ating are adopted. In some cases, especially in dry
distillations, the furnace flames play directly on the retorts, in
others, such as in the case of nitric acid, the whole still comes
under the action of the furnace gases to prevent condensation on the
upper part of the still, while in others the furnace gases do not play
directly on the base or upper portion of the still but are conducted
around it by a system of flues (see COAL-TAR). Steam heating, dry or
live, is employed alone and also as an auxiliary to direct firing.

The condensing plant varies with the volatility of the distillate. Air
cooling is adopted whenever possible. For example, in the less modern
methods for manufacturing nitric acid the vapours were conducted
directly into double-necked bottles (_bombonnes_) immersed in water. A
more efficient arrangement consists of a stack of vertical pipes
standing up from a main or collecting trough and connected at the top
in consecutive pairs by a cross tube. By an arrangement of diaphragms
in the lower trough the vapours are circulated through the system. As
an auxiliary to air cooling the stack may be cooled by a slow stream
of water trickling down the outside of the pipes, or, in certain
cases, cold water may be injected into the condenser in the form of a
spray, where it meets the ascending vapours. Horizontal air-cooling
arrangements are also employed. A common type of condenser consists of
a copper worm placed in a water bath; but more generally straight
tubes of copper or cast iron which cross and recross a rectangular
tank are employed, since this form is more readily repaired and
cleansed. Wood-spirit, petroleum and coal-tar distillates are
condensed in plant of the latter type. In cases where the condenser is
likely to become plugged there is a pipe by means of which live steam
can be injected into the condenser. The supply of water to the
condenser is regulated according to the volatility of the condensate.
When the vapours readily condense to a solid form the condensing plant
may take the form of large chambers; such conditions prevail in the
manufacture of arsenic, sulphur and lampblack: in the latter case
(which, however, is not properly one of distillation) the chamber is
hung with sheets on which the pigment collects. Large chambers are
also used in the condensation of mercury.

Dephlegmation of the vapours arising from such mix