1859.] HUNT FORMATION OF GTPSUMS AND DOLOMITES. 153 



another occasion the discussion of the theories which have been pro- 

 posed to account for the origin and association of these substances, 

 I propose to mention some recent observations of mine which serve 

 to throw light upon the question. 



When a solution of bicarbonate of hme in carbonic-acid water is 

 mingled with one of sulphate of magnesia, there are formed sulphate 

 of lime and bicarbonate of magnesia ; so that in presence of an ex- 

 cess of bicarbonate of lime there is obtained a saturated solution of 

 gypsum, from which this salt may be thrown down by the addition 

 of alcohol. 



In the same way I have found that a solution of sulphate of soda 

 is decomposed by bicarbonate of lime, whose solubility in water is 

 much augmented by the presence of the alkaline sulphate. From 

 such a solution the addition of alcohol precipitates gypsum, leaving 

 bicarbonate of soda in solution. 



"When a mixed solution of sulphate of magnesia and bicarbonate 

 of lime is evaporated at a gentle heat (from 80° to 120° F.), the 

 gypsum, being much less soluble than the bicarbonate of magnesia, 

 is first deposited in a crystalline form, and at a later period the mag- 

 nesia separates as a hydrous carbonate. The presence of sea-salt 

 does not prevent this reaction, nor the precipitation of gypsum by 

 evaporation ; so that by continued additions of bicarbonate of lime 

 to a basin of sea-water, its sulphate of magnesia might at length, 

 under favourable conditions, be entirely converted into gypsum. 



The hydrated carbonate of magnesia deposited from solutions of 

 the bicarbonate is very soluble in dilute acids ; but when heated to 

 300°-400° F. under pressure to prevent the escape of carbonic acid, 

 it is converted into a crystalline, anhydrous, and difficultly soluble 

 carbonate, already obtained by De Senarmont, and apparently iden- 

 tical with magnesite. When the hydrous carbonate is thus heated 

 in presence of carbonate of lime, the two combine to form a double 

 carbonate, which, by taking advantage of its sparing solubility in 

 cold dilute acetic acid, maybe separated from any admixture of free 

 carbonate of lime. This double carbonate of lime and magnesia is 

 dolomite. As prepared under various conditions, I found it to contain 

 from 35 to 45 per cent, of carbonate of lime, the excess of magnesia 

 being due apparently to a variable admixture of magnesite. Its forma- 

 tion by the direct union of the carbonates of lime and magnesia takes 

 place alike in the presence of solutions of earthy chlorides and alkaline 

 carbonates. In Von Morlot's experiment, however, which consists 

 in heating a mixture of two equivalents of carbonate of lime and 

 one of sulphate of magnesia in sealed tubefl to 200°C, the sulphate 

 of lime whieh Lb formed seems to prevent the union of the two car- 

 bonates, and the product I have found to be chiefly magnesite mixed 

 with carbonate of time. In Ifarignac's experiment, where chloride 

 of magnesium is substituted for the sulphate, the newly limned 

 carbonate of magnesia unites with the carbonate of lime, and a 

 porl ion of dolomite is formed. 



Bui the solutions of bicarbonate of magnesia whieh give rist 

 deposits of hydrous carhonate of magnesia, that, according ai they 



