in
takes place. Now if there was present in the paraffin oil some substance
soluble in the oil, but more soluble in water, such as alcohol, we should
by the operation described wash the alcohol out of the oil, and when the
liquids separated into layers after agitation the watery layer would
contain the alcohol. By drawing off the oil or the water the former would
then be obtained free from alcohol--it would have been "washed." This
operation is precisely what the manufacturer does on a large scale with
the coal-tar oils. These oils contain certain impurities of which some are
of an acid character and dissolve in alkalies, while others are basic and
dissolve in acid. The oil is therefore agitated in a suitable vessel
provided with mechanical stirring gear with an aqueous solution of caustic
soda, and after separation into layers the alkaline solution retaining the
acid impurities is drawn off. Then the oil may be washed with water in the
same way to remove the lingering traces of alkali, and then with
acid--sulphuric acid or oil of vitriol--which dissolves out basic
impurities and certain hydrocarbons not belonging to the benzene series
which it is desirable to get rid of. A final washing with water removes
any acid that may be retained by the oil.

The total product containing the benzene hydrocarbons is put through such
a series of washing operations as above described, and is then ready for
separation into its constituents by another and more perfect process of
fractional distillation. This final separation is effected in a piece of
apparatus somewhat complicated in structure, but simple in principle. It
is a development on a large scale of the apparatus used by Mansfield in
his early experiments. The details of construction are not essential to
the present treatment of the subject, but it will suffice to say that the
vapours of the boiling hydrocarbons ascend through upright columns, in
which the compounds of high boiling-point first condense and run back into
the still, while the lower boiling-point compounds do not condense in the
columns, but pass on into a separate condenser, where they liquefy and are
collected. But even with this rectification we do not get a perfect
separation--the hydrocarbons are not perfectly pure from a chemical point
of view, although they are pure enough for manufacturing purposes. Thus
the first fraction consists of benzene containing a small percentage of
toluene, then comes over a mixt