they give a very secure joint, impervious to acids and gases. But rubber should not be employed where it might come in contact

FIG. 143.

Retorts with Adapters.

with such solvents as chloroform or carbon disulphide. Corks, before being used, should be gently pressed and softened, then

perforated by means of the cork borer (Fig. 147). Cork borers are brass cylinders, the lower edges of which are sharpened, the

DISTILLATION.

FIG. 148.

milled heads affording a firm hold. When dull, the edges are
readily sharpened on a whetstone or by a file. In
boring a cork, place it on a block of wood, then with a
tube of the desired size, moistened with water, push
through with a twisting motion, taking care to keep the
instrument perpendicular. In boring through a thick
cork, the bored portion is apt to break off, before the
borer is through. If this happens, the borer should be
withdrawn, cleared of the broken piece, then wetted and
reinserted, when it will readily cut its way further.
Never try to bore a hole by starting it from both sides;
it will rarely come true. In piercing rubber stoppers,
the end of the tube should be moistened with alcohol
or oil of turpentine. When we desire to connect two
tubes of equal diameter or where one just fits over the

FIG. 147.

Mohr's Cork-borer.

Cork-File.

other, it is best to slip a short piece of firmly-fitting rubber tube
over the joint; this will render it air-tight and secure.

Inverted or Upright (or Reflux) Condenser.-When it is desired to subject a substance to the action of a boiling liquid solvent, we

employ a flask with inverted condenser, arranged as shown in Fig. 151. The flask is heated on a water,- sand- or oil-bath, or over the free flame, according to the nature and boiling point of the solvent. Fig. 152 illustrates a Soxhlet's spherical condenser, which possesses the great advantage of occupying a very little space. The cold

FIG. 152.

water enters at a into
the condensing space c,
flowing out through b.
The vapor enters in
from below, circulates
around and condenses
in the space d.

Flask with Upright or
Reflux Condenser.

Distillation in a Current of Steam.-This is applicable to such substances as are injured a when heated or distilled alone, as the volatile oils or organic bases; we also employ it in the separation of volatile from non-volatile bodies. The substance is placed in the distilling flask d (Fig. 153) with a little water, then steam (generated in the can a) is blown through. This carries the volatile matter over, to be condensed in the cooler e. Distillation is usually continued until oily globules cease to be driven over.

Soxhlet's Spherical Condenser.

DISTILLATION.

Fractional Distillation is the separation of a mixture of liquids of different degrees of volatility, by means of distillation. Mixed volatile liquids have no constant boiling point. The complete separation of two liquids, which boil at different temperatures, can only be carried out easily, when the

interval between their boiling points
is a large one. If they only differ by
10 to 30 degrees, then on distilling,
one will observe a continuous rise of
the thermometer without its remain-
ing stationary at any given boiling
point, and the change in the compo-
sition of the distillate will be gradual
instead of abrupt. In such cases, the
distillate must be collected in separate
"fractions," according to the
rise in boiling point, i. e.,
every 5 or 10 degrees. This
must be repeated until the

d. Distilling flask. e. Condenser. FIG. 153.-Apparatus for Distillation in Current of Steam. a. Can in which steam is generated. b. Safety valve. c. Exit tube for steam.

middle fractions have been separated into the higher and lower boiling constituents. Thus, for instance, on distilling a mixture of two liquids, their boiling points being respectively 100° C. and 150° C., the liquid of lower boiling point does not come over alone at 100° C., nor the higher alone at 150° C., but the thermometer gradually rises from 100° C. up, and we obtain a mixture of both liquids, in different proportions. In this case, we collect that

portion which comes over between 100° and 110°, which consists almost entirely of the lower boiling fraction, then another fraction boiling between 110° and 140°, which consists of a mixture of both constituents in about equal proportions; finally, the fraction from 140° to 150°, which contains mainly the higher boiling liquid. The middle fraction (110° to 140°) is then again fractioned, the respective distillates between 100° and 110° and 140° and 150° being set aside; this is again repeated if necessary, and finally the

high and low boiling fractions (i. e., those from 100° to 110°, and those from 140° to 150°), are in turn distilled, the receiver being changed at every 5 degrees. These products on repeated distillation in the above manner, finally yield the two pure products. For carrying on fractional distillation, the apparatus as illustrated in Fig. 84, should be employed.

Rectification is repeated distillation, whereby a distillate of