were decorated with rilievi; the side walls of the cella were unornamented, and undoubtedly bold relief would have been less adapted for them. The Temple of Theseus was built about thirty years before the Parthenon; and it is not impossible that the satisfactory effect of the flat rilievi on the cella of the latter might have suggested a similar treatment, or some modification of it, in the Temple of Theseus, had it been erected later. It may be observed in general, that alto-rilievo can seldom be fit for interiors, not only from its liability to accident, but from the difficulty of displaying it by the full light which it requires. A superficial light, especially if in a lateral direction, necessarily throws the shadows of one figure on another. Instances of this occur in some of the palaces in Rome where works of sculpture have been injudiciously placed. A room, for example, lighted in the the windows) adorned with a frieze in considerable relief; the figures nearest the light consequently project their shadows so as to half conceal the next in order. occupying the external frieze was more or less crossed by the shadow of the cornice. This precaution necessarily limits the attitudes, for many actions equally natural with those adopted would have projected shadows on the figure itself, thus tending to confuse the forms. A statue which can be seen from various points, and sometimes in various lights, might thus be unfit as to its composition for that intelligible display in one view and under a constant light which rilievo requires. On the principle that high relief is fittest for the open light, the rilievi of the temple of Phigaleia, which are also preserved in the British Museum, are bold in their projections. These works adorned the interior of the cella, but as the temple was hypäthral, or lighted from the open sky, the principles of external decoration were applicable. Had the temple been imperfectly lighted, a flatter kind of relief would have been preferable, and this leads us to consider the style of basso-ordinary way, will have its walls (at right angles with that occupied by rilievo, properly so called, the most perfect existing specimen of which is also in the British Museum. It adorned the external wall of the cella of the Parthenon, within the peristyle or colonnade, and was consequently always in shade: the strongest light it could ever receive would probably be the reflection from the pavement below when the sun was highest; but as reflected lights are uncertain, and may proceed from various points, the sculptures in question were calculated to be equally distinct in whatever direction the light was thrown. Their great elevation, and the peculiar angle at which they were seen, owing to the narrowness of the space between the exterior columns and the cella, may also be mentioned in considering the reasons which rendered projection unadvisable. That this confined view was not how ever the sole reason, may appear from the bold relief of the Phigaleian marbles, which, in the interior of the narrow cella of the temple they adorned, must have been seen on the side walls at a very inconsiderable distance compared with their height. The Phigaleian temple was built, according to Pausanias, by Ictinus, the chief architect of the Parthenon; and although the sculptures are inferior as works of art to the generality of Greek specimens, their style of relief is precisely the point where the architect may be supposed to have influenced their execution. As projection commands shade, so flatness commands light, and the flattest relief is hence fittest for an invariably dark situation. The same principle is observable in architecture in the treatment of mouldings in interiors, the form and projection of which differ materially from the corresponding members in the open light, and which are intended to be seen at a distance. The flatness which insures light would, however, be altogether indistinct and formless unless the outlines were clear and conspicuous at the first glance. The contrivance by which this is effected is by abruptly sinking the edges of the forms to the plane on which they are raised, instead of gradually rounding and losing them. The mass of the relieved figure being sometimes very little raised in its general surface, its section would thus almost present a rectangular projection. In many instances the side of this projection is even less than rectangular; it is undercut, like some mouldings in architecture which require to be particularly distinct, and thus presents a deeper line of shade. But if the figure can thus command distinctness of outline, notwithstanding the inconsiderable light it may receive, it is obvious that its lowness or flatness of relief will in such a light greatly aid its distinctness: above all, this contrivance gives the work thus seen in an obscure situation the effect of rotundity. Indeed, it is a great mistake to suppose that the flat style of relief was intended to appear flat, and it is a great mistake to apply it in situations, as in the open air, where it must appear so, and be indistinct besides. The conventions of the arts are remedies, adopted in certain situations and under particular circumstances, and are supposed to be concealed in their results: their ultimate resemblance to nature, and their successful effect in those circumstances, are the test of their propriety and necessity. The absence of all convention in alto-rilievo (as opposed to the flat style), thus fits it for near situations, if not too near to expose it to accidents. The excellent sculptures which decorate the pronaos and posticum of the Temple of Theseus, Lateral portico of the End portico of the Temple Parthenon. of Theseus. although under the portico, are in bold relief. They were not only nearer the eye, and seen at a more convenient angle than the flat rilievi of the cella of the Parthenon, but the reflected