sebacic, the properties of which are quite distinct from those which had been for merly described. The process by which Thenard obtained the sebacic acid is the following. He distilled a quantity of hogslard, and washed the product several times with hot water. He then dropped into it acetate of lead; there was formed a flakey precipitate, which was collected and dried, put into a retort with sulphuric acid, and heated. The liquor in the receiver had no acid character; but there appeared in the retort a melted matter analogous to fat. This is carefully separated; and after being washed, is boiled with water. By the action of heat the whole is dissolved by the water, and when it cools, crystals in the shape of needles are deposited. These are the sebacic acid which has the following properties. It has no smell, a slight acid taste, and reddens strongly the tincture of turnsole. When heated it melts like tallow. It is much more soluble in warm than in cold water. Alcohol dissolves it in large quantities. Boiling water saturated with this acid forms a solid mass on cooling. It crystallizes in small needles, but with certain precautions may be obtained in the form of long, large, and very brilliant plates. It precipitates the acetate and nitrate of mercury and lead, and nitrate of silver; it neutralizes the alkalies, and forms with them soluble salts.

SEBATES, in chemistry, salts formed of the sebacic acids, and alkalies, earths, &c.; they are soluble in water.

SECALE, in botany, rye, a genus of the Triandria Digynia class and order. Natural order of Gramina or Grasses. Essential character: calyx opposite, two-valved, two-flowered, solitary. There are four species, viz. the villosum, orientale, creti. cum, and cercale. S. villosum, or wood ryegrass, is distinguished by a calyx with wedge-shaped scales, and by the fringe of the glume being woolly. The glumes of the S, orientale are shaggy, and the scales of the calyx are shaped like an awl. The glumes of the S. creticum are fringed on the outside. The S. cereale, or common rye, has glumes with rough fringes. It is a native of the island of Candia, was introduced into England many ages ago, and is the only species of rye cultivated in this kingdom. There are, however, two varieties, the winter and spring rye. The winter rye, which is larger in the grain than the spring rye, is sown in autumn at the same time with wheat, and sometimes mixed with it; b

as the rye ripens sooner than the wheat, this method must be very exceptionable. The spring rye is sown along with oats, and usually ripens as soon as the winter rye ; but the grain produced is lighter, and it is therefore seldom sown except where the autumnal crop has failed. Rye is commonly sown on poor, dry, limestone, or sandy soils, where wheat will not thrive. By continuing to sow it on such a soil for two or three years, it will at length ripen a month earlier than that which has been raised for years on strong cold ground.

SECANT, in geometry, is a line that cuts another, or divides it into two parts.

In trigonometry, the secant denotes a right line drawn from the centre of a circle, which cutting the circumference, proceeds till it meets with a tangent to the same circle.

SECHIUM, in botany, a genus of the Monoecia Syngenesia class and order. Natural order of Euphorbiæ, Jussieu. Essential character: calyx half, five-cleft; corolla five-cleft, with ten hollows in the upper part of the tube; nectary: male, filaments five, connected: female, stigma very large, peltate reflexed, five-cleft; pericarpium large, ovate, turbinate, one-seeded. There is only one species, viz. S. edule, the chocho vine. It is a native of the West Indies; flowering and fruiting in December.

SECOND, in geometry, chronology, &c, the sixtieth part of a prime or minute; whether of a degree, or of an hour; it is denoted by two small accents, thus ( ́ ́).

SECRETARY, an officer who by his master's orders writes letters, dispatches, and other instruments, which he renders authentic by his signet. Of these there are several kinds; as, 1. Secretaries of State, who are officers that have under their management and direction the most important affairs of the kingdom, and are obliged constantly to attend on the King: they receive and dispatch whatever comes to their hands, either from the crown, the church, the army, private grants, pardons, dispen sations, &c. as likewise petitions to the sovereign, which, when read, are returned to them; all which they dispatch according to the King's direction. They have authority to commit persons for treason and other offences against the state, as conservators of the peace, at common law, or as justices of the peace throughout the kingdom. They are members of the Privy Council, which is seldom or never held without one of them being present; and as to the business and correspondence in all parts of this kingdom,

it is managed by either of the secretaries, without any distinction; but with respect to foreign affairs the business is divided into two provinces or departments, the southern and the northern, comprehending all the kingdoms and states that have any intercourse with Great Britain; each secretary receiving all letters and addresses from, and making all dispatches to, the several princes and states comprehended in his province. 2. Secretary of an Embassy, a person attending an embassador for writing dispatches relating to the negociation. There is a great difference between the secretary of an embassy, and the embassador's secretary; the last being a domestic or menial of the embassador, and the first a servant or minister of the prince. 3. The Secretary of War, an officer of the War Office, who has two chief clerks under him, the last of which is the secretary's messenger. There are also secretaries in most of the other offices.

