BAEIUM, STRONTIUM, AND CALCIUM. 227 



315. Carbonate of Calcium, CaCO 3 . This salt presents it- 

 self in various forms chalk, common limestone, and the 

 beautiful granular marble. The mineral calcite, which 

 sometimes appears in large, magnificent crystals of various 

 colors, is one of the forms of this salt. The variety of 

 form which this salt presents is analogous to the variety 

 that we so commonly see in sugar, which is perfectly crys- 

 talline in rock -candy, like calcite, imperfectly so in the 

 granular loaf-sugar, like marble, and without any trace of 

 crystallization when pulverized finely, like common chalk. 

 Carbonate of lime is very abundant, and is in fact one of 

 the chief constituents of our earth. There are hills and 

 ridges of mountains built up with limestone. In a pul- 

 verized state it exists extensively in the soil, in some dis- 

 tricts being very prominent, making what is called a cal- 

 careous soil. Oyster- shells, and the shells of shell-fish 

 generally, are composed almost entirely of carbonate of 

 lime. So are the shells or frame-work of many very 

 small animals, some of them exceedingly minute, and yet, 

 by their numbers, occupying much space in the earth. 

 The skeletons of the coral animals, of which so much of 

 some portions of the earth has been built up, are made 

 chiefly of this salt. 



316. Depositions of Carbonate of Calcium. If you breathe 

 into lime-water there will be precipitated carbonate of cal- 

 cium, or chalk, as you learned in 125. If, after this pre- 

 cipitate is formed, however, you continue to breathe into 

 the lime-water, some of the precipitate will disappear, a part 

 of it being dissolved again. How is this, when carbonate 

 of calcium is insoluble in water ? It is because you have 

 now something more than water in the vessel ; it is water 

 considerably charged with carbonic anhydride. Now, 

 while pure, simple water can not dissolve carbonate of 

 calcium, water charged with this gas can do it. Hence the 