light which displayed them would necessarily be much stronger. It is also to be remembered that only the end porticoes, where the sculpture could be more conveniently seen and was better lighted, The conditions of proximity and distance, as well as the quantity and direction of light, were carefully attended to by the Greek sculptors, and suggested new varieties of relief. The end of the art, as far as relates to execution, is accomplished when the work is distinct and intelligible at the distance whence it is intended to be viewed. Hence the conventions which are intended to correct the defects of distance, of material, want of light, &c., are evidently unnecessary where the work admits of close inspection. The style of mezzo-rilievo, which in its boldest examples presents about half the thickness of the figure, is, on many accounts, least fit for a distant effect: the figure is nowhere detached from its ground; at a very little distance its shadowed side is lost in its cast shade, and its light side in the light of its ground; the outline, in short, soon becomes indistinct, but the semi-roundness of the forms is directly imitative, and thus again the absence of all conventional treatment fits the work for near situations. The style was preferred to alto-rilievo in such cases, as the latter would have been more liable to accidents, and would besides in some measure deform the outline or profile of any object which is circular in its plan. The figures which adorn sculptured vases are thus in mezzo-rilievo: these works probably ornamented interiors where any indistinctness in their distant effect or in an unfavourable light might be obviated by closer inspection. Two specimens may be seen in the second room of the Gallery of Antiquities in the British Museum. The celebrated Medicean and Borghesan vases, the finest known examples, are in like manner ornamented with mezzo-rilievo. The same consideration applies to all works, however unfit for a distant effect, which can, or in their original situation could, only be seen near. Even the mixed style of relief in the sculptures which occupy the internal sides of the Arch of Titus at Rome, would hardly be objected to, since the objects represented are distinctly seen, and can only be seen, at the distance of a few feet. The style of semi-relief (much purer than that of the Arch of Titus) adopted by Flaxman in front of Covent Garden Theatre, may be de fended on the same principle, since the utmost width of the street is hardly a more distant point than a spectator would naturally retire to in order to see them conveniently. The still flatter style which has been introduced on the exterior of several buildings in London cannot, however, be defended on any grounds, and there can be no doubt, from the reasons adduced, that bold relief is generally fittest for the open light. The mezzi-rilievi on the miniature choragic monument of Lysicrates (casts from them are in the British Museum) may be admitted to have been fitly calculated for their situation because they must have been seen near; but there was in this case an additional consideration to be attended to; the building is circular, and alto-rilievo was avoided in order to preserve the architectural profile: on the other hand, the frieze of the small Temple of Victory, which was rectangular, was adorned with alti-rilievi; and in this case it appears that they did not even extend to the angles. The objections to sculpture on monumental columns will be obvious from these considerations; it has been observed, that in attempting to preserve the architectural profile, as in the Trajan column, and its modern rival in the Place Vendôme at Paris, the sculpture thus slightly relieved soon becomes indistinct, nor indeed would this indistinctness be obviated at a considerable height even by alto-rilievo, the figures being necessarily small, while the evil is only increased by substituting the dark material of bronze for marble. We proceed to consider the varieties of style in this art as affecting composition. In rilievo, and in sculpture generally (a colourless material, or a material of only one colour being always supposed), it is evident that shadow is the essential and only source of meaning and effect. In works placed in the open air, and visible in one point only, as in the case of alto-rilievo, a certain open display of the figure is generally adopted; the shadows, or rather the forms which project them, are so disposed as to present at the first glance an intelligible and easily recognised appearance, and the impossibility of changing the point of view, or changing the light, as before observed, limits the attitudes more than in a statue, and, as will also appear, more than in a basso-rilievo. For in the latter, however distinct the outline is in which the chief impression and meaning of the figure reside, the shadows within the extreme outlines are in a great measure suppressed; it is, in fact, by their being so suppressed that the general form becomes so distinct. This is also the case when one form is relieved on another; it will be seen that the nearest object is very much reduced and flattened in order that its shadow may not interfere with the more important shadows of the outlines on the ground, and hence it may often happen that the nearest projection is least relieved. It will thus be evident that, owing to this power of suppressing the accidental shades and preventing them from rivalling or being confounded with the essential ones, the choice of attitudes becomes less limited, and many a composition which in full relief would present a mass of confusion from its scattered and equally dark shades, may be quite admissible and agreeable in basso-rilievo. Accordingly the attitudes of statues, which are generally unfit for alto-rilievo, frequently occur in the flat style. Visconti even supposes that certain figures in the bassirilievi