and the formation of Gypsum and Magnesian Rocks. 185 
change as we have seen in the experiments of de Senarmont 
(§ 83) takes place at from 155° to 175° C. The hydrated carbon- 
ates of magnesia formed at low temperatures and readily soluble 
in dilute acids, are in like manner, when heated under pressure, 
to prevent the loss of carbonic acid, converted into magnesite; 
if under these conditions carbonate of lime be present the two 
combine to form a double salt, possessing the chemical characters 
of dolomite.* 
88. In his researches on the double carbonates, H. Deville has 
described an anhydrous crystalline salt composed of one equiva- 
lent each of the carbonates of magnesia and soda. This double 
carbonate is insoluble in cold water, but readily dissolves in 
acetic acid. When itis heated with a solution of chlorid of mag- 
nesium in sealed tubes to 200° C. chlorid of sodium and spar- 
4 ingly soluble magnesite are obtained. When warmed with a 
_ solution of chlorid of calcium this double carbonate is decom- 
and soda were added to two ie of chlorid of calcium dissolved 
»f carbonates, of which the carbonate of lime 
The undissolved portion esced. 
from a consideration of the den 
n to. a com 
Spars, that supposing them to possess 
resent calcite by 15(CoM p00) while dolomi 
Magnesite and carbonate of zine (smit) 
€al 
