1865.] CHEMISTRY OF NATURAL WATERS. 13 



magnesia. This reaction I have been unable to verify. A solu- 

 tion of gypsum in distilled water was made to percolate slowly 

 through a column of several inches of finely powdered dolomite, 

 and after ten nitrations, occupying as many days, no perceptible 

 amount of sulphate of magnesia had been formed. Solutions of 

 gypsum were then digested for many months with pulverized 

 dolomite, and also with crystalline carbonate of magnesia, but with 

 similar negative results ; nor did the substitution of a solution of 

 chlorid of calcium lead to the formation of any soluble magnesian 

 salt. Solutions of gypsum were then impregnated with carbonic 

 acid, and allowed to remain in contact with pulverized dolomite 

 and with magnesite, as before, during six months of the warm 

 season, when only inappreciable traces of magnesia were taken 

 into solution. These experiments show that no decomposition of 

 dissolved gypsum is effected by native carbonate of magnesia, or 

 by the double carbonate of lime and magnesia, at ordinary tem- 

 perature. 



§ 21. I find however that hydrated carbonate of magnesia readily 

 and completely decomposes a solution of gypsum when agitated with 

 it, with formation of carbonate of lime and sulphate of magnesia ; 

 and the same result is produced with the native hydrate of mag- 

 nesia when mingled with a solution of gypsum in presence of car- 

 bonic acid. Now there may be dolomites which contain an admix- 

 ture of hydro-carbonate of magnesia, as there certainly are others 

 which like predazzite, are penetrated with hydrate of magnesia. 

 The reaction between solutions of gypsum and such magnesian 

 limestones, (with the intervention, in the case of predazzite, of 

 atmospheric carbonic acid,) would suffice to explain the results 

 obtained by Mitscherlich, and the appearance in certain cases of 

 sulphate of magnesia as an efflorescence on dolomites. In the 

 experiments above described, the nearly pure crystalline dolomites 

 from the Guelph and Niagara formations were made use of. 



§ 22. When sea-water is exposed to spontaneous evaporation, the 

 whole of the lime which it contains separates in the form of sulphate, 

 gypsum being insoluble in a concentrated brine, and subsequently 

 the greater portion of the chlorid of sodium crystallises out in a 

 nearly pure state. The mother-liquor of specific gravity 1.24, 

 having lost about four fifths of its chlorid of sodium, still contains a 

 arge proportion of sulphate of magnesia. If the evaporation is 

 continued at the ordinary temperature, till a density of 1.32 is 

 attained, about one half of the magnesian sulphate separates, mixed 



