170 FIRST YEAR COURSE IN GENERAL SCIENCE 



oxides of carbon and leaving the iron nearly pure. The 

 melted metal sinks to the bottom of the furnace and the 

 slag, being lighter than the metal, floats upon it. At 

 intervals each is drawn off through .openings in the side 

 of the furnace. The iron runs into trenches in sand, where 

 it cools in bars, and these bars are then broken into con- 

 venient lengths called "pigs." 



191. Gems. Gems are minerals valued for their beauty. 

 This quality of beauty depends on color, hardness, and bril- 

 liance. The color is often due to a slight trace of some metal 

 present as an impurity, like a stain, which is not a part of the 

 compound. 



"Precious stones," or jewels, are cut from gem stones, 

 which are usually crystals. A jewel is generally very much 

 smaller than the crystal from which it is cut, because only a 

 part of the crystal has the clear, uniform color desired and 

 much has to be cut away. The chips are used for jewels in 

 watches and for ornamental settings of other stones. As the 

 diamond is the hardest gem, diamond chips are often bedded 

 in a metal tool for cutting other gems or glass. 



192. Quartz. The most abundant minerals are quartz, 

 feldspar, mica, and calcite. Quartz, also called silica, is 

 composed of the two elements, silicon and oxygen. It occurs 

 in different forms and colors, and so has many names. Rock 

 crystal is a kind of quartz that is colorless and transparent; 

 amethyst is a crystal of purple color; agate is of various 

 colors, sometimes in bands; and flint is dark and horny 

 looking. Rock crystal and amethyst are in the crystalline 

 form ; the others are not. All have the same degree of hard- 

 ness, insolubility in water and acids, and infusibility in fire. 

 Quartz is therefore a very durable mineral. Quartz melts 

 if mixed with soda or potash and heated to a high tem- 

 perature, and the product is common glass. Finer grades 

 of glass and colored glass are made by adding other mineral 

 compounds. 



