MAG 



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MAG 



tive state. It enters into the compo- 

 sition of some of the primary rocks, 

 to which it usually imparts a sap- 

 onaceous feel, producing also a 

 striated texture, and frequently a 

 greenish shade. Magnesia first 

 became known ahout the begin- 

 ning of the eighteenth century. 

 Little, however, was known con- 

 cerning its nature, till Dr. Black 

 published his celebrated experi- 

 ments on it 1755. Magnesia may 

 be thus prepared : sulphate of 

 magnesia, a salt, composed of mag- 

 nesia and sulphuric acid, is to be 

 dissolved in water, and half its 

 weight of potass added. The potass 

 having a stronger affinity for the 

 sulphuric acid than the magnesia 

 has, seizes the sulphuric acid, and 

 the magnesia is precipitated. Mag- 

 nesia is often present in chalk; 

 some of the strata in Prance are 

 said to contain ten per cent, of 

 magnesia. Magnesia is present 

 in all the inferior stratified rocks, 

 with the exception of quartz rock, 

 (without mica,) and certain eurites, 

 or compact felspars. In the de- 

 trital rocks it is also common, 

 particularly when mica forms any 

 considerable portion of them. 

 There are few limestones which do 

 not contain magnesia. It is an 

 essential ingredient of dolomite, 

 carbonate of magnesia constitutes 

 more than 40 per cent, of that 

 rock. Magnesia is also dissemi- 

 nated through the waters of the 

 ocean, muriate of magnesia form- 

 ing from -004 to -005 of their 

 mass. 



MAGNE'SIA MICA. Called also Biotite. 

 See Biotite. 



MAG^E'SIAN LI'MESTONE. (The Zech- 

 stein of the German, the Calcaire 

 Alpin of the French geologists.) 

 The Magnesian limestone, though 

 somewhat extensively deposited in 

 England and Germany, appears but 

 little known in France. A marine 

 deposit, belonging to the new red 



sandstone group. It lies above the 

 red conglomerate and below the 

 variegated sandstone. It is com- 

 posed of carbonate of magnesia, the 

 proportion of the latter amounting 

 to nearly one half in some instances. 

 It effervesces much more slowly 

 and feebly with sulphuric, nitric, 

 or muriatic acids than does the 

 common limestone. The magnesian 

 limestone of this country is a 

 dolomite of a yellow or yellowish- 

 brown colour ; it is distinctly 

 stratified, the strata varying from 

 a few inches to several feet in 

 thickness. This deposit is fossili- 

 ferous, and certain shells, products, 

 appear for the first time in the 

 magnesian limestone. Magnesian 

 limestone forms the most durable 

 building-stone, and it is of this that 

 the two new houses of parliament 

 are built. It is to be lamented 

 that Waterloo Bridge was not built 

 of magnesian limestone instead of 

 felspathic granite, a very perishable 

 kind of stone. Where the mag- 

 nesia is in excess the land is sterile, 

 but when it is not in excess, the 

 soil is fruitful, and, as a subsoil, 

 healthful. 



MA'GNESITE. A mineral of a white 

 or yellowish-white colour. It 

 occurs massive, tuberose, reniform, 

 and versicular : fracture is con- 

 choidal : opaque : sp. gr. 2 '881 : 

 infusible, and before the blow-pipe 

 becomes so hard as to scratch glass. 

 It is composed of one equivalent of 

 carbonic acid and one of mag- 

 nesia. 



MAGNE'SIUM. The metallic basis of 

 magnesia. 



MAGNETIC IEOJT QBE. The Fer oxy- 

 dule of Haiiy ; Magnet eisenstein 

 of Werner. A black ore, possess- 

 ing a slight metallic lustre. Occurs 

 regularly crystallized; in granular 

 concretions ; massive and dissemi- 

 nated. It is magnetic, sometimes 

 sufficiently so to take up a needle. 

 It occurs in beds in primary and 



