ALUM, a compound salt employed by dyers and other artists in their different processes. It is soluble in water, has an astringent, acid, and sweetish taste; reddens vegetable blues, and crystallizes in regular octahedrons. When heated, it liquefies; and if the heat be continued, the water of crystallization is driven off, the salt frothes and swells, and at last a white matter remains, known by the name of burnt alum.

Its constituents are sulphuric acid, alumina, an alkali, and water. The alkali may be either potash, soda, or ammonia. Hence there are three distinct species of alum, depending upon the nature of the alkali which each contains. Potash alum (in which the alkali is potash) is the common alum of this country. In France both potash and ammoniacal alum are manufactured; while soda alum is met with native in different states, and probably in considerable quantity, in South America; for it is curious that, on that continent, soda almost uniformly replaces potash. Instead of nitrate of potash, which occurs in the old continent, there are great depositos of nitrate of soda in South America. It is likely that albite will be found replacing felspar in the granite of South America.

The progress made by chemists in the discovery of the constitution of alum was very slow. The species first investigated was potash alum. That it contained sulphuric ≡cid as a constituent, was known even to the alchemists. Pott and Margraaf demonstrated that alumina was another constituent. Mr Pott, in his Lithogeognosia, showed that the earth of alum, or the precipitate obtained when an alkali is poured into a solution of alum, is quite different from lime and chalk, with which it had been confounded by Stahl. Margraaf went much farther. He not only showed that alumina is one of the constituents of a .⅛ n, but that this earth possesses peculiar properties, is different from every other substance, and is one of the ingredients in common clay. (Experiences faites sur la Terre i lAlwι.∙ Margraaf s Opusc. ii. 111.) Margraaf showed likewise, by many experiments, that crystals of alum cannot be obtained by dissolving alumina in sulphuric acid, and evaporating the solutions. The crystals formed are always soft, and quite different in their appearance from alum crystals. But when a solution of potash or ammonia is dropt into this liquid, it immediately deposits perfect crystals of alum. (Sur la Régénération de l’Alun ; Margraafs Opusc. ii. 86.) He mentions likewise, that manufacturers of alum in general were unable to procure the salt without a similar addition, and that at first it had been customary to add a quantity of putrid urine, and that afterwards a solution of carbonate of potash was substituted in its place. But subsequent chemists do not seem to have paid much attention to these important observations of Margřaaf: they still continued, without any rigid examination, to consider alum as a sulphate of alumina.

Bergman indeed had observed, that the addition of potash or ammonia made the alum crystallize, but that the same effect was not produced by the addition of soda or of lime. (De Confectione Aluminis : Bergman’s Opusc. i. 225.) He had observed likewise, that sulphate of potash is frequently found in alum. He decomposed the solution of alum by means of ammonia, evaporated the filtered liquid to dryness, and exposed the residue to a red heat. A quantity of sulphate of potash often remained behind in the crucible. (Ibid. p. 326.) From these facts he drew the conclusion that sulphate of potash readily combines with sulphate of alumina. Yet it is obvious, from the whole of his dissertation, that he had no conception that alum is a double salt. He ascribed the difficulty of crystallization to the excess of sulphuric acid present He thought that the only use of the potash was to saturate this excess; and advises manufacturers to substitute clay instead of potash, as a method which would not only saturate the excess of acid, but increase the quantity of alum. This very bad advice was not, we presume, followed by any alum-makers. If they had tried it, they would soon have been convinced of its injurious consequences. Instead of alum, they would have obtained an insoluble, tasteless powder, well known by the name of alum saturated with its earth.

After Klaproth had discovered the existence of potash as an ingredient in leucite and lepidolite, it occurred to Vauquelin, that it was probably an ingredient likewise in many other minerals. He recollected that alum crystals often make their appearance during the analysis of stony bodies; and, considering that alum cannot be obtained in crystals without the addition of potash, he began to suspect that this alkali constituted an essential ingredient in the salt. A set of experiments, undertaken on purpose to elucidate this important point, soon satisfied him that his conjecture was well founded. Accordingly, in the year 1797 he published a dissertation, demonstrating that alum is a double salt, composed of sulphuric acid, alumina, and potash. (Annales de Chimie, xxii. 258.) Soon after, Chaptai published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and alum manufactured by himself. This analysis led to the same result as that of Vauquelin. (Ann. de Chim. xxii. 280.)

