CHAPTER XXI 



CARBON AND SOME OF ITS COMPOUNDS 



Forms Of Carbon. Carbon is an element which is very widely 

 distributed in nature, being present in all living matter, and in 

 most products resulting from vital activity. 



EXPT. 290. Heat a series of organic substances, such as 

 meat, wood, potato, egg, &c., in a crucible, and notice in all 

 cases the production of a black residue, consisting largely of 

 carbon. Heat more strongly, and observe that it burns away, 

 leaving an almost colourless ash. 



Carbon occurs also in many rock masses, being a constituent of 

 all the minerals known as carbonates. Combined with oxygen as 

 carbon dioxide, it occurs in the atmosphere and certain natural 

 gases, or dissolved in spring waters. 



In the pure state it exists in various allotropic forms. Of 

 these the purest and the most valuable is the Diamond. This 

 form of carbon is crystalline and very hard, being capable of 

 scratching all other minerals. Its refractive index (p. 171) is 

 very high, and on this depends its brilliancy as a gem. Diamond 

 is proved to consist of carbon by burning it, when only carbon 

 dioxide results. 



Blacklead or graphite is another form of almost pure carbon, 

 with properties totally different from those of the diamond. It 

 is opaque and black, and so soft that it will mark paper. It is 

 really a crystalline form of carbon, although good crystals are 

 not very common. It occurs naturally in mines, chiefly in Cali- 

 fornia, and was formerly largely obtained from Cumberland. 

 Besides its use for lead pencils, it is also used as a lubricant. 



Other forms of more or less pure carbon in an uncrystallised 



