334 SALTS SULPHATES. 



(/.) Also by calcining the magnesian limestones; treating them 

 with muriatic acid to dissolve the lime, and then with sulphuric acid, 

 or sulphate of iron, to form the sulphate of magnesia.* 



SULPHATE OF ALUMINA AND ALUM. 



Common alum. 



1. PREPARATION. Always prepared in the large way; rarely 

 by the chemist , unless in analysis. 



2. PROPERTIES. 



(a.) Its properties are always shown by the alum of commerce, 

 which is a triple salt, and not mere sulphate of alumina, which has 

 characters entirely different. 



(b.) Crystals formed from a hot concentrated solution, filtered ; 

 a frame of sticks or some hairs or strings or wires are often suspended 

 in it, for the crystals to adhere to ; they form a beautiful group, and 

 are handsomely exhibited in a bottle. 



(c.) Aqueous fusion and subsequent desiccation by heat, on an ignited 

 iron ; the product was formerly called alumen ustum ; there is a par- 

 tial expulsion of the acid so that the solution of the desiccated alum 

 does not easily redden blue vegetable colors. f By a very violent 

 heat, most of the acid is expelled. The solution of the crystals red- 

 dens litmus liquor decidedly, cabbage liquor slightly, " but blue tinc- 

 tures, from the petals of plants, are generally turned by it green."f 



(?.) Air has generally no action sometimes produces a slight 

 efflorescence. 



(e.) Taste, sweetish, acid, and astringent; rather agreeable to 

 most persons. Specific gravity 1.71. 



(/.) Water 5 parts at 60 dissolves 1 of the salt; at 212 1 part 

 of water dissolves three fourths of its weight. 



Or-) Pyrophorus. Take 3 parts of alum and 1 of flour or brown 

 sugar, heat the mixture, and stir it constantly, in an iron pot or ladle, 

 tiU it has ceased to swell, and has become dry ; powder the mixture 

 finely, and introduce it into a vial coated with clay ; set this in a 

 sand heat, and continue the heat till gas ceases to be inflamed, by 

 bringing a lighted paper to the mouth ; we are usually directed to in- 

 troduce a small tube, through a perforated cork, into the vial's mouth ; 

 when the operation is over this may be removed and a cork substituted. 



(h.) This pyrophorus fires in the air ; more vividly, in ajar of 

 oxygen gas ; it fires also in chlorine and nitric oxide gas. 



* Id. and Ann. de China, et de Phys. T. VI, p. 86, and Gray's Op. Chem. 



t It is suggested that the effect of alum on blue colors, may be owing to a feeble 

 affinity between the acid and the earth, and of course to an attraction between the 

 acid and the coloring matter, rather than to an excess of acid. 



t Quarterly Jour. XVIII, 396. 