Since that time alŋm has been admitted by chemists to be a triple salt; tt∏α various analyses of it have been made to determine its constituents. Vauquelin (Ann. de Chim. 1. 167), Thenard and Roard (ibid. tom. lix. 72), Curaudau (Journal de Physique, lxvii. 1), and Berzelius (Ann. de Chim. lxxxii. 258), successively published the results of their experiments. These analyses gradually led to an accurate knowledge of the composition of this [7:2:572] salt. The atomic weights of the constituents of alum are as follows :—
[table]

Every atom of the sulphuric acid is combined with an atom of base; so that potash alum is a compound of sulphate of potash and sulphate of alumina, in the proportion of one atom of the former salt to three atoms of the latter. We may therefore state the constituents of potash alum as follows :—
[table]

Soda alum differs from potash alum in containing sulphate of soda instead of sulphate of potash. Its constituents are as follows :—
[table]

The weight of an integrant particle of this alum is only 58∙875; because the alum of soda weighs only 4, while that of potash is 6.

The constituents of ammoniacal alum are as follows :—
[table]

The weight of the integrant particle is only 57, because ammonia has an atomic weight of only 2 ’ 125.

One of the most remarkable differences between these three species of alum is the śolubility of each in water. At the temperature of 60°, 100 parts of water dissolve 9∙37 parts of ammoniacal alum,

14∙79 parts of potash alum,

327∙6 parts of soda alum.

This great solubility of soda alum renders the manufacture of it very difficult It does not easily cryallize; indeed, when the weather is hot, cryals of it can hardly be obtained. This great solubility, together with the inferior weight of its integrant particle, would render it more convenient and more economical for dyers and calico printers, provided it could be furnished at the same rate with common alum. But the greater difficulty attending the making of it would probably prevent it from being saleable at a price sufficiently low to make it available as a mordant.

Soda alum was first mentioned by Mr Winter in 1810, in his account of the Whitby alum processes (Nicholson’s Jour. XXV. p. 254, 255); but before that time it had been made by Charles Macintosh, Esq. of Crossbasket Mr William Wilson, at Hurlet, .near Glasgow, afterwards made it in considerable quantities. Specimens of it have been still more recently sent by Dr Gillies from the neighbourhood of Mendoza, in South America, where it occurs native in considerable quantity.

These three different species of alum differ also somewhat from each other in their specific gravities, which are as follows :—
[table]

The word alumen, which we translate alum, occurs in Pliny’s Natural History. In the 15th chapter of his 35th book, he gives us a detailed description of it By comparing this with the account of crmrr∣ζia given by Dioscorides in the 123d chapter of his 5th book, it becomes quite obvious that he alludes to the same substance. Hence it follows, that ΰτυντηοια is the Greek name for alumen. Pliny informs us that alumen was found naturally in the earth. He calls it salsugo terrae. Different substances, he informs us, were distinguished by the name of alumen; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and in medicine. The light-coloured alumen was useful in brilliant dyes, the dark-coloured only in dyeing black or very dark colours. One species of alumen was a liquid, which was apt∙to be adulterated; but, when pure, it had the property of striking a black with the juice of the pomegranate. This property seems to characterize a solution of sulphate of iron in water. It is quite obvious that a solution of our alum would possess no such property. Pliny says that there is another kind of alum which the Greeks call schistos. It forms in white threads upon the surface of certain stones. From the name schistos, and the mode of formation, there can be little doubt that this species was the salt which forms spontaneously on certain slaty minerals, as alum slate and bituminous shale, and which consists chiefly of sulphate of iron and sulphate of alumina. Possibly in certain places the sulphate of iron may have been nearly wanting, and then the salt would be white, and would answer, as Pliny says it did, for dyeing bright colours. Several other speeies of alumen are described by Pliny, but we are unable to make out towhat minerals he alludes.

The alumen of the ancients, then, was not the same with the alum of the moderns. It was most commonly a sulphate of iron, sometimes probably a sulphate of alumina, and usually a mixture of the two. But the ancients were unacquainted with our alum. They were acquainted with sulphate of iron in a crystallized state, and distinguished it by the names of misy, sory, chalcanthum. (JPliny, xxxiv. 12.) As alum and green vitriol were applied to a variety of purposes in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two salts accurately from each other. In the writings, of the alchemists we find the words misy, sory, chalcanthum, applied to alum as well as to sulphate of iron; and the name atramentum sutorium, which ought to belong, one should suppose, exclusively to green vitriol, applied indifferently to both.

