1865.] CHEMISTRY OP NATURAL WATERS. 279 



and bicarbonate of magnesia co-exist. When such a solution is 

 submitted to evaporation at ordinary temperatures, provided there 

 is present a sufficient amount of chlorid of calcium, carbonate of 

 lime alone is deposited, and chlorid of magnesium remains in 

 solution. 



In case the chlorid of calcium is insufficient, the lime is still first 

 deposited as carbonate, and the more soluble magnesian carbonate 

 is precipitated by further evaporation. When however such a 

 water is boiled, a reverse process takes place ; the carbonate of lime 

 slowly decomposes the magnesian chlorid. and carbonate of mag 

 nesia is deposited, while chlorid of calcium remains in solution. 

 Hence if the amount of chlorid of magnesium be great enough, and 

 the ebullition sufficiently prolonged, the precipitate will at length 

 contain only carbonate of magnesia ; while an equivalent of chlorid 

 of calcium, now found in the solution, represents the carbonate of 

 lime which the analysis of the precipitate at an earlier stage of the 

 ebullition would have furnished. 



As an example of this may be cited the analysis of the water of 

 Ste. Genevieve ( §42, No. 8), where the precipitate after a few 

 minutes boiling contained carbonates of lime and magnesia in the 

 proportion 12 : 750. When however another portion was boiled 

 down to one sixth, the precipitate was found to be pure carbonate 

 of magnesia. Again, the Plantagenet water gives, by ebul- 

 lition, the results set forth in § 42, No. 1 ; showing chiefly carbonate 

 of magnesia, together with a portion of chlorid of calcium. When 

 however this water is left to spontaneous evaporation, the whole of 

 the lime separates as carbonate, and the liquid remains for a time 

 charged with carbonate of magnesia, probably as sesqui-carbonate- 

 This solution is however after a time spontaneously decomposed 

 even in closed vessels, with deposition of a portion of crystalline 

 hydrated carbonate of magnesia ; another portion remains in solu- 

 tion, together with chlorid of magnesium, but is precipitated by 

 ebullition. (Silliman's Journal [2], xxvii. 173.) 



§ 55. Bicarbonate of magnesia and chlorid of calcium, when 

 brought together in solution, undergo mutual decomposition with 

 separation of carbonate of lime if the solutions are not too dilute. 

 At the ordinary temperature and pressure, water saturated 

 with carbonic acid will not hold in more than about one gram 

 of carbonate of lime to the litre (1 : 1000) ; equal to only 0.88 grams 

 of carbonate of magnesia. {The solubility of carbonate of lime in 

 pure water is well known to be much less, and is, according to 



