CHAPTER XIX. 



CALCIUM, BARIUM, STRONTIUM, MAGNESIUM AND ZINC. 



CALCIUM (Ca) is a very abundant element, as calcium carbon- 

 ate it forms the limestones, the marbles and the chalk of the 

 earth. Calc spar, the marbles and many limestones are crystal- 

 line forms of this mineral. It occurs in bones, corals and the 

 shells of mollusks are nearly pure calcium carbonate. 



Calcium oxide called lime or quick lime is formed by heating 

 the carbonate which drives off the carbon dioxide, leaving the 

 oxide as a white infusible solid, formula CaO. The oxide, com- 

 bines vigorously with water, forming the calcium hydroxide 

 Ca0 2 H 2 . In this process a high degree of heat is evolved, often 

 enough to ignite wood, the oxide swelling up and crumbling to a 

 white powder, called slacked lime, which is somewhat soluble in 

 water forming what is called lime water. If exposed to the air 

 the oxide taking water from the air slowly changes to the hydrox- 

 ide. The hydroxide is the most important of the bases, and is 

 used in almost every art or manufacture which involves chemical 

 changes. It is used in making mortar, glass and soap. 



Calcium sulphate (CaS0 4 -|-H 2 0) occurs in nature as the trans- 

 parent selenite and the opaque gypsum, which is often beautifully 

 colored, sometimes it is pure white when it is called alabaster. 

 When heated to a temperature of 350 to 400 F. the water of the 

 gypsum is driven off and it becomes, what is called, plaster of 

 Paris. When this powder is mixed with water a combination 

 takes place forming a solid nearly as compact and strong as the 

 original gypsum. In the act of setting the plaster expands 

 slightly which makes it especially valuable for taking casts of 

 objects. It is much used in the reproduction of statues and other 

 sculptural forms. The walls and ornamentation of the Colum- 

 bian Fair buildings were made of light wood covered with 



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