ALUMINO-SOLPHATE OF POTASH.
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not present any regular geometrical form; but, by immersion in waterduring a few days, octahedral and rectangular forms are developed in itssurfaces. (Daniel, Quart. Journ. i. 24.) Alum has an astringent andsweetish acid taste : its reaction on vegetable colours is that of an acid.Its sp. gr. is 17. By exposure to the air it slowly and slightly efflo-resces. Its transculent or diathermanous power is very slight.
When heated, alum undergoes the watery fusion, swells up, gives outits water of crystallization, and becomes a white spongy mass, calleddried or burnt alum, (Alumen Exsiccatum, Ph . L., alumen ustum), whichhas a more astringent taste, and does not so quickly dissolve in water asthe crystallized salt. If too much heat be applied a portion of the acidis expelled, and escapes, partly as sulphuric acid, partly in the form ofoxygen and sulphurous acid, and the residue consists of alumina and sul-phate of potash: the acid liquor obtained by heating alum w r as formerlytermed spirit of alum. In the preparation of burnt alum care should betaken not to apply too great a heat in order that the acid may not bedriven off. On this account a shallow earthen vessel is preferable to acrucible.
When alum is calcined with charcoal or some carbonaceous substance,as sugar, we obtain a spontaneously inflammable substance called Hom-berg's Pyrophorus, composed of sulphur, potassium, alumina, and charcoal.
Alum dissolves in 18 times its weight of cold and less than its ownweight of boiling water.
The alum procured at Tolfa and other parts of Italy , and calledRoman alum (alumen Romanian ) is covered with a pale, rose-colouredefflorescence, composed of oxide of iron and an aluminous sulphate ofpotash. Under the name of Roche or Rock alum (alumen rufeum, senalumen de Rochi, so called from Roccha, in Syria , whence a red-coloured alum was formerly brought) we find in English commerce crys-talline fragments of alum, not larger than almonds, coloured externallywith bole or rose-pink.
Characteristics. —That alum is a sulphate is shewn by the tests forthe soluble sulphates already mentioned (p. 265). It reddens litmus, andforms sulphate of lead when mixed with pure carbonate of lead : in theseproperties it agrees with the supersulphates. The nature of its basicconstituents is shewn by the following tests:—The ferrocyanides, theoxalates, and hydrosulphuric acid, occasion no precipitate in a solutionof alum. Hydrosulphuret of ammonia, the caustic alkalies and theircarbonates, and phosphate of soda, throw down white precipitates: thatproduced by- the alkalies is soluble in an excess of alkali, but is insolublein solutions of the carbonated alkalies: these characters shew the pre-sence of alumina. Potash is recognised by perchloric acid and chlorideof platinum (vide p. 274.) Lastly, the crystalline form of the salt assistsin recognising it.
Composition. —The composition of alum is as follows:—
Crystd. Potasli-Alum
Eq.
Eq. Wt. PerCent. Thomson. Berzel.
Eq. PerEq. Wt. Cent.
3 .. 54 .. 11-088 .. 11*09 10*76
1 .. 48 .. 9856 .. 9*86 .. 9‘95
4 ..160 .. 32-854 .. 32-85 .. 33*74
25 ..225 .. 46-201 .. 46*20 .. 45'55
1 ..487 .. 99*999 ..100-00 ..lOO'OO
Sulphate of Alumina 3 .. 174 .. 35-728Sulphate of Potash.. I .. 88 .. 18*069Water. 25 .. 225 .. 46-201
Crystd. A1 amino- ) ,Sulph te . Potash 1 *
487 .. 99-998
In the above table I have assumed, with Thomson, Braude, and Phil-
B B