it ceases to rise the agitators are thrown out of gear, and the mixture
allowed some hours to cool and settle. The acid is then drawn off, and the
nitro-benzene is well washed with water, and sometimes distilled with wet
steam, to recover a little unconverted benzene and a trace of paraffin
(about .5 per cent. together). At many English works, 100 to 200 gallons,
or 800 to 1,760 lbs., are nitrated at a time, and toluene is often used
instead of benzene, especially if the nitro-benzene is for use as essence
of myrbane. The waste acids, specific gravity 1.6 to 1.7, contain a little
nitro-benzene in solution and some oxalic acid. They are concentrated in
cast-iron pots and used over again.

~Di-nitro Benzene~ is obtained by treating a charge of the hydrocarbon
benzene with double the quantity of mixed acids in two operations, or
rather in two stages, the second lot of acid being run in directly after
the first. The cooling water is then shut off, and the temperature allowed
to rise rapidly, or nitro-benzene already manufactured is taken and again
nitrated with acids. A large quantity of acid fumes come off, and some of
the nitro- and di-nitro-benzol produced comes off at the high temperature
which is attained, and a good condensing apparatus of stoneware must be
used to prevent loss. The product is separated from the acids, washed with
cold water and then with hot. It is slightly soluble in water, so that the
washing waters must be kept and used over again. Finally it is allowed to
settle, and run while still warm into iron trays, in which it solidifies
in masses 2 or 4 inches thick. It should not contain any nitro-benzol, nor
soil a piece of paper when laid on it, should be well crystallised, fairly
hard, and almost odourless. The chief product is meta-di-nitro-benzene,
melting point 89.8, but ortho-di-nitro-benzene, melting point 118°, and
para-di-nitro, melting point 172°, are also produced. The melting point of
the commercial product is between 85° to 87° C.