When our alum was discovered is entirely unknown. Beckman devoted a good deal of time to trace the history of this salt, and published a curious dissertation on the [7:2:573]subject; but his attempts to trace its origin were unsuccessful. The manufacture of it was discovered in the East, but at what time or place is totally unknown. It would appear that, about four or five hundred years ago, there was a manufactory of it at Edessa in Syria, at that time called Rocca; hence, it is supposed, the origin of the term Roch alum, commonly employed in Europe: though there are others who pretend that the term originated at Civita Vecchia, where alum is made from a yellow mineral in the state of a hard rock.

Different alum works existed in the neighbourhood of Constantinople. About the time of the fall of the Grecian empire, the art of making alum was transported into Italy, at that period the richest and most manufacturing country in Europe. Bartholomew Pernix, a Genoese merchant, discovered alum ore in the island of Ischia, about the year 1459. Nearly at the same time John di Castro, who was well acquainted with the alum works in the neighbourhood of Constantinople, suspected that a mineral fit for yielding alum existed at Tolfa, because it was covered with the same trees that grew on the alum mineral near Constantinople. His conjecture was verified by trials, and the celebrated manufactory at Tolfa established. Another was begun in the neighbourhood of Genoa; and the manufacture flourished in different parts of Italy. To this country it was confined for the greater part of a century. Various manufactories of it were established in Germany by the year 1544. In the time of Agricoa there was a manufactory of it at Commotau, in Bohemia. About the same time an alum work was established at Alemaron, near Carthagena, in Spain.

England possessed no alum works till the reign of Charles I. Thomas Chaloner, Esq. son of Dr Chaloner, who had been tutor to Charles, while hunting on a common in Yorkshire took notice of the soil and herbage, and tasted the water. He found them similar to what he had seen in Germany, where alum works were established. In consequence of this, he got a patent from Charles for an alum work. This manufactory was worth two thousand a year, or perhaps more. But some of the courtiers thinking this too much for him, prevailed with the king, notwithstanding the patent, to grant a moiety of it to another person. This was the reason why Mr Cha-loner was such a partisan of the parliament, and such an enemy of the king, that, at the end of the civil war, he was one of those who sat in judgment upon his majesty and condemned him.^[2. Letters written by Eminent Persons in the Seventeenth and Eighteenth Centuries, &c.; and Lives of Eminent Men, by John Aubrey, Esq. vol. ii∙ p. 281. Lond. 1813. ] Since that time various alum works have been established in different parts of England and Wales; but no one at present exists in Britain except the Whitby works, originally established by Mr Chaloner, and two works at Hurlet and Campsie, both in the neighbourhood of Glasgow.

Several alum works likewise exist in Sweden, particularly in West Gothland. There is one, for example, at Haensaeter, near the borders of the Wene Lake, on the west side of the mountain called Kinnekulle. But, for a description of the Swedish works, we refer to Bergman’s Opuscula, vol. i. p. 284, or the English translation, vol. i. p. 342. We do not know if any alum works exist in Poland or Russia; but as the greater part of these extensive countries consists of alluvial soil to a great depth, it is probable that little alum ore will be found in them.

Various minerals are employed in the manufacture of alum, but by far the most important of them are the following three; alum-stone, alum-slate, bituminous shale.

Alum-stone was first observed at Tolfa, in the neighbourhood of Rome; afterwards in Hungary; and Cordier has shown that it is very common in volcanic rocks, but that it never occurs anywhere else. {Annales des Mines, tom. iv. p. 205, and tom. v. p. 303.)

The colour is white, greyish-white, or sometimes yellowish-white; most commonly amorphous; but it occurs also crystallized in rhomboids approaching to cubes, the angles being about 89° and 91°. In some crystals the apex of the rhomboid is replaced by a tangent plane. The size of these crystals varies from 0∙03937 to 041811 of an inch in length.

