it is used for
roofing-felt and other purposes.

We shall now give an outline of the processes used in the distillation
of tar.

_Dehydration._--The first operation in coal-tar distilling is the
removal of the mechanically enclosed water. Some water is chemically
combined with the bases, phenols, &c., and this, of course, cannot be
removed by mechanical means, but splits off only during the
distillation itself, when a certain temperature has been reached. The
water mechanically present in the tar is separated by long repose in
large reservoirs. Very thick viscous tars are best mixed with thinner
tars, and the whole is gently heated by coils of pipes through which
the heated water from the oil-condensers is made to flow. Sometimes
special "tar-separators" are employed, working on the centrifugal
principle. The water rises to the top and is worked up like ordinary
gas-liquor. More water is again separated during the heating-up of the
tar in the still itself, and can be removed there by a special
overflow.

[Illustration: FIG. 1.--Tar-Still (sectional elevation).[1]]

_Tar-Stills._--The tar is now pumped into the tar-still, fig. 1. This
is usually, as shown, an upright wrought-iron cylinder, with an arched
top, and with a bottom equally vaulted upwards for the purpose of
increasing the heating surface and of raising the level of the pitch
remaining at the end of the operation above the fire-flues. The fuel
is consumed on the fire-grate a, and, after having traversed the holes
bb in the annular wall e built below the still, the furnace gases are
led around the still by means of the flue d, whence they pass to the
chimney. Cast-iron necks are provided in the top for the outlet of the
vapours, for a man-hole, supply-pipe, thermometer-pipe, safety valve,
and for air and steam-pipes reaching down to the bottom and branching
out into a number of distributing arms. Near the top there is an
overflow pipe which comes into action on filling the still. In the
lowest part of the bottom there is a running-off valve or tap. In some
cases (but only exceptionally) a perpendicular shaft is provided, with
horizontal arms, and chains hanging down from these drag along the
bottom for the purpose of keeping it clean and of facilitating the
escape of the vapours. This arrangement is quite unnecessary where the
removal of the vapours is promoted by the injection of steam, bu