Alum is colourless, inodorons, has an as tringent taste, and crystallizes generally in regular octahedrons ; but by the addition of alumina, and particular management, it may be made to crystallize in cubes. It is brittle, and easily reduced to powder ; its specific gravity is about 1.731; water, at Fahren heit, dissolves about one-eighteenth of its weight of alum, and boiling water about three fourths of its weight. The solution reddens vegetable blue colours strongly; when exposed to dry air, alum effloresces slightly on the sur face, but it remains long without undergoing any change internally. When moderately heated, alum dissolves in its water of crystal lization ; if more strongly heated, the water is evaporated, and when exposed to a very high temperature, sulphuric acid is expelled, and there remains a mixture of alumina and sul phate of potash.
The above-named properties relate to com mon or potash alum. Soda alum is produced by the action of sulphate of soda on sulphate of alumina. Their properties are very closely allied to those of potash alum.
Alum is decomposed, not only by heat, but by many of the acids, alkalies, and earths. It is employed for a vast number of purposes. It is used in lake colours, dyeing, calico print ing, leather dressing ; and by candle-makers to harden the tallow and render it white. It
is an ingredient in making pyrophorus ; and in medicine it is employed as an astringent.
Mr. Cliff obtained a patent in 1845 relating to the alum manufacture, in which a double useful result is sought to be obtained. Many kinds of fire-clay used for making retorts, cru cibles, and similar articles, contain an excess of alumina, which injures the quality of the articles made from them when at a high heat.
2. Cliff proposes to take a very aluminous clay, such as is found at Wortley near Leeds ; to grind it, calcine it, mix it with sulphuric or some other acid, dissolve the alumina out of it, evaporate the solution, and obtain alum from the residue by the usual chemical means. The earthy sediment, which results when all or nearly all the alumina has been removed, is then to be mixed up with ground native clay, to make a fire-clay fitted for retorts, &c. Many other patents have been recently taken out, for various improvements in the alum manufacture.
The proprietors of some of the chemical works near Newcastle intend to exhibit, at the great display of 1851, specimens of alum, in large masses, procured by the direct chemical union of the component ingredients.