ting of any part of the
still not protected inside by liquid tar, or, at the end of the
operation, by liquid pitch. The outlet pipe is equally protected
against overheating and also against any stoppage by pitch solidifying
therein. The capacity of tar-stills ranges from 5 to 50 tons. They
hold usually about 10 tons, in which case they can be worked off
during one day.

The vapours coming from the still are condensed in coolers of various
shapes, one of which is shown in figs. 2 and 3. The cooling-pipes are
best made of cast-iron, say 4 in. wide inside and laid so as to have a
continuous fall towards the bottom. A steam-pipe (b) is provided for
heating the cooling water, which is necessary during the later part of
the operation to prevent the stopping up of the pipes by the
solidification of the distillates. A cock (a) allows steam to be
injected into the condensing worm in order to clear any obstruction.

[Illustration: FIG. 2.--Condensing Worm (Plan)]

The cooling-pipe is at its lower end connected with receivers for the
various distillates in such a manner that by the turning of a cock the
flow of the distillates into the receivers can be changed at will. In
a suitable place provision is made for watching the colour, the
specific gravity, and the general appearance of the distillates. At
the end of the train of apparatus, and behind the vacuum pump or
ejector, when one is provided, there is sometimes a purifier for the
gases which remain after condensation; or these gases are carried back
into the fire, in which case a water-trap must be interposed to
prevent explosions.

_Distillation of the Tar._--The number of fractions taken during the
distillation varies from four to six. Sometimes a first fraction is
taken as "first runnings," up to a temperature of 105 deg. C. in the
still, and a second fraction as "light oil," up to 210 deg. C., but more
usually these two are not separated in the first distillation, and the
first or "light oil" fraction then embraces everything which comes
over until the drops no longer float on, but show the same specific
gravity as water. The specific gravity of this fraction varies from
0.91 to 0.94. The next fraction is the "middle oil" or "carbolic oil,"
of specific gravity 1.01, boiling up to 240 deg. C.; it contains most of
the carbolic acid and naphthalene. The next fraction is the "heavy
oil" or "creosote o