The specific gravity is 2*7517; but the amorphous specimens, owing probably to cavities, are rather lighter. Haüy states the specific gravity to be 2*587; harder than calcareous, but softer than fluor, spar; fracture foliated in a direction perpendicular to the axis of the rhomboid; in all other directions the fracture is conchoidal; fragments irregular, with blunt edges; easily pulverized; feels harsh, and does not stain; decrepitates before the blowpipe; gives out sulphureous acid when heated on platinum-foil; and tastes of alum when applied to the tongue. In a strong heat it loses its acid, and becomes tasteless.

The constituents of the pure crystals, according to the analysis of Cordier, are—-
[table]

We see from this constitution that alum-stone contains in itself all the ingredients of alum. The absence of iron accounts for the superior purity for which Roman alum was so long celebrated.

In the alum manufactory at Campsie, near Glasgow, the alum is made from a shale taken out of the old abandoned coal-pits in the neighbourhood. At first this shale furnished alum by simple lixiviation with water. This process having been continued for a number of years, a great quantity of washed shale gradually accumulated in the neighbourhood of the works. This shale, when burnt, was found to yield a new crop of alum. Now, in this burnt shale thin bands of a greyish-white matter occasionally make their appearance, intermixed with portions having a yellow colour, and which are unequally distributed. The fracture is earthy; the matter is opaque, friable, and has an astringent, acid, and sweetish taste. The specific gravity is 1 ’ 887L

It occurred to the writer of this article, that this substance bore a considerable analogy to alum-stone. This induced him to subject it to chemical examination.

When digested in water it dissolves, with the exception of a white powder, amounting to 15*31 per cent, which is a subsulphate of alumina.
[7:2:574]

When heated it melts sotnewhat like alum, and gives out ' pure water. WheiĻ heated to redness it swells out like alum, and finally leaves a yellowish-white, porous, tasteless matter, nearly similar to what would be left by alum treated in the same manner, making allowance for the difference of colour.

The constituents were found to be—
[table]

These constituents are equivalent to

24 atoms sulphate of alumina,

9 atoms bipersulphate of iron,

1 atom bisulphate of potash; and each of these atoms is combined with about 5 atoms of water.

From this analysis we see that the substance which appears after burning the Campsie shale is not the same with alum-stone; but it constitutes an excellent article for the manufacture of alum, being highly productive; and is consequently much valued by the manufacturers.

Alum-slate is a much more abundant mineral thqn alum-stone. It is said to alternate with primitive clayslate. It occurs abundantly along with transition-slate; and there can be little doubt that it occurs likewise in the floetz formations. In West Gothland in Sweden, it constitutes a part of different hills, as Kinnekulle, Hunne-berg, and Halleberg; in all of which it appears to alternate with floetz trap rocks. It occurs likewise abundantly at Whitby, in Yorkshire. We have never ourselves been upon the spot ;· but, from the general structure of Yorkshire and the neighbouring counties, indeed of the whole east coast of England, there can be very little doubt that, in this position, it is also a floetz rock.

Alum-slate, as the name implies, is a slaty rock, though sometimes it occurs in balls. The colour is bluish-black, with a strong shade of grey; fracture straight slaty; fragments tabular; its internal lustre is glimmering; it retains its colour in the streak, but acquires more lustre; soft; not particularly brittle; when exposed to the air it effloresces, and acquires an aluminous taste.

This mineral has never been accurately analyzed; but there can be no doubt , that it contains silica, alumina, iron, sulphur, charcoal, and often likewise potash. Probably this was the mineral upon which the alumen scissile of the ancients was found.

Bituminous shale, the Brandschiefer of the Germans, is a slaty mineral, which almost constantly accompanies beds of coal, and accordingly is very common in Great Britain. Its colour is brownish-black; its fracture is thin slaty; fragments tabular; internal lustre glimmering, but the colour is not altered; very soft; rather sectile; feels rather greasy; easily frangible. When heated it burns with a pale flame and sulphureous odour, and becomes white. It has never been accurately analyzed; but it is probably nothing more than slate-clay, which occurs so abundantly in the independent coal formation ' impregnated with the matter of coal. Its other principal constituents must be silica, alumina, and iron pyrites.

Slate-clay itself, at least not sufficiently impregnated with coaly matter to deserve the name of bituminous shale, is frequently employed in the making of alum. This is the case in the neighbourhood of Glasgow.

