enser is most frequently used. In its original form, this
consists of a long tube surrounded by an outer tube so arranged that
cold water circulates in the annular space between the two. The
vapours pass through the inner tube, and the cold water enters at the
end farthest from the distilling flask. For more efficient
condensation--and also for shortening the apparatus--the central tube
may be flattened, bent into a succession of V's, or twisted into a
spiral form, the object in each case being to increase the condensing
surface. Of other common types of condenser, we may notice the
"spiral" or "worm" type, which consists of a glass, copper or tin worm
enclosed in a vessel in which water circulates; and the ball
condenser, which consists of two concentric spheres, the vapour
passing through the inner sphere and water circulating in the space
between this and the outer (in another form the vapour circulates in a
shell, on the outside and inside of which water circulates). A very
effective type is shown in fig. 2. The condensing water enters at the
top and is conducted to the bottom of the inner tube, which it fills
and then flows over the outside of the outer tube; it collects in the
bottom funnel and is then led off. The vapours pass between the inner
and outer tubes.

[Illustration: FIG. 2.]

Practically any vessel may serve as a receiver--test tube, flask,
beaker, &c. If noxious vapours come over, it is necessary to have an
air-tight connexion between the condenser and receiver, and to provide
the latter with an outlet tube leading to an absorption column or
other contrivance in which the vapours are taken up. If the substances
operated upon decompose when heated in air, as, for example, the zinc
alkyls which inflame, the air within the apparatus is replaced by some
inert gas, e.g. nitrogen, carbon dioxide, &c., which is led in at the
distilling flask before the process is started, and a slow current
maintained during the operation.

2. _Distillation under Reduced Pressure._--This method is adopted for
substances which decompose at their boiling-points under ordinary
pressure, and, generally, when it is desirable to work at a lower
temperature. The apparatus differs very slightly from that employed in
ordinary distillation. The "receiver" must be connected on the one
side to the condenser, and on the other to the exhaust pump. A safety
vessel