178 NON-METALS AND THEIR COMBINATIONS. 



14. CARBON. SILICON. BORON. 

 O = 12 (11.91). Si* = 28.3. B' = 10.9. 



Occurrence in nature. Carbon is a constituent of all organic 

 matter. In a pure state it is found crystallized as diamond and 

 graphite, amorphous in a more or less pure condition in the various 

 kinds of coal, charcoal, boneblack, lampblack, etc. As carbon 

 dioxide, carbon is found in the air; as carbonic acid, in water; as 

 carbonates (marble, limestone, etc.), in the solid portion of our earth. 



Properties. The three different allotropic modifications of carbon 

 differ widely from each other in their physical properties. 



Diamond is the purest form of carbon, in which it is crystallized in 

 regular octahedrons, cubes, or in some figure geometrically connected 

 with these. Diamond is the hardest substance known ; when heated 

 intensely in the presence of oxygen it burns, forming carbon dioxide. 



Graphite, plumbago, or black-lead, is carbon crystallized in short 

 six-sided prisms ; it is a somewhat rare, dark-gray mineral, having 

 an almost metallic lustre. It feels soft and greasy between the fingers 

 and leaves a black mark when drawn over a white surface. It is 

 used to make lead pencils, and also as a lubricator, in stove polish, 

 as an admixture with clay used for crucibles, etc. 



Amorphous carbon is always a black solid, but the hardness and 

 specific gravity of the different kinds of amorphous coal differ widely. 

 Amorphous carbon in the various kinds of coal is the chief agent for 

 generating heat by combustion. In the form of lamp-black it is used 

 in printer's ink ; in bone-black it serves for decolorizing sugar syrups 

 and other liquids. 



Neither form of carbon is soluble in any of the common solvents, 

 but it dissolves to some extent in melted iron. On cooling, under 

 ordinary conditions, most of the dissolved carbon separates in the 

 form of graphite ; when cooling takes place under high pressure 

 small diamond-like crystals may be obtained. By the intense heat 

 produced by electricity carbon becomes softened and in small quan- 

 tities also volatilized. 



Carbon is a quadrivalent element ; it has little affinity for metals, 

 though at high temperatures it combines with many, forming com- 

 pounds, termed carbides. It does not enter into combination with 

 oxygen at ordinary temperature, but at red heat it combines eagerly 

 with free or combined oxygen, serving in many cases as a deoxidizing 

 agent. Compounds of carbon with other non-metallic elements are 

 mostly formed by indirect processes. 



