379 
double carbonate of liine and magnesia, have shown that wlien the bi-carbonates 
of lime and magnesia are dissolved in pure water, in solutions of sea-salt, of 
chlorid of magnesium, or of carbonate of soda, and evaporated at the ordinary 
temperature, or heated to 100 degrees Fahrenheit, the carbonate of lime is de- 
posited as in the previous experiments, carrying witli it only traces of the mag- 
nesiau carbonate, which is afterwards separated by elevating the temperature 
nearly to boiling point or by farther evaporation. 
Tlie aildition of chlorid of calcium sutfices, even at ordinary temperatures to 
decoin]iose tiic magnesian bi-carbonate and to precipitate carbonate of lime ; 
but when tlie solution of the two bi-carbonates is boiled, even in the presence 
of cldorid of calcium, a portion of the magnesia falls 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 w'e in them any evidence of the formation of 
a true dolomite. 
Mr. Hunt has found in the course of his experiments that the introduction 
of a soluble s\dphate modifies, in an unsuspected manner, the results already 
described, ilitscherlich found gypsum to be incompatible at ordinary tempe- 
ratures with carbonate of magnesia, but it is no longer so iu the presence of an 
excess of carbonic acid ; iu fact, gji^sum may be crystallized from a solution of 
bi-carbonate of magnesia. 
If to a solution of bi-carbonate of lime, sulphate of magnesia is added, and 
the liquid allowed to evajiorate at any ordinary temperature to a small volume, 
the whole of tlie lime is deposited in the form of crystaUiue gypsum. The same 
result is obtained wlien bi-carbonate of lime is added to a solutiou containing 
sea-salt, chlorid of magnesium, and suljihates. By evaporation at a temperature 
of from 90 to 100 degrees Tahreidicit the g^'psum is entirely deposited before 
the separation of the sea-salt commences, wliile the bi-carbonate of magnesia 
remains in solution, and is only separated by evaporation to complete dryness, 
or by ebullition. This reaction may help to explain the frequent association of 
gypsum and dolomite, as well as m the occurrence of both in fresh-water for- 
mations ; but " it is evident," Mr. Hunt says in conclusion, " that with 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 lime upon waters containing both sulphates and 
magnesiau 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 
daring the concentration of sea-water. Prom these solutions beds of carbonate 
of magnesia may readily be formed by evaporation iu Limited basins, precisely 
as we conceive gypsimi and rock-salt to have been deposited ; and if we suppose 
an admixture of carbonate of lime deposited from alkaline waters or any other 
source, w^e have aU the elements of dolomite, although not chemically combined 
as a double salt. M. St. Claire DeviUe in his beautiful researclies on the 
double carbonates, found that when a mixture of basic carbonate of magnesia 
with bi-carbonate of soda and water is exposed to a geutle heat, a slow combi- 
nation ensues, and the mixture is transformed into a mass of small transparent 
crystals, which are an anliydrous double carbonate of soda and magnesia, 
insoluble in water — in fact a soda-dolomite (Ami. de Cliim. et Phys., vol. 
xxxiii., p. 89). 
" A similar reaction betw-een the mingled carbonates of lime and magnesia, 
under conditions not yet understood, may probably result in their gradual trans- 
format ion into dolomite." 
