| Gas Carbon or Retort Carbon is formed as a lining on the upper parts of the retorts in which coal is heated in the manufacture of coal-gas. It owes its origin to the thermal dissociation of the hydrocarbon vapours evolved from the coal; and the process may be copied by passing ethylene (C2H4) through a red-hot porcelain tube, when gas carbon is formed. It contains little or no hydrogen and about 3 per cent, of ash; it is therefore a moderately pure form of amorphous carbon. Gas carbon is grey in colour and very hard; it approaches graphite in its density, which is over 2.0, and it is a good conductor of heat and electricity. On this account it is used, when ground together with charcoal and graphite, for making electric-light carbons and the carbon plates or rods of Bunsen batteries. |