SECRETION, In the course of the circulation the blood is conveyed to certain organs named glands, and is there entirely changed in its chemical composition, so as to form various products not pre-existing in the mass of blood, and which form some of the most important varieties of animal mat. ter. See PHYSIOLOGY.

SECTION, in geometry, denotes a side or surface appearing of a body or figure cut by another; or the place where lines, planes, &c. cut each other. The common section of two planes is always a right line; being the line supposed to be drawn on one plane by the section of the other, or by its entrance into it.

SECTION of a building, in architecture, is the same with its profile; or a delineation of its heights and depths raised on a plane, as if the fabric was cut asunder to discover its inside.

SECTOR, in geometry, is a part of a circle, comprehended between two radii and the arch; or it is a mixed triangle, formed by two radi and the arch of a circle.

SECTOR is also a mathematical instrument, of great use in finding the proportion between quantities of the same kind, as between lines and lines, surfaces and surfaces, &c. for which reason the French call it the compass of proportion.

The great advantage of the sector above common scales, &c. is, that it is adapted to all radii, and all scales. For, by the line of chords, sines, tangents, &c. on the sector, we have lines of chords, sines, tangents,

&c. adapted to any radius between the length and breadth of the sector, when opened. The sector is founded on the fourth proposition of the sixth book of Euclid, where it is demonstrated, that similar triangles have their homologous sides proportional. See MATHEMATICAL INSTRUMENTS.

SECUNDINES, after birth, in anatomy, the several coats or membranes wherein the fœtus is wrapped up in the mother's womb, as the chorion and amnios, with the placenta, &c. See MIDWIFERY.

SECURIDACA, in botany, a genus of the Diadelphia Octandria class and order. Natural order of Papilionaceæ, or Leguminosæ. Essential character: calyx threeleaved; corolla papilionaceous, with the standard two-leaved within the wings; legume ovate, one-celled, one-seeded, ending in a ligulate wing. There are three species.

SEDUM, in botany, stonecrop, a genus of the Decandria Pentagynia class and order. Natural order of Succulenta. Sempervivæ, Jussieu. Essential character: calyx five-cleft; corolla five-petalled; scales nectariferous, five, at the base of the germ; capsules five. There are thirty species, all of which are hardy, herbaceous, succulent perennials, durable in root, but mostly annual in stalk, &c. which, rising in spring, flower in June, July, and August, in different sorts; the flowers consisting univer. sally of five spreading petals, generally crowning the stalks numerously in corymbose and cymose bunches and spikes, ap. pearing tolerably conspicuous, and are succeeded by plenty of seeds in autumn, by which they may be propagated, also abundantly by parting the roots, and by slips or cuttings of the stalks in summer; in all of which methods they readily grow, and spread very fast into tufted bunches: being all of succulent growth, they consequently delight most in dry soils, or in any dry rubbishy earth. As flowering plants, they are mostly employed to embellish rock-work, ruins, and the like places; planting either, the roots or cuttings of the shoots in a little mud or any moist soil at first, placing it in the crevices, where they will soon root and fix themselves, and spread about very agreeably.

SEED, in botany, the essence of the fruit of every vegetable. Linnæus denominates it to be a deciduous part of the plant, containing the rudiments of the new vegetable, and fertilized by the sprinkling of the male dust. Plants are furnished with

one seed as the sea-pink; or two as in umbelleferous plants; or three as in the spurge: or many as in the ranunculus, &c. The shape, structure, and sides of seeds are various, Linnæus denominates seeds the eggs of plants; and the fecundity of plants is often astonishing: there are 4,000 seeds in a single sun-flower; more than 30,000 in a poppy; and in a single tobacco plant 360,000 have been enumerated. The annual produce of a single stalk of spleenworth has been estimated to be a million of seeds. Plants are disseminated in various methods: some are carried along by rivers and torrents many hundred miles from their native soil, and cast upon a very different climate, to which, however, by degrees they render themselves familar. Some are formed by wings to be borne before the wind to distant places. Birds, squirrels, &c. swallow seeds, and void them whole and fit for vegetation, and thus disseminate them. There are others that disperse themselves by an elastic force, that resides either in the "calyx", as in oats and the ferns in their "pappus", as in the "centaurea crupina", or in their capsule, as in the geranium.

SEED. See SEMEN.

SEGMENT of a circle, in geometry, that part of the circle contained between a chord and an arch of the same circle.

SEGMENT of a sphere, is a part of a sphere terminated by a portion of its surface, and a plane which cuts it off, passing somewhere out of the centre; being more properly called the section of a sphere. The base of such a segment, it is evident, is always a circle. And the convex surfaces of different segments, are to each other as their altitudes, or versed sines. And as the whole convex surface of the sphere is equal to four of its great circles, or four circles of the same diameter; so the surface of any segment is equal to four circles on a diameter equal to the chord of half the arc of the segment. So that if d denotes the diameter of the sphere, or the chord of half the circumference, and c the chord of half the arc of any other segment, also a the altitude or versed sine of the same; then, 3.1416d is the surface of the whole sphere, and

3.1416, or 3.1416d, the surface of the

segment.

