STATE GEOLOGIST. 35 
Calcium carbonate is dimorphous, crystallizing as calespar in the hex- 
agonal and as aragonite in the rhombic system. Calcium carbonate is 
nearly insoluble in water, 1,000 grams of water dissolving .0018 gram of 
the compound, but it is more soluble in water containing carbon dioxide ; 
I,000 grams of water charged with this gas will dissolve, at 0°C.,0.7 gram 
of calcium carbonate; by increasing the pressure 3 grams of the carbonate 
may be dissolved. ‘This calcium salt is readily decomposed by inorganic 
and even weak organic acids with the evolution of carbon dioxide. When 
exposed to higher temperatures the calcium carbonate is decomposed 
according to the reaction: 
CaCO,=—CaO-+CO,. 
On enclosing a piece of calcium carbonate in a tube and connecting 
the latter with a mercury manometer it is found that for every temper- 
ature the evolution of the carbonic acid gas remains stationary at a certain 
pressure, no more calcium carbonate being decomposed. This was shown 
experimentally by Le Chatelier,* who thus determined the dissociation 
tensions of calcium carbonate for a number of temperatures, which are 
shown in the accompanying table: 
Dissociation of Calcium Carbonate. 
Temperature. Pressure. Temperature. Pressure. 
547° C. 27mm. Mercury. 745° C. _ 289 mm. Mercury. | 
610 oe 46 66 (79 810 (13 678 a3 (79 
625 66 56 66 66 812 66 753 66 (73 
740 “ 205 cc 865.“ 13ep & © 
The reaction is a reversible one; that is, carbon dioxide is reabsorbed 
as soon as the tension of the gas approaches the dissociation tension at 
the given temperature and calcium carbonate is again. formed. This point 
is well illustrated in limekilns where it is observed that when the draft of 
the kiln is sluggish and the temperature low the calcium oxide, being sur- 
rounded by the carbon dioxide, reverts to calcium carbonate. 
The decomposition of calcium carbonate is accelerated by the presence 
of steam, so that the burning temperature is reduced greatly. Professor 
Herzfeld has made extensive experiments with the use of superheated 
steam and has proven that it affords an excellent means of producing a high 
grade porous calcium oxide. | 
*Compt. rend. 102 1283 (1886.) 