. Several native varieties of sulphate of alumina and soda alum occur in South America, some of the most remarkable of which it may be proper to specify.

1. Sulphate .of alumina from Rio Saldana. It is said to occur in nests in the transition-slate of the Andes. The colour is white, here and there tinged yellow, obviously from external impurities. It occurs in fine crystalline needles; lustre silky; taste that of alum, but stronger; specific gravity 1.6606; soft; before the blowpipe behaves like alum. The writer of this article subjected it to a chemical analysis, and found its constituents as follows :—
[table]

This is equivalent to

1 atom sulphate of alumina,

6 atoms water, ∙τ⅞θ atom sulphate of soda, ∙ s 1 θ atom persulphate of iron.

So that if the sulphate of soda and persulphate of iron be only accidental ingredients, the mineral is a compound of
[table]

2. Polcura, or alum-earth, found near the summit of a lofty ridge near El Paso de las Damas, in the Chilian Andes, and used as a mordant by the inhabitants in dyeing red. This mineral was brought over to Great Britain by Dr Gillies, who was kind enough to favour the writer of this article with a specimen. It occurs in hard masses, partly earthy, partly fibrous. The fibres have a silky lustre. The colour is white, and the taste that of alum.

When digested in water, a portion was dissolved, and a portion remained in the form of a white insipid earth. The quantity of insoluble matter differed very much in different parts of the specimen. The least was 8∙62, and the greatest 34∙65 per cent. This insoluble matter contained a little sulphur; for, when heated, it emitted a blue flame with the smell of sulphureous acid. The rest of it was a mixture of alumina and silica, tinged yellow by peroxide of iron. The portion dissolved in water, being subjected to analysis, was found to consist of
[table]

3. Soda-alum. It occurs native in the province of St .Tuan, situated to the north of Mendoza, on the east side of the Chilian Andes, at about lat. 30° S. The alum is white, and composed of fibres adhering longitudinally.
[7:2:575]

and having a certain breadth, but very thin. It bears some resemblance to fibrous gypsum, but is harder, not being scratched by the nail, though the knife scratches it with great ease. It is sectile. The outer fibres are white and only slightly translucent, as if they had lost a portion of their water; but the internal fibres are transparent, and have a silky aspect.

It tastes precisely like alum, and is very soluble, water at the temperature of 62° dissolving 3∙773 parts of it, and boiling water dissolving any quantity whatever. When exposed to heat, it behaves very nearly as common alum.

Its constituents were found to be—
[table]

It will be observed that the sulphuric acid constitutes just four atoms, the soda one atom, and the alumina just 0∙29 less than three atoms. But the quantity of lime and oxides of iron present is exactly equivalent to 0*29 atom of alumina. Hence these substances appear to have displaced a small∙quantity of alumina in the salt. The water amounts to very nearly twenty atoms. It is obvious from all this, that the true constitution of the salt is—
[table]

It contains five atoms less water than soda-alum artificially crystallized.

4. There is a mineral called aluminite, which was observed in the environs of Halle many years ago, and which was afterwards detected by Mr Webster in the chalk rocks of Ncwhaven in Sussex, which, if it were sufficiently abundant, would constitute an excellent material for the manufacture of alum.

Its colour is snow-white. It occurs in reniform pieces of greater or smaller size; fracture fine earthy; dull; streak glistening; opaque; adheres feebly to the tongue; soils very slightly; very soft; feels fine, but meagre; specific gravity 1∙7054. Its constituents, as determined by three several analyses of Stromeycr, are—
[table]

It is therefore a hydrous trisulphate of alumina.

Four different processes are employed in the manufacture of alum, according to the nature of the mineral from which the alum is to be extracted.

The process employed at Tolfa is the simplest of all. If the Tolfa stone be kept constantly moistened with water for about two months, it falls to powder of itself, and yields alum by lixiviation. But this is not the process employed by the manufacturers. The alum-stone is broken into small pieces, and piled on the top of a perforated dome, in which a wood fire is kindled. The smoke and flame of the wood penetrate through the pieces of alum-stone, and a sulphureous odour is disengaged, owing to the decomposition of a portion of the sulphuric acid in the stone. This roasting is twice performed; the pieces of ore which, the first time, were at the edge of the dome, being the second time put in the middle. The process of roasting this stone requires considerable attention. If the heat be too great, the quality of yielding alum is destroyed: if the heat be too small, the stone does not readily fall to powder. There can be little doubt that the unroasted stone would yield more alum than the roasted; but probably the additional labour requisite in the latter case would more than swallow up the increase of product.

