I pointed to benzene
or benzol in the table as a hydrocarbon, C_{6}H_{6}, which forms a
principal colour-producing constituent of coal-tar. If you desire to
produce chemical appetite in benzene, you must rob it of some of its
hydrogen. Thus C_{6}H_{5} is a group that would exist only for a moment,
since it has a great appetite for H, and we may say this appetite would
go the length of at once absorbing either one atom of H (hydrogen) or of
some similar substance or group having a similar appetite. Suppose, now,
I place some benzene, C_{6}H_{6}, in a flask, and add some nitric acid,
which, as we said, is NO_{2}OH. On warming the mixture we may say a
tendency springs up in that OH of the nitric acid to effect union with
an H of the C_{6}H_{6} (benzene) to form HOH (water), when an appetite
is at once left to the remainder, C_{6}H_{5}--on the one hand, and the
NO_{2}--on the other, satisfied by immediate union of these residues to
form a substance C_{6}H_{6}NO_{2}, nitro-benzene or "essence of
mirbane," smelling like bitter almonds.
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