SALTS OF MAGNESIA. 505 



nate of potash at the boiling point. The precipitate is diffused 

 in pure water, and a stream of carbonic acid sent through it, by 

 which the carbonate of magnesia is dissolved. On allowing 

 this solution to evaporate spontaneously the excess of carbonic 

 acid escapes, and carbonate of magnesia is deposited in small 

 hexagonal prisms with right summits. These crystals contain 

 3 eq. of water. They effloresce in dry air, and then lose 2 eq. of 

 water, according to my own observations. Carbonate of mag- 

 nesia has also been obtained in crystals with 5 eq. of water. 

 There are, consequently, three hydrates of this salt, of which 

 the formulse are, Mg O, CO 2 , HO ; 



MgO, CO 2 , HO + 2HO; 

 MgO, C0 2 , HO + 4HO. 



The fact that this salt dissolves in carbonic acid water is not 

 to be held as proof of the existence of a bicarbonate of magnesia. 

 Various insoluble salts, such as phosphate of lime and fluoride 

 of calcium dissolve in the same liquid, which appears to possess 

 a specific solvent power. In the analogous solution of carbo- 

 nate of lime in carbonic acid water, the proportion of the car- 

 bonate was found by Berthollet to have a variable and indefinite 

 relation to the acid. On theoretical grounds, supersalts of 

 magnesia, or the magnesian family of oxides, of the ordinary 

 constitution, are not to be expected, as they would be double 

 salts of water and another magnesian oxide. 



Magnesia alba, or the subcarbonate of magnesia of pharmacy, 

 is prepared by precipitating a boiling solution of sulphate of 

 magnesia or chloride of magnesium, by means of carbonate of 

 potash. Carbonate of soda is not so suitable as a precipitant 

 of magnesia, as a portion of it is apt to go down in combination 

 with the magnesian carbonate, but it may be used provided the 

 quantity applied be less than is required to decompose the 

 whole magnesian salt in solution. Magnesia alba, when well 

 washed with hot water is very white, light and bulky. A por- 

 tion of carbonic acid is lost, the magnesia not being in combi- 

 nation with a full equivalent of that acid. Berzelius found 

 magnesia alba to contain, in 100 parts, 35.77 carbonic acid, 

 44.75 magnesia, and 19.48 water, or to consist of 3 eq. of car- 

 bonic acid, 4 eq. of magnesia, and 4 eq. of water. He views it 

 as a combination of hydrate of magnesia with hydrated car- 

 bonate of magnesia, of which the formula is MgO, HO + 

 3(Mg O, CO 2 , HO). This compound is said to require 2493 

 parts of cold; and 0000 of hot water for solution. 



