CARBONATE OF LIME.

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er simple mineral, and is by some supposed to constitute one eighthpart of the exterior crust of the globe. In fact, calcareous minerals,belonging to this species, are every day produced ; arising either fromthe disintegration of Carbonate of lime, which had previously existedin the mineral kingdom, or proceeding from the decomposition of cal-careous substances, once attached to animals. This mineral surpas-ses all others in the facilities, which it affords for the study of crys-tallography, by the frequent occurrence of its crystals, the diversityof their forms, the regularity of their structure, and the ease, withwhich they yield to mechanical division.

This species presents an unusual number of varieties, differing ex -ceedingly from each other in their external characters. Hence, inmany cases, it is necessary to depend chiefly on the chemical charac-ters. It exists not only in extensive, amorphous masses, and underalmost every imitative form, known in the mineral kingdom, but veryfrequently in crystals.

These crystals permit an easy separation of their component lam-in® ; and mechanical division obtains for the primitive form a rhomb(PI. Ill, fig. 12), that is, a solid, whose sides are rhombs, hav- ving their two acute angles 78° 28', and their two obtuse angles 101°32'. The faces of this rhomb are inclined to each other at angles of75° 31' and 104° 29'.* Its integrant particles have the same form,according to Haiiy; but Bournon says they are triedral pyramidswith inclined bases.—In some hexaedral prisms the primitive rhombis visible to the eye, its summits touching the terminal faces of theprism.

Carbonate of lime is harder than sulphate of lime, but may alwaysbe scratched by iron, and usually by fluate of lime. When pure andcrystallized, its specific gravity varies from 2.68 to 2.74, Whentransparent, it possesses the property of double refraction in a highdegree, which may be observed by looking through two parallel facesof a rhomboidal crystal or fragment at a black circle, drawn on whitepaper.

Phosphorescence is not essential to this species; but some varie-ties phosphoresce, either by projecting them in powder on burning-coals, or by friction, or indeed by both methods.

CChemical characters.J Carbonate of lime is soluble in nitricacid.; and, by the escape of the carbonic acid, more or less efferves-cence is produced; some varieties, however, effervesce very slowly.Before the blowpipe it decrepitates, and, if pure, is perfectly infusi -ble; but, by a strong heat, its carbonic acid is driven off - , and quick-

* According' tq Dr. Wollaston, the angle last mentiened is 105° 05'.