plication of heat, and merely by pressure, is the purest; but the quantity which seeds in general yield is comparatively small, and some seeds which contain a considerable portion of oil scarcely afford any when treated in this way. It therefore becomes necessary for extracting the oil from seeds of the latter description, and to have it in greater abundance from all seeds, to employ heat to facilitate the separation of the oil from the mucilage, or other matters with which it is combined. For this purpose heat is applied, either to the apparatus which is employed in pressing out the oil, or the bruised seeds are exposed to the vapour of water, and sometimes they are boiled in the water itself; by which means those substances which are soluble in water are separated, and thus the oily part which adhered to these substances is disengaged. The oils which are obtained in this manner are very impure. They are mixed with mucilage, and other parts of the substances from which they have been extracted. Many of these matters separate from the oils when they are left at rest. They are sometimes mechanically purified by filtration through coarse cloths, by which means the grosser parts are separated. Different oils too, it is said, undergo different kinds of purification by different manufacturers, but these processes are kept secret. After they have remained at rest for some time, they are filtered and agitated with water, by which the parts that are soluble in this fluid are separated from the oil. Sometimes they are gently heated, for a shorter or longer time, according to the nature of the substances with which the oil is contaminated. Acids diluted with water are employed to separate the mucilage; lime and the alkalies are also used to combine with an acid which holds this mucilage in solution, and thus to favour its precipitation. Alum, chalk, clay, and ashes, are also employed in the purification of oils.

Fixed oils are generally liquid, but of a thick, viscid consistence, and in general they are lighter than water. The specific gravity varies from 0.91, which is that of olive oil, to 0.94, that of linseed oil. The boiling point of the fixed oils is not under the temperature of 600°. When exposed to cold they congeal, and even crystallize. There is, however, a considerable variety in this respect among fixed oils: some become solid at the temperature of a few degrees above the freezing point of water; while others, on the contrary, require a degree of

cold=5o; and some remain fluid when exposed to the greatest cold. Those oils, it has been observed, which most readily become solid, such as olive oil, are least subject to change; while those which congeal with difficulty have a greater tendency to spoil and become rancid. When fixed oil is exposed to heat it does not evaporate, till it is raised to the temperature of boiling, or 600°; but when it is thus raised in vapour its properties are changed. It is decomposed by the separation of some of its principles. The part that is volatilized has a greater proportion of hydrogen; charcoal is deposited, and water and sebacic acid are formed, while carbonated hydrogen gas is disengaged. When oil is exposed to the open air, and a burning body is brought in contact with it, it readily takes fire, and burns rapidly, with a yellowish white flame. It is on this conversion of oil into vapour, and the inflammation of this vapour, that the application of oil in lamps and candles depends. The oil is gradually and in small quantities - brought in contact with the burning part of the wick; it is converted into vapour, which is immediately inflamed, and continues to burn till new portions are supplied to undergo the same change, and thus keep up a constant and uniform light and heat. According to the analysis of olive oil by Lavoisier, it is composed of hydrogen and carbon, viz.

The fixed oils are insoluble in water. When it is necessary to combine them with this liquid, it is by means of mucilaginous substances, in which case the mixture is known under the name of emulsion, or with alkaline substances, when it is distinguished by the name of soap. Some of these oils become thick, opaque, white, granulated, and are analogous in appearance to tallow. Oils subject to this change are called fat oils; such, for instance, is olive oil, almond oil, and rape-seed oil. This change is more or less rapid in different circumstances. If a thin layer of oil be spread on the surface of water, and exposed to the air, it takes place in a few days, and this effect is owing to the absorption of oxygen, which combines with the oils. But other oils, when they are exposed to the air, dry altogether, yet have the property of retaining their transparency, Oils which have this peculiar property are

called drying oils. The oil of poppies, hemp seed oil, and particularly linseed oil, are possessed of this property. The nature of the change which takes place in these drying oils is supposed to depend on the absorption of oxygen; and this oxygen combining with the hydrogen of the oil forms water. This opinion is supported by the practice which is followed to increase the drying property of linseed oil. It is usually boiled with litharge, before it is employed by painters. The litharge in this case is partly reduced to the metallic state, by being deprived of its oxygen, which is supposed to combine with the oil. Phosphorus combines with oils, with the assistance of heat. A small portion of the phosphorus is dissolved, which communicates a luminous property to the oils, so that when they are spread upon any surface they shine in the dark. Hence some twenty years ago a person exhibited in London, as the everlasting lamp of the ancients, a vessel containing phosphorns immersed in oil.

