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 ove in common distillation, with a greater degree of heat than that of boiling water, or by the application of a strong heat to substances 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 Sapote, Jussieu. Calyx entire; corolla funnel-form, trifid; nectarium four; berry three-celled, manyseeded. There is but one species, viz. O. Zeylanica, a native of Ceylon. OLD age. See LONGEVITY. OLDENBURG, (HENRY,) in biography, 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; in 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 applied 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 corre spondent of Mr. Robert Boyle, and he translated many of that, ingenious gen. tleman's works into Latin. About the year 1674, he was drawn into a dispute with Mr. Hook, who comhim justice, in the History of the Transplained that the Secretary had not done actions, 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 termifor pursuant to an open representation nated to the honour of Mr. Oldenburg; of the affair to the Royal Society, the Council thought fit to declare, in behalf thing of Mr. Hook having printed a book, of their Secretary, that they knew nopublisher of the "Transactions" had conentitled "Lampas," &c., but that the ducted 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 fifty-two 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 re conciling differences, and promoting peace. OLDENLANDIA, in botany, a genus of the Tetandria Monogynia class and order. Natural order of Stellatæ. Rubiaceæ, Jussieu. Essential character: calyx fastened to the pericarpium with four awl-shaped teeth at top; corolla onepetalled, 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 four-cleft, with sub-ovate segments; drupe one-seeded. 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, broadleaved 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 of Heaven; it is still considered as emblematic of peace and plenty; the great quantity of oil which it produces in some countries, effectually realizes the latter of these blessings. Unripe olives pickled, especially the Provence and Lucca sorts, are to many persons extremely grateful; they are supposed to promote digestion. OLERON laws, laws relating to maritime affairs, and so called, because made when King Richard I. was at the Isle of Oleron in Aquitaine. OLFACTORY nerves, the first pair of the head; so called from their being the immediate instruments of smelling. OLIFIANT gas, a name given by the Dutch chemists to carburated hydrogen, or heavy inflammable gas. See GAs. OLIGARCHY, a form of government wherein the administration of affairs is lodged in the hands of a few persons. See GOVERNMENT. OLIVE. See OLEA. OLIVINE, in mineralogy, a species of the Chrysolite family, found in the form OLYRA, in botany, a genus of the Monoecia Triandria class and order. Natural order of Gramina, Gramineæ, or Grass es. Essential character: male, calyx glume one-flowered, awned; corolla glume awnless. Female, calyx glume oneflowered, 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 usually called omnium. As the 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 dis count, 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 194, and on the 3d September was at 224; it soon after fell considerably, and on the 14th of October was at 44, 24, 34; but on the 18th No. vember it had got up again to 12 per cent. premium. The omnium of the year 1801 rose, on the signing of preliminaries 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 12 per cent. discount. OMPHALEA, in botany, a genus of The Monoecia Monadelphia class and order. Natural order of Tricocc.e. Euphorbiæ, Jussieu. Essential character: male, calyx four-leaved; corolla none; filaments columnar, with the anthers inserted into it: female, calyx fiveleaved; corolla none; stigma trifid; cap. sule fleshy, three-celled; nut 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 describ. ed in the transactions of the Linnean Society. It inhabits Bengal, on the leaves of the typha elephantina, about an inch long, and not quite so broad, but linear, and longer when creeping. In appearance it very much resembles a limax, but differs principally in wanting the shield and lateral pore, and in being furnished with a vent behind. Body above convex; head small and placed beneath, which when the animal is in motion is perpetually changing its form and size, and drawn in when at rest; mouth placed lengthwise, and continually varying its shape from circular to linear: feelers retractile, resembling those of the slug, and apparently tipt with eyes; arms dilatable, solid, compressed, and palmate when fully expanded. ONION, in botany, see ALLIUM. Considered chemically it may be observed, that as it possesses most of the proper ties of GARLIC (which see) though not in so large proportions, a volatile oil, on which its activity depends, might be expected, but this has not been found. Water distilled from it yields no oil; if therefore there is any oil, it must be in very small quantities, and soluble in waThe active principle of the onion acts upon the tin of the alembic in which experiments have been made. ter. ONISCUS, in natural history, a genus of insects of the order aptera. Jaw truncate, denticulate; lip bifid; antennæ from two to four, setaceous; body