For the solid content of a segment, there are two rules usually given; viz. 1. To three times the square of the radius of its

base, add the square of its height; multiply the sum by the height, and the product by .5236. Or, 2dly, From three times the diameter of the sphere, subtract twice the height of the frustrum; multiply the remainder by the square of the height, and the product by .5236.

SEGUIERIA, in botany, so named in honour of Jean François Seguier, a genus of the Polyandria Monogynia class and order. Essential character: calyx, fiveleaved; corolla none; capsule, one-seeded, terminated by a large wing, having small lateral ones. There are two species, viz. S. Americana and S. Asiatica.

SEISIN, is two-fold, seisin in law, and seisin in fact. Seisin in fact, when an actual possession is taken; seisin in law, when something is done, which the law accounts a seisin. See ESTATE, FEE, &C.

SELAGO, in botany, a genus of the Didy. namia Gymnospermia class and order. Natural order of Aggregatæ. Vitices, Jussieu. Essential character: calyx, five-cleft; corolla, tube capillary, border almost equal; seed one or two. There are twenty species. These plants are herbaceous or shrubby; all natives of the Cape of Good Hope. Leaves alternate; flowers in most of the species, allied to those of eranthemum and verbena, irregular, tubular, in alternate terminating spikes, which are simple or manifold; in a few, the flowers are regular, with a short two seeded tube, in a sort of terminating corymb.

SELENITE, in mineralogy, a species of the Calc genus, is of a snow white colour, passing into different shades of grey. It is most generally massive, and not unfre quently crystallized; the crystals are seldom large; internally, its lustre shining and splendent. The fracture is perfectly foliat ed. It is transparent, soft, and not particularly frangible; specific gravity 2.3; it is composed of lime, sulphuric acid, and water, and is found in Germany, France, and England. On account of its great purity it is employed in taking the most delicate impressions: it is also used for crayons, and when burnt and powdered, it is used for cleansing silver. It is the same with GYPSUM, under which word will be found some other particulars relating to it.

SELEUCIDE, in chronology. Era of the Seleucidæ, or the Syro Macedonian æra, is a computation of time, commencing from the establishment of the Seleucidæ, a race of Greek kings, who reigned as successors of Alexander the Great, in Syria, as the

Ptolemies did in Egypt. This æra we find expressed in the book of the Maccabees, and on a great number of Greek medals, struck by the cities of Syria, &c. The rabbins call it the æra of contracts; and the Arabs therik dilkarnain, that is, the æra of the two horns. According to the best accounts, the first year of this æra falls in the year 311 before Christ, being twelve years after Alexander's death.

SELINUM, in botany, a genus of the Pentandria Digynia class and order. Natural order of Umbellata or Umbelliferæ. Essential character: petals, cordate, equal; involucre, reflex; fruit oval, oblong, compressed, flat, striated in the middle. There are nine species.

SELL, in building, is of two kinds, viz. ground-sell, which denotes the lowest piece of timber in a timber building, and that on which the whole superstructure is raised; and the window-sell, is the bottom piece in a window-frame.

SEMECARPUS, in botany, a genus of the Pentandria Trigynia class and order. Essential character: calyx inferior, fivecleft; corolla, five petalled; nut, kidneyform, inserted into a large fleshy, flattened receptacle. There is but one species, viz. S. anacardium, marking nut tree. It is a native of all the mountainous parts of India, flowering in July and August.

SEMEN. See PHYSIOLOGY.

Semen

is secreted in the testes of male animals; but when it is ejected it is composed of two substances; the one is fluid and milky, and the other of a thick mucilaginous consistence, in which appear a great number of white silky filaments, especially if it be agitated in cold water. It has a disagreeable odour, and an acrid irritating taste. The specific gravity varies considerably, but is always greater than that of water. When it is rubbed in a mortar, it froths up, and acquires the consistence of pomatum from the air with which it mixes. It converts the flowers of violets, to a green colour, and it precipitates the calcareous and metallic salts, which shows, that it contains an uncombined alkali. The thick part of the semen as it cools, becomes transparent, and assumes a greater degree of consistence; but it afterwards becomes entirely liquid, even without absorbing moisture from air. If semen be exposed to the air after it has become liquid at the temperature of sixty degrees, it becomes covered with a transparent pellicle, and at the end of three or four days deposits fine

transparent crystals of a line in length, crossing each other like radii from a centre. When they are magnified, they appear to be four-sided prisms terminated by long four-sided pyramids. When semen is exposed to a warm air, in considerable quantity, it is decomposed; it assumes the colour of the yolk of egg, and becomes acid, either by absorbing the oxygen from the atmosphere, or by a different combination and arrangement of its own constituent principles. Heat accelerates the liquefaction of semen; and when it has undergone this change it is no longer susceptible of coagulation. It is decomposed by the application of strong heat. Water is first separated; it then blackens, swells up, and emits yellow fumes, having an empyrenmatic, ammoniacal odour. A light coal remains behind, which burns readily to white ashes.

