MAGNESIUM 177 



purpose of decomposing the horn-silver (chloride of silver) present. For an account 

 of this process of reduction, see SILVEB. 



MAGMA is the generic name of any crude mixture of mineral or organic matters 

 in a thin pasty state. 



MAGNANIER is the name given in the southern departments of Franco to tho 

 proprietor of a nursery in which silkworms are reared upon the great scale, or to the 

 manager of the establishment. The word is derived from magnans, which signifies 

 silkw6rms in the language of the country people. See SILK. 



MAGNESIA (Magnesie, Fr. ; Bittererde, Talkerde, Ger.) is one of the earths, 

 first proved by Sir H. Davy to be the oxide of a metal, which he called magnesium. 

 It is a fine, light, white powder, without taste or smell, which requires 5,150 parts 

 of cold water, and no less than 36,000 parts of boiling water, for its solution. Its 

 specific gravity is 2'3. It is fusible only by the heat of the hydroxygen blowpipe. 

 A natural hydrate exists which contains 30 per cent, of water. Magnesia changes 

 the purple infusion of red cabbage to a bright green. It attracts carbonic acid from the 

 air, but much more slowly than quicklime. It consists of 61 -21 parts cf metallic basis and 

 38'79 of oxygen ; and has, therefore, 20 for its equivalent upon the hydrogen scale. 

 Its only employment in the arts is for the purification of fine oil, in the preparation of 

 varnish. 



Magnesia, popularly known as Calcined Magnesia, may be obtained by precipitation 

 with potash or soda from its sulphate, commonly called Epsom salt ; but it is usually 

 procured by calcining the artificial or natural carbonate. There is a heavy calcined 

 magnesia prepared by burning tho dense carbonate. Mr. Lockyor has shown, however, 

 that a very dense and pure magnesia could be obtained by calcining the ordinary 

 pure carbonate in large masses, and at a very high temperature. 



MAGNESIA, CARBONATE! OP ; properly speaking, a subcarbonate, con- 

 sisting of 44'69 magnesia, 35'86 carbonic acid, and 19-45 water. It is prepared by 

 adding to the solution of, the sulphate, or the chloride (the bittern of sea-salt evapora- 

 tion works), a solution of carbonate of soda, or of carbonate of ammonia distilled from 

 bones in iron cylinders. Mr. Hugh Lee Pattinsou introduced the manufacture of car- 

 bonate of magnesia from the dolomite rocks, availing himself of the different rates of 

 solubility of the carbonates of lime and magnesia in water saturated with carbonic 

 acid. (See DOLOMITE.) The subcarbonate, or magnesia alba of the apothecary, has 

 been proposed by Mr. E. Davy to be added by the baker to damaged flour, to counter- 

 act its acescency. 



MAGNESIA, NATIVE HYDRATE OP, or Brucite. This mineral consists 

 of magnesia, 68'97, water, 3T03, according to analyses by Bruce. It accompanies 

 other magnesian minerals in serpentine at Swinaness in Unst, one of the Shetland 

 Isles, in the Ural Mountains, in France, and opposite to New York. 



MAGNESIA, SILICATES OF. Compounds of this character are abundant 

 in the mineral kingdom. Meerschaum, French Chalk or Steatite, Talc, Serpentine, 

 and many other minerals are silicates of magnesia. (See these articles.') 



MAGNESIA, SUIiPHATE OP, (Epsom Salts,) is generally made by acting 

 upon magnesian limestone with somewhat dilute sulphuric acid. The sulphate of lime 

 precipitates, while the sulphate of magnesia remains in solution, and may be made to 

 crystallise in quadrangular prisms, by suitable evaporation and slow cooling. Where 

 muriatic acid may be had in profusion for the trouble of collecting it, as in the soda- 

 works in which sea-salt is decomposed by sulphuric acid, tho magnosian limestone 

 should be first acted upon with as much of the former acid as will dissolve out the lime, 

 and then, the residuum being treated with the latter acid, will afford a sulphate at the 

 cheapest possible rate ; from which magnesia and all its other preparations may be 

 readily made. Or, if the equivalent quantity of calcined magnesian limestone be boiled 

 for some time in bittern, the lime of the former will displace the magnesia from the 

 muriatic acid of the latter. This is the most economical process for manufacturing 

 magnesia. See DOLOMITE. 



MAGNESIAN XiIMESTONE. See DOLOMITE ; LIMESTOXB. 



MAGNESITE. Carbonate of Magnesia ; Rhomb Spar. This native carbonate of 

 magnesia, consisting of magnesia 47'6, carbonic acid 52-4, is found with serpentine 

 and other magnesian rocks. 



MAGNESIUM. The metal obtained from magnesia. It was first procured by 

 Bussy, although previously shown to exist by Davy. Ib is now made by placing 

 potassium or sodium in a platinum crucible, covering them with chloride of mag- 

 nesium, fastening down the cover of the crucible, and exposing it to the heat of a 

 spirit-lamp. It has been prepared by Bunsen by the action of the voltaic current ; 

 and the late Dr. Matthiessen obtained it by the electrolysis of fused chloride of mag- 

 nesium. 



We are much indebted to M. Sonstadt for removing the obstacles in the w&y of 

 VOL. HI. N 



