which also renders the decomposition more regular by increasing the
conductivity of the mass. The most favourable retort is a shallow iron
pan heated in a sand bath, and provided with a screwed-down lid
bearing the delivery tube. Sidney Young has suggested conducting the
operation in a current of carbon dioxide which sweeps out the vapours
as they are evolved, and also heating in a vapour bath, e.g. of
sulphur.

One of the earliest red-hot tube syntheses of importance was the
formation of naphthalene from a mixture of alcohol and ether vapours.
Such condensations were especially studied by M. P. E. Berthelot, and
shown to be very fruitful in forming hydrocarbons. Sometimes reagents
are placed in the combustion tube, for example lead oxide (litharge),
which takes up bromine and sulphur. In its simplest form the apparatus
consists of a straight tube, made of glass, porcelain or iron
according to the temperature required and the nature of the reacting
substances, heated in an ordinary combustion furnace, the mixture
entering at one end and the vapours being condensed at the other.
Apparatus can also be constructed in which the unchanged vapours are
continually circulated through the tube. Operating in a current of
carbon dioxide facilitates the process by preventing overheating.

7. _Distillation in Chemical Technology._--In laboratory practice use
is made of a fairly constant type of apparatus, only trifling
modifications being generally necessary to adapt the apparatus for any
distillation or fractionation; in technology, on the other hand, many
questions have to be considered which generally demand the adoption of
special constructions for the economic distillation of different
substances. The modes of distillation enumerated above all occur in
manufacturing practice. Distillation in a vacuum is practised in two
forms:--if the pump draws off steam as well as air it is termed a
"wet" air-pump; if it only draws off air, it is a "dry" air-pump. In
the glycerin industry the lyes obtained by saponifying the fats are
first evaporated with "wet vacuum" and finally distilled with closed
and live steam and a "dry vacuum." Two forms of steam distillation may
be distinguished:--in one the still is simply heated by a steam coil
wound inside or outside the still--this is termed heating by dry
steam; in the other steam is injected into the mass within the
stil