ALUM, in chemistry and the arts, is denotninated thp sulphate of alumina, though it is not merely a combination of alumina with the sulphuric acid. It pos sesses the following characters: I: It has a sweetish astringent taste. 2. It rs aolu: ble in warm water, and the solution red. dens vegetable colours, which proves the acid to be in excess. S. When mixed with a solution of carbonate of potash, an effervescence is produced by the un combined acid, which prevents the first portions of alkali that are added to the solution of sulphate of alumina from occa sioning any precipitate. 4. When sulphate of alumina is heated, it swells up, loses its regular form, and becomes a dry spongy mass ; but, according to the experiments of Vauquelin, the whole of its acid cannot be thus expelled. 5. The combination of sulphuric acid with alumina is incapable of crystallizing without an addition of .potash, which makes a constituent part of all the alum of commerce. 6. It is de composed by charcoal, which combines with the oxygen of the acid, and leaves the sulphur attached to the alumina.
Dr. Thomson says there are four varie ties of alum, all of which are triple salts ; two neutral, and two he calls sUper-salts. These are this denominated : 1. Sulphate of alumina and potash.
2. Sulphate of alumina and ammonia.
3. Super-sulphate of alumina and potash.
4. Super-sulphate of alumina and am monia.
The discovery of alum was made in Asia, from whence it continued to be im ported till the end of the fifteenth centu ry, when a number of alum works were established in Italy. In the sixteenth century it was manufactured in Germany and Spain; and during Queen Elizabeth's reign an alum manufactory was establish ed in England. The alum of commerce is usually obtained from native mixtures of pyrites and clay, or sulphuriC acid and clay. Bergman has published a very com plete dissertation on the process usually followed. The earth from which it is pro cured is usually called *luminous shistus, because it is slaty. lts colour is blackish, because it contains some bitumen. In most cases it is necessary to burn it be fore it can be employed: this is done by means of a slow smothered fire. Some times long exposure to the weather is sufficient to produce an efflorescence of alum on the surface. It is then lixiviated, and the water concentrated by evapora tion, and mixed with putrid urine, or mu riate of potash; crystals of alum and of sulphate of iron usually form together. The composition of alum has been but lately understood with accuracy. It has been long known, indeed, that one of its ingredients is sulphuric acid ; and the ex periments of Pott and Margraff proved in contestibly that alumina is another ingre dient. But sulphuric acid and alumina are incapable of forming alum. Manu facturers knew that the addition of a quan tity of.potash or of ammonia, or of some substance containing these alkalies, is al most always necessary, and it was proved, that in every case in which such additions are unnecessary, the earth from which the alum is obtained contains already' a quan tity of potash. Various conjectures were made about the part which potash acts in this case ; but Vauquelin and Chaptal ap pear to have been the first chemists that ascertained, by decisive experiments, that alum is a triple salt, composed of sulphu ric acid, alumina, and potash or ammonia united together. Alum crystallizes in re gular octahedrons, consisting of two four sided pyramids applied base to base. The sides are equilateral triangles. The form of its integrant particles, according to natty, is the regular tetrahedron. Its taste is, as we have observed, astringent. It always reddens vegetable blues. Its specific gravity is 1.7109. At the tempe rature of 60° it is soluble in from 15 to 20 parts of water, and in 4ths of its weight of boiling water. When exposed to the air it effloresces slightly. When exposed to a gentle heat it undergoes the watery fusion A strong heat causes it to swell and foam, and to lose about 44 per cent. of its weight, consisting chiefly of water of crystallization. What remains is call ed calcined or burnt alum, and is some. times used as a corrosive. By a violent heat, the greater part of the acid may be driven oft: Though the properties of alum are in all cases pretty nearly the same, it has been demonstrated by Van quelin that three varieties of it occur in commerce. The first is, super-sulphate
of alumina and potash; the second, super sulphate of alumina and ammonia ; the third, is a mixture or combination of these two, and contains both potash and ammo nia. It is the most common of all ; doubt less, because the alum-makers use both urine and the muriate of potash to crys tallize their alum. Vauquelia has lately analysed a number of specimens of alum manufactured in different countries. The result was, that they all contain very near ly the same proportion of ingredients The mean of all his trials was as follows • Acid -30.52 Alumina 10.50 Potash 10.40 Water . 48.58 100.00 When an unusual quantity of potash is added to ;limn liquor, the salt lases its usuld form, and crystallizes in cubes. This constitutes a fourth variety of alum, usually distinguished by the name of cu bic alum. It coutains an excess of alkali. When he potash is still further increased, Uhaptal has observed, the salt loses the property of crystallizing altogether, and falls down in flakes. This constitutes a fifth variety of alum, consisting of sul phate of potash combined with a small proportion of alumina.. If three parts of alum and one of flour or sugar be melted torther in an iron ladle, and the mixture dried till it becomes blackish and ceases to swell ; if it be then pounded small, put into a glass phial, and placed in a sand bath till a blue flame issues from the month of the phial, and after burningfor a rninute or two be allowed to cool, R sub stance is obtained, known by the name of Ilomberg's pyrophorus, which has the property of catching fire whenever it is exposed to tlie open air, especially if the air be moist. -This substance was acci dentally discovered by Homberg about the beginning of the eighteenth century, while he was engaged in his experiments on the human faces. lie had distilled a mixture of human faces and alum till he could obtain nothing more from it by means of heat ; and four or five days after, while lie was taking the residuum out of the retort, he was surprised to see it take fire spontaneously. Soon after, Lemery the younger discovered that honey, sugar, flour, or almost any animal or vegetable matter, could be substituted for hturnan faces; and afterwards Mr. Lejoy de Su viry shewed that several other salts con taining sulphuric acid may be substituted for alum. Scheele proved that alum de prived of potash is incapable of forming pyrophortis, and that sulphate of potash may be substituted for alum. And Mr. Proust has shewn that a number of neu tral salts, composed of vegetable acids and earths, when distilled by a strong fire in a retort, leave a residuum which takes fire spontaneously on exposure to the air. These facts have thrown a great deal of light on the nature of lIomberg's pro. phorns, and enabled us in some measure to account for its spontaneous inflamma tion. It has been ascertained, that part (-lithe sulphuric acid is decomposed (lu ring the formation ofthe pyrophorus, and of course a part of the alkaline base be comes tincombined with acid; and the charcoal, which gives it its black colour, is evidently divided int() very minute par ticles. It has been ascertained, that du ring the coinbustion of the pyrophorus a quantity of oxygen is absorbed. The in flammation is probably occasioned by the charcoal; the sulphuret of potash also acts an essential part. Perhaps it pro duces a sudden increase of temperature, by the absorption and solidification of wa ter from the atmosphere.
A new process for making alum is used at some works, for which We are indebt. ed to Mr Sadler, which is as follows : The boilers are filled with prepared liquor of 10 pennyweights, to which sulphate of potash is added, and boiled together, un til it weighs 16 pennyweights, hy which t ime the r,.% hole of the superfluous alumina and the oxyde of iron is precipitated. The fluid is then run into a settler,where it remains until clear, after which it is plimped into a second boiler, and evapo rated up to 26 pennyweights, let into the coolers, and left to cry-stallize. -By this process, it is said, he gains the whole of the alum at one evaporation, and from the mother liquor remaining there is a pro duct, the sulphate of iron.