt is occasionally found native, when it is sometimes termed hair-salt.

Preparation. It is usually prepared from clay, as free from iron as possible. The clay is calcined at a dull red heat, reduced to a fine powder, and mixed with half its weight of sulphuric acid of spec. gr. 1.45. The mixture is heated till the acid begins to go off, then left to itself for a day or two; after this time it is extracted with water, when a solution is obtained, containing sulphate of alumina and sulphate of sesquioxide of iron. The solution is now mixed with a solution of ferrocyanide of potassium1 (yellow prussiate of potassa), as long as any blue precipitate (sesquiferrocyanide of iron, Prussian blue) is obtained, and the filtered liquid, which is now free from iron, evaporated to a syrupy consistence, and allowed to solidify in shallow leaden pans.

Properties.-The sulphate thus obtained is a white mass, which resists a high temperature without decomposition. It is very soluble in water, requiring only twice its weight for complete solution, but very sparingly soluble in alcohol. The aqueous solution has a sweet astringent taste, and an acid reaction; a hot saturated solution, on cooling, deposits small tabular crystals, of the formula A1,0,350,+18Aq.

These crystals fuse when heated, intumesce greatly, and are ultimately decomposed, losing the greater part of their acid. Sulphate of alumina is extensively

used as a mordant.

A basic Sulphate of Alumina, of the formula 2A1,0.3SO,, may be obtained by digesting a solution of the neutral sulphate with hydrate of alumina. It has not been crystallized. Another basic sulphate occurs in nature, in the mineral aluminite (AlO3S0,2A10,9HO). It may be prepared by adding a little ammonia to a solution of the neutral sulphate, when it falls down as a crystalline precipitate.

Sulphate of alumina is capable of combining with the sulphates of the alkalies, giving rise to certain double salts, which may be taken as the types of the class of salts termed alums.

AN ALUM is a double-salt, composed of a sulphate of a protoxide combined

On the large scale, ferrocyanide of sodium is commonly employed; the precipitated Prussian blue is afterwards decomposed with solution of caustic soda, in order to reproduce the ferrocyanide of sodium, which is used to precipitate a fresh portion of iron.

with the neutral sulphate of a sesquioxide, thus the general formula of an alum may be written :

MO.SO,,M,O,.3SO;;

where MO represents the basic protoxide, and M,O, the basic sesquioxide. The alums all crystallize in cubes or octohedra containing 24 eqs. of water. In order to render the above definition more intelligible, we subjoin the formulæ of some of the principal alums:—

SULPHATE OF ALUMINA AND POTASSA, POTASSA-ALUMINA-ALUM, COMMON ALUM, A1,0,.3SO,,KO.SO, +24 Aq.1

§ 211. This salt exists native in the neighborhood of Naples, where it is extracted by simply treating the rock with water."

It may be prepared by mixing together hot concentrated solutions of sulphate of alumina and sulphate of potassa, when crystals of alum are deposited as the solution cools.

In Italy, alum is prepared from alum-stone, which is found at Tolfa, near Civita-Vecchia. The composition of this mineral is KO.SO,,3(A1,O,.SO)+9Aq; when this mineral is calcined, a portion of the alumina is converted into the insoluble form; and when the mineral is afterwards treated with water, a quantity of alum is dissolved out;3 the washings are evaporated, when they yield cubical crystals of alum; these crystals have a reddish color, from the presence of a little sesquioxide of iron in the insoluble state (?). This variety of alum is known as Roman alum, or Roch alum, and is preferred by dyers, since its aqueous solution is always free from iron. It is at present prepared artificially, by mixing solution of alum with a small quantity of carbonate of potassa, which precipitates any sesquioxide of iron which may be present, evaporating to crystallization, and coloring the crystals thus obtained with brick-dust, or Armenian bole, to make them appear like true Roman alum.

Alum is, however, most extensively prepared from alum slate, or shale, which is a mineral of very common occurrence. Alum slate contains silicate of alumina, together with finely-divided iron-pyrites, and more or less bituminous matter. The mineral is coarsely broken up, and subjected to a process of oxidation, which generally consists either in exposing it to the continued action of air and moisture, or in throwing it into pyramidal heaps, with or without an additional quantity of combustible, and setting fire to these in different parts, letting them smoulder away till all the pyrites are oxidized. The produce is better, the more slowly and uniformly this operation is conducted. The roasted heaps are then allowed to remain exposed to the air till they are in a fit state for extraction.

According to Jacquelain, 22Aq.

The natural formation of alum may be easily explained, where iron-pyrites occurs associated with feldspathic rocks; the oxidation of the iron-pyrites (FeS2) gives rise to sulphate of iron and free sulphuric acid, which combines with the alumina and potassa contained in the feldspar.

3 The calcination of the mineral continues until sulphurous vapors begin to pass off; the mineral is then transferred to cisterns, and repeatedly moistened with water during 3 or 4 months, when it crumbles down to a sort of mud.

