spiders), a class of invertebrate animals, including spiders, mites, and scorpions, and comprehending Cuvier's second-class of the Articulata. Arachni'tis, in pathology, inflammation of the arachnoid membrane of the brain.

Arach'noid (Gr.), in zoology and botany, a term applied to objects which are like a spider's web.-In anatomy, a tunic of the vitreous humour of the eye.-The Arachnoid membrane is a cobweb-like membrane, situate between the pia and the dura-mater of the brain. Arachnology (Gr.), the science of the Arachnida; that part of natural history which treats of spiders. Aræos'tyle (Gr. width between columns), in architecture, an arrangement of columns with certain widths between them.

Aræosys'tile (Gr.), in architecture, an arrangement of columns with an interval of half a diameter between the coupled columns, and of three diameters and a half between the piers. Aragoa'cea (from Arago, the French physicologist), in botany, a natural order of beautiful exogenous plants, of the class Corollifloræ, natives of New Granada.

Aragonite, in mineralogy, a columnar crystal, of snowy whiteness. Aragu, in commerce, a scientific term applied to crude stick-lac, as extracted from the tree. Aralea'ceæ, in botany, a natural order

of Exogens, of which the Aralia, a shrub of N. America, is the type. Aranei'dans (Lat. and Gr. like spiders),

a family of Arachnides, comprehending the various genera of spiders. Arauca'ria (so called from Araucaros, a tribe of Chili Indians), in botany, a genus of gigantic firs, which are found fossil in the coal formation. Arbor (Lat. a tree), in mechanical science, the part of a machine which sustains the rest.

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Arbor-Dia'na (Lat. tree of Diana), in chemistry, the name given to a beautiful arborescent appearance, which takes place in a vessel containing a solution of nitrate of silver, when mercury has been thrown into it.Arbor Martis (tree of Mars) is duced by dissolving iron filings in aquafortis, until the acid is saturated, and then pouring gradually into the liquid a solution of fixed alkali, when a strong effervescence takes place, and the iron, instead of falling to the bottom of the vessel will often rise to cover its sides, and present the appearance of vegetable florescence. Arbores'cent Star-fish (Lat. growing like

a tree), a genus of Asterias, the Caput Medusa. Arboriculture (Lat.), the art of cultivating trees and shrubs. Arborization (Lat.), in mineralogy, the figuration of a tree or plant in minerals or fossils.

Arbor-Vitae (Lat. tree of life), in botany, the Thuya occidentalis.In anatomy, a term applied to the medullary ramifications of the brain. Arbutus (Lat.), a genus of plants, with fruit similar to the strawberry; order Ericaceæ.

Arc (Lat. a bow), in geometry, the segment of a circle; an arch.In astronomy, a diurnal arc is that part of a circle described by a celestial body between its rising and setting, as the nocturnal arc is that described between its setting and rising.-Arc of progression or direction, an arc of the zodiac which a planet appears to pass over when its motion is according to the signs.Equal arcs are those which contain the same number of degrees, and whose radii are equal.

Arca (Lat. an ark), a genus of bivalved Mollusca, the shells of which are transverse, and nearly equal in their valves.

Arcade (Lat.), in malacology, a fam. of marine Mollusca, which includes the Arca, Nucula, Pentunculas, Byssoarcea, and Trigonia. Arcbou'tant (Fr.), in architecture, a flying buttress.

Arch (Lat. a bow), in mathematics, the part of a circle or eclipse (see Arc).— In architecture any solid work formed into an arc of a curve, supported at the two extremities.-There are various terms of art connected with the arch of a building; thus, the pedestals upon which an arch rests, are called its piers; the portions of the pedestal from which the arch is said to spring are termed the flanks; the lower tier of the arch-stones is called the intrados or soffit; the upper, the extrados or back; the arch-stones are termed voussoirs; and the highest stone, the keystone, the top of which is termed the crown.-Triumphal Arch is a stately gate, adorned with sculptures, &c., and erected in honour of some distinguished individuals who had deserved a tribute of honour; as, for instance, the Arc de Triomphe at Paris. Archeology (Gr. archaios ancient), the science which describes the objects of antiquity. Archan'gel (Gr. chief), in botany, the name of the death nettle and other species of the genus Lanium.

