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 acid as a constituent was known even to the alchemists. Pott and Marggraff demonstrated that alumina was another constituent. 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. Marggraff went much farther. He not only showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, is different from every other substance, and is one of the ingredients in common clay (“Expériences faites sur la Terre d’Alun,” Marggraff’s Opusc. ii. 111). Marggraff 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,” Marggraff’s Opusc. ii. 86). He mentions likewise that manufacturers of alum in general were unable to procure the salt without a similar addition, that at first it had beer 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 Marggraff: they still continued, without any rigid examination, to consider alum as a sulphate of alumina.

Bergmann indeed had observed that the addition of potash or ammonia made the alum crystallise, but that the same effect was not produced by the addition of soda or of lime (“De Confectione Aluminis,” Bergmann’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.

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, Chaptal 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 alum has been admitted by chemists to be a triple salt, and various analyses of it have been made to determine its constituents. Vauquelin (Ann. de Chim. 1. 167), Thenard and Board (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 salt.

One of the most remarkable differences between the three species of alum is the solubility 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 crystallise; indeed, when the weather is hot, crystals of it can hardly be obtained. Its great solubility 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. pp. 254, 255); but before that time it had been made by Mr Charles Macintosh of Crossbasket. Mr William Wilson, at Hurlet, near Glasgow, afterwards made it in considerable quantities. Specimens of it have been 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 :—

Ammoniacal alum 1 ∙ 56

Potash alum...'. 1 ∙ 75

Soda alum 1 ∙ 88

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 στvπτηρίa given by Diosco-rides in the 123d chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it salsugo terroe. Different substances, he informs us, were distinguished by the name of alumen; but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. The light-coloured alumen was useful in brilliant dyes, the dark-coloured only in dyeing black or very dark colours. One species 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 characterise 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 species of alumen are described by Pliny, but we are unable to make out to what 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 crystallised state, and distinguished it by the names of misy, sory, chalcanthum (Pliny, 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 would suppose, exclusively to green vitriol, applied indifferently to both.

When our alum was discovered is entirely unknown. Beckmann devoted a good deal of attention to the history of this salt, and published a curious dissertation on the 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 rock alum, commonly employed in Europe; though others allege that the term originated at Civita Vecchia, where alum is made from a yellow mineral which occurs 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.

England possessed no alum works till the reign of Charles I. Thomas Chaloner, 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. Since that time various alum works have been established in different parts of Great Britain,— the most important now in operation being the Whitby works, originally established by Mr Chaloner; and the works at Pendleton, near Manchester, and Goole, Yorkshire, and at Hurlet and Campsie, both in the neighbourhood of Glasgow.

Several alum works exist in Sweden, particularly in West Gothland. There is one, for example, at Haensaeter, near the borders of the Wener Lake. But for a description of the Swedish works we refer to Bergmann’s Op uscula, i. 284, or English translation, i. 342.

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 or Alunite was first observed at Tolfa, near Rome, in the 15th century, and afterwards in Hungary and several other places, chiefly in trachyte or other volcanic rocks. It appears to be produced by the action of sulphureous vapours on the felspars they contain, and generally occurs in compact, granular, or earthy masses, mixed with quartz or felspar. Small crystals are found in cavities, and are either rhombohedrons with angles of 89° 10', and thus nearly cubes, or these with the polar angles replaced by the basal plane. The specific gravity ranges from 2∙58 to 2∙752, the compact varieties being the lighter. Its hardness is 3 ∙ 5 to 4, or rather softer than fluor spar. It has a distinct cleavage perpendicular to the axis of the rhombohedron, and conchoidal fracture in other directions. The pure varieties are white and colourless, but it is often coloured greyish, yellowish, or reddish. The crystals decrepitate before the blowpipe, but are infusible, as well as the compact alunite. The alum is extracted from this mineral by repeated roasting and treating with water. The absence of iron accounts for the superior purity for which the Roman alum was long celebrated.

Alum-slate is a far more abundant substance, occurring in beds in different formations. Thus it is common in the older Palaeozoic or Silurian strata of Scandinavia and Scotland. Generally it is distinctly slaty, but sometimes forms rounded balls or concretions. It contains much carbonaceous matter, and hence its colour is greyish or bluish-black. It has a dull lustre, is soft and sectile. It contains much disseminated iron pyrites, and on decomposition in the air yields sulphate of iron, and alum as an efflorescence on the surface.

Many of the shales or slate clays in the coal formation also contain much iron pyrites, and thus also produce alum when acted on by the atmosphere. Such are those used for manufacturing alum at Campsie and other places near Glasgow. Where they contain much bituminous matter they show a shining resinous streak and greyish-black colour, and are named bituminous shales. These burn when heated, with a pale flame and sulphureous odour.

The alum slates at Whitby in Yorkshire belong to the Lias, and are used in the alum works in that neighbourhood. In other places, as in many parts of Germany, similar beds are found in Tertiary formations, particularly in connection with the brown coal deposits. When fresh dug they often show no trace of alum, which only appears after exposure to the air, or when the decomposition of the iron pyrites is assisted by the action of heat.

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, or Alunogene, was first found at Rio Saldanha, but is now obtained from several places in Europe and America. 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∙6 to 1∙7; soft; before the blowpipe behaves like alum.

2. Soda-alum. It occurs native in the province of St Juan, situated to the north of Mendoza, on the east side of the Chilian Andes, at about 30° S. lat. The alum is white, and composed of fibres adhering longitudinally, and having a certain breadth, but very thin. It bears some resemblance to fibrous gypsum, but it 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.

3. 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 clay resting on chalk at Newhaven in Sussex. This, 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. It consists of alumina, sulphuric acid, and water.

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.

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 crystallises, and attaches itself to the sides and bottom of the vessel. The mother liquid is then 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.

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. 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 oxidised into sulphate...

Aluminium is a white metal resembling silver in appearance. It is very malleable and ductile, and may be beaten and rolled into thin sheets, or drawn into fine wire. By hammering in the cold it becomes as hard as soft iron, but may be softened again by fusion. Being highly sonorous, it has been used for making bells. It is very light, being only 2½ times heavier than water, and is thus four times lighter than silver. After fusion it has a specific gravity of 2 ∙ 56; by hammering this may be increased to 2 ∙ 67. It melts at a red heat, and is non-volatile at very high temperatures. The metal conducts heat and electricity as well as silver. Aluminium does not oxidise in air, even at a red heat, has no action on water at ordinary temperatures, and is not acted upon by sulphuretted hydrogen or sulphide of ammonium, and thus preserves its lustre where silver would be tarnished and blackened. It is not attacked by nitric acid, even when concentrated, and is not soluble in dilute sulphuric acid, but is readily soluble in dilute or concentrated hydrochloric acid with evolution of hydrogen. Solutions of caustic potash or soda dissolve the metal with great ease, forming aluminate of potash or soda, and giving off hydrogen. Aluminium forms alloys with most metals. The copper alloy called aluminium-bronze is the most important because of its colour, hardness, and malleability, and is largely used for articles of jewellery, for mounting sextants and other astronomical instruments, and for making balance beams.