The various purposes to which fixed oils are applied, are too well known to require particular enumeration. They are employed in domestic economy, either as articles of food, and for this purpose are used alone, or in combination with other substances; or they are employed for giving light, by being burnt in lamps. They are used in medicine, either on account of the properties which peculiar oils possess, or on ac. count of the properties they communicate to other substances with which they are combined. In this state the use of oils is well known in the form of unguents, plasters, and liniments. In the arts, fixed oils are of the most extensive utility. They are employed in the fabrication of soaps, for mixing colours in painting, for some kinds of varnish, and for defending substances from the action of air and moisture.

Volatile oils are distinguished from the fixed oils by their volatility, fragrance, and acrid taste. They are also known under the name of aromatic oils, from their odour; or essential oils, or simply essences, from being supposed to constitute the essence or the existence of the vegetable matters which furnish them. Volatile oils are not limited to particular parts of plants, but are found to exist in every part of the plant, excepting in the seed, which furnishes the fixed oils. A great number of roots, which are generally distinguished by an aromatic odour, and have more or less of an acrid taste, afford volatile oils. They are furnished also

by many woods, such as those of the pine and fir tribe, and by many of those which are natives of warm climates. The leaves of a great number of plants belonging to the Didynamia class also afford volatile oil, as well as many of the umbelliferous plants. It is obtained also from many flowers of vegetables, and also from the covering of many fruits, as the skin of oranges and lemons. It is likewise obtained from a great number of seeds; but it is never found in the cotyledons or lobes themselves, but only in the external covering. The quantity of volatile oil which is obtained from vege tables, varies according to the age, the soil in which they grow, and the state of the plant. Some plants while green furnish it in greatest abundance, while others yield most when they are dry. There are two processes by which volatile oil may be obtained. When it exists in plants in great abundance, and in vesicles in a fluid state, it may be separated by mechanical means. Thus, by simple expression, the volatile oils are extracted from many plants, as, for instance, from the fruit of the orange and the lemon. From the outer rind of these fruits, when they are fresh, the volatile oil is obtained in the liquid form; but in general the volatile oils of plants are neither so abundant, nor do they exist in that state of fluidity by which they can be procured by so simple a process. In most cases they are subjected to the process of distillation; and for this purpose they are macerated for some hours in water. They are then introduced into a still with the water; a moderate heat is applied and continued till the fluid boil, when a great quantity of vapour of water, mixed with the volatile oil, passes over, and is received in proper vessels. The oil collects on the surface of the water, from which it may be easily separated. The water itself is of a milky colour, on account of a small quantity of oil suspended in it; and even after the water becomes transparent by the particles of the oil separating from it, and rising to the top, it is still loaded with the peculiar odour of the plant. The volatile oils are particularly distinguished by their fragrance, which varies in the oils extracted from different plants. The consistence of the volatile oils also va ries considerably. Sometimes they are as fluid as water, which is the case with those oils obtained by expression. Some are thick and viscid, as those generally are which are extracted from woods, roots, barks, and fruits of the warmer regions. Some con

geal, or assume a granulated solid consistence at different temperatures. Of these last some are always found to be in the concrete state. Several of the volatile oils are susceptible of crystallization, depositing in the remaining portion of the oil, which continues liquid, transparent polyhedrons, more or less of a yellow colour, which are found to be pure oil. This last change is probably owing to an incipient oxydation; for it never takes place unless oils have been kept for some time. There is great variety of colour among volatile oils. Some indeed are nearly colourless, as the oil of turpentine; but in general they are of different shades of colour. Some are yellow, as the oil of lavender; some are of a reddish yellow or brown, as the oil of cinnamon or of rhodium; some are blue, as the oil of chamomile; and some are green, as that of parsley. But the most prevailing colour among volatile oils is yellow or reddish.

Volatile oils have almost always an acrid, hot, and even barning taste. It is observed that the most acrid vegetable matters do not yield an oil possessed of this quality. The specific gravity of volatile oils is generally less than that of water. Some volatile oils, however, as those of sassafras and canella, have a greater specific gravity. The specific gravity of oils varies from 0.87 to 0.99, in those which are lighter than water; but those which are heavier are from 1.03 to 1.40. When volatile oils are exposed to the light, the colour becomes considerably deeper; they become thicker, and increase in specific gravity. When volatile oils are exposed to heat, they evaporate very readily. They are much more combustible than the fixed oils; and in burning give out a great quantity of smoke, a very bright white flame, and a good deal of heat. They require a greater proportion of oxygen than the fixed oils, and yield a greater quantity of water. This arises from a greater proportion of hydrogen, and a smaller quantity of carbon, which they contain. The volatile oils are in some degree soluble in water. When they are agitated with this liquid they combine with it, and communicate a very strong odour, and a slightly acrid taste. Phosphorus and sulphur are soluble in volatile oils. With phosphorus the solu tion is luminous in the dark, is extremely fetid, and gives ont by the force of heat phosphorated hydrogen gas. Some of these oils are employed in medicine. They are used also for the solution of those substances which are to be employed as varnishes;

