llation, (7) distillation in chemical
technology and (8) commercial distillation of water.

1. _Ordinary Distillation._--The apparatus generally used is shown in
fig. 1. The substance is heated in a retort a, which consists of a
large bulb drawn out at the top to form a long neck; it may also be
provided with a tubulure, or opening, which permits the charging of
the retort, and also the insertion of a thermometer b. The retort may
be replaced by a distilling flask, which is a round-bottomed flask
(generally with a lengthened neck) provided with an inclined side
tube. The neck of the retort, or side tube of the flask, is connected
to the condenser c by an ordinary or rubber cork, according to the
nature of the substance distilled; ordinary corks soaked in paraffin
wax are very effective when ordinary or rubber corks cannot be used.
Sometimes an "adapter" is used; this is simply a tapering tube, the
side tube being corked into the wider end, and the condenser on to the
narrower end. The thermometer is placed so that the bulb is near the
neck of the retort or the side tube of the distilling flask. It
generally happens that much of the mercury column is outside the flask
and consequently at a lower temperature than the bulb, hence a
correction of the observed temperature is necessary. If N be the
length of the unheated mercury column in degrees, t the temperature of
this column (generally determined by a small thermometer placed with
its bulb at the middle of the column), and T the temperature recorded
by the thermometer, then the corrected temperature of the vapour is T
+ 0.000143 (T - t) N (T. E. Thorpe, _Journ. Chem. Soc._, 1880, p.
159).

[Illustration: FIG. 1.]

The mode of heating varies with the substance to be distilled. For
highly volatile liquids, e.g. ether, ligroin, &c., immersion of the
flask in warm water suffices; for less volatile liquids a directly
heated water or sand bath is used; for other liquids the flask is
heated through wire gauze or asbestos board, or directly by a Bunsen.
The condensing apparatus must also be conditioned by the volatility.
With difficulty volatile substances, e.g. nitrobenzene, air cooling of
the retort neck or of a straight tube connected with the distilling
flask will suffice; or wet blotting-paper placed on the tube and the
receiver immersed in water may be used. For less volatile liquids the
Liebig cond