Elements of chemistry, in two volumes / by John Murray.

  • Murray, John.
Date:
1828
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, in two volumes / by John Murray. Source: Wellcome Collection.

    113/768 (page 101)
    of acid and one of earth, with 7 atoms of water. A sub- sulphate of alumina occurs native, containing 3 atoms of earth to 1 of acid. This salt, known by the name of Alum, is a ternary com- pound of alumina, potash, and sulphuric acid. In some of the varieties of the alum of commerce, ammonia even en- ters into the composition, and it is often contaminated with a little sulphate of iron. Alum is a natural product, oc- curring efflorescent in the layers of what has been named Alum Slate : it is also formed by an artificial process, from what is named Alum Ore, which appears to be a variety of slate impregnated with sulphur or sulphui’et of iron ; this slate is calcined, and afterwards exposed to the at- mosphere in a humid state ; the sulphur absorbing oxy- gen is converted into sulphuric acid, which acts on the ar- gillaceous earth of the slate. The saline matter is extract- ed by lixiviation, and when a little potash has been added, alum is obtained by crystallization. The alum of com- merce is in large masses, white and transparent. When regularly crystallized, the form of the crystals is an octae- dron. It is soluble in 15 parts of cold, and in two parts of boiling water : its crystals are slightly efflorescent: its taste is sweetish and astringent: its solution reddens the vegetable colours. When exposed to a moderate heat it melts, from the water of crystallization which it contains dissolving the solid salt •, this soon evaporates, and leaves a light white powder, named Burnt or Calcined Alum. By a strong heat the acid is partly expelled, partly decom- posed, a quantity of oxygen being afforded by its decom- position. This salt is decomposed by the alkalis and al- kaline earths, which attract the greater part of the acid, and precipitate the earth with a small quantity of acid com- bined with it. With regard to the constitution of alum, there is a difference of opinion arising from the uncertain-