Archetype (Gr. arche beginning), in mechanical science, an original model or pattern.

Archil, a sort of lichen; the name of a violet-red paste, used as a dye stuff. Archime'dean Screw (so called from Archimedes, the Greek mechanician), a spiral machine for raising water, consisting of a tube rolled in a spiral form round a cylinder, a modification of which has been introduced for propelling steam-vessels. Architect (Gr. archos, and tekton an artificer), a professor of the science and art of building; a person capable of designing and superintending the erection of any great building or edifice.-Architecture is the art or science of building. It is divided into Civil Architecture, which comprehends the erection of public edifices and domestic buildings;Military Architecture, or Fortification, and Naval Architecture, which (besides the building of ships and smaller vessels) includes the erection of docks, ports, moles, &c., though generally classed under the science of civil engineering. In the erection of public edifices, of an ornamental character, there are different orders of architecture, as the Tuscan, Doric, lonic, Corinthian, and Composite, which are noticed under their respective heads.

Architrave (Gr. archein to govern, and Lat. trabs a beam), in architecture the chief beam, or that part of a column which is immediately upon the capital.

Arc'hivolt, in architecture the ornamented band of mouldings round the arch-stones of an arch, terminating horizontally on the imposts. In the Tuscan order, the architrave has only one face; in the Doric and Ionic it has two crowned; and in the Corinthian and Composite the mouldings are the same as those of the architrave.

Arc'hograph (Lat. arcus a bow, and Gr. grapho to describe), an instrument for drawing a circular arch without the use of a central point. Arch-stone, in architecture, the keystone that binds an arch. Arctic (Gr. northern bear), belonging to the arctic regions.-Arctic circle is one of the lesser circles of the sphere, described as being 23 from the north pole.

Arctic'tis (Gr. a weasel-bear), a genus of marsupial animals, consisting of two Indian species, A. albifrons, about the size of a large cat, and A. ater, the size of a large terrier dog; both having long prehensile tails.

Arc'tomys (Gr. a bear-rat), in zoology, a genus of Rodents, the marmot, or bear-rat, which passes the winter in a state of torpor.

Arc'tonyx (Gr. bear-clawed), a genus of omnivorous Pachyderms, the pigbear.

Arctostaph'ylus (see Arbutus). Arcturus (Gr. the bear's tail), a brilliant star of the first magnitude, in the constellation Boötes. Ar'deada (Lat.), in ornithology, a family of large wading birds, including the herons and cranes; of which the Ardea, or heron, is the type. Ardisie's (Gr. spear-like or pointed), in botany, a tribe of plants, of which Ardisia is the type; order Myrsiniaceæ or Myrrh plants.

A'rea (Lat.), in geometry, the superficial contents of any figure.

Are'ca (Malabar), the betel-nut tree.A. oleracea is a remarkable genus of lofty palm trees, known as the cabbage tree.

Are'na (Lat. sand), the space of an amphitheatre or circus covered with sand, for the exhibition of combats or equitation.

Arena'ria (Lat. arena), in ornithology, a genus of wading birds, the sanderling. In botany, a genus of plants, of the order Caryophylleæ. Aren'dalite (from Arendal, in Norway, and lithos a stone), in mineralogy, the name of a fine crystal, a variety of epidote.-Dana.

Areng, a genus of palm trees, from which palm-wine and sago are produced.

Aren'icole (Lat. sand inhabitants), in zoology, a genus of Dorsibranchiata annulata or sand-worms, which are about a foot in length, and inhabit the sea-shore.

