Alum, a name given to a remarkable series of double salts, of which potash alum may be taken as the type. The alums are more or less soluble in water, crystallize in regular oc-tahedra, and differ from the normal compound in the fact that the alumina and potash are replaced in whole or in part by their isomorphs. We can replace the alumina by the sesquioxide of iron, of manganese, or of chromium, and the potash by soda, the oxide of ammonium, the oxides of ammonium compounds, the oxides of rubidium, caesium, and thallium. Lithia is the only one of the alkalies that does not form an alum. Selenic acid, isomorphous with sulphuric acid, and (it is believed) telluric acid, can be substituted for sulphuric acid in the composition of alums. The following is a list of alums actually known to chemists, together with their chemical formulas: 1. Ordinary potash alum. (So4)3A12 + SO4K2 + 24H2O.

2. Soda alum,. (SO4)3Al2 + SO4Na2 + 24H2O.

3. Ammonia alum, (SO2Al2 + SO4(NH4)2 + 24H2O. 4. Rubidium alum, (SO4)3A12 + SO4 Rb2 + 24H2O. 5. Caesium alum, (SO4)3A12 + SO4Cs2 + 24H2O. 6. Thallium alum, (SO4)3 Al2 + SO4T12 + 24H2O. 7. Manganese alum, (SO4)3Mn2+ SO4K2+ 24H2O. 8. Chrome alum, (SO4)3Cr2 + SO4K2 + 24H2O. 9. Chrome-ammonium alum, (SO4)3Cr2 + SO4 (NH4)2 + 24H2O. 10. Iron alum, (SO4)3Fe2 + SO4K2 + 24H2O. 11. Iron-ammonium alum, (SO4)3Fe2 + SO4 (NH4)2 + 24H2O. 12. Thallium-iron alum, (SO4)3Fe2 + SO4T12 + 24H2O. 13. Selenic alum, (SeO4)3Al2 + SeO4K2 + 24H2O. To this list may be added alum of triinethy-lamine, alum of ethylamine, alum of methy-lamine, and alum of amylamine. - The history of alum dates back to a remote antiquity. Pliny in his Historia Naturalis mentions several kinds of alumen, and says that a black and a white occur in Cyprus, the former being-used for dyeing dark wool and the latter for light fabrics. As he afterward says that alumen liquidum is colored black by nut galls, he either refers to an impure alum or confounds the substance with sulphate of iron or green vitriol.

It is, however, safe to assume that theAlum 100236 of the Greeks and the alumen of Pliny have reference to impure varieties of what is now called alum. It was manufactured some centuries since at Racca in Mesopotamia, whence Leibnitz traced the name alumen roccae, or rock alum. In the 13th century the business was established near Smyrna, and in 1248 extended to Italy, where it was protected against foreign importations by the pope. From thence it spread over Germany, and in the reign of Queen Elizabeth was undertaken in England by Thomas Chaloner, and successfully prosecuted notwithstanding the anathemas of the pope. At first only potash alum was manufactured, but since the introduction of ammonia as a refuse product in the production of illuminating gas large quantities of ammonia alum have been made. The more recent development of the potash deposits of the Stassfurt mines has again brought back the production of potash alum to the first rank, and its manufacture is now conducted on an immense scale. - Potash alum occurs ready formed in nature, especially among volcanic rocks near Naples, on the Rhine, on the island of New York, and in numerous other localities.

Where the quantity is sufficient to pay for the trouble, the decomposed rock is leached and the resulting liquor left to crystallize; but only a very small proportion of the alum of commerce is obtained in this way. There are three classes of raw material from which potash alum is manufactured:

4. Such as contain the constituents of alum in the native state; e. g., native alum, alum stone, alum slates. 2. Such as only contain sulphate of alumina, and require the addition of an alkali; e. g., a majority of the alum slates, alum earths, clay, and pyritous bituminous shales. 3. Such as only contain the alumina and require the addition of both the sulphuric acid and the alkali; e. g., clay, cryolite, bauxite, and feldspar. By far the greater proportion of alum is made from alum earths and shales, although the employment of clay, cryolite, and feldspar is on the increase. The mineral alum stone or alunite forms seams in trachytic and allied rocks, where it has been formed as a result of the alteration of the rock by means of sulphurous vapors. It is met with at Tolfa near Civita Vecchia, at Monti-oni in Tuscany, in Hungary, and in other local-ities. The compact varieties of Hungary are so hard as to admit of being used for millstones. It was first observed at Tolfa in the loth century, by J. de Castro, a Genoese, who had been engaged in the manufacture of alum from an alum stone or "rock alum " found near the Euphrates. The composition of alunite is very variable.

One specimen from Tolfa was found to contain sulphuric acid 35.50, alumina 39.65, potash 10.02, and water 14.83. It is only when the alunite has been heated to 450° C. that the alum can be extracted from it by water. The stone is roasted in heaps on calcining kilns until it begins to give oft1 sulphurous acid fumes. when the operation is suspended. The calcined material is then placed in troughs of masonry and sprinkled with water until it forms a slimy paste; this is leached in shallow pans with hot water, the lye concentrated and crystallized. Roman alum has a reddish hue, and has long been preferred on account of its freedom from soluble foreign substances. Much of what is now called Roman alum is colored red by the addition of brick dust. Most of the alum of commerce is made by the calcination of aluminous schists, which are argillaceous rocks, containing considerable quantities of sulphide of iron. This is converted by exposure to the air into ferrous sulphate and free sulphuric acid, FeS2 + O7 + H2O = FeSO4 + H2SO4; and the sulphuric acid, acting on the alumina contained in the clay, forms sulphate of aluminum.

These aluminous schists are found in two different geological positions, viz.: in the transition strata (alum slate), in which position they are largely impregnated with bitumen; and in the lower tertiary strata, just above the chalk (alum earth). The latter are much less compact than the former; consequently their oxidation is easier, and sometimes takes place spontaneously. - The most extensive alum manufactory in Great Britain (1871) is at Hurlett, near Paisley. The next in magnitude is at Whitby, of whose state and processes an instructive account was published by Mr. Winter in the 25th volume of "Nicholson's Journal."" The stratum of aluminous schist is about 29 miles in width, and is covered by strata of alluvial soil, sandstone, ironstone, shell, and clay. The alum schist is generally found disposed in horizontal lamina?. The upper part of the rock is the most abundant in sulphur, so that a cubic yard taken from the top of the stratum is five times more valuable than the same bulk 100 feet below. If a quantity of the schist be laid in a heap and moistened with sea water, it will take tire spontaneously, and continue to burn till the whole inflammable matter is consumed. Its color is bluish gray; sp. gr. 2.4=8. It imparts a bituminous principle to alcohol.