90 CHEMISTRY OF DOLOMITES AND GYPSUMS. [VIII. 



as it appears to be at ordinary temperatures, the filling up by it 

 of such cavities both in magnesian and in pure limestones, not 

 less than its deposition in veins and druses, indicates that dolo- 

 mite is under certain conditions soluble in water. 



Conclusions. 



1. The action of solutions of bicarbonate of soda upon sea- 

 water separates in the first place the whole of the lime in the 

 form of carbonate, and then gives rise to a solution of bicar- 

 bonate of magnesia, which by evaporation deposits hydrous 

 magnesian carbonate. 



2. The addition of solutions of bicarbonate of lime to sul- 

 phate of soda or sulphate of magnesia gives rise to bicarbonates 

 of these bases, together with sulphate of lime, which latter may 

 be thrown down by alcohol. By the evaporation of a solution 

 containing bicarbonate of magnesia and sulphate of lime, either 

 with or wifhout sea-salt, gypsum and hydrous carbonate of mag- 

 nesia are successively deposited. 



3. When the hydrous carbonate of magnesia is heated alone 

 under pressure, it is converted into magnesite ; but if carbonate 

 of lime be present, a double salt is formed, which is dolomite. 



4. Solutions of bicarbonate of magnesia decompose chloride 

 of calcium, and, when deprived of their excess of carbonic acid 

 by evaporation, even solutions of gypsum, with separation of 

 carbonate of lime. 



5. Dolomites, magnesites, and magnesian marls have lia<l 

 their origin in sediments of magnesian carbonate formed l>y 

 the evaporation of solutions of bicarbonate of magnesia. These 

 solutions have been produced either by the action of bicarbon- 

 ate of lime upon solutions of sulphate of magnesia, in which 

 case gypsum is a subsidiary product, or by the decomposition 

 of solutions of sulphate or chloride of magnesium by the waters 

 of rivers or springs containing bicarbonate of soda. The sub- 

 sequent action of heat upon such magnesian sediments, either 

 alone or mingled with carbonate of lime, has clum^-d them 

 into magnesite or dolomite. 



