366
ELEMENTS OF MATERIA MEDICA.
Genoa, sulphate of magnesia is manufactured from schistose minerals,containing sulphur, magnesia, copper, and iron. After being roasted, andmoistened to convert them into sulphates, they are lixiviated, and thesolution is deprived, first, of copper by refuse iron, and afterwards of ironby lime (Dr. Holland, Phil. Trans. 1816, p. 294). In Bohemia, sulphateof magnesia is procured, by evaporation, from the waters of Seidlitz andSaidschiitz. Hermann (Poggendorff’s Annalen, xi. 249) extracts it fromliquids containing chloride of magnesium, by means of sulphate of soda.At Baltimore , sulphate of magnesia is procured from the siliceous hydrateof magnesia or marmolite, by reducing the mineral to powder, saturatingwith sulphuric acid, and calcining the dried mass to peroxidize the iron.It is then re-dissolved in water (from which solution the remaining ironis separated by sulphuret of lime), and crystallized. By a second crys-tallization it is obtained nearly pure (D. B. Smith, in the Dispensatoryof the U. S. of America ).
Properties. —The impure sulphate obtained from bittern is calledsingles, and when purified by re-crystallization, doubles. The sulphateusually met with in the shops is in small acicular crystals. By solutionand re-crystallization we readily obtain tolerably large four-sided prisms,with reversed diedral summits, or four-sided pyramids: the primaryform of the crystals is the right rhombic prism. Both large and smallcrystals are colourless, transparent, and odourless, but have an extremelybitter taste. When heated they undergo the watery fusion, then give outtheir water of crystallization, become anhydrous, and at a high tempera-ture undergo the igneous prism, and run into a white enamel, but withoutsuffering decomposition. Exposed to the air the crystals very slowly andslightly effloresce. It dissolves in its own weight of water at 60°, and inthree-fourths of its weight of boiling water. It is insoluble in alcohol.
Characteristics. —It is known to contain sulphuric acid by the testsfor the sulphates already mentioned (p. 265). The nature of its base isshown by the tests for magnesia before described (p. 361).
Composition. —The following is the composition of crystallized sul-phate of magnesia:—
Magnesia.1 . . . . 20 ... . 16-26 .... 16-04 .... 16-86
Sulphuric Acid. 1 . . . . 40 ... . 32'52 .... 32"53 .... 30-64
Water. 7 ... . 63 ... . 51'22 .... 5P43 .... 52'50
Cryst d . Sulphate of Magnesia 1 .... 123 .... lOO'OO .... lOO'OO .... 100-00Purity. —The sulphate of magnesia met with in the shops is usuallysufficiently pure for all medicinal and pharmaceutical purposes. Itshould be colourless, and undergo no change when mixed with ferro-cyanides or hydrosulphurets. When obtained from bittern it is some-times contaminated with chloride of magnesium, which, by its affinityfor water, keeps the sulphate in a damp state. By digestion in alcoholthe chloride is dissolved, and by evaporating the spirituous solution, maybe obtained in the solid state. It is said, that occasionally small crystalsof sulphate of soda are intermixed with those of sulphate of magnesia—a fraud I have never met with in English commerce, nor is it likely tooccur at the present low price of the magnesian salt. Should such anadulteration be suspected, there are several methods of detecting it: thesophisticated salt would effloresce more rapidly than the pure salt, andwould communicate a yellow tinge to the flame of alcohol. Boiled with