Are'ola (Lat.), the coloured circle round the nipple of woman; also, an inflamed ring round pustules arising from variola or vaccinia. Are'ole (Lat.), in entomology, the smaller spaces into which the wings of insects are divided by the nervures. -In botany, the areas or small spaces on the surface of certain plants, as in the fossil genera Sigillaria. Areom'etry (Fr.), art of measuring the specific gravity of fluids.-Areometer is the instrument by which they are weighed.

Argali, in zoology, a species of wild sheep, found in the elevated steppes of Siberia, and the mountain chains of Central Asia.

Argand Lamp, in practical science, a lamp so named from the inventor, in which the burner is in the form of a hollow cylinder, with a series of per

forations around the circumference for the egress of gas. A free supply of oxygen being thus admitted to the interior as well as to the exterior of the flame, a very brilliant light is obtained.

Argem'one (Gr. argemon, a disorder in

the eye), a genus of Mexican plants, the prickly poppy.

Argent (Lat. silver), in heraldry, the white colour used in armorial bearings.

Argen'tan (Lat. silvery), in metallurgy, an alloy of nickel with copper and zinc; German silver.

Argen'tate, a term applied to fulminating silver, as argentate of ammonia. Argen'tite, in metallurgy, sulphuret of

silver.

Argil (Gr. whiteness), in mineralogy, pure clay or alumina.-Argillite is the name of clay-slate. Argillaceous schist is indurated laminated clay or slate.-Argil has various combinations in natural history: as, Argillo-arenaceous, consisting of clay and sand; Argillo-calcareous, a species of calcareous earth; Argillocalcite, a species of calcareous earth, with a large proportion of clay; Argillo-murite, a variety of magnesite. Argilliformis, resembling clay; Argillo-ferruginous, containing clay and iron; Argillo-gypseus, containing clay and gypsum; Argillo-silicius, containing clay and silex.

Argo, a ship or constellation; a shell

fish.

Argol (old Fr.), crude tartar, or tartar of wine, obtained from the inside of wine vessels.

Argonau'ta (argo, and Lat. nauta a sailor), a genus of shell-fish, of which the principal species is the Argo; the nautilus.

Argus-shell, a species of beautifully variegated porcelain shell. Argyre'ia (Gr. silvery whiteness), in botany, a tribe of exogenous plants, of the natural order Convolvulacea.

Argyrtes (Gr. argyros silvery), a genus of coleopterous insects, belonging to the Mycetophagi or Mice-eaters. Argyros is of frequent occurrence in natural history, as a prefix, denoting silvery whiteness: as Argyranthemus, having flowers of a silvery whiteness; Argyrocephalus, having a white silver-like head; Argyrophthalmus, having silver-like eyes; Argyrophyllus, having silver leaves; Argyropygus, having the lower part of the abdomen white; Argyrostigma, having the flowers spotted with silverlike spots.

Ariad'ne, the name of one of the re

cently-discovered planets, first observed by Pogson in 1857.

Aricine, in chemistry, an alkali found in the Arica bark, containing one atom more of oxygen than quinine. A'ries (Lat. a ram), in astronomy, a constellation figured as a ram, which is the first sign of the zodiac, and is marked thus Y.

Aril, or Aril'lus (Lat.), in botany, a peculiar wrapper of certain seeds, formed by a fleshy expansion of the umbilical cord.

Ario'so (Ital.), in instrumental music, a sustained vocal style.

Aris'ta (Lat.), in botany, the beard of corn, or other grasses. Aristolo'chis (Gr. best parturition), in botany, a natural order of plants, with hermaphroditic flowers. -The genus Aristolochia includes different species, which for their medical virtues have obtained a prominent place in our pharmacopoeias: as A. clematis, which is slightly diaphoretic ; A. anguicida, snake-killing birthwort; and A. serpentaria, which in cases of protracted ague increases the efficacy of cinchona. Arith'metic (Gr. measurement or calculation), the science of numbers, or the art of computation by figures.Integral arithmetic is the science of whole numbers.-Fractional arithmetic is divided into Vulgar and DecimalFractions.-Arithmetical complement is that which a number wants of the next highest decimal denomination.-Arithmetical complement of logarithm is the sum or number which a logarithm wants of 10,000,000. Arithmetical mean is that number or fraction which lies between two others.-Arithmetical progression is a series of numbers which increase or decrease by equal steps. -Arithmetical ratio is the difference between any two terms in arithmetical progression. Arithmom'eter (Gr.), a circular logarithmic scale for the mechanical performance of arithmetical calculations.

