433 



SCIENTIFIC RECREATIONS. 



is the anhydrous sulphate of calcium. The hydrated sulphate is called gypsum. 

 One variety of the hydrated sulphate is selenite, another is known as alabaster. 

 Apatite, or asparagus stone, and pharmacolite are in this group. 



Carbonate of lime, not content with one system of crystals, makes its 

 appearance in two. It is therefore divided into two minerals namely, calca- 

 reous spar and arragonite. In the former it possesses various forms, as will 

 be observed in the accompanying diagrams. It is a very important mineral, 

 as will be readily acknowledged ; it enters largely into the composition of 

 all shells and bones. The minute shells, deposited by millions at the bottom 

 ' of the sea, have combined to raise our chalk cliffs. Carbonate of lime is a 

 constituent of water> as the deposits at the bottoms of kettles, upon the sides 

 and bottoms of water-bottles, and the stalactites all testify. A little good 



Primary rhombohedron (r). 



Rhombohedron (r). 



Six-sided prism 

 regular. 



CLluse ihcmlchcdion. 



Primitive rhombo- 

 hedron (r), with 

 acute form (r\ 



Equal six-sided prism (a), 

 ending in regular (r). 



Obtuse rhomtrahe- 

 dron (yi), ending 

 in prism (g). 



Fig. 461. CRYSTALS OF CARBONATE OF LIME. 



vinegar will quickly dissolve this deposit. Calc-spar is crystallized, and the 

 Iceland spar is celebrated. Marble, which is another form of carbonate of 

 lime, is white, hard, and granular. It is sometimes varied, but the pure 

 white is the most valuable. Chalk, we know well, is soft, and is useful for 

 writing. We have also aphrite, schiefcr spar, compact limestone in various 

 forms, and finally, arragonite, called from the place of its nativity, Arragon, 

 a colourless and somewhat transparent vitreous crystal. 



BARYTES. The sulphate of baryta is known as 

 heavy spar ; the crystals are of tabular forms, but 

 numerous modifications exist. One of the forms is 

 represented in the margin. 



Fig. 462. Tabular form of 

 heavy spar. 



STRONTIUM is the metallic basis of strontia. 