oval, consisting of about fourteen transverse segments; fourteen legs. These insects feed on animal and vegetable matter, and they cast their skin. There are nearly fifty species, divided into sections. A. without feelers; four antennæ, sessile. B. feelers unequal, the hind-ones longer; antennæ filiform. The most common species is the O. asellus, or common woodlouse, found in great quantities under the bark of decayed trees, beneath stones in damp situations.. It preys on minuter inescts. O. armadillo, the medical woodlouse, is of a darker colour than the for mer, but found in similar situations. When suddenly disturbed or touched, it rolls itself up into a round form in the manner of the armadillos; frequently remaining in that state for a considerable length of time. This insect was formerly considered as a specific in many disorders, but is now rarely used. Among marine insects of this genus, is the O. gaudeloupensis, measuring about one inch and three quarters in length; anpair rather longer; abdomen covered tennæ very short, compressed, inferior with six scales, beneath; tail somewhat oval, flat, furnished with a lateral style fid; the five segments of the body before on each side, the last joint of which is bithe tail are much narrowed, and destitute of feet. The female protects her young, for a considerable time after their exclusion, under the abdominal scales; in this respect resembling the opossum amongst the quadrupeds. They are often found in the mouth of the Clupea menhaden or mossbanker, attached to the palate, and partaking of its form. Cryptogamia Filices class and order. NaONOCLEA, in botany, a genus of the tural order of Filices or ferns. Generic character; capsules under the recurved and contracted pinnules of the frond, resembling pericarps. There are two species, viz. O. sensibilis, and O. polypodioides; the former is a native of Virginia, the latter was found by Koenig, in the fissures of the rocks near the top of the Table Mountain at the Cape of Good Hope. ONONIS, in botany, restharrow, a genus of the Diadelphia Decandria class and order. Natural order of Papilionacea or Leguminosa. Essential character: calyx, five-parted, with linear segments; banner striated; legume turgid, sessile; filaments connate, without a fissure. There are thirty-eight species; these are herbaceous plants or under shrubs; leaves ternate, with the leaflets often serrulate; stipules fastened to the bottom of the petiole; flowers yellow or purple, one or many flowered. Es ONOPORDUM, in botany, a genus of the Syngenesia Polygamia Equalis class and order. Natural order of Composite Capitate. Cinarocephale, Jussieu. sential character: calyx scales mucronate: receptacle honey-combed. There are seven species. ONOSMA, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Asperifoliæ. Borragineæ, Jussieu. Essential character: corolla, bell-shaped, with the throat pervious: seeds four. There are three species. ONYX. See CHALCEDONY. OPACITY, in philosophy, a quality of bodies which renders them impervious to the rays of light. It has been suppos ed that opacity consists in this, that the pores of the body are not all straight. This doctrine, however, is deficient: for though to have a body transparent, its pores must be straight, or rather open every way, yet it is inconceivable how it should happen, that not only glass and diamonds, but even water, whose parts are so very moveable, should have all their pores open and pervious every way; while the finest paper, or the thinnest gold leaf, should exclude the light, for want of such pores. So that another cause of opacity must be sought for. Now all bodies have vastly more pores or vacuities than are necessary for an infinite number of rays to pass freely through them in right lines, without striking on any of the parts themselves. For, since water is nineteen times lighter or rarer than gold, and yet gold itself is so very rare that magnetic effluvia pass freely through it, without any opposition, and quicksilver is readily received within its pores, and even water itself by compression, it must have more pores than solid parts; consequently water must have at least forty times as much vacuity as solidity. The cause, therefore, why some bodies are opaque, does not consist in the want of rectilinear pores, pervious every way, but either in the unequal density of the parts, or in the magnitude of the pores, and their being either empty, or filled with a different matter; by means of which the rays of light, in their passage, are arrested by innumerable refractions and reflections, till at length, falling on some solid part, they become quite extinct, and are utterly absorbed. Hence, cork, paper, wood, &c. are opaque; while glass, diamonds, &c. are pellucid. For in the confines or joining of parts alike in density, such as those of glass, water, diamonds, &c. among themselves, no refraction or reflection takes place, because of the equal attraction every way; so that such of the rays of light as enter the first surface pass straight through the body, excepting such as are lost and absorbed, by striking on solid parts; but in the bordering of parts of unequal density, such as those of wood and paper, both with regard to themselves, and with regard to the air, or empty space in their larger pores, the attraction being unequal, the reflections and refractions will