The roasted stone, which has now acquired a reddish colour, is placed in rows between trenches filled with water. This liquid is so frequently sprinkled on it, that the stone is always moist. In two or three days it falls to powder, like slacked quicklime; but the daily watering is continued for a month. The success of this part of the operation is said to depend very much on the weather. When the weather is rainy, the alum is all washed out, and little or nothing left for the manufacturer to extract. In such cases, it is obvious that the alum-stone should be protected from the rain by a shed.

When the stone has by this process been reduced to a sufficiently fine powder, it is thrown into a leaden boiler filled two thirds with water. During the boiling, the powder is frequently stirred up, and the water that evaporates is replaced. When the boiling has been continued for a sufficient time, the fire is withdrawn, and time allowed for the earthy matter to subside to the bottom. A cock is then opened, which allows the clear liquor to flow out into deep wooden square vessels, so made that they can be easily taken to pieces. Here the alum gradually crystallizes, and attaches itself to the sides and bottom of the vessel. The mother liquid is now drawn off into shallower wooden troughs, where more alum crystals are deposited. The liquid has now a red colour, and is muddy; and the last alum crystals are mixed with this red matter. They are washed clean in the mother liquor, which is finally pumped into a trough, and used in subsequent processes.

The alum obtained at Tolfa is known by the name of Roman alum, and is in very high estimation. It is always mixed with a little reddish powdery matter, which is easily separated from it. What this red matter is, has not been ascertained; but it is not peroxide of iron. To the eye it has very much the appearance of a vegetable matter. We have some notion that it is added artificially by the sellers of Roman alum. Probably Roman alum at first had a red tinge, in consequence of the red matter in the mother liquor remaining partially attached to it. The goodness of the alum may have given the purchasers a partiality to the red colour, and induced the sellers to add a red powder artificially. We have never had an opportunity ourselves of examining this matter, but have been informed by those who have, that it contains no iron.

It is not improbable that this process would be improved by grinding the Tolfa stone to a fine powder in a mill, without any previous roasting, and then keeping the powder moistened with water for a considerable time. It the residual earth, after the alum is extracted, be boiled with sulphuric acid, the liquid yields alum crystals by evaporation. This is a demonstration that neither the alumina nor the potash is exhausted, and that the sulphuric acid driven off by the roasting is so much alum lost to the manufacturer. Indeed the quantity of sulphuric acid in the alum-stone is not sufficient to occupy the whole of the potash in the formation of alum. It would be necessary to add about one tenth of the weight of the alum-stone of sulphuric acid, if it were wanted to employ the whole potash present in the stone. The consequence of this addition, supposing no loss, would be an additional quantity[7:2:576] of alum, amounting to rather more than one fifth of the weight of the alum-stone employed.

Alum-slate, being very different in its composition, requires a different treatment to fit it for yielding alum. If the alum-slate contain a notable quantity of lime or magnesia, it does not answer the purposes of the manufacturer so well. ' Indeed the proportion of lime present may be conceived to be such that no alum whatever would be obtained. As alum-slate has never been subjected to accurate analysis, we do not know in what proportion these two earths exist in it, or whether they may not, in many cases, be absent altogether. The essential ingredients in alum-slate, for the alum-makers, are alumina and iron pyrites.

The first process is to roast the ore. In Sweden, where the fuel is wood, and consequently expensive, it is customary to use the alum-slate itself as fuel for roasting the ore. For this purpose a small layer of brushwood is covered with pieces of alum-slate, and set on fire; and, as the combustion proceeds, new layers of alum-slate are added. It is usual to place alternate layers of roasted and unroasted alum-slate. The combustion continues for a month or six weeks. At Whitby, coal is employed for roasting the alum-slate. Indeed the alum-slate of Whitby is lighter coloured than that of Sweden, and probably would not burn of itself. So great is the quantity of combustible matter in the Swedish alum-slate, that it is employed as fuel for burning limestone. Great quantities of limestone are burnt in this manner at Hunneberg, near the south side of the lake Wener. The roasted ore has usually a brown colour. When it is red, the quantity of alum which it yields is considerably diminished.

