CARBONATE OF LIME.
357
Single system of rings seen by looking through a slice of calcareous spar (cut perpendicularto the axis of the crystal) placed between two plates of tourmaline (cut parallel to
the axis of the crystal).
Fig. 54.
Fig. 54 is seen when the plane of the axis ofcular to the plane of polarization. Fig. 55 i
Fig. 55.
the calcareous spar is parallel or perpendi-s seen when the calcareous spar is turned 45°.
Granular limestone (of which white marble is the purest kind) is mas-sive, and consists of small grains or minute crystals, presenting a lamellarstructure and brilliant lustre, but intersecting each other in every direc-tion, and thereby giving a glimmering lustre to the mass.
Chalk is massive, opaque when pure white, and has an earthy fracture.It is usually soft to the touch, and adheres to the tongue.
Chahacteristics. —Carbonate of lime is recognized as a carbonateby the tests already mentioned for this class of salts (p. 191). As a cal-careous salt it is known by the characters before described (p. 343) forlime.
Composition. —Carbonate of lime has the following composition:—
Eq. Eq.Wt. PerCent. Marcet. Stromeyer. Ure.
Lime. 1 .... 28 ... . 56 ... . 56*1 .... 56*35 .... 56*4
Carbonic Acid. . . 1 .... 22 ... . 44 ... . 43*9 .... 43*65 .... 43*6
Carbonate of Lime 1 . . . . oO . . . . 100 .... 100*0 .... 100*00 .... 100*0
Purity. —Pure marble or chalk should be perfectly soluble, with effer-vescence, in hydrochloric acid, by which the absence of silica is shown.Ammonia should not cause any precipitate with this solution, by whichits freedom from alumina, oxide of iron, &c. may be inferred: nor shoulda solution of sulphate of lime throw down any thing by which theabsence of baryta and strontian is proved.
Physiological Effects. —The local effects of chalk are those of anabsorbent, antacid, and mild desiccant. When swallowed it neutralizesthe free acid of the gastric juice, and in this way alone must, by conti-nued use, injure the digestive functions. It causes constipation, an effectcommonly observed from the use of a few doses in diarrhoea. By theaction of the free acids (acetic and hydrochloric) of the alimentary canal,it is converted into two soluble calcareous salts (acetate of lime andchloride of calcium), which become absorbed. Hence the continueduse of carbonate of lime is attended with the constitutional effects of thecalcareous salts, and consequently the statements which have been madeas to the influence of chalk over the lymphatic vessels and glands, and