of the Parthenon suggested the attitudes of celebrated statues afterwards executed; as, for instance, the Jason, or Cincinnatus, and the Ludovisi Mars. As a remarkable proof how much the attitudes were limited in alto-rilievo compared with the flat style, it may be observed, that the contrasted action of the upper and lower limbs, which gives so much energy and motion to the figure, is perhaps never to be met with in the fine examples of alto-rilievo, whereas in the flat style it is adopted whenever the subject demands it. In the annexed sketch of an early Greek basso-rilievo, representing Castor managing a horse (from the third room of the gallery of the British Museum), the action of the upper and lower limbs is contrasted, as is the case in all statues which are remarkable for energy and elasticity of movement: the statue called the Fighting Gladiator may be quoted as a prominent example. This disposition of the lower limbs, or the alternate action in which one of the arms would cross the body, never occurs in altorilievo, because the shadow of the arm on the body or of one of the lower limbs on the other could then no longer be suppressed, as it is in this case, but would rival the shadows of the whole figure on the ground. Among the metopes of the Parthenon, the Phigaleian marbles, and the alti-rilievi of the Temple of Theseus, there is not a single instance of the contrasted action alluded to; while in the two latter examples, the contrary position, or open display of the figure, repeatedly It must however be admitted, that this recurs, even to sameness. open display of the figure, although not presenting the most energetic action, is as beautiful as it is intelligible, and hence the finest exhibitions of form were quite compatible with the limited attitudes to which the sculptors thus wisely confined themselves. The objections which compelled this limitation being however entirely obviated in basso-rilievo, by the power of suppressing at pleasure the shadows within the contour, we find the fullest advantage taken of the latitude which was thus legitimately gained. A better example cannot be referred to than the flat rilievi already mentioned from the cella of the Parthenon. (See the next Illustration.) The subject represents the Panathenaic procession, and although no perspective diminution is admitted, several equestrian figures are sometimes partly relieved one upon the other. The confusion which results from the number of similar forms in the repetition of the horses' limbs, as well as in the actions of the horsemen, must be admitted; but perhaps the subject is thus better expressed than by a simpler arrangement, and this treatment contrasts finely with the single figures. In a procession of horsemen moving two or three abreast, we are at once aware that the figures are similar, and the eye is satisfied, as it would be in nature, not in searching out each individual figure as if it had a separate principle of action, but in comprehending the movement and the mass, for one indicates the whole. Where the figures thus cross each other they are treated as a mass; the outline of the whole group is distinct and bold, being more or less abruptly sunk to the ground, but the outlines which come within the extreme outline are very slightly relieved. In short, the principle here applied is precisely the same as that observable in a single figure in the same style of relief: the outline of the whole form is distinct, or rather most distinct where it is most important, and the internal markings are seldom suffered to rival it, but are made subservient to this general effect. The relative importance of the objects is, indeed, the only consideration which is suffered to interfere with this principle: thus loose drapery is sometimes slightly relieved on the ground, while a significant form is now and then strongly relieved even on another figure. In comparing the slight varieties of treatment in these rilievi, it is to be remembered that the end porticoes were a little wider than the lateral colonnades. It is undoubtedly to this circumstance that the difference of treatment alluded to is to be referred; the figures in the end friezes are more separated from one another, and consequently somewhat more relieved than the compact processions on the side walls. The fact that these bassi-rilievi, as well as most of the sculpture of the ancients, were partially painted, has been purposely left out of the account, because the very contrivances resorted to are calculated to supply the absence of colour. The custom in the best age of Grecian art of painting architecture and sculpture may be defended or excused elsewhere; it may be however here remarked, that while the ancient sculptors added colour after having employed every expedient which could supply its want, the moderns, in altogether rejecting it, often fail to make use of those very conventions which its absence demands. It appears that the principle of suppressing the relief within the extreme contour which, with the strong marking of the outline itself, mainly constitutes the style of basso-rilievo, was employed by the ancients in works of considerable relief, in interiors, in particular lights, and probably at some distance or elevation. The real projection which works thus strictly belonging to the class of bassi-rilievi may sometimes present, points out the essential difference between basso and mezzo rilievo: a work, even if in very slight general relief, which has the parts that are nearest the most relieved, belongs to mezzo-rilievo; while a work which has the nearest parts least relieved, constitutes bassorilievo, whatever its general projection may be. In the former, the outline is thus less apparent than the forms within it; in the latter, the outline is more apparent than the forms within it. The early Greek and Etruscan rilievi, which, however flat, have the nearest parts