SALTS OF MAGNESIA. 509 







some time upon this precipitate, at a temperature only a few 

 degrees above 32. The precipitate re-dissolves, and there are 

 formed on the sides of the vessel thin crystalline needles, 

 transparent, brilliant, hard, and having much of a mineral 

 character. They are insoluble in hot or cold water. They 

 lose by heat 58.4 per cent of water, or 8 atoms, and leave a 

 compound of single equivalents of boracic acid and magnesia, 

 Mg O, BO 3 . After the deposition of this neutral borate, the 

 liquid affords large crystals of a double borate of soda and mag- 

 nesia, containing 52.5 per cent of water, but of which the 

 proportions of the constituent salts have not been determined. 



The mineral boracite, which occurs in the cube and its allied 

 forms, is an anhydrous compound of magnesia and boracic acid, 

 in the extraordinary ratio of 3 eq. of magnesia to 4 of boracic 

 acid, or its formula is 3MgO + 4BO 3 . This mineral becomes 

 electrical by heat. The rare mineral, hydroboracite, is, ac- 

 cording to Hess, a compound of a borate of lime and borate of 

 magnesia, in both of which the acid and base are in the same 

 ratio as in boracite, with 18 eq. of water. 



Silicates of magnesia. Magnesia is found combined with 

 silicic acid in various proportions, forming several mineral 

 species, of which the formulee are as follows : 

 Steatite . . . MgO,SiO 3 . 

 Meerschaum . . MgO,SiO 3 + H(X 

 Rcrosmine and pyrallo- J 3Mg o + 2gi Qy 



Periote (olivine, or chry- J 3Mg Q + Si Og> 



erpentine (hydrate ofl 



magnesia with subsili- >3Mg O,6HO + 2(3MgO + 2Si O 3 ). 



cate of magnesia) 



magnesia) 

 Pyroxene or augite (si- "I 



licate of lime and mag- > 3Ca 0, 2Si O 3 -f 3Mg O, 2Si O 3 . 

 nesia) . . .J 

 Amphibole, or horn-"| 



blende (silicate of lime > Ca O, Si O 3 + 3Mg O, 2Si O 3 . 

 and magnesia) . . J 



In these minerals, particularly the two last, the magnesia is 

 often replaced in whole or in part by protoxide of iron, which 

 gives them a green, and sometimes black colour. Fine crystals 

 of pyroxene are often found among the scoriae of blast furnaces. 

 Serpentine is easily decomposed by acids, and may be em- 

 ployed in the preparation of sulphate of magnesia. 



