/|t)6 CALCIUM. 



heated gently on a plate of metal, it becomes very luminous in 

 the dark, or phosphoresces. Fluoride of calcium is insoluble in 

 water, and is obtained in a granular condition, when hydrofluoric 

 acid is neutralised by freshly precipitated carbonate of lime. 

 But when a neutral salt of lime is mixed with a soluble fluoride, 

 the fluoride of calcium appears as a translucent gelatinous mass. 

 This fluoride, whether artificial or natural, is not decomposed by 

 sulphuric acid at a low temperature, but imbibes that acid, and 

 forms a thick ropy liquid. At 104 (40 cent.) this mixture be- 

 gins to decompose and emits hydrofluoric acid. 



SALTS OF LIME. 



Carbonate of time ; CaCO 2 ; 632.5 or 50.68. This is one of 

 the most abundantly diffused salts in nature, forming the basis 

 of limestones, marbles, marles, coral-reefs, shells, &c. It is 

 always anhydrous, and occurs in two incompatible crystalline 

 forms, the rhomboidal crystal of calc-spar, which with its nume- 

 rous modifications is much the most abundant, and the six- 

 sided prism of arragonite, isomorphous with carbonate of stron- 

 tian, which may be readily recognized by falling to powder 

 when heated. The grains of this powder have the form of calc- 

 spar. The density of carbonate of lime in these two forms is 

 sensibly different, that of calc-spar being 2.719, and of arrago- 

 nite 2.949 (G. Rose). Carbonate of lime consists in 100 parts, 

 of 56.29 lime and 43.7 1 carbonic acid. 



Carbonate of lime may also be obtained in the state of a hy- 

 drate, by heating together 1 part of hydrate of lime, 3 of sugar, 

 and 6 of water, filtering the solution and leaving it to cool in a 

 shallow vessel. In twenty-four hours crystals appear upon 

 the surface of the liquid, and in fifteen days the whole lime is 

 generally converted into hydrated carbonate, in the form of 

 sharp transparent rhombs. The carbonic acid is absorbed from 

 the atmosphere. These crystals contain 5 eq. of water. By 

 boiling them in anhydrous alcohol, a second definite hydrate is 

 obtained containing 3 eq. of water, as ascertained by Pelouze. 

 These correspond in composition with two crystalline hydrates 

 of carbonate of magnesia. 



Carbonate of lime is considered an insoluble salt, although 

 according to Bucholz it dissolves in 1 6 or 24 thousand times its 

 weight of pure water. But it is soluble in water containing 

 carbonic acid, and is generally present in the water of wells, and 