the fullest, while the outline is scarcely, if at all, rectangular in its section, have thus the principle of mezzo-rilievo. They are even fitted for near inspection, and cannot be said to present any unsatisfactory convention; for the bulk, however really thin, is proportionate in its relief, and is so far directly imitative; inasmuch as the eye consents to a diminished scale of bulk as easily as to a diminished scale of height, while the indistinctness of the outline has the effect of rounding the form. Such works are besides fitted for near examination, because they can scarcely command any shadow. Various specimens may be seen in the British Museum. The antique vases of Arezzo were ornamented with figures in this kind of relief. Certain silver vases mentioned by Pliny were of the same description. The Egyptian intaglio, for so it may be called rather than rilievo, belongs to the same style. The Egyptian artists, instead of cutting away the background from the figure, sunk the outline, and slightly rounded the figure, on the principle of mezzo-rilievo within. Thus no part of the work projected beyond the general sur face, and the architectural profile was preserved. There are, however, many very ancient examples at Thebes of figures slightly relieved from the ground, somewhat on the principle of basso-rilievo as practised by the Greeks.—that is, with the nearest parts least relieved, and with out lines rectangular in the section. Many of them, probably, in their original situations, and when the buildings were entire, ornamented interiors. Some Persian rilievi, in the British Museum, approach the same style. The Egyptian rilievi were painted in brilliant colours, and would have been ineffective in the open light without such an addition. The distinctions of the three styles of relief, according to the Greek examples, may now be thus recapitulated. In the highest relief, however decided the shadows may and must of necessity be, on the plane to which the figure is attached, the light on the figure itself is kept as unbroken as possible, and this can only be effected by a selection of open attitudes; that is, such an arrangement of the limbs as shall not cast shadows on the figure itself. In basso-rilievo, the same general effect of the figure is given, but by very different means: the attitude is not selected to avoid shadows on the figure, because, while the extreme outline is strongly marked, the shadows within it may be in a great measure suppressed, so that the choice of attitudes is greater. Mezzo-rilievo differs from both: it has neither the limited attitudes of the first, nor the distinct outline and suppressed internal markings of the second on the contrary, the outline is often less distinct than the forms within it, and hence it requires, and is fitted for, near inspection. Its imitation may thus be more absolute, and its execution more finished, than those of either of the other styles. Most of the coins of antiquity are executed on the principle of mezzo-rilievo; and though often far bolder in this relief than modern works of the kind, are treated in a mode corresponding with their minute dimensions, which require close examination. The outline thus gradually rounds into the ground, and is never abruptly sunk, while the nearest parts are most relieved. Thus, conventional methods are always wanting in works that admit of close inspection, where the eye can be satisfied without such expedients. The comparatively strong relief of the heads on the ancient medals is again a contrivance for their preservation, and presents a new variety in the style of rilievo. Coins are exposed to friction, and the forms they bear are thus liable to be soon effaced. The earliest means adopted to prevent this was by sinking the representation in a concavity, in which it was thus protected. This plan was soon abandoned, for obvious reasons; and the method ultimately adopted was that of raising the least important parts most. Accordingly, the parts that are rubbed away in many fine antique coins are precisely those which can best be spared; the hair has generally a considerable projection, so that the face and profile are often perfectly preserved after 2000 years: a better specimen cannot be adduced than the celebrated Syracusan coin representing the head of Arethusa or Proserpine. In addition to the propriety of its style, this head is remarkable for its beauty, and is classed by Winkelmann among the examples of the highest character of form. The ordinary style of mezzo-rilievo was also used for gems, and indeed for all works in this branch of sculpture which required close inspection, and needed no conventional contrivance. A flat style of relief, which is sometimes observable in cameos, was adopted only for the sake of displaying a subject on a different coloured ground; the layers of colour in the stone employed, generally the sardonyx, being very thin. The difference of colour in the ground has, however, the effect of giving roundness to the figures relieved on it, as if, their whole effect becoming apparent, the internal markings disappeared. The figures on the Portland Vase are treated on this principle; and as it was intended to imitate a precious stone (for which indeed it was at first taken), the thinness of the outer layer of colour is also imitated. Such works, however, reduced to one colour in a cast or copy, are totally wanting in effect and style. The impressions from intagli, or engraved gems, which were used for seals, are never in the flat style of relief, but however slightly raised, are on the principle of mezzo-rilievo as above defined. The gems of Dioscorides, the finest of antiquity, are in mezzo-rilievo, and often of the fullest kind; as for instance, the heads of Demosthenes and Io, and the figures of Mercury and Perseus. The same may be observed of other celebrated gems, such as the Medusa of Solon, the Hercules of