One of the most noteworthy
substances produced by the polymerisation of acetylene is benzene, the
formula of which is C_6H_6, and this is formed in the manner indicated by
the equation--
(4) 3C_2H_2 = C_6H_6.
Now benzene also contains 92.3 per cent. of carbon and 7.7 per cent. by
weight of hydrogen in its composition, but its molecule contains 6 atoms
of each element. When the chemical formula representing a compound body
indicates a substance which is, or can be obtained as, a gas or vapour,
it convoys another idea over and above those mentioned on a previous
page. The formula "C_2H_2," for example, means 1 molecule, or 26 parts by
weight of acetylene, just as "H_2" means 1 molecule, or 2 parts by weight
of hydrogen; but both formulae also mean equal parts by volume of the
respective substances, and since H_2 must mean 2 volumes, being twice H,
which is manifestly 1, C_2H_2 must mean 2 volumes of acetylene as well.
Thus equation (4) states that 6 volumes of acetylene, or 3 x 26 parts by
weight, unite to form 2 volumes of benzene, or 78 parts by weight. If
these hydrocarbons are burnt in air, both are indifferently converted
into carbon dioxide (carbonic acid gas) and water vapour; and, neglecting
for the sake of simplicity the nitrogen of the atmosphere, the processes
may be shown thus:
(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.
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