By this roasting the pyrites is decomposed. The sulphur is converted into sulphuric acid, while the iron is oxidized. In what manner this change is produced it is not easy to say. Indeed it does not seem certain that pyrites is a constant ingredient in alum-slate. We have never been able to detect any, by the eye, in any specimens of Whitby alum-slate which we have examined. At Haensaeter, in Sweden, no sulphate of iron crystallizes when the liquid is evaporated; yet, if pyrites had been present, it is difficult to see any reason that should prevent this salt from being formed. Hence it is probable, that in alum-slate the sulphur is sometimes at least combined with other substances than iron. It must always be in a state of combination; for, if it were in a loose state, it would be driven off by the roasting. This point deserves to be elucidated by analyzing different varieties of alum-slate.

The roasted ore has an astringent taste, owing to the sulphate of iron and sulphate of alumina which it contains. The next process is to lixiviate it with water, in order to dissolve these salts. For this purpose it is put into reservoirs made of wood or masonry, with a stopcock at the bottom to draw off the water. The usual method is to keep the water for twelve hours in contact with ore that has been twice lixiviated; then to draw it off, and allow it to remain for an equal period on ore that has been once lixiviated. Lastly, it is run upon fresh ore, and allowed to remain on it for twelve hours longer. If the specific gravity of the liquid thus treated be 1∙25 at the temperature of 55°, it may be considered as saturated with sulphate of alumina and sulphate of iron. But we presume that this specific gravity is not often obtained.

The liquid, thus impregnated with salt, is now boiled down in leaden vessels to the proper consistency for crystallization. In Sweden, the fuel employed for this purpose is alum-slate. By this means a double effect is produced; the liquid is evaporated, and the alum-slate is roasted. During the boiling, abundance of oxide of' iron falls mixed with selenite, if lime be one of the constituents of the alum-slate. When the liquid is sufficiently concentrated, it is let into a square reservoir, in order to crystallize. Great quantities of sulphate of iron crystals are usually deposited in this vessel. These are collected by drawing the liquid off into another reservoir. When all the sulphate of iron that can be obtained has been separated, a quantity of sulphate of potash, muriate of potash, or putrid urine, is mixed with the liquid. The sulphate of potash is procured from the sulphuric acidmakers, and the muriate of potash from the soap-makers. By this addition, alum is formed in the liquid, and it gradually deposits itself in crystals on the sides of the vessel. These crystals are collected, and dissolvedin the smallest quantity of boiling water that will take them up. This solution is poured into large wooden casks. In a fortnight or three weeks the alum crystallizes, and covers the side’s and bottom of the cask. The hoops are now taken off, and the staves of the cask removed. A mass of alum crystals, having the shape of the cask, remains. This mass is pierced, the mother liquor allowed to run out, and preserved for a subsequent process. The alum, being now broken in pieces, is fit for sale.

The manufacture of alum from bituminous shale and slate-clay bears a considerable resemblance to the manufacture from alum-slate, but differs in several particulars. There are two works of this kind in the neighbourhood of Glasgow, managed with great skill, and excellent in every respect. We shall give a sketch of the processes followed in these works. The bituminous shale and slate-clay employed are obtained from old coal-pits, which are very extensive in the neighbourhood of Glasgow. The air in these coal-pits is moist, and its average temperature about 62°. The shale, having been exposed for many years, has gradually opened in the direction of its slaty fracture, so as to resemble in some respects a half-shut fan; and all the chinks in it are filled with a saline efflorescence in threads. This salt is white, with a shade of green; has a sweetish astringent taste; and consists of a mixture of sulphate of iron and sulphate of alumina. In order to obtain these salts in a state of solution,’ nothing more is requisite than to lixiviate this shale with water. The lixiviated ore being left exposed to the weather, forms more salt, which is gradually washed out of it by the rain-water, and this water is collected and preserved for use.

