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Alum

The alum of commerce is a sulphate of alumina and potassa, obtained by lixiviation (see No. 23) from crude alum ore, or schist. It is obtained in large crystals, slightly efflorescent. It is applied in the arts portion, holding the bromine in solution, floats upon the surface. By decanting, and evaporating the ether, a crude bromine may be obtained at once. To get it pure, the ethereal solution is carefully decanted, and agitated with a solution of potassa, by which means bromide of potassium and bromate of potash are formed. The whole is next evaporated to dryness, and submitted to a dull red heat; the residuum is then powdered, mixed with pure peroxide of manganese, and placed in a retort; sulphuric acid, diluted with half its weight of to a great variety of purposes. When deprived water, is now poured in. Red vapors immeof its water of crystallization by heat, it be- diately arise, and condense into drops of brocomes burnt or dried alum. Pure red or roche mine, and are collected by plunging the neck alum was originally imported from Italy, where of the retort to the bottom of a small receiver it is found in a native state. This has a red- containing cold water. The bromine forms a dish tinge, which extends more or less through stratum beneath the water, and may be colthe crystals. lected and further purified by distillation from