and many of them are used in perfumery. As many of the volatile oils are produced but in small quantity, they are consequently high priced. There is therefore some temptation to adulterate them with fixed oils, with cheaper volatile oils, or with other substances, to increase the quantity. It is therefore of some importance to be able to detect such frauds. When a volatile oil is adulterated with a fixed oil, there is a very easy test to discover it. Let a single drop of the oil that is suspected fall on clean pa per, and expose it to a gentle heat. If the oil is pure, the whole will be evaporated, and no trace remain on the paper; but if it has been mixed with a fixed oil, a greasy spot remains behind. Volatile oils are frequently adulterated with oil of turpentine; but this can only be detected by its peculiar odour, which continues for a longer time than most of the other volatile oils. When they are adulterated with alcohol, it is easily detected by mixing a little of the oil with water, which immediately produces a milkiness, by the abstraction of the alcohol from the oil, and its combination with the water. There is another class of oils, known under the name of empyreumatic oils, which have different properties from those which have been described. These oils are acrid and stimulating, with a strongly fetid and disagreeable odour. It would appear that these properties are owing to a partial decomposition of other oils. These oils are produced, as the name imports, by the action of fire. They are obtained when oils are forced to rise in vapour, and pass over in common distillation, with a greater degree of heat than that of boiling water, or by the application of a strong heat to sub- . stances from which no oil was previously extracted. These empyreumatic oils agree in some of their properties with the volatile oils. They combine in small proportion with water, and they are soluble in alcohol; and probably any difference that exists between them is owing to a partial decomposition; for when they are distilled, the oil is

restored to a state of purity, and the carbonaceous matter which had been separated remains behind. See Thomson's Chemistry. OINTMENT. See PHARMACY.

OLAX, in botany, a genus of the Triandria Monogynia class and order. Natural order of Sapota, Jussieu. Calyx entire ; corolla funnel-form, trifid; nectarium four; berry three-celled, many-seeded. There is but one species, viz. O. Zeylanica, a native of Ceylon.

in

OLD age. See LONGEVITY. OLDENBURG, (HENRY), in biogra. phy, who wrote his name sometimes Grubendol, reversing the letters, was a learned German gentleman, and born in the duchy of Bremen, in Lower Saxony, about the year 1626, being descended from the counts of Aldenburg in Westphalia; whence his name. During the long English Parliament, in the time of Charles I., he came to England as consul for his countrymen; which capacity he remained at London in Cromwell's administration. But being discharged of that employment, he was engaged as tutor to Lord Henry O'Bryan, an Irish nobleman, whom he attended to the University of Oxford; and in 1656, he entered himself a student in that university; chiefly to have the benefit of consulting the Bodleian Library. He was afterwards appointed tutor to Lord William Cavendish, and became intimately acquainted with Milton the poet. During his residence at Oxford, he became also acquainted with the members of that society there which gave birth to the Royal Society; and upon the foundation of this latter, he was elected a member of it; and when the society found it necessary to have two secretaries, he was chosen assistant to Dr. Wilkins. He appli. ed himself with extraordinary diligence to the duties of this office, and began the publication of the "Philosophical Transactions," with Number 1, in 1664. In order to discharge this task with more credit to himself and the Society, he held a correspondence with more than seventy learned persons, and others, upon a great variety of subjects, in different parts of the world. This fatigue would have been insupportable, had he not, as he told Dr. Lister, managed it so as to make one letter answer another; and that, to be always fresh, he never read a letter before he was ready immediately to answer it; so that the multitude of his letters did not clog him, nor ever lie upon his hands. Among others, he was a constant correspondent of Mr. Robert Boyle, and he translated many of that ingenious gentleman's works into Latin.

About the year 1674, he was drawn into a dispute with Mr. Hook, who complained, that the Secretary had not done him justice, in the History of the Transactions, with respect to the invention of the spiral spring for pocket-watches: the contest was carried on with some warmth on both sides, but was at length terminated to the honour of Mr. Oldenburg; for, pursuant to an

open representation of the affair to the Royal Society, the Council thought fit to declare, in behalf of their Secretary, that they knew nothing of Mr. Hook having printed a book entitled "Lampas," &c., but that the publisher of the "Transactions" had conducted himself faithfully and honestly in managing the intelligence of the Royal Society, and given no just cause for such reflections.