Cneius, &c. It is supposed that the same artists who engraved on gems, and who frequently ARTS AND SCI. DIV. VOL. I. | inscribed their names, also executed the dies for coins. The latter are among the finest antique works of art; but of the many thousand existing specimens there is but one which bears the name of the artist, namely, the coin of Cydonia in Crete, the inscription on which proves it to be the work of Nevantus. It was observed, that in the antique coins, the least important parts are the most raised, and the reasons which dictated this practice limited the view of the head to the profile; but as the same reasons were no longer applicable in engraved gems, the impressions from which could be renewed at pleasure, the front, or nearly front view of the head was occasionally attempted, and seems to have been preferred by Dioscorides and his school. The head of Io before mentioned, considered with reference to this specific propriety of its style, as well as with regard to its general merits, is placed by Visconti in the first class of antique engraved gems. Thus the most skilful artists of antiquity seemed to consider the style of any one of the arts to consist chiefly in those points which were unattainable by its rivals. It may be here observed too, that they generally limited their representation to the most worthy object, namely, the human figure, when the dimensions on which they were employed were necessarily confined. Indeed, the principles of imitation itself were, as it were, condensed, and true character often exaggerated as the materials appeared less promising; so that the genius of ancient art is as conspicuous in minute engraved gems as in colossal sculpture. Mezzo-rilievo of the fullest kind was also fitly employed (as well as alto-rilievo, when in situations not exposed to accidents) to ornament tombs and sarcophagi. These works, placed in the open air, decorated 8 sulphate of potash or sulphate of ammonia. These two descriptions of Such natural alum constitutes, however, but a very small proportion 1. A thin layer of brushwood is laid upon the ground, and on this alum schist is piled in a pyramidal form to the height of 90 or 100 feet. The brushwood is then ignited, and the combustion gradually communicates itself to the coaly matter of the schist, which is present in sufficient quantity to afford heat enough for the calcination of the whole mass. Great care is taken to cause the combustion to proceed very slowly, otherwise the mass would become partially fused, and the objects of the operation be defeated. From one to two years are therefore required for the burning of one of these heaps. During the calcination the bisulphide of iron (iron pyrites) is decomposed, and converted at the expense of atmospheric oxygen into protosulphate of iron and free sulphuric acid, the latter uniting immediately with the alumina of the shale to form sulphate of alumina. These chemical changes may be thus expressed : the approaches to cities, as the sepulchres were always without the walls. The Appian Way was the most magnificent of these streets of tombs in the neighbourhood of Roine, and must have exhibited, literally, thousands of sepulchral monuments. Though generally the work of Greek artists, and often interesting from being copies of better works now lost, the haste and inattention with which such prodigious numbers were executed, tended to degrade the style of their sculpture. In these rilievi, even in the better specimens, buildings and other objects are occasionally introduced behind the figures, thus approaching the spurious style of relief in which the effects of perspective are attempted to be expressed: a great variety, of various degrees of excellence, are to be seen in the British Museum. The greater part of what are called Roman bassi-rilievi are of this kind, and may be considered a middle style between the pure Greek rilievo and the modern Italian. It was from antique sarcophagi, fine in execution, but with these defects in style, that Niccola da Pisa, in the 13th century, first caught the spirit of ancient art. Many of the works from which he is believed to have studied are still preserved in Pisa. D'Agincourt gives a representation of one of the best. In imitating the simplicity of arrangement, and, in a remote degree, the purity of forms which these works exhibited, the artist was not likely to correct the defects alluded to, which had been already practised in Italy and elsewhere. Various degrees of relief, background figures and objects, and occasional attempts at perspective, are to be found in the works of the Pisani and their scholars, yet their works, which are to be regarded as the infancy of Italian art, and which undoubtedly are rude enough in workmanship and imitation, are purer in style than those of the succeeding Florentine masters, who attained so much general perfection in sculpture. The rilievi of Donatello are mostly in the style called by the Italians stiacciato, the flattest kind of mezzo-rilievo, according to the definition before given, which he probably adopted, as he worked in bronze, from the facility of casting; yet in such a style, commanding little distinctness from its inconsiderable projection, he introduced buildings, landscape, and the usual accessories of a picture. But this misapplication of ingenuity was carried still farther by Lorenzo Ghiberti, in the celebrated bronze doors of the baptistery, or church of San Giovanni, at Florence, which exhibited such skilful compositions, in which the stories are so well told, and in which the single figures are so full of appropriate action. In these works the figures gradually emerge from the stiacciato style to alto-rilievo. They are among the best specimens of that mixed style, or union of basso-rilievo with the principles of painting, which the sculptors of the fifteenth century and their imitators imagined to be an improvement