MAGNESIA. 



617 



large scale, the bittern or liquor remaining after the 

 crystallization of sea-salt, which is principally a solu- 

 tion of muriate and sulphate of magnesia, is substi- 

 tuted for the pure sulphate, and this is precipitated 

 by a solution of pearlash or of carbonate of ammonia. 

 Carbonate of magnesia is perfectly white, friable, 

 and nearly tasteless. It is very sparingly soluble in 

 water, requiring at least 2000 times its weight at 

 60. When acted on by water impregnated with 

 carbonic acid, it is dissolved ; and from this solution, 

 allowed to evaporate spontaneously, the carbonate of 

 magnesia is deposited in small prismatic crystals, 

 which are transparent and efflorescent. 



Nitrate of magnesia has a taste bitter and acrid. 

 Its crystallization exhibits a mass of needle-like 

 crystals, deliquescent, soluble in half their weight of 

 water at 60. 



Sulphate of magnesia, generally known by the 

 name of Epsom salt, is made directly by neutralizing 

 dilute sulphuric acid with carbonate of magnesia ; 

 but in the large way, by the action of dilute sul- 

 phuric acid on magnesian limestone, and the 

 native carbonate of magnesia. It is possessed of a 

 saline, bitter, and nauseous taste. It crystallizes 

 readily in small quadrangular prisms, which efflor- 

 esce in a dry air. It is obtained also in larger six- 

 sided prisms, terminated by six-sided pyramids. Its 

 primary form is a right rhombic prism, the angles of 

 which are 90 30' and 89 30'. It is soluble in an 

 equal weight of water at 60, and in three-fourths of 

 its weight of boiling water. It undergoes the watery 

 fusion when heated. On mixing solutions of sulphate 

 of magnesia and sulphate of potash in atomic propor- 

 tion, and evaporating, a double salt is formed, which 

 consists of one equivalent of each of the salts, and six 

 equivalents of water. A similar double salt (isomor- 

 phous with the preceding) is formed by spontaneous 

 evaporation from the mixed solutions of sulphate of 

 ammonia and sulphate of magnesia. 



Phosphate of magnesia, formed from the combina- 

 tion of the acid and the earth, crystallizes in prisms, 

 which are efflorescent, soluble in about 15 parts of 

 cold water, and which, by heat, melt into a glass. 



A triple phosphate of magnesia and ammonia 

 exists, which is formed by adding phosphoric acid 

 with ammonia, in excess, to a magnesian salt. It is 

 insoluble, and is precipitated in a soft white powder 

 of shining lustre. It forms one variety of urinary 

 calculus, and its formation affords one of the best 

 tests for the discovery of magnesia. 



Muriate of magnesia has such an affinity to water, 

 that it can be obtained in acicular crystals only by 

 exposing its concentrated solution to sudden cold. 



Chloride of magnesia may be formed in the same 

 manner as chloride of lime. Chloride of magnesia 

 may be obtained by heating the magnesia in chlorine 

 gas, when the oxygen escapes, and the chlorine com- 

 bines with the metal. It has the same bleaching 

 power, and it has been proposed to apply it to the 

 same purpose. When the chloride of lime is used, 

 a small quantity of lime is left on the cloth : this, in 

 the last operation of washing the cloth with water 

 acidulated with sulphuric acid, is converted into sul- 

 phate of lime, which, being insoluble, remains, and 

 affects the colours, when the cloth is dyed. The 

 advantage of employing the chloride of magnesia is, 

 that, if sulphate of magnesia is formed, it is so 

 soluble as to be easily removed by washing. Mag- 

 nesia is a very useful article of the materia medico. 

 It is used as an antacid and cathartic. It is, how- 

 ever, nearly inoperative, unless there is acid in the 

 stomach, or unless acid is taken after it. The car- 

 bonate and sulphate are the most frequently used of 

 the preparations of magnesia ; but the pure earth, 

 sold under the name of calcined magnesia, is some- 



times preferred ; it is liable, however, to form large 

 and dangerous accumulations in the bowels, of 

 several pounds weight, when its use has long been 

 persevered in. 