be very great; and thus the rays will not be able to pass through such bodies, being continually driven about, till they be come extinct. That this interruption or discontinuity of parts is the chief cause of opacity, Sir Isaac Newton argues, appears from hence, that all opaque bodies immediately begin to be transparent, when their pores become filled with a substance of nearly equal density with their parts. Thus, paper, dipped in water or oil, some stones steeped in water, linen cloth dipped in oil or vinegar, &c. become more transparent than before. OPAL, in mineralogy, a species of the Quartz family, found in many parts of Europe, especially in Hungary. When first dug out of the earth it is soft, but it hardens and diminishes in bulk by expo. sure to the air. The specific gravity varies from 19 to 2.5. There are four subspecies, viz. the precious, the common, the semi, and the wood opal. Some specimens have the property of emitting vari ous coloured rays, with a particular effulgency, when placed between the eye and the light. The opals that possess this property are distinguished by lapidaries by the epithet oriental or nobilis. It is esteemed the most beautiful of the gems by Eastern nations; but in Europe it is not quite so highly valued, on account of its liability to split on a sudden change of temperature: it is principally used for necklaces, ear rings and finger rings. The most beautiful opals known are in the Imperial cabinet of Vienna; one is five inches long and two and a half in diameter; another is of the size and nearly of the shape of a hen's egg. The noble opal consists of silica and water in the proportion of 9 to 1. Specimens of the common and semiopal have been analysed, and found to consist as follows: OPATRUM, in natural history, a genus of insects of the order Coleoptera. Antennæ moniliform, thicker towards the tip; head projecting from a cavity in the thorax; thorax a little flattened, margined; shells immarginate, longer than the abdomen. There are twenty-eight species, O. sabulosum is brown, shells with three indented raised lines; thorax emarginate. Inhabits Europe and America, on sand. OPERA, a dramatic composition set to music, and sung on the stage, accompa nied with musical instruments, and enriched with magnificent dresses, machines, and other decorations. OPERA-glass in optics, so called from its use in theatres, &c.; it is sometimes called a "diagonal perspective" from its construction. It consists of a tube about four inches long, in each side of which there is a hole exactly against the middle of a plane mirror, which reflects the rays falling upon it to the convex glass, through which they are refracted to the concave eye-glass, whence they emerge parallel to the eye at the hole in the tube. This instrument is not intended to magnify objects more than about two or three times. The peculiar artifice is, to view a person at a small distance, so that no one shall know who is observed: for the instrument points to a different object from that which is viewed; and as there is a hole on each side, it is impossible to know on which hand the object is situat ed, which you are looking at. OPERCULARIA, in botany, a genus of the Tetandria Monogynia class and order. Essential character: flower compound; calyx common, one-leafed, unequally toothed, closed by a common receptacle, flowering above, seeding below, falling when ripe. There are three species. OPHIDIUM, in natural history, a genus of fishes of the order Apodes. Generic character: the head rather naked; teeth in the jaws, palate, and throat; gill membrane seven-rayed; body in the form of a sword. There are four species. We shall notice only O. barbatum, or the bearded Ophidium: this is generally about eight inches long, and is a native of the Adriatic and Mediterranean Seas, and is not much valued as food. It subsists on small fishes and crabs. OPHIOGLOSSUM, in botany, adder's tongue, a genus of the Cryptogamia Filices class and order. Natural order of Filices, or Ferns. Generic character: capsules numerous, connected by a membrane into a distich spike, subglobular, when ripe opening transversely, without any elastic ring seeds very many, extremely minute. There are nine species. OPHIORHIZA, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Stellatæ. Gentianæ, Jussieu. Essential character: corolla funnel-form; germ bifid; stigmas two; fruit two-lobed. There are three species. OPHIOXYLUM, in botany, serpentine wood, a genus of the Polygamia Monoecia class and order. Natural order of Apocinex, Jussieu. Essential character: hermaphrodite, calyx five-cleft; corolla fivecleft, funnel-form; stamens five; pistil one male calyx bifid; corolla five-cleft, with a funnel-form mouth; nectary cylindric; stamens two. There is but one species; viz. O. serpentinum, scarletflowered ophioxylum, a native of the East Indies. OPHIRA, in botany, a genus of the Octandria Monogynia class and order. Natural order of Onagræ, Jussieu. Essential character; involucre two-valved, three-flowered; corolla four-petalled, superior; berry one-celled. There is but one species: viz. O. stricta, a native of Africa. OPHRYS, in botany, a genus of the Gynandria Diandria class and order. Natural order of Orchidea. Essential character: nectary somewhat keeled underneath. There are thirty-four species. These plants are of the same natural ge |