Oils And Fats, an important natural group of organic compounds found in the various parts of plants, particularly the seeds, and in animals, principally in the adipose tissues. (See Adipose Substances.) In vegetables there are two kinds of oils, totally distinct and having a different chemical formation, viz.: the fixed, which are analogous to the animal oils and fats, and the volatile or essential oils; and there is also a class of oils and fats which are the result of destructive distillation. These last and the volatile oils will be found treated under the heads Coal Products, Paraffixe, Petroleum, and Essential Oils. The natural oils and fats, which alone are the subjects of this article, are now regarded as the compound ethers of glycerine, a triatomic alcohol (see Glycerine), and may be artificially formed by the action of this alcohol upon certain monobasic acids. The principal elements in their composition are carbon and hydrogen, oxygen entering as a constituent in smaller proportions; the solidity of the fatty body being generally in proportion to the amount of carbon, and its fluidity in proportion to that of oxygen. When separated from the organism the fatty bodies which are solid at ordinary temperatures are called fats, while those which are liquid are called oils.

The fatty bodies taken from warm-blooded animals are generally solid at ordinary temperatures, but those obtained from fish and other cold-blooded animals are principally liquid. One of the distinguishing characteristics of the oils and fats is that they arc lighter than water, the specific gravity varying from 0.91 to 0.94. They are chiefly composed of three proximate principles, stearine, palinitine, and oleine, the first two being solid at ordinary temperatures, the last liquid. The mixture of the three therefore varies in softness according to the proportion of oleine which it contains. These proximate principles are compounds of the triatomic alcohol glycerine, acting as a base, and stearic, palmitic, and oleic acids, and may therefore be regarded as organic salts. It is to the investigations of Chevreul, made about 1820, that we owe our fundamental knowledge of the fatty bodies. Since then others, and particularly Berthelot, have extended his researches, and in the main confirmed their correctness. The nature of these bodies was well defined by Chevreul, but he regarded them as compounds of stearine, oleine, and margarine.

It has however been shown by Heintz that ChevreuPs margarine is not a simple fat, but a mixture of palmitine and stearine; for when it is saponified, the acid obtained from the soap is found to be a mixture of palmitic and stearic acids. The natural oils and fats may be heated to nearly 500° F. without much change; but they cannot be distilled without decomposition, by which they are distinguished from the volatile oils, the latter evaporating and distilling at various temperatures. At about 500° they begin to evolve acrid and offensive vapors, and at about 600° they are decomposed with evolution of gaseous hydrocarbons. When heated with caustic alkalies they undergo a peculiar change called saponification or conversion into soap, during which process glycerine is liberated, while the alkali combines with the oleic, stearic, and palmitic acids. (See Soap.) All the natural oils and fats are soluble in ether, and to a certain extent in alcohol. Oil of turpentine and benzole also readily dissolve them, and they mix with each other in all proportions. - The fixed oils are divided into drying and non-drying oils. Drying oils when exposed to the air thicken from absorption of oxygen, being converted when spread upon surfaces into a tough transparent membrane or varnish.

Linseed, nut, hemp, and poppy oils belong to this class, and contain an oleine which differs from that of the non-drying oils, yielding by saponification, instead of oleic, lin-oleic acid or one similar to it. (See Drtixg-Oils, and Lixseed Oil.) The non-drying oils are also gradually altered by exposure to the air, but in a different way; they lose much less fluidity, become acid, and acquire an acrid, disagreeable taste. This alteration, however, never takes place in pure glycerides, as pure stearine, palmitine, or oleine, or mixtures of them; but only when other organic matters, such as the cellular substance of the plant or animal in which the oil naturally exists, are present. These substances contain nitrogen, and act as ferments, producing decomposition of a part of the fatty matter with which they are mixed; by this action stearic, palmitic, and oleic acids are set free, and small quantities of certain volatile acids, as butyric, valerianic, and caproic, are formed, probably from atmospheric oxidation. By treatment with boiling water, and afterward in the cold with a weak alkaline solution, rancid oils may be purified and restored to their original condition. - The uses of the fatty oils are extensive. The drying oils are used in the preparation of paints, varnishes, and cements.

They are also used in medicine, often in the forms of liniment, as linseed oil in the lime-water liniment, or the linimentum calcis of the pharmacopoeia, an excellent application to burns and abrasions of the skin. The non-drying oils are used in the manufacture of soap, for lubricating machinery, for illumination by various methods, for the preparation and preservation of food, and also in medicine.