_Distilled, Volatile, or Essential Oils._
The British pharmacopœiæ direct them to be obtained by distillation
only; the French codex orders several of them to be prepared by
expression. QUALITIES. _Form_, liquid, sometimes viscid; _specific
grav._ various; oil of turpentine, which is the lightest, being only
0·792, whilst the oil of cloves, cinnamon, and allspice, exceed 1·030,
and that of sassafras, which is the heaviest, amounts to 1·094; these
latter oils hold resin in solution, and of course sink in water.
_Odour_, penetrating and fragrant; _Taste_, acrid; they are volatilized
at a temperature somewhat below that of boiling water; they are very
inflammable. SOLUBILITY. Very soluble in alcohol, forming what are
termed in perfumery _Essences_; in water they are very sparingly
soluble; the solutions are known in pharmacy under the title of
_distilled waters_; they are also dissolved by æther, and the _fixed_
oils; when digested with ammonia, some of the less odorous acquire a
considerable degree of fragrance, whilst on the contrary, fixed alkalies
universally impair their odour; they are rapidly decomposed by nitric
and sulphuric acids, and their action is sometimes attended with instant
inflammation. Volatile oils, from continued exposure to the air, absorb
oxygen, and become resinous, by which they lose their volatility,
fragrance, and pungency, hence they should be preserved in small opaque
phials, completely full and well stopped. MED. USES. They act as
powerful stimulants and aromatics; they remove nausea and flatulence,
correct the griping of certain purgatives, and cover the offensive taste
of various remedies. See _Aquæ destillatæ_. They, moreover, have the
property of defending certain animal and vegetable preparations from
mouldiness. This curious fact has been already noticed, see _page 177_.
The following is a list of the species admitted into our British
pharmacopœiæ; those designated in _italics_ are principally for internal
use. OLEA _Anisi_, _Anthemidis_, _Carui_, _Juniperi_. Lavandulæ, _Menthæ
Piperitæ_, _Menthæ viridis_, Origani, _Pimentæ_, _Pulegii_, _Rosmarini_,
L. OLEA VOLATILIA, _Juniperi communis_, Juniperi Sabinæ, Lavandulæ
Spicæ, Lauri Sassafras, _Menthæ Piperitæ_, _Myrtæ Pimentæ_, _Pimpinellæ
Anisi_, Rorismarini Officinalis. E. OLEA _Juniperi_, _Pimento_, Corticis
et Ligni Sassafras, _e Seminibus Anisi_, _Carui_, _Fœniculi dulcis_,
Florum Lavendulæ, Foliorum Sabinæ, _Herbæ florescentis Menthæ Sativæ_,
Origani, _Pulegii_, _Rorismarini_, _Rutæ_, D. ADULTERATIONS. _Fixed
Oils_ may be detected by moistening writing paper with the suspected
article and holding it before the fire: if the oil be entirely
essential, no stain of grease will remain; as castor oil is more soluble
in spirit than the others, it is the one generally selected for this
fraudulent purpose, and the addition of alcohol restores the
sophisticated oil to its proper degree of consistency. _Alcohol_ is
discovered by adding water, which, if it be present, occasions a
milkiness, and at the same time, an increase of temperature; a decrease
of bulk also takes place, which may easily be ascertained by measuring
the oil and water separately, and then transferring them, in a state of
mixture, into a tube of small diameter. _Cheaper oils_, as that of
turpentine, are recognised by their peculiar odour, which may be
developed by rubbing a drop upon the hand and holding it to the fire,
or, by the dense black smoke with which they burn. The oil of aniseed,
as it crystallizes at 50°, is frequently sophisticated with wax,
spermaceti, or camphor; the fraud is detected by warming the oil, when
the crystals, if genuine, will dissolve.[575] In some cases the
refractive power of the oil affords a test of its purity. _See my work
on Chemistry in its relations to Medicine, § 318._