te of very
minute division.

EMILY.

Pray what is sublimation?

MRS. B.

It is the evaporation, or, more properly speaking, the volatilisation of
solid substances, which, in cooling, condense again in a concrete form.
The process, in this instance, must be performed in a closed vessel,
both to prevent combustion, which would take place if the access of air
were not carefully precluded, and likewise in order to collect the
substance after the operation. As it is rather a slow process, we shall
not try the experiment now; but you will understand it perfectly if I
show you the apparatus used for the purpose. (PLATE XI. fig. 1.) Some
lumps of sulphur are put into a receiver of this kind, which is called a
_cucurbit_. Its shape, you see, somewhat resembles that of a pear, and
is open at the top, so as to adapt itself exactly to a kind of conical
receiver of this sort, called the head. The cucurbit, thus covered with
its head, is placed over a sand-bath; this is nothing more than a vessel
full of sand, which is kept heated by a furnace, such as you see here,
so as to preserve the apparatus in a moderate and uniform temperature.
The sulphur then soon begins to melt, and immediately after this,
a thick white smoke rises, which is gradually deposited within the head,
or upper part of the apparatus, where it condenses against the sides,
somewhat in the form of a vegetation, whence it has obtained the name of
flowers of sulphur. This apparatus, which is called an _alembic_, is
highly useful in all kinds of distillations, as you will see when we
come to treat of those operations. Alembics are not commonly made of
glass, like this, which is applicable only to distillations upon a very
small scale. Those used in manufactures are generally made of copper,
and are, of course, considerably larger. The principal construction,
however, is always the same, although their shape admits of some
variation.

[Illustration: Plate XI. Vol. I. p. 237.

Fig. 1. Sublimation of Sulphur.
A Alembic.
B Sand-bath.
C Furnace.

Fig. 2. Eudiometer.

Fig. 3. Decomposition of water by Carbon.
A Retort containing water.
B Lamp to heat the water.
C.C Porcelain tube containing Carbone.
D Furnace through which the tube passes.
E Receiver for the gas produced.
F Water bath.]

CAROLINE.

What is the use of that neck, or tube, which bends down from the upper
piece of the apparatus?

MRS. B.

It is of no use in sublim