378 



THE GEOLOGIST, 



crystalliue, but contain no magnesia. Delanone has pointed out examples of a 

 similar error in the crystalline limestones of the calamine mines in Belgium, 

 where in cases of supposed dolomitizatiou by contact with igneous rocks, he 

 found no increase in the proportion of magnesia. 



These facts show that dolomities have been formed under conditions where 

 tlie theory of the intervention of volcanic and metaniorphic agencies is inad- 

 missible, and we are to conclude that they have been deposited as magnesian 

 sediments in seas, or sometunes in lacustrine basins, from waters which often 

 permitted the development of animal life. The conditions required for the 

 separation of carbonate of magnesia from the sea or other waters, therefore, 

 naturally claim oui- attention as a fii'st step towards the solution of the problem 

 before us. Mr. Hunt has akeady shown in a previous report, that the precipi- 

 tate produced by carbonate of soda in water containing soluble salts of lime 

 and magnesia consists in great part of carbonate of lime, the magnesium 

 salts being decomposed only after the lime has been removed ; and some ex- 

 periments since made with carbonated waters serve further to illustrate this 

 geologicaUy important fact. 



If to an artificial sea-water, containing, besides common salt, chlorids of cal- 

 cium and magnesium in the proportion of one equivalent of each, we add a so- 

 lution of bi-carbonate of soda in water saturated with carbonic acid, a gelatinous 

 precipitate separates, which immediately becomes crystalline. This precipitate 

 being separated after a few houi-s, and submitted to analysis, gave three succes- 

 sive precipitations from the same liquid of 2"20, 2'00, and 1"23 per cent, of car- 

 bonate of magnesia, the remainder being carbonate of lime ; the proportion of 

 magnesia thus diminishing as the magnesian salt became predominant in the so- 

 lution, which now gave no further precipitate with bi-carbonate of soda, but 

 deposited by evaporation to dryness, a granular residue of hydrated carbonate 

 of magnesia with a little carbonate of lime. 



Bineau has shown that if we evaporate solutions containing bi-carbonates of 

 lime and magnesia in presence of sulphate or muriate of Hme either at the or- 

 dinary temperature or by artificial heat, the carbonate of lime is deposited 

 with but a trace of magnesia. Erom this he concludes that the carbonates 

 of magnesia exhibit, with all the soluble salts of lime, the sam.e reactions of 

 incompatibility as the corresponding carbonate of potash and soda (Ann. de 

 Chhn. et de Phys., vol. li., p. 302). 



Another cause wliich prevents the precipitation of carbonate of magnesia 

 with the carbonate of lime, even when other salts of lime no longer exist in the 

 solution, is found in the great solubility of bi-carbonate of magnesia as com- 

 pared wdth the bi-carbonate of lime. According to Bischoif, carbonate of Ume 

 requiries for its solution about 1,000 parts of water saturated with carbonic 

 acid ; and Mr. Hunt states that he has found it quite easy to obtain solutions 

 containing lO'O grams of magnesia, equal to 21 "0 grams of carbonate of mag- 

 nesia to a litre of water, or 2-1 per cent. Bineau found tliat by the aid of a 

 current of carbonic acid prolonged for several days, a solution might be obtained 

 containing 11*2 grains of magnesia, combined with nearly two equivalents of 

 carbonic acid, in a litre of water. Such solution by spontaneous evaporation in 

 the open air loses carbonic acid and deposits carbonate of magnesia, finally re- 

 taining only 0"10S grains of magnesia in a litre, Avith carbonic acid sufiicient to 

 form a sesqui-carbonate. 



Wlien recently precipitated hydrated carbonate of magnesia is added to a 

 solution of bi-carbonate of lime, it immediately dissolves, but the transparent 

 solution soon after becomes troubled from the precipitation of carbonate of 

 lime. This reaction is precisely analogous to that produced by carbonate of 

 soda, which, with bi-carbonate of lime, gives a precipitate of neutral carbonate. 

 The results of a variety of experiments, undertaken in the hope of producing a 



