oftening point by re-melting it with
heavy coal-tar oils.

We now come to the treatment of the various fractions obtained from
the tar-stills. These operations are frequently not carried out at the
smaller tar-works, which sell their oils in the crude state to the
larger tar-distillers.

_Working up of the Light-Oil Fraction._--The greatest portion of the
light-oil fraction consists of aromatic hydrocarbons, about one-fifth
being naphthalene, four-fifths benzene and its homologues, in the
proportion of about 100 benzene, 30 toluene, 15 xylenes, 10
trimethylbenzenes, 1 tetramethylbenzene. Besides these the light-oil
contains 5-15% phenols, 1-3% bases, 0.1 sulphuretted compounds,
0.2-0.3% nitriles, &c. It is usually first submitted to a preliminary
distillation in directly fired stills, similar to the tar-stills, but
with a dephlegmating head. Here we obtain (1) first runnings (up to
O.89 spec. grav.), (2) heavy benzols (up to O.95), (3) carbolic oil
(up to 1.00). The residue remaining in the still (chiefly naphthalene)
goes to the middle-oil fraction.

The "first runnings" are now "washed" in various ways, of which we
shall describe one of the best. The oil is mixed with dilute caustic
soda solution, and the solution of phenols thus obtained is worked up
with that obtained from the next fractions. After this follows a
treatment with dilute sulphuric acid (spec. grav. 1.3), to extract the
pyridine bases, and lastly with concentrated sulphuric acid (1.84),
which removes some of the aliphatic hydrocarbons and "unsaturated"
compounds. After this the crude benzol is thoroughly washed with water
and dilute caustic soda solution, until its reaction is neutral. The
mixing of the basic, acid and aqueous washing-liquids with the oils is
performed by compressed air, or more suitably by mechanical stirrers,
arranged on a perpendicular, or better, a horizontal shaft. Precisely
the same treatment takes place with the next fraction, the "heavy
benzols," and the oils left behind after the washing operations now go
to the steam-stills. The heaviest hydrocarbons are sometimes twice
subjected to the operation of washing.

The washed crude benzols are now further fractionated by distillation
with steam. The _steam-stills_ are in nearly all details on the
principle of the "column apparatus" employed in the distillation of
alcoholic liquids, as represented in fig. 4