The next step in the process is to boil down the liquid to a sufficient state of concentration. At Campsie, all these boilers are composed of stone, and the heat is applied to the surface. This is a great saving, as leaden vessels are not only much more expensive, but require more frequent renewal. When the liquid is raised to a sufficiently high temperature in the stone reservoir, pounded sulphate of potash, or muriate of potash, as they can be procured, is mixed with it; and there is an agitator in the vessel, by which it is continually stirred about. This addition converts the sulphate of alumina into alum. The liquid is now let into another trough, and allowed to remain till it crystallizes. In this liquid there are two salts contained in solution, viz. sulphate of iron and alum; and it is an object of great consequence to separate them completely from each other. The principal secret consists in drawing off the mother liquor at the proper time; for the alum is much less soluble in water than the sulphate of iron, and therefore crystallizes first. The first crystals of alum formed are very impure. They have a yellow colour, and seem to be partly impregnated with [7:2:577] sulphate of iron. They are dissolved in hot water, and the solution poured into troughs, and allowed to crystallize a second time. These second crystals, though much purer, are not quite free from sulphate of iron; but the separation is accomplished by washing them repeatedly with cold water; for sulphate of iron is much more soluble in that liquid than alum. These second crystals are now dissolved in as small a quantity of hot water as possible, and the concentrated liquid poured while hot into large casks, the surface of which is covered with two cross beams. As the liquor cools, a vast number of alum crystals form on the sides and surface. The casks are allowed to remain till the liquid within is supposed to be nearly of the temperature of the atmosphere. This, in winter, requires eleven days; in summer, fourteen or more. We have seen the liquid in a cask that had stood eleven days in summer, still more than blood-hot. The hoops are then removed, precisely as in the manufacture of alum from alum-slate.

There always remains in the boilers a yellowish substance, consisting chiefly of peroxide of iron. This is exposed to a strong heat in a reverberatory furnace, and it becomes red. In this state it is washed, and yields more alum. The red residue is ground to a fine powder, and dried. It then answers all the purposes of Venetian red as a pigment. By altering the temperature to which this matter is exposed, a yellow ochre is obtained instead of a red.

In France, where alum-ores are by no means abundant, alum is manufactured from clay. This method of making the salt was first put in practice by Chaptal, when professor of chemistry at Montpellier. His methods have been since gradually improved, and brought to a state of considerable perfection. The first process tried was this: the clay was reduced to a fine powder in a mill, and then mixed with sulphuric acid. After remaining some days, it was exposed for twenty-four hours to a temperature of about 130°. It was then lixiviated, and the liquid mixed with urine or potash. This method being found inconvenient, was abandoned for the following: the clay being well ground, was mixed with half its weight of the saline residue from a mixture of sulphur and nitre. This residue is little else than sulphate of potash. The mixture was formed into balls about five inches in diameter, which were calcined in a potter’s furnace. They were then placed on the floor of a chamber in which sulphuric acid was made. The acid vapour caused them to swell, and to open on all sides. In about a month they were sufficiently penetrated with the acid. They were then exposed to the air, under shades, that the saturation might become more complete. Finally, they were lixiviated, and the liquid being evaporated, yielded pure alum.

This process has been considerably improved by Berard, the present proprietor of the Montpellier alum work. Instead of exposing the calcined balls to the fumes of sulphuric acid, he sprinkles them with a quantity of sulphuric acid of the specific gravity 1∙367, equal to the weight of the clay employed: but it is obvious that the proportion must vary with the nature of the clay. The solution takes place with the greatest facility, and crystals of alum are obtained by evaporating the liquid.

Another process was put in practice by Chaptal, in the neighbourhood of Paris, and is still followed, or was at least followed some years ago, by Μ. Bouvier. A mixture is made of 100 parts of clay, 50 parts of nitre, and 50 parts of sulphuric acid of the specific gravity 1∙367; and this mixture is put into a retort, and distilled. Aquafortis comes over, and the residue in the retort being lixiviated with water, yields abundance of excellent alum.

We may mention another process described by Caraudau, and certainly practicable, and even easy, though we do not believe that it would be attended with profit? He forms 100 parts of clay into a paste with water, holding 5 parts of common salt in solution. This paste is formed into cakes, and calcined in a reverberatory furnace. The calcined mass is reduced to powder, and well mixed with the fourth part of its weight of concentrated sulphuric acid. When the muriatic acid vapours are dissipated, as much water is added as there had been employed of acid, and the mass is carefully kneaded. A strong heat is produced; the composition swells; more water is added; and at last a solution of potash, in which the alkali amounts to one fourth of the acid employed. The liquor is now drawn off, and, on cooling, it yields a copious deposite of alum crystals. (l.)