The processes are divided into four distinct operations; viz.--

1. _Expression_; 2. _Distillation_; 3. _Maceration_; 4. _Absorption_.

1. _Expression_ is only adopted where the plant is very prolific in its
volatile or essential oil,--_i.e._ its odor; such, for instance, as is
found in the pellicle or outer peel of the orange, lemon, and citron,
and a few others. In these cases, the parts of the plant containing the
odoriferous principle are put sometimes in a cloth bag, and at others by
themselves into a press, and by mere mechanical force it is squeezed
out. The press is an iron vessel of immense strength, varying in size
from six inches in diameter, and twelve deep, and upwards, to contain
one hundred weight or more; it has a small aperture at the bottom to
allow the expressed material to run for collection; in the interior is
placed a perforated false bottom, and on this the substance to be
squeezed is placed, covered with an iron plate fitting the interior;
this is connected with a powerful screw, which, being turned, forces the
substance so closely together, that the little vessels containing the
essential oils are burst, and it thus escapes. The common tincture press
is indeed a model of such an instrument. The oils which are thus
collected are contaminated with watery extracts, which exudes at the
same time, and from which it has to be separated; this it does by itself
in a measure, by standing in a quiet place, and it is then poured off
and strained.

[Illustration: Pipette to draw off small portions of otto from water.]

2. _Distillation._--The plant, or part of it, which contains the
odoriferous principle, is placed in an iron, copper, or glass pan,
varying in size from that capable of holding from one to twenty gallons,