Magnesian Minerals. Of these, the hydrate of 

 magnesia, or native magnesia, deserves to be men- 

 tioned in the first instance. It is a rare substance, 

 having hitherto been met with only at two localities 

 Swinaness in Uist, one of the Shetland Isles, and 

 Hoboken, in New Jersey ; in the latter place, occur- 

 ring in thin seams, traversing serpentine. It exhi- 

 bits a lamellar, or broad columnar structure ; is but 

 little above talc in hardness, or in the difficulty of its 

 cleavage ; sectile ; thin laminae flexible ; specific 

 gravity 2 - 350. Its colour is white, inclining to green; 

 lustre pearly; translucent. Before the blow-pipe, 

 it loses its transparency and weight, and becomes 

 friable. In acids, it is dissolved without ef- 

 fervescence, and consists of 70 magnesia, and 30 

 water. 



The siliceous hydrate, or Deweylite, is a compact, 

 white, or yellowish-white mineral, found in the 

 serpentine of Middlefield, Massachusetts, and near 

 Baltimore, Maryland. It has a hardness between 

 calc-spar and fluor, and is composed of silica 40, 

 magnesia 40, and water 20. It appears to be identi- 

 cal with the kerolite of Breithaupt. 



Carbonate of magnesia, or magnesite, is found 

 crystallized in radiating and parallel fibres, reniform, 

 tuberose, and massive ; fracture, when massive, flat 

 conchoidal. It also occurs pulverulent ; fracture flat 

 conchoidal, sometimes earthy; dull ; colour yellowish- 

 gray, cream-yellow, yellowish and grayish-white ; 

 streak white ; opaque ; adheres to the tongue. Some 

 of the compact varieties are very tough, giving fire 

 with the steel, though too soft to impress fluor; 

 specific gravity, 2 -808. It is infusible before the 

 blow-pipe ; dissolves with a slow effervescence in 

 the dilute nitric and sulphuric acids. It consists of 

 magnesia 48-00, carbonic acid 49'00 and water 3-00. 

 It is found in Stiria, Silesia, and Spain. A variety 

 of it, possessing an earthy fracture, and containing 

 about four per cent, of silex, is found in the islands 

 of Samos and Negropont, in the Archipelago, and is 

 called, by the Germans, Meerschaum, and by the 

 French, Ecume de Mer. It is soft when first dug, 

 and, in that state, is made into pipes, but hardens by 

 exposure to the air. The most remarkable deposit 

 of this mineral, however, is found at Hoboken, in 

 New Jersey, where it occurs disseminated, in seams, 

 through a serpentine rock ; and is sometimes crys- 

 tallized, at others pulverulent. 



Sulphate of magnesia is found in crystalline fibres, 

 parallel and divergent, and in the shape of crusts ; 

 more rarely, also, it has been found pulverulent. It 

 is easily recognised by its bitter saline taste. Specific 

 gravity, 1-75; colour white; lustre vitreous, trans- 

 lucent, or transparent. It dissolves very easily in 

 water, deliquesces before the blow-pipe, but is 

 difficultly fusible, if its water of crystallization has 

 been driven off. It effloresces from several rocks, 

 both in their original repository and in artificial walls, 

 and then it is a product of their decomposition. It 

 forms the principal ingredient of certain mineral 

 waters. It occurs at Freiberg and its vicinity, 

 efflorescing upon gneiss, also at the quicksilver mines 

 of Idria, in Carniola, and various other places in 

 Europe. Its most remarkable depositories, however, 

 are the limestone caves of Kentucky, whose floors are 

 often covered with it, in delicate crystals, to a con- 

 siderable depth, imtermingled with a dry earth, which 

 has come from the decomposition or disintegration 

 of the limestone rock : this earth is leached, in very 

 considerable quantities, by the inhabitants of the 

 country, who obtain from it their supply of Epsom 