on the well-considered simplicity of the ancients. In these and similar specimens, the unreal forms of perspective build-insoluble basic persulphate. A certain portion of the iron, however, ings, and diminished or foreshortened figures, which in pictures create illusion, when aided by appropriate light and shade and variety of hue, are unintelligible or distorted in a real material, where it is immediately evident that the objects are all on the same solid plane. Even Vasari, who wrote when this mixed style of rilievo was generally prac tised, remarks the absurdity of representing the plane on which the figures stand ascending towards the horizon, according to the laws of perspective; in consequence of which, "we often see," he says, "the point of the foot of a figure, standing with its back to the spectator, touching the middle of the leg." owing to the rapid ascent or foreshortening of the ground. Such errors, he adds, are to be seen "even in the doors of San Giovanni." Lorenzo Ghiberti, like other Florentine sculptors, first learnt the practice of his art from a goldsmith, and the designs of the artists who competed with him for the honour of executing the doors of San Giovanni were submitted to the judgment of goldsmiths and painters as well as sculptors. The taste of the Florentines in basso-rilievo was thus greatly influenced by the prevalence of a style most applicable to the precious metals, in which a general sparkling effect is best insured by avoiding uniformly violent relief, which projects considerable shadows, and especially by avoiding unbroken flatness. The background is thus filled with slightly relieved distant objects, so as to produce everywhere a more or less roughened or undulating surface. The same end seems to have been attained in the antique silver vases, by the introduction of foliage. The style continued to be practised with occasionally greater absurdities than those before alluded to, and perhaps less redeeming excellence, till the close of the last century. The sculptor Falconet says of the antique bassi-rilievi, that "however noble their composition may be, it does not in any way tend to the illusion of a picture, and a basso-rilievo ought always to aim at this illusion." He leaves no doubt as to the literal meaning he intends by citing the Italian writers who applied the term quadro indiscriminately to picture and basso-rilievo. Sculpture in this country was indebted principally to Flaxman for the revival of a purer taste in the application of bassorilievo to architecture. In works of decoration, intended to be executed in the precious metals, in which, as before observed, moderately embossed and general richness of surface is so desirable, in order to display the material as well as the work, he, however, united his own purity of taste and composition with an approach to the mixed style of relief practised by the Florentine masters, who, in this branch of sculpture, perhaps never equalled his shield of Achilles. ALUM. (KÔ, SO, + A103, 380, +24 aq) or (NH,O, SO, + Al,O,, 3SU2+24 aq.) Alum as met with in commerce is a double salt, one of the constituents of which is sulphate of alumina, and the other either 3FeSO1 + Al,Og Al2O3, 3SO3 + 3FeOSO.. Iron pyrites. Protosulphate of iron. Alumina. Sulphate of 2. When the whole mass has been exposed to the necessary temperature, and has again become nearly cold, it is sprinkled with water from time to time, so as to moisten it thoroughly without interrupting the circulation of air through its interstices; the object being to oxidise the protosulphate of iron and to convert it as far as possible into still remains as neutral persulphate, but this probably suffers double decomposition with the silicate of alumina in the shale, producing soluble sulphate of alumina and insoluble silicate of iron. The mass is now lixivated with water in stone cisterns, when sulphate of alumina nearly free from persulphate of iron dissolves out. 3. The aluminous liquor thus obtained is now concentrated by evaporation until it attains a density of about 14. This operation is performed in brickwork cisterns lined with lead, thirty-six feet long, six feet wide, and two or three feet deep, heated by the reverberatory flame from a furnace placed at one end and fed with highly bituminous coal. Much soot is deposited in the pans, but it subsides, leaving the liquor above it clear. The latter is then run off into tanks and allowed to cool. 4. The cool concentrated liquor is now ready for conversion into alum, which is effected by adding to it a sufficient quantity of a saturated solution of sulphate of potash or sulphate of ammonia; in the former case potash alum is produced, whilst in the latter ammonia alum is the result. As sulphate of ammonia is now, in almost all localities, a more abundant and consequently cheaper salt than sulphate of potash, it has almost entirely superseded the last-named salt in the alum manu. facture, nearly all the alum now met with in commerce being ammonia alum. As alum is far less soluble in cold water than sulphate of alumina, it is precipitated in the form of a granular crystalline powder, termed "alum-meal," when either of the sulphates just mentioned is added to the concentrated solution of sulphate of alumina. Thus the alum at the moment of its production becomes separated from a number of other salts, such as the sulphates of iron, lime, magnesia, &c., which remain in solution, and the remaining operations have for their object the rendering of this separation more complete by removing the impure liquor from the interstices of the alum-meal, which is effected by washing the latter twice with small quantities of cold water and then recrystallising it from a boiling saturated solution. The latter operation is performed in vessels termed roaching casks, which consist of a sufficient number of staves, six feet long, lined with lead, and fitting