4 Alum-earth differs from alum-slate more in its texture, which is soft and friable, than in its composition.

Sometimes the beaps take fire spontaneously from the heat evolved by the critation of the pyrites; the workmen then socher the fre, to prevent loss of sulphur in the form of sulphames acid

The alum-shale is often asseiated with so math coal, that any further addition of fuel to the beaps is found unecessary: bdeed, in some cases, it is requisite to add a certain quantity of shale which is poor in coal in order to eeromite the fiel

The change which takes place during the cridation is easily inligible.

The inc-prites is covered into sulphate of iron, whilst the excess of sulphur Fields a quantity of free salpharie acid

FeS,+0,=Fe0.80,+80,

The latter, acting upon the state of alumina, gives rise to sulphate of alumina. The wiphate of alumina and sulphate of iron are then extracted by water,3 and the volation evaporated to the requisite degree of eccentration.

The principal substances contained in the role alam-üquor are, sulphates of alamina, ind, magnesia, and soda, together with small quantities of the sulphates of manganese, potassa, and lime, the chlorides of magnesium and aluminum, seaquehloride of iron, and free sulphurie and hydrochloric acids. The liquor always contains a certain amount of potassa-alem the potassa being derived from the scale itself, or from the wood employed as foel and of ammonia-alum (atless too high a temperature has been employed in the process of oxidation, the ammonia being derived from the destructive distillation of the coal ̧.

The crude alum-iiquor generally contains a suficient amount of green vitriol (eulphate of iron, to pay for extraction, hence the manufacture of this salt is menalty carried on simultaneously with that of alum. Any persalt of iron which the Siquor may contain is reduced by metallie iron, and the sclation is then bosed down to the point at which the green vitriol crystallizes out. In this manner a mother-liquor is obtained, which is saturated with sulphate of alumica The green vitriol is parised by recrystallization.

The liquor is now mixed with a strong solution of chloride of potassium,* which converts the sulphates (with the exception of sulphate of alumina into chlorides, which, being much more sclable, are more easily removed from the resulting alum than the sulphates could be. The due regulation of the amount of chloride of potassium added, is of considerable importance, since an excess of that salt would give rise to the production of ebloride of aluminum.

The solution of the chloride is gradually added to the liquor, with constant agitation, and the alum-flour thus produced is then allowed to subside; it is afterwards drained, and washed with a little cold water. It is then redissolved, by exposing it, in a perforated leaden funnel, to the action of steam, and the saturated solution thus obtained is allowed to flow into wooden casks, or roaching-tuns, where it crystallizes.

1 It would appear that the sulphurous acid which is produced in those parts of the heap where the temperature is very high, is absorbed by the alumina, and subsequently converted into sulphuric acid, by the sulphate of sesquioxide of iron formed on exposing the protosulphate to the air.

A certain quantity of sulphuric acid is also liberated, in consequence of the formation of a basic sulphate of sesquioxide of iron when the protosulphate is oxidized by exposure. It is found advantageous to cover the heaps with exhausted ore, which retains a quantity of sulphuric acid that would otherwise be lost.

The oxidation of the heap requires from 10 to 24 months, according to the nature of

the ore.

2 Part of the sulphur also sublimes upon the outer and cooler portion of the heap.

3 The lixiviation of the alum-earth is carried out upon the same principles as those already explained in the manufactures of nitre and borax (22 145 and 173).

This salt is obtained as soap-boilers' waste, also from the saltpetre refineries and glass-houses.

The mother-liquor from the alum-crystals receives different applications, depending upon its composition. If it contain much sulphate of iron, it is digested with metallic iron, to neutralize any free sulphuric acid, and to deoxidize any persulphate of iron, and green vitriol is then crystallized from it. If much chloride of iron be present, the liquor is evaporated to dryness, and heated to redness, when sesquioxide of iron is left, which is employed as a pigment. mother-liquors are also sometimes employed for the preparation of sulphate of ammonia, from the ammoniacal liquor of the gas-works, or, when they contain much magnesia, for the manufacture of Epsom salts.

The

[In some places, alum is manufactured by allowing the sulphurous acid, produced in metallurgic operations, to act upon rocks containing alumina, in the presence of air and moisture, thus giving rise to sulphate of alumina, from which alum may be manufactured, in the manner above detailed. Alum is also made from clay, which is converted into sulphate of alumina by a process which has already been described (§ 210). It has been proposed to manufacture alum from feldspar, by fusing it first with sulphate, and afterwards with carbonate of potassa; when the resulting mass is boiled with water, an insoluble silicate of alumina and potassa is left, which is converted into alum by treatment with sulphuric acid.]

Properties.-Alum is found in commerce in large octohedral crystals, the angles of which are sometimes truncated by the faces of a cube; these crystals are frequently aggregated in large masses, which retain the form of the casks in which the crystallization has taken place.

