506 SALTS OF MAGNESIA. 



Bicarbonate of potash and magnesia. This salt was formed 

 by Berzelius by mixing a solution of nitrate of magnesia or 

 chloride of magnesium, with a saturated solution of bicarbonate 

 of potash in excess, and allowing the liquor to rest. In the 

 course of a few days, the double salt is deposited in large regular 

 crystals. These crystals are insipid. They are insoluble in 

 pure water, but are slowly decomposed by it. The composi- 

 tion of this salt corresponds with 1 eq. of potash, 2 of magnesia, 

 4 of carbonic acid, and 9 of water. As a compound of 1 eq. 

 of bicarbonate of potash and 2 of carbonate of magnesia, it may 

 be represented thus : 



MgO, CO 2 , HO + 2HO 

 HO, CO 2 , (KG, CO 2 ) +2HO 

 MgO, CO 2 , HO + 2HO 



But a compound like this, of three salts of similar constitution, 

 is not easily reconciled with the ordinary laws of saline combi- 

 nation. This salt loses 8HO, at 212, or all its combined water., 

 except the single basic equivalent. A corresponding bicarbo- 

 nate of soda and magnesia also exists. 



Dolomite, a magnesian lime-stone, very extensively diffused 

 in nature, is a mixture or combination of the carbonates of lime 

 and magnesia, having the crystalline form of calc-spar. The 

 two salts are almost always in the proportion of single equiva- 

 lents. It is remarkable that when this rock is exposed to the 

 solvent action of water containing carbonic acid, the carbonate 

 of lime is dissolved exclusively, and a magnesian limestone 

 remains in the form of a porous and crystalline mass. It is not 

 unusual to find whole mountains of magnesian limestone thus 

 altered. 



Sulphate of magnesia Mg O, SO 3 , HO -(-6HO ; 759.6 + 787.5, 

 or 60.8fi-f63. This salt exists in many mineral springs, in 

 the waters of Epsom, of Seidlitz in Bohemia, &c., from which 

 it was first procured by evaporation. It is now generally ob- 

 tained from the bittern of sea water, which consists principally 

 of chloride of magnesium and sulphate of magnesia, and is con- 

 verted wholly into sulphate by the addition of sulphuric acid. 

 Or magnesia is precipitated from seawater confined in a tank, 

 by means of hydrate of lime, and the earth thus obtained, after- 

 wards neutralised by sulphuric acid. Magnesian limestone is 

 also had recourse to for magnesia. It is burnt and slaked with 

 water, to obtain it in a divided state, and then neutralised by 