tightly around a circular flagstone, four to five feet in diameter, which thus forms the bottom of a cask or vat; the staves are held together by strong iron hoops, which can be tightened or relaxed at pleasure by screws joining their extremities, thus permitting the casks to be put together or taken to pieces with great facility. Into these casks the boiling saturated solution of alum is run; and as it cools most rapidly where it is in contact with the staves, a thick crystalline encrustation of alum soon attaches to the sides of the cask, and in three or four days becomes strong enough to retain the interior liquor without the assistance of the staves, which are accordingly removed to facilitate the refrigeratory process. It takes from eight to fourteen days for the liquor to cool and deposit its crystals; the mother liquor remaining in the centre is then run off through a hole bored in the side of the crystalline mass, which is then broken up into irregular square blocks and sent into the market. By the use of a purer raw material than the alum shale above mentioned, the processes may be much simplified, so far at least as regards the production of the sulphate of alumina. Thus, in some manufactories, pipe-clay or china-clay containing little else than silicate of alumina, contaminated with only a small quantity of oxide of iron, is gently roasted in a reverberatory furnace, and then decomposed by hot sulphuric acid which combines with the alumina, expelling the silicic acid chiefly in the insoluble form. The mass thus obtained is now lixiviated with water, and there is thus obtained a concentrated solution of sulphate of alumina, which is subsequently converted either into potash or ammonia alum in the manner already described. A large quantity of alum is now made by Spence's process, which is peculiarly fitted for coal districts, inasmuch as the whole of the materials employed are derived from coal and the coal strata. Thus the iron pyrites so frequently met with amongst coal in lenticular masses or distinct strata, furnishes sulphuric acid [SULPHURIC ACID] which, when, heated in large shallow leaden pans with roasted coal shale (a material very similar in composition to alum shale), yields sulphate of alumina, containing an excess of sulphuric acid. A quantity of the ammoniacal liquor of gasworks [Gas] is now boiled in a close vessel, and the vapours of carbonate of ammonia and sulphide of ammonium arising therefrom are conducted into the hot acid solution of sulphate of alumina, where they are converted into sulphate of ammonia, which immediately forms ammonia alum with the sulphate of alumina present, whilst carbonic acid and sulphuretted hydrogen gases escape. On cooling, the solution deposits a copious crop of alum crystals, which are purified by washing and recrystallisation, in the manner already described. Properties.-Alum presents the appearance of colourless, tolerably transparent octohedral crystals, which possess a sweetish astringent taste, and are soluble in about 18 parts of cold water and less than their own weight of boiling water. These crystals contain 24 equivalents of water, which they lose on exposure to a heat of about 400°, and are converted into a light, porous, white, insoluble mass termed burnt alum. At a bright red heat ammonia alum is totally decomposed; ammonia and sulphuric acid are volatilised, whilst pure and anhydrous alumina is left behind. For the chemical relations of alum, see ALUMS. Uses.-Alum is largely employed in dyeing and calico-printing, and in the manufacture of paper; it is also used in the production of pig. ments called lakes, and is sometimes mixed with the flour used for bread. A minute quantity of alum thus employed renders the bread, especially if made from the inferior kinds of flour, lighter, whiter, and probably more digestible; it also enables the bread to retain a larger amount of water after baking. It is easy to perceive that these properties may, on the one hand, be made fraudulently subservient to the production of bread apparently of the best quality from inferior flour; and on the other, may be legitimately used as means whereby such flour, when it must be employed as food, may be converted into a more palateable and wholesome bread. The amount of alum (about To of the weight of the flour) required to produce this effect, is too minute to exercise any deleterious effect upon the consumer of such bread. In all these applications it is immaterial whether potash alum or ammonia alum be employed; weight for weight, however, ammonia alum is rather more valuable than potash alum, since it contains 11.90 per cent. of the active ingredient, alumina, whilst potash alum contains only 10-82 per cent. About 20,000 tons of alum are now annually manufactured in the United Kingdom. ALU'MINA (Al, O.). The oxide of aluminium; it constitutes a large proportion of all clays, which in a great measure owe to it their plastic property. The name of alumina is derived from alumen, the Latin for alum, the salt from which it is generally obtained in a pure state, by means which will presently be mentioned. Alumina is widely diffused in nature; the adamantine spar or corundum, the ruby, and sapphire, are alumina nearly pure and crystallised, and are among the hardest substances in nature. The diaspore is a crystallised mineral, which consists almosts entirely of alumina and water; and in North America another hydrate of alumina has been found, and called gibbsite. Emery, likewise remarkable for its hardness, and hence much used for grinding and polishing, is also alumina coloured with the oxides of iron and manganese. The following is the process recommended by Berzelius (Traité de Chimie,' ii. 369) for procuring pure alumina: Dissolve and crystallise alum repeatedly, to deprive it of the peroxide of iron which it usually contains; when thus rendered pure, a portion of the alum dissolved in