name to that fraction. Here we have only half a dozen
distinct chemical compounds to deal with, and if we confine our attention
to these for the present, we shall be enabled to gain a good general idea
of what chemistry has done with these raw materials. The products
separated during the processes which have to be resorted to for the
isolation of these raw materials have also their uses, which will be
pointed out incidentally.

Beginning with the first runnings and the light oil, from which the
hydrocarbons of the benzene series are separated, we have to make
ourselves acquainted with the treatment to which these fractions are
submitted by the tar-distiller. The light oil is first distilled from an
iron still, similar to a tar-still, and the first portions which come over
are added to the oily fraction brought over by the water of the first
runnings. The separation of the oil from the water in this last fraction
is a simple matter, because the hydrocarbons float as a distinct layer on
the water, and do not mix with it. We have at this stage, therefore, four
products to consider, viz. 1st, the oil from the first runnings; 2nd, the
first portions of the light oil; 3rd, the later portions of the light oil;
and 4th, the residue in the still. The first and second are mixed
together, and the third is washed alternately with alkali and acid to
remove acid and basic impurities, and can then be mixed with the first and
second products. The total product is then ready for the next operation.
The last portion of the light oil which remains in the still is useless as
a source of benzene hydrocarbons, and goes into the heavy oil of the later
tar fractions.

The process of purification is thus far one of fractional distillation
combined with chemical washing. In fact, all the processes of purification
to which these oils are submitted are essentially of the same character.
The principle of fractional distillation has already been explained
sufficiently for our present purpose. The process of washing a liquid may
appear mysterious to the uninitiated, but in principle it is extremely
simple. If we pour some water into a bottle, and then add some liquid
which does not mix with the water--say paraffin oil--the two liquids form
distinct layers, the one floating on the other. On shaking the bottle so
as to mix the contents, the two liquids form a homogeneous mixture at
first, but on standing for a short time separation into two layers aga