Mr. Oldenburg continued to publish the "Transactions" as before, to Number 136, June 25, 1677; after which, the publication was discontinued till the January following, when they were again resumed by his successor in the secretary's office, Mr. Nehemiah Grew, who carried them on till the end of February, 1678. Mr. Oldenburg died at his house at Charlton, between Greenwich and Woolwich, in Kent, August 1678, and was interred there, being fiftytwo years of age.

He published, besides what has been already mentioned, twenty tracts, chiefly on theological and political subjects; in which he principally aimed at reconciling differences and promoting peace.

OLDENLANDIA, in botany, a genus of the Tetandria Monogynia class and or des. Natural order of Stellatæ. Rubia. ceæ, Jussieu. Essential character: calyx fastened to the pericarpium with four awlshaped teeth at top; corolla one-petalled, four cleft; capsule inferior, two-celled; receptacle free, fastened to the partition by the base only. There are sixteen species.

OLEA, in botany, olive, a genus of the Diandria Monogynia class and order. Natural order of Sepiariæ. Jasmineæ, Jussieu. Essential character: corolla fourcleft, with sub-ovate segments; drupe oneseeded. There are seven species, of which the O. longifolia, long-leaved European olive, is chiefly cultivated in the south of France, from which they make the best oil. O. latifolia, broad-leaved European olive, is principally cultivated in Spain, where the trees grow to a much larger size than the preceding; the fruit is nearly the size of a Provence olive; but of a stronger flavour, for which reason it is not so grateful to an English palate. The olive seldom becomes a large tree; two or three stems frequently rise from the same root, from twenty to thirty feet in height, putting out branches almost their whole length, covered with a greyish bark.

The olive, in all ages, has been held in peculiar estimation, as the, bounteous gift

OLYRA, in botany, a genus of the Monoecia Triandria class and order. Natural order of Gramina, Gramineæ, or Grasses. Essential character: male, calyx glume oneflowered, awned; corolla glume awnless. Female, calyx glume one-flowered, spreading, ovate; style bifid; seed cartilaginous. There are two species, viz. O. paniculata, and O. pauciflora, both natives of Jamaica.

OMENTUM, the cawl, in anatomy, a membranaceous part, usually furnished with a large quantity of fat; being placed under the peritoneum, and immediately above the intestines. See ANATOMY.

OMNIUM, a term in familiar use among

stock-brokers and speculators in the funds, to express the whole of the articles which the subscribers to a loan receive from government. Thus if the subscribers, according to their agreement with government are to have for every hundred pounds advanced a certain sum in 3 per cent. consols, a further sum in 4 per cents, and a proportion of the long annuities, the blank receipts which they receive for making the instalments on the several articles, are, when disposed of independent of each other, as the 3 per cent. consols only, called scrip, but when the receipts are sold together as originally received, they are As the omnium usually called omnium.

of every loan is the subject of extensive speculations, it generally is liable to considerable variations with respect to its current price, sometimes selling at a high premium, at other times at a discount, according to the circumstances which take place between the agreement for the loan and the day fixed for paying the last instalment. Thus the omnium of the year 1799, was at first at 4 and 5 per cent. premium; on the 20th of August it had risen to 193, and on the 3d September was at 224; it soon after fell considerably, and on the 14th October was at 41, 24, 31; but on the 18th November it had got up again to 12 per cent. premium. The omnium of the year 1801 rose on the signing of preli minaries of peace, to 18 per cent., and was soon after at 25 per cent. premium: the omnium of the following year was at one time at 12 per cent, discount.

OMPHALEA, in botany, a genus of the Monoecia Monadelphia class and order. Natural order of Tricoccæ. Euphorbiæ, Jussieu. Essential character; male, calyx four-leaved; corolla none; filaments columnnar, with the anthers inserted into it: female, calyx five-leaved; corolla none; stigma trifid; capsule fleshy, three-celled; nuts solitary. There are four species, all natives of Jamaica.

ONCHIDIUM, in natural history, a genus of the Vermes Mollusca class and order. Body oblong, creeping, flat beneath, mouth placed before; two feelers, situate above the mouth; two arms at the side of the head; vent behind and placed beneath. There is but a single species, viz. O. typha, the onch, which is described in the Transactions of the Linnæan society. It inhabits Bengal, on the leaves of the typha elephantina about an inch long, and not quite so broad, but linear, and longer