CHAPTER XXVII 

 CARBON AND SOME OF ITS COMPOUNDS. 



79. FORMS OF CARBON. 



i. Carbon is contained in organic substances. Heat a series of organic 

 substances, such as meat, wood, potato, egg, etc., 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. 



ii. Properties of carbon. Examine and write down the properties of 

 as many of the following forms of carbon as you can obtain : diamond, 

 blacklead, wood-charcoal, bone-black and soot. 



iii. Charcoal is porous. (a) Show that charcoal floats in cold water. 

 In boiling water charcoal sinks after a time, and then will not float again 

 unless thoroughly dried. This is because air is driven out of the charcoal 

 by the warmth of the water. 



(6) Fill a large test-tube with ammonia gas over mercury and show that 

 it can be absorbed by introducing a small lump of charcoal. This is a 

 striking proof of the porosity of carbon. 



Forms of carbon. Carbon is a substance which is very widely 

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

 products resulting from vital activity. 



Carbon occurs, combined with other substances, 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 dissolved in spring waters. 



Diamond. In the pure state carbon 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 materials. Its refractive index is very high, and 

 on this depends its brilliancy as a gem. Diamond is proved to consist of 

 carbon by burning it in air or oxygen, 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 



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