_Expressed or Fixed Oils._
These are obtained from animal matter by fusion, and from vegetable by
expression, or decoction with water. QUALITIES. _Odour_, none; _Taste_,
mild; they boil at 600°, but undergo decomposition, becoming acrid and
empyreumatic; the oil, in this state, was formerly used in medicine
under the name of _philosopher’s oil_.[576] By exposure to air they
absorb oxygen and become rancid; they congeal at a temperature of 32°,
and some even above that. When the oil is expressed by heating the
plates of the press, or by previously roasting the seeds, it is more
disposed to become rancid; _cold drawn_ oils are on this account to be
preferred for the purposes of pharmacy. SOLUBILITY. They are insoluble
in water, and, except castor oil, nearly so in alcohol and æther; with
caustic alkalies they combine and form soaps; when aided by heat they
readily unite with oxide of lead, forming the solid compound well known
by the term _plaister_. They unite also very readily with each other,
and with volatile oils. SOLVENT POWERS. They dissolve sulphur, and form
a kind of balsam with it; they also possess the power of extracting and
dissolving the narcotic and acrid principles of several vegetable and
animal substances, in consequence of which, the French pharmacopœia
directs a series of preparations under the term “_Olea Medicata_;” thus
there are olea Cicutæ, Hyoscyami, Solani, Stramonii, Nicotianæ;[577]
which are made by digesting with a gentle heat, one part of the subject
in two parts of olive oil.[578]