t this
steam must be carefully dried beforehand, or, better, slightly
superheated, in order to prevent explosions, which might be caused by
the entry of liquid water into the tar during the later stages of the
work, when the temperature has arisen far above the boiling-point of
water. The steam acts both by stirring up the tar and by rapidly
carrying off the vapours formed in distillation. The latter object is
even more thoroughly attained by the application of a vacuum,
especially during the later stage of distillation. For this purpose
the receivers, in which the liquids condensed in the cooler are
collected, are connected with an air pump or an ejector, by which a
vacuum of about 4 in., say 1/8 atmosphere, is made which lowers the
boiling process by about 80 deg. C.; this not merely hastens the process,
but also produces an improvement of the quality and yield of the
products, especially of the anthracene, and, moreover, lessens or
altogether prevents the formation of coke on the still-bottom, which
is otherwise very troublesome.

Most manufacturers employ ordinary stills as described. A few of them
have introduced continuously acting stills, of which that constructed
by Frederic Lennard has probably found a wider application than any of
the others. They all work on the principle of gradually heating the
tar in several compartments, following one after the other. The fresh
tar is run in at one end and the pitch is run out from the other. The
vapours formed in the various compartments are separately carried away
and condensed, yielding at one and the same time those products which
are obtained in the ordinary stills at the different periods of the
distillation. Although in theory this continuous process has great
advantages over the ordinary style of working, the complication of the
apparatus and practical difficulties arising in the manipulation have
deterred most manufacturers from introducing it.

The tar-stills are set in brickwork in such a manner that there is no
over-heating of their contents. For this purpose the fire-grate is
placed at a good distance from the bottom or even covered by a brick
arch so that the flame does not touch the still-bottom at all and acts
only indirectly, but the sides of the still are always directly
heated. The fire-flue must not be carried up to a greater height than
is necessary to provide against the overhea