OILS. Under this head is ranged a group of organic compounds of great interest, both on account of their great economic value, and from the fact that they occur abundantly both in animals and plants. They consist chiefly of carbon and hydrogen, but more or less of oxygen is generally associated with them, and causes considerable
variation in their qualities. They are either solid or liquid, and in the former condition are more frequently termed fats. These fats are more abundant in the animal than in the vegetable kingdom. Oils, whether liquid or solid, usually consist of three other substances, two of which, stearine (stear, suet), and margarine (margar, a pearl), are solid; and the other, elaine or oleine, is liquid at ordinary temperatures. They are all from 6° to 9° lighter than water, and their liquid or solid condition depends upon the proportion in which their component parts are mixed. Thus, in the fats the oleine exists in small quantity, and in the liquid oils it is the chief constituent. A certain degree of heat is necessary to the mixture, for at low temperatures there is a tendency to separation; the stearine and margarine are precipitated and solidified, and, if pressed, can be entirely freed from the oleine. Both oils and fats, when boiled with water and alkali, undergo the peculiar process of saponification, or, in other words, solidify, and become converted into soap; during this process a liquid of a sweet taste, called glycerine, is given off. (See GLYCERINE). Glycerine exists in oil and fats as a base to which stearic, margaric, and oleic acids are united, forming stearine, margarine, and oleine.
The principal uses to which oils and fats are applied are soap-making, illumination by candles or oil, lubricating machinery, and dressing cloth. They are easily separated by moderate pressure from the animal or vegetable tissues which contain them, but are not usually pure until they are rendered so by clarifying.
Of the animal oils, those are chiefly solid which are yielded by the mammals and birds; whilst those derived from reptiles and fishes are for the most part liquid at the ordinary temperature. The true oils and fats are unchanged when heated even to a temperature above 400°; but there is another group of compound substances, termed essential oils, yielded by the vegetable kingdom, which are volatile at ordinary temperatures; hence the term fixed oils is often applied to the former, and volatile oils to the latter. The volatility of the essential oils renders distillation a ready means of procuring many of them. They resemble the fixed oils in many respects, but differ materially in others; for instance, they do not undergo saponification when treated with alkalies; like them, however, they often separate at low temperatures into solid and liquid portions,—the former called stearopten, the latter elaopten. They are very slightly soluble in water, and they differ materially in the sensation they produce on the skin. Instead of the smooth soft feel of the true oils, they are harsh and rough to the touch. The essential oils are mostly pure hydro-carbons, but many are capable of absorbing oxygen when exposed to the air, which darkens their colour, and renders them resinous in appearance, a result which may be seen generally around the mouths of bottles in which they are kept. Some are obtained already oxidized, and some are found to contain sulphur: hence they have been classified as pure hydro-carbons, when free from oxygen; oxidized essences, when obtained in combination with oxygen; and sulphuretted essences, when combined with sulphur. The combinations which the essential oils enter into render them peculiarly interesting to the student in organic chemistry. They appear to be the cause of all the more remarkable odours and flavours which characterize plants; and as they can generally be separated easily, they are very valuable in an economic point of view, affording us the means of concentrating and retaining the perfumes of the most evanescent flowers, and in the same way of preserving the most pungent and delicate flavours.
Essential oils are in some instances procured by simple pressure, as those from the rind of the orange tribe; others are distilled with water, and float upon the condensed water in the receiver. Some, however, are so easily destroyed by these processes, that they can only be obtained by the power
Oils. which the fixed oils have of absorbing them. Thus, essence of jessamine is obtained by placing layers of the freshly-gathered flowers between layers of cotton-wool saturated with the fixed oil of almonds or of poppy seed, both of which are themselves odourless. They, however, soon absorb the essential oil naturally emitted from the flowers, and become highly perfumed. Fresh layers of flowers are supplied until the fixed oil is saturated, when it is pressed out from the cotton-wool.
