The distinctive characters of oil are unctuosity and inflammability, insolubility in water, and fluidity at moderate temperatures. Oils are distinguished into fixed, or fat oils, which do not rise in distillation, at the temperature of boiling water; and volatile, or essential oils, which do rise at that temperature with water, or under 320° by themselves. The latter having been treated of under the word Essential, in the preceding part of this work, we shall here confine our attention to the former class chiefly.
Fixed oils are generally contained in the seeds and fruits of those vegetables of which they are the products, and are formed principally at the period of maturity. They are extracted by pressure, sometimes with, and sometimes without, the aid of heat. They are usually impregnated with the mucilaginous or extractive matter of the vegetable, whence they acquire colour, odour, and taste; and if heat has been employed to favour their extraction, they acquire acrimonious qualities, and undergo a change in some of their chemical properties. The purest oils are those expressed from the fruit of the olive, or the seeds of the almond; others less pure are extracted from linseed, hemp-seed, and numerous other seeds of plants. Fixed oils are usually fluid, but of a thick consistence, and they congeal at moderate temperatures; some are even naturally concrete. When fluid, they are transparent, colourless, or of a yellowish or greenish tinge, inodorous, and insipid; they are lighter than water. The following table exhibits the specific gravities of the principal sorts of commerce; water being 1.000: -
Cacao . . . . . . . . . . . . . . . . . .
Rape seed . . . . . . . . . . . . . . . .
Olives . . . . . . . . . . . . . . . . . .
Ben . . . . . . . . . . . . . . . . . .
Beech-nut . . . . . . . . . . . . . . . .
Walnut . . . . . . . . . . . . . . . . . .
Almond . . . . . . . . . . . . . . . . .
Linseed . . . . . . . . . . . . . . . . .
Poppy . . . . . . . . . . . . . . . . . .
Hazel-nut . . . . . . . . . . . . . . . .
Fixed oils are incapable of combining with water; and are very sparingly soluble in alcohol, in the cold, with the exception of castor, which is abundantly dissolved by rectified alcohol, and of linseed oil, which is dissolved, though more sparingly; boiling oil dissolves it, and also the others in sensible quality. Expressed oils cannot be volatilized by heat, without a change of their properties. At temperatures below 600° of Fahr. they remain fixed, if the heat has not been for a long time continued. At the temperature mentioned, they are converted into vapour, but the oil condensed therefrom is altered in it3 properties; it has lost its mildness, and has become more limpid and volatile, a portion of carbon having been deposited. Transmitted through an ignited tube, oil is converted into carbonic acid, and carburetted hydrogen, with a small portion of acid liquor, and a residium of charcoal. Exposed to a warm atmosphere, expressed oils gradually acquire a sharp taste and smell, and become thick. This change, termed rancidity, is owing to absorption of oxygen. Drying oils, as those expressed with the aid of heat are named, do not become rancid, but by absorbing oxygen, are partially converted into a resinous kind of matter.
At the temperature of ignition, at which it is converted into vapour, oil burns in atmospheric air, a large quantity of light and heat being extricated by its combustion. When the access of the air to the vapour of the oil is insufficient, it burns with a black smoke, and a quantity of carbonaceous matter which has escaped the combustion is deposited. Hence the utility of a slender wick, which draws up the oil by capillary attraction, and when kindled, produces sufficient heat to convert it into vapour. In a hollow cylindrical wick, like that in the Argand lamp, through which an internal circulation of air is established, the supply of air is more abundant, and the whole of the oil is consumed; the illumination therefore is greater, though there is some diminution of it in consequence of the light from the internal surface having to pass through the flame. Expressed oils combine with the alkalies, and form soap, which see. Expressed oils dissolve phosphorus by the aid of heat, forming a liquid, which Decomes luminous when exposed to the air. They combine with a number of the metallic oxides, and acquire thereby a drying property. Boiled with oxide of lead, expressed oil forms a compound of firm consistency, constituting the "common plaster " of the apothecaries.
Expressed oils form the basis of paints (see Painting), and are hence called oil-colours, (see also, Oil-colour Cakes, subjoined to this article.) Expressed oils, combined with resins and turpentine, form varnishes; (see Varnish.) Combined with lamp-black, they form printing-ink; (see Ink.) For most of these uses, however, the drying-oils are employed. There are two distinct processes of obtaining oil by pressure; one cold, the other warm; the cold-drawn oil being preferable for one purpose, and the warm for another. In the former, the substances are submitted to pressure, without increasing their natural temperature; in the latter, heat is artificially applied, generally through the medium of steam or air. The application of heat to seeds and most oleaginous matters, causes a great quantity of the oil to flow out, without pressure; and heat softens them so much, that less mechanical force becomes necessary to expel the remainder It is therefore an indispensable point of economy to make use of heat whenever the application of it does not deteriorate the quality of the oil; for more oil is thus obtained with less labour.
In the large manufactories, Unseed and rape-seed are the chief vegetable substances from which oil is obtained in this country; heat is usually employed before pressure, and the separate products of oil in the different stages of manu facture are preserved, as distinct qualities. The ordinary "mill " for this purpose consists usually of an extensive range of machinery, and is usually distinguished by the denomination of the Dutch Mill, as the industrious people of Holland were the inventors, or chief improvers of it In these, the seeds are put into bags, and covered with envelopes, consisting of hair-cloth, and sheepskin sewn together; in this state, they are subjected to pressure by the force of wedges, that are continually being struck by perpendicular stampers. These stampers are raised by cams fixed to a revolving axis, (worked by a steam-engine, or other adequate power,) and fall from the height they arc thus raised upon the wedges. The oil thus expressed, runs off, and is conducted to a cistern, and the seed in the bag is reduced to a very hard solid cake, which is sold for the feeding of cattle, as it retains a considerable portion of farinaceous and other nutritive matter.