ain definite temperatures,
the boiling-point of a chemical compound being an inherent property. If a
mixture of substances boiling at different temperatures is heated in a
suitable vessel the compounds distil over, broadly speaking, in the order
of their boiling-points. The separation by this process is not absolute,
because compounds boiling at a certain temperature have a tendency to
bring over with them the vapours of other compounds which boil at a higher
temperature. But for practical purposes it will be sufficient to consider
that the general tendency is for the compounds of low boiling-point to
come over first, then the compounds of higher boiling-point, and finally
those of the highest boiling-point. This is the principle made use of by
the tar-distiller. The tar-still is a large iron pot provided with a
still-head from which the vapours boil out into a coil of iron pipe kept
cool in a vessel of water (see Fig. 6). The still is heated by a fire
beneath it, and the different portions which condense in the iron coil are
received in vessels which are changed as the different fractions of the
tar come over. The process is what chemists would call a rough fractional
distillation. The first fractions are liquid at ordinary temperatures, and
the water in the condenser is kept cold; then, as the boiling-point
rises, the fraction contains a hydrocarbon which solidifies on cooling,
and the water in the condenser is made hot to prevent the choking up of
the coil. Every one of these fractions of coal-tar, from the beginning to
the end of the process, has its story to tell--all the chief constituents
of the tar separated by this means have by chemical science been converted
into useful products.

[Illustration: FIG. 6.--Sectional diagram of tar-still with arched bottom.
The fireplace is at _i_; the hot gases pass over the bridge _k_ and
through _g_ into the flues _h_, _h_. The pipe at _c_ is to supply the
still with tar; _a_ is the exit pipe connected with the condenser, and _b_
a man-hole for cleaning out the still. The condenser and bottom pipe for
drawing off pitch have been omitted to avoid complication.]

It is customary at the present time to collect four distinct fractions
from the period when the tar begins to boil quietly, _i.e._ from the
point when the small quantity of watery liquor which is unavoidably
entangled with the tar has distilled over, by which time the temperature
in the still is about 110 deg. C. T