56 DE. THOMAS STEERY HUNT ON THE 



slight excess of carbonate of soda, was found to contain more than two per cent, of the latter, 

 but it was not certain whether this did not proceed from an admixture of the hydrous 

 double carbonate of lime and soda, gaylussite. The new composition itself was described 

 as having the composition of a gayhissite, in which magnesium replaces sodium. The 

 production of crystals of true gaylussite, as observed by Fritzsche, by the slow crystalliza- 

 tion of the gelatinous precipitate got when a strong solution of carbonate of soda in excess 

 is mingled with one of calcium-chlorid, is another remarkable example of the phenomenon 

 under consideration. 



Fritzsche moreover observed that it is not necessary that the lime-carbonate should be 

 in its gelatinous form in order to produce this compound, since the previously j^recipitated 

 carbonate when digested with a solution of carbonate of soda, slowly combines with it to 

 form the crystalline hydrous double salt. More remarkable still is the observation of H. 

 Ste. Claire Deville, which I haA^e repeatedly verified, that a paste of magnesia alba and 

 bicarbonate of soda, with water, is slowly changed, at a temperature of from 60° to *70° C, 

 into a transpai'ent crystalline anhydrous double carbonate of lime and soda, hexagonal in 

 form, and called by its discoverer a soda-dolomite."' 



§ 113. In this connection, it should be said that we have here an explanation of the 

 formation of the double carbonate of lime and magnesia which constitutes ordinary dolo- 

 mite. The origin of this mineral species, which so often constitutes rock-masses, is still 

 generally misunderstood. The baseless notion of its production by a metasomatosis or 

 partial replacement of the lime in ordinary limestone, imagined by the older geologists, is 

 still repeated, and holds its place in the literature of the science, despite the facts of geog- 

 nosy and of chemistry. I have long since shown, by multiplied examples, that the ordinary 

 mode of the occurrence of dolomite in nature is not in accordance with this hypothesis 

 of its origin, since beds of dolomite, or more or less magnesian limestone, are found 

 alternating, sometimes in thin and repeated layers, with beds of non-magnesiau carbonate 

 of lime. Moreover, beds of crystalline dolomite, conglomerate in character, are found to 

 enclose pebbles and fragments of prire non-magnesian carbonate of lime. I have also ex- 

 plained at length the natural reactions by which precipitates consisting of a greater or less 

 proportion of hydrous carbonate of magnesia, mixed with carbonate of lime, must, in past 

 ages, have been laid down in the waters of lakes and inland seas, in some cases with, and 

 in others without, the simultaneous formation of sulphate of lime. 



It was, moreover, found that the reaction at an elevated temperature in presence of 

 water, between sulphate of magnesia and an excess of carbonate of lime, supposed by 

 Haidinger and A^on Morlot to explain the frequent association of gypsum and dolomite, 

 does not yield the double carbonate, since the carbonate of magnesia separates in an anhy- 

 drous form, and does not combine with the carbonate of lime. Finally, it was shown that 

 mixtures of hydrous carbonate of magnesia and carbonate of lime, when heated together in 

 presence of water, unite to form the anhydrous double carbonate which constitutes dolo- 

 mite. In my experiments, their combination, with the formation of dolomite, was effected 

 rapidly, at 120° C, but many considerations lead to the conclusion that its production in 



*'Hunt, Contributions to the History of Lime aud Magnesia Salts, part ii., 1866. Amer. Jour. Science, vol. 

 xlii., pp. 54-57. 



