CHAPTER XVI. 



CARBON, SILICON AND BORON 



CARBON (C) is an essential constituent of all animal and vege- 

 table bodies. It is found in the air, in many kinds of rocks and 

 soils, and in many natural waters, but is especially the element of 

 organic bodies. Every living body from the microscopic animal- 

 cule up to the giant whale, from the tiniest vegetable to the lofty 

 red- wood contains carbon. It occurs in its purest form in the 

 crystalline transparent diamond, the hardest substance known. 

 Another form of nearly pure carbon is graphite, and another 

 still is common charcoal. Diamonds are found in South America, 

 South Africa, in the East Indies and in Australia, and to some 

 extent in other localities. They are usually found in alluvial de- 

 posits as water-worn pebbles whose crystalline form and trans- 

 parency have been obscured by abrasions. It is the most beauti- 

 ful and highly prized of all the gems. It is a powerful refractor of 

 light, and is sometimes employed in making the lenses of micro- 

 scopes. Small, dark stones are much used for cutting glass, 

 drilling gems and in making the cutting edges of core drills. If 

 heated to a very high temperature in oxygen gas it burns up, the 

 sole product of the combustion being carbon dioxide. 



Graphite, another form of carbon, occurs in great quantities 

 among the older rocks of Canada and other localities, and is often 

 associated with iron ores. It sometimes occurs in six-sided plate- 

 like crystals, while the diamond usually has the form of an octa- 

 hedron. Graphite has a grayish black color with a metallic 

 luster ; has an oily feel, and is often used in place of oil to prevent 

 friction in machinery. Graphite resists a very high degree of 

 heat, and is used in manufacturing crucibles for melting steel and 

 other refractory substances. When subjected to a high degree of 

 heat in oxygen it burns, evolving intense heat nnd light, but the 



(127) 



