Vegetables yield two distinct species of oil, one of which is volatile at a high temperature, but the other cannot be volatilised without suffering decomposition. The first of these is termed volatile oils, the second, fixed oils. The latter name is properly adopted by the Edinburgh College, and is more suitable than expressed oils, the epithet given to this class of substances by the Dublin College.

Fixed Oils are obtained from fruits and seeds either by expression or decoction with water. The dicotyledons, or seeds with two seed-lobes, yield the greatest proportion of oil. When the first process is employed, the fruit or seed is put into a strong hempen or hair bag, and subjected to the press; during the action of which the oil is forced out, generally combined with some other of the vegetable principles, which are afterwards separated by subsidence. The process is facilitated, and the quantity of oil increased, by heating the plates of the press, or previously roasting the seeds; but the oil thus obtained is more liable to become rancid, and hence the cold-drawn oils are always preferred for medicinal purposes. When the oil is to be obtained by decoction, the fruits or seeds are to be bruised previously to being boiled; and the oil which is separated is to be skimmed off from the surface of the water on which it swims.

Fixed oils have different degrees of consistence; they are, 1. Fluid at the ordinary temperature of the atmosphere, congealing in a temperature a little higher than the freezing point of water. 2. Concrete at the ordinary temperature of the atmosphere, and require a higher degree for their liquefaction. The first are denominated fluid oils; the second, vegetable butters.

1. Fluid fixed Oils are generally inodorous and nearly insipid, or have a mild taste. They are transparent, viscid, so as to run in streaks upon the sides of glass vessels, and have generally a slight tinge of colour, which may be removed by digestion with charcoal. They are generally lighter than water, but differ from each other in specific gravity. At about 600° of Fahrenheit they boil, and are then volatilised, but in a state of partial decomposition; the vapour readily catches fire, and burns with a yellow flame. When exposed to the atmosphere at a high natural temperature, such as exists in summer, or in heated rooms, the fixed oils expressed without heat become thick, lose much of their transparency, acquire a sharp taste and a disagreeable odour, and are then said to be rancid; but when heat has been used in their expression, they only become thick, and acquire resinous properties. In both cases the changes are produced by the absorption of oxygen; but in the first case, owing to the combination of the oxygen with some of the vegetable mucilage present in the cold-drawn oil, sebacic acid is formed; and by its diffusion through the oil, the change in its properties is produced.

Fixed oils are insoluble in water; but they may be mingled through water, and kept suspended in it by means of mucilage or yolk of egg. They are, with one or two exceptions, nearly insoluble in alcohol and ether; but unite readily with each other, with volatile oils, and with resinous substances. They dissolve sulphur, and form what was formerly regarded a balsam with it. With the alkalies they combine, and form soaps; but with the acids undergo decomposition: and when boiled with some of the metallic oxides, tough, solid compounds, or plasters, are produced.

2. Concrete fixed oils possess nearly the same properties as the fluid fixed oils. They are, however, more soluble in alcohol and ether, but are not capable of entering so readily into combinations with the alkalies. The ultimate constituents of fixed oil are carbon, hydrogen, and oxygen.1

For medicinal purposes, these oils are required to be free from rancidity; consequently, they must be preserved in closed vessels, and carefully excluded from the air.