water, and added to a solution of potash in excess, gives a precipitate at first, which is afterwards completely redissolved. To a boiling solution of the purified alum, add one of carbonate of potash, as long as precipitation takes place; then a slight excess of the carbonate being used, digest with a gentle heat to decompose the subsulphate of alumina formed. Wash this carefully on a filter, and redissolve it in hydrochloric acid; precipitate the clear solution with ammonia or car bonate of ammonia, and wash the precipitate, which, when dried with a gentle heat, is hydrate of alumina, and when heated to redness becomes pure alumina, by losing its water. If intended for the purpose of solution in acids, it is better to keep the alumina in the state of hydrate; for when once rendered anhydrous, acids act upon it slowly and with great difficulty. Pure anhydrous alumina may also be obtained by heating ammonia alum to redness. Hydrate of alumina, when recently precipitated, presents the ap pearance of a white, bulky, semi-transparent, gelatinous substance; on drying it contracts greatly and forms a white powder, which adheres to the tongue strongly. Hydrate of alumina readily combines with acids, except carbonic acid, forming salts which are generally soluble and uncrystallisable. They all possess an acid reaction. If the acid be volatile, a portion of it is generally expelled on boiling a solution of the salt, and a basic compound is precipitated. With acetate of alumina this takes place at ordinary temperatures; as, for instance, in the ageing of calico, which has been mordanted with acetate of alumina, acetic acid passes off during the ageing process, and a basic and insoluble acetate becomes fixed in the fibre, forming what is termed the mordant. Hydrate of alumina, as usually prepared, is insoluble in water; but Walter Crum has described a curious modification of it, which dissolves in large quantity in water containing a minute amount of acetic acid. Hydrate of alumina is readily soluble in solutions of the fixed alkalies, but insoluble in ammonia. If its solution in caustic potash or soda be exposed to the air, carbonic acid is gradually absorbed, and terhydrate of alumina (Al, O,, 3 HO) deposited in small but well-defined crystals. When strongly heated, hydrate of alumina becomes suddenly incandescent, contracts greatly, and loses its water of hydration. It is then nearly insoluble in acids, but may be fused before the oxyhydrogen blowpipe, yielding an exceedingly hard vitreous mass, resembling corundum. Alumina is an important substance, whether regarded as the ore of the metal aluminium, as a constituent of soils, or with respect to its extensive employment in the operations of the dyer, calico-printer, and colour-maker. It is also a necessary ingredient in all kinds of porcelain, earthenware, bricks, and tiles. ALUMINA, SULPHATE OF. (A1,0, 3SO2+18aq.) A salt formed by dissolving hydrate of alumina in dilute sulphuric acid, and then evaporating the solution. It crystallises in colourless flexible scales, containing eighteen atoms of water. They are soluble in twice their weight of cold water. Sulphate of alumina has an astringent and acid taste; it is permanent in the air, and is nearly insoluble in alcohol. It is used in dyeing and calico-printing, in the place of alum, for the production of mordants; and, as a general rule, it may be employed for all the purposes to which alum is applicable. For these applications sulphate of alumina is manufactured on a large scale by treating the white previously calcined clays of Devonshire and Dorsetshire with their own weight of dilute sulphuric acid, specific gravity 1.200 at the temperature of boiling water; and then, after separating the insoluble silica, precipitating the small amount of peroxide of iron, which is dissolved as sulphate, with ferrocyanide of potassium. The Prussian blue thus formed is allowed to subside, and the supernatant liquor is concentrated by evaporation, until it solidifies on cooling into a white mass, which is nearly pure sulphate of alumina, containing from 13 to 14 per cent. of alumina. The commercial salt thus manufactured is sometimes called patent alum. A less pure, but almost equally efficacious, sulphate of alumina is made by a still more simple process, and is sold under the name of aluminous cake [ALUMINOUS CAKE]. ALUMINIUM, the metal contained in alumina. Aluminium was first obtained in a state approaching to purity by Wöhler, in 1827, by decomposing chloride of aluminium by means of potassium. Its preparation has since been simplified and rendered practicable on the large scale by Bunsen and by Deville. The former chemist prepares it by the electrolytic decomposition of the double chloride of aluminium and sodium (AICI, + NaCl). The melted salt is maintained at a temperature of about 400°; and as the metal separates at the negative pole, which consists of a plate of gas-carbon, it sinks to the bottom of the crucible containing the melted salt, and is thus protected from oxidation. On subsequently raising the temperature, the particles of aluminium coalesce into a large globule. M. Deville, to whom we are indebted for a process capable of preparing the metal on a manufacturing scale, obtains aluminium by decomposing its chloride with sodium. The chloride of aluminium he prepares as follows:-anhydrous alumina is mixed with charcoal, made into a paste with oil or tar, and ignited in a covered crucible. When cold, the mass, which now consists of alumina and carbon, is broken into fragments and placed in an earthenware retort, with the short neck of which a glass receiver is connected. The retort and its contents being heated to dull redness, a current of dry chlorine gas is introduced through the tubulare, and solid chloride of aluminium immediately begins to condense in the receiver. The following is the reaction:= AlCl3 + SCO. Al2O3 + 3C + Cla |