when, by
a further increase of heat, what are known as the _heavy oils_ finally
pass over, and when the last of these, _green grease_, as it is called,
distils over, pitch alone is left in the still. Pitch is used to a large
extent in the preparation of artificial asphalte, and also of a fuel
known as "briquettes."

The products thus obtained at the various stages of the process are
themselves subjected to further distillation, and by the exercise of
great care, requiring the most delicate and accurate treatment, a large
variety of oils is obtained, and these are retailed under many and
various fanciful names.

One of the most important and best known products of the fractional
distillation of crude coal-naphtha is that known as _benzene_, or
benzole, (C_{6}H_{6}). This, in its unrefined condition, is a light
spirit which distils over at a point somewhat below the boiling point of
water, but a delicate process of rectification is necessary to produce
the pure spirit. Other products of the same light oils are toluene and
xylene.

Benzene of a certain quality is of course a very familiar and useful
household supplement. It is sometimes known and sold as _benzene collas_,
and is used for removing grease from clothing, cleaning kid gloves, &c.
If pure it is in reality a most dangerous spirit, being very inflammable;
it is also extremely volatile, so much so that, if an uncorked bottle be
left in a warm room where there is a fire or other light near, its vapour
will probably ignite. Should the vapour become mixed with air before
ignition, it becomes a most dangerous explosive, and it will thus be seen
how necessary it is to handle the article in household use in a most
cautious manner. Being highly volatile, a considerable degree of cold is
experienced if a drop be placed on the hand and allowed to evaporate.

Benzene, which is only a compound of carbon and hydrogen, was first
discovered by Faraday in 1825; it is now obtained in large quantities
from coal-tar, not so much for use as benzene; is for its conversion, in
the first place, by the action of nitric acid, into _nitro-benzole,_ a
liquid having an odour like the oil of bitter almonds, and which is much
used by perfumers under the name of _essence de mirbane_; and, in the
second place, for the production from this nitro-benzole of the far-famed
_aniline_. After the distillation of benzene from the crude coal-naphtha
is completed, the chief impurities in the residue