d temperature, which
makes the tar more fluid. The water then rises to the top, and is
removed in the ordinary way or by special "separators."

The tar itself is a mixture of exceedingly complex character. The great
bulk of its constituents belongs to the class of "aromatic"
hydrocarbons, of very different composition and degrees of volatility,
beginning with the simplest and most volatile, benzene (C6H6), and
ending with an entirely indistinguishable mass of non-volatile bodies,
which compose the pitch left behind in the tar-stills. The hydrocarbons
mostly belong to the benzene series CnH2n-6, the naphthalene series
CnH2n-12, and the anthracene and phenanthrene series CnH2n-18. Small
quantities of "fatty" ("aliphatic") hydrocarbons are never absent, even
in pure tars, and are found in considerable quantities when shales and
similar matters have been mixed with the coal in the gas-retorts. They
belong mostly to the paraffins CnH2n+2, and the olefines CnH2n. The
"asphalt" or soluble part of the pitch is also a mixture of
hydrocarbons, of the formula CnH2n; even the "carbon," left behind after
treating the pitch with all possible solvents is never pure carbon, but
contains a certain quantity of hydrogen, although less than any of the
volatile and soluble constituents of the tar.

Besides the hydrocarbons, coal-tar contains about 2% of the simpler
phenols CnH2n-7OH, the best known and most valuable of which is the
first of the series, carbolic acid (q.v.) C6H5OH, besides another
interesting oxygenized substance, cumarone C8H6O. The phenols,
especially the carbolic acid, are among the more valuable constituents
of coal-tar. Numerous sulphur compounds also occur in coal-tar, some of
which impart to it their peculiar nauseous smell, but they are of no
technical importance or value.

Still more numerous are the nitrogenated compounds contained in
coal-tar. Most of these are of a basic character, and belong to the
pyridine and the quinoline series. Among these we find a somewhat
considerable quantity of aniline, which, however, is never obtained from
the tar for commercial purposes, as its isolation in the pure state is
too difficult. The pyridines are now mostly recovered from coal-tar, but
only in the shape of a mixture of all members of the series which is
principally employed for denaturing alcohol. Some of these nitrogenated
compounds possess considerable antiseptic properties, but on the whole
they are only considered