The acids readily dissolve semen, and this solution is not decomposed by the alkalies; nor indeed is the alkaline solution of semen decomposed by the acids. Wine, cider, and urine, also dissolye semen, but it is in consequence of the acid which is combined with these liquids. The crystals which form in semen by spontaneous evaporation in the open air, and which are entangled in the viscid matter, may be separated by adding water.

These crystals have neither smell nor taste. They melt under the blow-pipe into a white opaque globule, which is surrounded with a yellowish flame. This salt is insoluble in water, and is not acted on by the alkalies; but is soluble in the mineral acids without effervescence, from which solutions, lime-water, the alkalies, and oxalic acid throw down a precipitate. The component parts of semen are found to be

wherein the cubes of the ordinates are as the squares of the abscisses. Its equation is axx = y'.

The area of the space A PM (Plate XIV. Miscel. fig. 1), is = xyAP X PM, or of the circumscribing rectangle. The content of the solid generated by the revolation of the space APM about the axis A P, is 4 px y2 = .7854 AP × P M2, or of the circumscribing cylinder. And a circle equal to the surface of that solid may be found from the quadrature of an hyperbolic space. Also the length of any arc, AM, of the curve may be easily obtained from the quadrature of a space contained under part of the curve of the common parabola, two semi-ordinates to the axis, and the part of the axis contained between them. This curve may be described by a continued motion, viz. by fastening the angle of a square in the vertex of a common parabola; and then carrying the intersection of one side of this square and a long ruler (which ruler always moves perpendicular to the axis of the parabola) along the curve of that parabola. For the intersection of the ruler, and the other side of the square will describe a semicubical para bola. Maclaurin performs this without a common parabola, in his Geometria Organica.

SEMIDIAMETER, half the diameter, or a right line drawn from the centre of a circle, or sphere, to its circumference; being the same with what is otherwise called the radius. The distances, diameters, &c. of the heavenly bodies, are usually estimated, by astronomers, in semidiameters of the earth; and the distances of the secondary planets from their respective primary ones, by semidiameters of the body of the primary planet.

Suppose the semidiameter of the Earth to be unity, then the measures of the Sun and planets will be as follow: The semidiameter of

bolas, y": ":: ax" :43

-1

-1

- ; or the powers of the semiordinates are, as the powers of the semiabscisses one degree lower; for instance, in cubical semiparabolas the cubes of the ordinates are as the squares of the abscisses; that is, y3; v3 : : x2 : z2.

SEMPERVIVUM, in botany, houseleek, a genus of the Dodecaudria Polygynia class and order. Natural order of Succulentæ. Sempervivæ, Jussieu. Essential character: calyx twelve-parted; petals twelve; capsule twelve, many-seeded. There are fourteen species.

SENECIO, in botany, groundsel, a genus of the Syngenesia Polygamia Superflua class and order. Natural order of Composite Discoidea. Corymbiferæ, Jussieu. Essential character: calyx cylindrical, calycled, with the scales mortified at the tip; down simple; receptacle naked. There are seventy-five species.

SENSATION. The brain is a soft pulpy mass, of a whitish colour on the inside, occupying all the cavity of the skull. Minute differences are observable in the substance of the brain in different parts of it, but it is unnecessary to enter upon a statement of them here. (See ANATOMY.) The spinal marrow is the continuation of the lowest part of the brain, which passes through the great opening of the skull down the hollow of the back bone. The substance contained in the hollow of the back bone is the same with that in the cavities of the skull; and it is sometimes convenient to comprehend both the portions under the same general name of brain. From the brain proceeds the nerves, which at first are fine fibres of the same substance with the brain: these fibres meet and form soft white pulpy cords, which afterwards spread themselves over various parts of the body, by splitting into innumerable and exceedingly minute branches. Anatomists count forty pairs of nerves, (for they come off in pairs, though they afterwards separate), and of those nine or ten only come from the brain at the bottom of the skull, and the rest from the spinal marrow. Those from the brain are distributed to various parts of the head; those from the spinal marrow are distributed over the trunk and extremities. The external organs of sense, the nerves, and the brain, are the organs of sensation. All, as we are at present constituted, are necessary to sensation. If the external organ is destroyed no sensation can be produced : where there are no nerves