Rock oil. Besides the fixed oils and fats, and the essential oils, there is a mineral product called rock oil; it is not, however, properly speaking, an oil, but is a variety of petroleum, which exists abundantly in some bituminous shales. Works have been established in Dorsetshire and other parts of the kingdom for obtaining this material, but without much success. Large quantities are, however, brought from Rangoon, in the Burmese empire, chiefly to Liverpool, where nearly 700 tons weight were imported in 1857. Works exist at St Helen's, near Liverpool, and other places, where from this material, which is of a dark greenish-brown colour, and nearly the consistency of butter, a light amber-coloured oil-like liquid is obtained, said to be very useful as a lubricant for machinery. A considerable proportion of paraffine is obtained from it, and it yields a very volatile naphtha.
Sources of oils. In enumerating the oils, those produced from the animal kingdom will be first mentioned, and in the order of zoological classification; then the fixed vegetable oils; and finally the three classes of essential oils. Of the twelve orders into which naturalists divide the mammals, only four comprise animals which yield oils of economic value. These orders are Carnivora, Cetacea, Ruminantia, and Pachydermata.
Animal oils. Of the carnivorous animals yielding oleaginous products, we have first the bear. The black bear (Ursus Americanus, Gmelin), a native of North America, yields an abundance of grease or soft fat; which is collected by the hunters who pursue the animal for its skin and hams, and is occasionally imported into this country, not, as some may suppose, to be used by the hair-dressers and perfumers for pomades, but for the more useful manufacture of candles, &c. But the principal oil-producing carnivora are the seals, several species of which are killed on purpose. The quantity of seal oil imported is very great. Most of it comes from Newfoundland. The species which chiefly yield it are Callocephalus Gruslandicus, Callocephalus vitulinus, Phoca barbata, and Arctocephalus Falklandicus. The part which yields the oil in these animals is the blubber, a peculiar layer of oil-cells which lies immediately under the skin of the animal, and in fact constitutes a portion of the skin itself. Seal oil is liquid, and, when pure, of a pale straw-colour. The first drawings from the blubber give the purest oil, which is obtained without pressure; but the succeeding drawings from the blubber-cask are more or less deeply coloured brown by the decomposition of the oil-cells.
The Cetacea are remarkable for the extent of this peculiar skin-development called blubber, which in some species is one or two feet in thickness. This is not, however, the only source of the whale oil; for in the great-headed cachalot or sperm-ceti whale (Cetodon macrocephalus, Cav.), the gigantic head, which nearly equals the body in bulk, has an enormous receptacle on the upper part of the skull, from which oil is obtained. This receptacle consists of a dense bag, divided into numerous large cells or compartments, in which the oil exists in a semi-fluid state, owing to the large quantity of stearine or spermaceti which it contains, and which can easily be separated from it by simple draining or slight pressure. The quantity of oil yielded by some of the larger whales, especially the cachalots, is enormous, but is often erroneously stated, owing to a misunderstanding of the fact that quantity in liquid oil is calculated by the ton measure, and not by the ton weight, as with solid oils or fats. A cachalot commonly yields 20 tons, or 6040 gallons; and single whales have been known to yield 30 tons (7560 gallons). The following oils derived from the whale tribe are known in commerce:—Sperm Oil, and its stearine, Spermaceti, from the cachalot; Triton or Common Whale Oil, from the Right Whale (Balaena mysticetus) and other species;
Porpoise Oil, from both Globicephalus deductor and Globicephalus Scirceus; and Porpoise Oil, from Phocaena communis. These oils are usually imported as train or sperm oils; but the brokers are well skilled in distinguishing them.
