OIL-REFINING. Several oils, from the mode of their extraction, are necessarily impure, and various means are taken for refining or purifying them: thus, the so-called fish-oils—that is, whale, seal, cod, etc.—are clarified either by mixing them with a chemi cal solution, or by passing steam through them and through coarse charcoal. The chemical solutions employed are various. One method is, to use a strong solution of oak bark, the tannic acid in which combines with the albuminous matters present in the oil, and precipitates them; another plan is, to agitate bleaching-powder, formed into a milk with water, with the oil; and then, after subsidence of the chloride of lime and water, to wash the oil with water, or jets of steam passed through it. A more simple and very effective plan, invented by Mr. Dunn, is to apply a steam heat not exceeding 200° F., and then pass a current of air of the same temperature through it continuously for some time: this effectually bleaches the oil.
Olive, and some other vegetable oils, are refined by agitating them with a saturated solution of caustic soda. This renders the whole soapy; but after a time the oil precip hates a saporaceons deposit, and the remainder becomes quite clear and pure, and is then poured off. The value of several of the most important oils of commerce is so greatly increased by refining, that this art has now become a very important branch of business, and is carried out on a large scale.
OILS (including fats). The fats and fixed oils constitute an important and well marked group of organic compounds, which exist 'abundantly both in the animal and vegetable kingdoms. They are not simple organic compounds, but each of them is a mixture of several such compounds to which the term glycerides is applied; and the glycerides which by their mixture in various proportions form the numerous fats and oils are mainly those of palmitic, stearic, and oleic acids—if we adopt the recent view that margaric acid (q.v.) has no independent existence—and to a less extent those of other fatty acids, which will be presently noticed, such as butyric, caproic, capulic, and capric acids, which are obtained from butter; myristic acid, which is obtained from cocoa-nut oil, etc. The members of this group may be solid and hard, like suet: semi solid and soft, like butter and lard; or fluid, like the oils. The solid and semi-solid are, however, generally placed together and termed fats, in contradistinction to the fluid oils.
The most solid fats are readily fusible, and become reduced to a fluid or oily state at a temperature lower than that of the boiling-point of water. They are not volatile, or, in other words, they cannot be distilled without decomposition, and it is not until a tem perature of between 500° and 600° is reached that they begin nearly simultaneously to boil and to undergo decomposition, giving off acroleine (an acrid product of the distil lation of glycerine) and other compounds. In consequence of this property, these oils are termed fixed oils, in contradistinction to a perfectly separate group of oily matters. on which the odoriferous properties of plants depend. and which, from their being able to bear distillation are known as volatile oils. These, which are also known as essential or ethereal oils, differ in. foto in their chemical composition from the compounds we are now considering, and will be separately noticed in the latter part of this article. All the fats and oils are lighter than water, and arc perfectly insoluble in that fluid. Their specific gravity ranges from about 0.91 to 0.94. They dissolve in ether, oil of turpentine (one of the volatile oils). benzoic, and to a certain extent in alcohol; while, on the other hand, they act as solvents for sulphur, phosphorus, etc. If a fatty matter be shaken with a watery solution of albumen, gum, or some other substance that increases the density of the water, and renders it viscid, the mixture assumes a milky appearance, in, consequence of the suspension of the fat or oil in the form of microscopic globules, and is termed au emulsion. These bodies possess the property of penetrating paper and other fabrics, rendering them transparent, and producing what is well known as a greasy stain. They are not readily inflammable unless with the agency of a wick, when they burn with it bright flame. In a pure and fresh state they are devoid of taste and smell, but on exposure to the air they become oxidized and acid, assume a deeper color, evolve a odor, and are acrid to the taste; or, in popular language, they become rancid. The rapidity with which this change occurs is considerably increased by the presence of mucilaginous or albuminous bodies. The rancidity may be removed by shaking the oil in hot water in which a little hydrated magnesia is suspended.