The crystals of ordinary alum are colorless, but those of Roch alum have (as before mentioned) a brownish-red color. They are somewhat efflorescent in dry air. When heated to 198° F. (92° C.), they undergo the aqueous fusion, and when further heated, lose their water with very considerable intumescence, yielding a spongy mass, which is known as burnt alum (alumen exsiccatum vel ustum). A high temperature is required to expel the whole of the water; even at 392° F. (200° C.) a small quantity of water remains. Alum, which has been heated to this temperature, redissolves with difficulty in water. At a much higher temperature, the salt itself is decomposed, sulphurous acid and oxygen are disengaged, and a mixture of alumina and sulphate of potassa remains.

The crystals dissolve in about ten parts of water at 50° F. (10° C.), and in less than one-third of their weight at the boiling-point, so that a hot saturated solution of alum deposits the greater part in crystals, on cooling.

The solution of alum has an acid reaction and a sweetish astringent taste. If alum be dissolved in hot concentrated sulphuric acid, crystals are deposited on cooling, of the formula Al,O, 3SO, KO.SO,+3Aq.

When potassa or its carbonate is gradually added to a solution of alum, until the precipitate which is formed is no longer dissolved on stirring, the liquid, when evaporated, gives cubical crystals, which have been noticed above as artificial Roman alum. These crystals are generally supposed to have the same composition as those of ordinary octohedral alum, but it has been asserted that they consist of a basic alum, which is very probable, since, when their solution is boiled, it deposits a white precipitate, and the filtered liquid, on evaporation, deposits octohedra of common alum.

If an intimate mixture of alum with carbon or sugar be calcined in a close vessel as long as any inflammable gas (carbonic oxide) is evolved, the residue will consist of a mixture of alumina, charcoal, and sulphide of potassium in a very finely-divided state. In consequence of the rapid oxidation of the last

According to Hertwig, alum loses 10 eqs. of water at 212° F. (100° C.).

2 If a very intense heat be employed, a compound of alumina and potassa is left.

mentioned substance, the mixture takes fire when thrown into the air, and has hence received the name of Homberg's pyrophorus.

Uses of Alum.-This salt is very largely employed in dyeing and calicoprinting, in paper-making, in the manufacture of colors, in rendering wood and paper incombustible, and in medicine.

For the first three of these uses, the presence of iron in a soluble form in the alum would be very injurious. Iron may be easily detected by mixing the solution of alum with ferrocyanide of potassium, which would produce a blue precipitate.

A sulphate of alumina and potassa having the same composition as the mineral alum-stone, viz., KO.SO,, 3(Al„O„.SO ̧)+9Aq, may be obtained as a crystalline precipitate by boiling a solution of alum with freshly-precipitated hydrate of alumina.

Soda-alum is much more soluble in water than potassa-alum. Like this salt, it is sometimes found native.

Ammonia-alum, which also occurs native, may be prepared by the direct combination of sulphate of alumina with sulphate of ammonia. On the large scale, ammonia-alum is prepared by processes similar to those employed in the manufacture of potassa-alum. It is very similar in its properties to potassaalum; when ignited, it leaves a residue of alumina.

Phosphate of Alumina occurs in nature as the mineral wavellite, the formula of which is 3A1,0,.2PO,+12Aq. Hermann has recently shown that the mineral gibbsite, formerly supposed to be a hydrate of alumina, is really a phosphate, the presence of the phosphoric acid having been overlooked by other analysts, in this mineral, as, at an earlier period, it was overlooked in wavellite. The formula of gibbsite is Al,O,.PO,+8Aq.

The precipitate produced by common phosphate of soda in solution of alum, formerly supposed to be Al,O,.PO,, has been found by Ludwig to contain 8A1,0.9PO. On dissolving this precipitate in hydrochloric acid, reprecipitating by ammonia, and igniting the washed precipitate, the compound Al,Ò ̧.PO, was obtained.

The existence of a carbonate of alumina is not certainly established. The precipitate produced by alkaline carbonates in solutions of alumina is generally considered as a hydrate, since its formation is attended with disengagement of carbonic acid.1

SILICATES OF ALUMINA.
ΜΙΝΑ.

§ 212. Combinations of alumina with silicic acid are very abundant in nature. The minerals known as andalusite, cyanite (disthene) and sillimanite, are silicates of alumina of the formula 3A1,0,.2SiO,. Allophane is the hydrate of this silicate.

The feldspars form a most important class of minerals, of which the neutral silicate of alumina Al,0,.3SiO, is always a constituent; indeed, the feldspars occupy much the same position among the silicates as the anhydrous aluminaalums occupy among the sulphates; for they consist of silicate of alumina combined with an alkaline silicate, or with silicate of lime or magnesia.

Potash-feldspar (orthoclase, adularia) is represented by the formula

KO.SIO,,A,O,.3SiO,.

This is the most common of the feldspars; it is found in very hard oblique

According to Muspratt, the formula of the precipitate produced by carbonate of ammonia in a solution of alum is 3A),0,.2CO2+16Aq.