Of the ruminating animals, we have only two species which yield oleaginous products; but they are unequalled in value by any others. They are the ox (Bos taurus, Linn.), and the sheep (Ovis aries, Linn.). The fat of both these animals melted down constitutes the tallow of commerce. They are so generally mixed together that there is no possibility of ascertaining the exact amount yielded by each. We receive our largest supply from Russia; but we import considerable quantities from Denmark, Prussia, the Hanse Towns, Holland, Turkey, South America (particularly Buenos Ayres and Monte Video), the Cape of Good Hope, the East Indies, Australia, &c. That imported from the ports of Monte Video and Buenos Ayres is chiefly, if not altogether, ox tallow; whilst that from Australia is principally from the sheep. Besides the enormous quantity imported, we have to take into consideration that which is produced in Great Britain, which has been computed to be equal to the amount imported.
From the bones of the feet of oxen a valuable oil is obtained. It contains comparatively little of the harder portion (stearine), and is in a fit state to be used for machinery. It is procured by boiling the bones, and skimming off the fatty oil as it rises to the surface of the water. It is called neat-foot oil; and from the fact that it remains liquid at a temperature below 32°, and is not liable to rancidity, it is peculiarly well adapted for turret clocks and other machinery much exposed to cold. The supply from such a source is necessarily limited.
Of the Pachydermata only two yield oily products of any commercial importance. The first of these in point of value is the common hog (Sus scrofa, Linn.), the fat of which, under the name of lard, is very extensively used. Considerable quantities are consumed in articles of food. Most of the ointments of the pharmacist have lard for their base; and when too rancid for these purposes, it is used for greasing machinery, especially the axles of railway carriages. In the United States the production of lard is immense; and its stearine, which is easily separable from the lard, is extensively used in the manufacture of candles. The liquid stearine, under the name of lard oil, is used for the finer parts of machinery, and for that purpose is extensively imported into this country from Europe and America. The fat of the horse does not, when melted, possess the same firm consistency as that of the ox, sheep, and swine. The proportion of stearine in it being comparatively small, it is only within the last eight years that it has attracted any attention; but now it constitutes an important article of trade with Buenos Ayres and Monte Video, whence it is imported under the names of horse or mare's grease. The latter name is, however, more generally applied. From its liquidity, it is extremely penetrating; hence the ordinary packages for grease and tallow were found to be insufficient, as the casks were frequently half empty on their arrival. This checked its introduction for some time; but it is now put into square boxes lined with tin, and arrives without loss. It is found to answer very well as a lubricant for machinery.
From the classes comprising the birds and the reptiles (Aves and Reptilia) neither oils nor fats of any importance are obtained, although the domesticated birds sometimes produce it in abundance. That of the goose, under the name of goose-grease, is occasionally heard of as a useful domestic remedy for various ailments.
The class of fishes (Pisces) is a considerable source of oil, always of a clear liquid quality. It is nearly all yielded by one species, the common cod-fish (Gadus Morhua, Linn.). The Cod Oil, and the Cod-liver Oil of commerce, are both obtained from the liver of the fish; the latter, which is now so extensively used medicinally, being only prepared with a little more care. Its principal value as a remedial agent appears to depend upon its nutritive qualities, and the digestive powers of a portion of pepsin, or biliary matter, which is always present, and which may be detected by the application of a drop of concentrated sulphuric acid, when, if the oil be really cod-liver oil, a beautiful purple colour will be immediately produced. The number of cod-fish captured is incalculable. The cod-fishers, in opening the fishes to salt and dry them, carefully preserve the livers, for which an extra boat is usually in at-
tendence; these are taken on shore, and piled up in immense masses exposed to the sun. The heat soon makes the oil run from the livers in considerable abundance, and for a short time it is very clear, and of a light straw colour,—this is the first quality, and is kept by itself. As the livers begin to decompose, however, they give a darker colour to the oil, and several qualities are obtained, the last of which is thick, turbid, and extremely offensive to the smell, and is known under the name of cod-pitchings. A small quantity of oil made from the common herring (Clupea harengus, Linn.) is imported from time to time from North America; but its strong and unpleasant odour prevents it from being much used.
The vegetable sources of oils are very numerous, and some are of great importance.