and a manometer are generally interposed between the pump and
receiver. For the purpose of collecting the distillates in fractions,
many forms of receivers have been devised. Bruhl's is one of the
simplest. It consists of a number of tubes mounted vertically on a
horizontal circular disk which rotates about a vertical axis in a
cylindrical vessel. This vessel has two tubulures: through one the end
of the condenser projects so as to be over one of the receiving tubes;
the other leads to the pump. By rotating the disk the tubes may be
successively brought under the end of the condenser. Boiling under
reduced pressure has one very serious drawback, viz. the liquid boils
irregularly or "bumps." W. Dittmar showed that this may be avoided by
leading a fine, steady stream of dry gas-air, carbon dioxide,
hydrogen, &c., according to the substance operated upon--through the
liquid by means of a fine capillary tube, the lower end of which
reaches to nearly the bottom of the flask. "Bumping" is common in open
boiling when the liquid is free from air bubbles and the interior of
the vessel is very smooth. It may be diminished by introducing
clippings of platinum foil, pieces of porcelain, glass beads or
garnets into the liquid. "Frothing" is another objectionable feature
with many liquids. When cold, froth can be immediately dissipated by
adding a few drops of ether. In boiling liquids its formation may be
prevented by adding paraffin wax; the wax melts and forms a ring on
the surface of the liquid, which boils tranquilly in the centre.

[Illustration: Wurtz. Linnemann. Le Bel-Henninger. Glynsky. Young.
Kreusler.

FIG. 3.]

3. _Fractional Distillation._--By fractional distillation is meant the
separation of a mixture having components which boil at neighbouring
temperatures. The distilling flask has an elongated neck so that the
less volatile vapours are condensed and return to the flask, while the
more volatile component passes over. The success of the operation
depends upon two factors: (1) that the heating be careful, slow and
steady, and (2) that the column attached to the flask be efficient to
sort out, as it were, the most volatile vapour. Three types of columns
are employed: (1) the elongation is simply a straight or bulb tube;
(2) the column, properly termed a "dephlegmator," is so constructed
that the vapours have to traverse a column of previously co