Olea Fixa. Fixed Oils, Fat Oils, Expressed Oils.
Oils of this class are obtained mainly from vegetables, and usually by the process of pressure–between either cold iron plates, or plates moderately heated to cause greater liquidity and a freer flow to the oil. But such oils possess many properties common to the animal oils; and a few of those of vegetable origin may, like the animal fats, be obtained by rendering with either boiling water or steam; whence the two classes are generally considered together under the general term of fixed or fat oils.
The fixed oils of vegetable origin are obtained mainly from the seeds of the plants; a few from the fleshy pulp surrounding the seeds, as the olive; and a limited number from the kernels, roots, and bark. The usual procedure is, to crush the seeds well in a suitable mill, put the meal in canvas sacks, and then subject them to very strong pressure between iron plates, by means of a hydraulic press. The best quality is first obtained by using the plates cold; but an additional amount, usually somewhat turbid and of an inferior quality, is afterwards obtained by heating the plates to the temperature of 200 deg. F., and subjecting the meal to a second pressure. Usually the cake from the first pressure is broken up and heated before being put under pressure the second time. Hot-pressed oils are more likely to become rancid than cold-pressed ones.
The fat oils, when first expressed without heat, taste merely unctuous on the tongue, and exhale the odor of their respective plants. Their fluidity is very various, some being solid at ordinary temperatures, as cocoa-nut oil; some congealing at about the freezing point, as olive oil; and some not congealing at less than four degrees below zero, F., as linseed oil. They are nearly all transparent when fluid, and have a yellowish tinge, but may be made quite colorless by treatment with animal charcoal. They are lighter than water, and vary but little in specific gravity, ranging from .892 (cocoa butter) to .968 (palm oil.) They can not be distilled; but burn at a temperature of about 600 deg. F., and give a more or less brilliant flame–with or without smoke, according as their combustive decomposition yields a large or small excess of carbon. In close vessels, out of contact of air, they may be preserved fresh for a very long time; but the presence of air changes most of them. Some will slowly thicken into yellowish, transparent, flexible, and dry substances; and when these are spread thickly upon a surface, they constitute the drying oils of the arts. Others do not thus grow dry, though they become thickened by age; and such are liable sooner or later to become rancid, and present some degree of acid reaction. These changes seem due to the absorption of oxygen from the atmosphere. When oils–especially animal oils–are added to wool or hemp, so as to expose a large surface to the air, they may absorb oxygen so rapidly as to generate enough heat to cause spontaneous combustion.
The fat oils are not soluble in nor miscible with water; but if shaken with the latter, will soon separate and rise to the top. If first incorporated with any thick mucilage, they can then be made miscible with water in the form of emulsion. Castor oil dissolves to a fair extent in cold absolute alcohol; but all the others are not acted on by this fluid, except it be hot. Ether, on the contrary, is an excellent solvent of the fixed oils; whence this menstruum is employed to separate such oils from other bodies, after which the oil may be obtained by evaporating the ether. The stronger acids decompose most of them, producing various results. They combine with the salifiable bases, including the alkalies and oxide of lead, the substance called glycerin being separated, and the other constituents of the oils forming a series of compounds of the soapy character. With the potassa and soda alkalies, the soap is soluble; with lime, insoluble; and with ammonia, fluid and milky, known as volatile liniment. The volatile oils, resin, and some other organic principles of plants, are soluble to a large extent in the fixed oils.
All fixed oils contain two distinct principles, one of which is fluid at ordinary temperatures, and the other concrete; some having a large excess of the former and others of the latter principle. The fluid quality is called olein; the solider principle is stearin or margarin. These principles are supposed to be of the nature of acids, whence they combine with the alkalies, setting free the glycerin which formerly acted the part of a base toward them. The acid principles are thus named oleic acid, and stearic or margaric acid. By cooling the fat down to a low point, and then subjecting it to pressure in linen bags, or between folds of blotting paper, the olein may be separated, while the stearin remains; or the olein may be dissolved in boiling alcohol, which leaves the stearin unaffected. The animal oils are variously treated in manufactories for the separation of these principles, which are applied to different purposes.
The Physiomedical Dispensatory, 1869, was written by William Cook, M.D.
It was scanned by Paul Bergner at http://medherb.com