REVIEWS. 



379 



double carbonate of lime and magnesia, have sliown that when the bi-carbonates 

 of lime and magnesia are dissolved in pm-e water, in solutions of sea-salt, of 

 chlorid of magnesium, or of carbonate of soda, and evaporated at the ordmary 

 temperature, or heated to 100 degrees Falu-enheit, the carbonate of Hme is de- 

 posited as in the previous experiments, carrying with it only traces of the mag- 

 nesian carbonate, which is afterwards separated by elevating the temperature 

 nearly to boiling point or by farther evaporation. 



The addition of chlorid of calcium suffices, even at ordinary temperatiu'cs to 

 decompose the magnesian bi-carbonate and to precipitate carbonate of lime ; 

 but when the solution of the two bi-carbonates is boiled, even iii the presence 

 of chlorid of calcium, a portion of the magnesia faUs down with the carbonate 

 of lime. In none of these conditions, however, do we obtain that double car- 

 bonate of lime and magnesia, insoluble in acetic acid, which forms the base of 

 the magnesian limestones ; nor have we iii them any evidence of the formation of 

 a true dolomite. 



Mr. Hunt has found in the coui'se of his experiments that the introduction 

 of a soluble sulphate modifies, in an unsuspected manner, the results already 

 described. Mitscherhch found gypsum to be incompatible at ordinary tempe- 

 ratures vdth. carbonate of magnesia, but it is no longer so in the presence of an 

 excess of carbonic acid ; in fact, gypsum may be crystallized from a solution of 

 bi-carbonate of magnesia. 



If to a solution of bi-carbonate of Hme, sulphate of magnesia is added, and 

 the liquid allowed to evaporate at any ordinary temperature to a small volume, 

 the whole of the lime is deposited in the form of crystalline u-ypsum. The same 

 result is obtained when bi-carbonate of luiie is added to a solution containing 

 sea-salt, chlorid of magnesium, and sulpliates. By evaporation at a temperatm-e 

 of from 90 to 100 degrees rahrenheit the gy]DSum is enth'ely deposited before 

 the separation of the sea-salt commences, while the bi-carbonate of magnesia 

 remains in solution, and is only separated by evaporation to complete dryness, 

 or by ebullition. This reaction may lielp to explain the frequent association of 

 gypsum and dolomite, as well as in the occurrence of both in fresh-water for- 

 mations ; but "it is evident," Mr. Hunt says in conclusion, "that \vith the 

 facts as yet before us we are not able to determine with certainty the manner 

 in which dolomites have been formed. 



" Bi-carbonate of magnesia may, however, be produced in two ways : — first, 

 by the action of bi-carbonate of lune upon waters containing both sidphates and 

 magnesian salts, gypsum being generated at the same time ; and secondly, by 

 the action of bi-carbonate of soda upon magnesian waters from which the lime 

 has previously been separated either as a carbonate by the previous action of 

 bi-carbonate of soda, or by evaporation in the form of sulphate, as takes place 

 during the concentration of sea-water. Erom these solutions beds of carbonate 

 of magnesia may readily be formed by evaporation in limited basms, precisely 

 as we conceive gypsmn and rock-salt to have been deposited ; and if we suppose 

 an admis.tiu'e of carbonate of lime deposited from alkahne waters or any other 

 source, we have all the elements of dolomite, although not chemically combined 

 as a double salt. M. St. Claire Devide in his beautiful researches on the 

 double carbonates, found that when a mixtui'e of basic carbonate of magnesia 

 with bi-carbonate of soda and water is exposed to a gentle heat, a slow combi- 

 nation ensues, and the mixture is transformed into a mass of small transparent 

 crystals, winch are an anhydrous double carbonate of soda and magnesia, 

 insoluble in water — in fact a soda-dolomite (Aun. de Chim. et Phys., vol. 

 xxxui., p. 89). 



" A sunilar reaction between the mmgled carbonates of lime and magnesia, 

 under conditions not yet understood, may probably result in then.' gradual trans- 

 formation into dolomite." 



