Illustration: PRESSING LINSEED FOR OIL.
Oils, whether animal or vegetable, have pretty much the same
properties. The vegetable oils are got by crushing and pressing
certain seeds, as linseed; the animal oils are obtained chiefly
from the whale and seal tribe, or from fish, as the cod and sturgeon.
They are viscid and of a pale yellow color, lighter than water, and
therefore float upon it, and are very combustible. Oils are used for
a multiplicity of purposes: for burning in lamps, for making paints
and varnishes, preventing the friction of machinery, in making soap,
and numerous other processes. Oils are generally divided into fixed
and volatile oils; as an example of the first, linseed oil may be
selected; of the last, oil of lavender.
Some of the fixed oils are called drying oils, and it is this
kind that are used for varnish making; as linseed oil possesses
the properties of this class very perfectly, it may serve as a
description of all. Linseed oil has the power of drying when spread
out in a thin layer, becoming of a resinous consistence like varnish,
and upon this quality depends the drying powers of paint, for the
other non-drying oils, as olive oil, may be exposed to the air for
months without drying at all; the drying powers of linseed oil are
greatly increased by boiling it with litharge (oxide of lead). This
forms the drying or boiled oil used in painting, and is employed
in making varnishes, printers’ ink, and for other purposes. Oiled
silk is formed by brushing silk over with this oil and exposing it
to the air till it is dry; this oiled silk is the same that is used
for sponge-bags and bathing-caps. Of the non-drying oils, the chief
are those used for burning in lamps, as sperm oil. Salad oil is
expressed from the olive, and is called olive oil. Of the volatile
oils, the most useful is oil of turpentine, commonly called spirit of
turpentine; it is got by distilling common turpentine (the concrete
juice of trees of the fir and pine tribe), it has a strong odour,
is very inflammable, and is volatile, that is to say, if spread out,
will evaporate, leaving nothing behind; this oil is capable of mixing
with drying oils, and it not only increases their drying powers, but,
by thinning them, makes them more applicable to many purposes. Oil
of turpentine dissolves resin and many resinous substances, and forms
useful varnishes with them. Mastic varnish is mastic dissolved in oil
of turpentine; it is the varnish always used for varnishing pictures.
Many of the volatile oils are used as perfumes, and the odour of
plants and flowers depends upon the volatile oil which evaporates
from them, as lavender, cloves, and others. Peppermint water, dill
water, and cinnamon water are produced by distilling water mixed with
the substances, and is simply water containing a small quantity of
the volatile oil of these substances in solution; but eau-de-Cologne
and lavender water, although called “waters,” are mixtures of
volatile oils and spirit of wine, and contain no more water than
is comprehended in the spirit used.