Armadillo (Sp.), in zoology, a quadruped covered with a bony shell, of the family Mammalia, which includes the three-banded and the six-banded armadillo, the giant armadillo, the Chlamyphorus, and the Touay. Ar'mature, in physics, applied to two pieces of soft iron fastened to the poles of a magnet, and connected at their ends by a third piece, so as to increase its power.

Arme'nian Bole, in the arts, a soft earth, of a red colour, employed in painting and gilding.

Armenian Stone, a mineral, of a greenish blue colour; a variety of blue carbonate of copper, used in medicine as a purgative.

Arme'ria (Lat. Sweet William), a genus of plants of the order Plumbagineæ. -A. vulgaris is the common thrift, which forms pretty borders for our gardens.

Armilla (Lat. a bracelet), in anatomy the Armilla membrosa is a circular ligament which binds the tendons of the wrist and hands.-Armillary sphere is a hollow artificial sphere geographically illustrative of imaginary lines surrounding the earth.Armillary trigonometry is an astronomical instrument consisting of five semicircles, which being divided and graduated assist in solving many useful problems.

Aroi'deæ, in botany, a natural order of plants, the Arum family, with which it agrees in all its essential properties. (See Arum.)

Aro'ma (Gr. and Lat.), in botany, the fragrant or spicy odour of flowers, roots, and vegetable substances. Aromadendron is a genus of plants, comprehending the elegant aroma tree of Japan; order Magnoliaceæ.Aromatic confection is a medicine for diarrhoea, composed of chalk and aromatics. Aromatic vinegar is acetic acid, flavoured with aromatics. Aromatite (Gr. aroma), a mineral resembling myrrh.

Arou'ra (Gr. ploughing), a Grecian measure of fifty feet; an Egyptian measure of 100 square feet. Arpeggio (Ital.), in music, the distinct

sound of the notes of an instrumental chord accompanying the voice; imitation of the harp, by striking the chords in rapid succession. Arquebusade (Fr.), an aromatic spirituous lotion applied to bruises and sprains.

Arquifoux, in the arts, a kind of lead ore used by potters to give their ware a green varnish.

Arragonite (from Arragon in Spain, where it was first discovered), in mineralogy, a species of carbonate of lime; sp. gr. 26 to 30. It occurs in Devonshire and Buckinghamshire, and also at Leadhills, in Scotland. Arris (Ital.), in architecture, the edge of two surfaces meeting each other.Aris fillet is a slight piece of timber used in raising slates against a wall or chimney, that obliquely cuts across a roof. Sometimes it is called eaves-lathe, eaves-catch, or eavesboards.-Arris gutter is the wooden gutter fixed to the eaves of a building. Arrow-headed Characters, in ancient

art, a term applied to certain marks found stamped on the bricks of Babylon, and cut upon the marble monuments at Persepolis. They have also been found at Nineveh and Susa, and also in Armenia. Arrow-root, a farinaceous substance, consisting of starch, albumen, vola tile oil, chloride of calcium, and water. It is obtained from the root of the Maranta arundinacea, and the tubers of Carcuma angustifolia. Dissolved in milk or water it forms a light and digestible food for children and invalids.

Arsen'iate (Gr. arsenikon), in chemistry, a neutral salt, formed by the union of arsenic acid with a metallic oxide. The principal arseniates are barytes, potassa, soda, lime, oxide of silver, and protoxide of lead.

Arsenic (Gr. arsenikos), in mineralogy, a soft brittle metal, which is a violent corrosive poison. Native arsenic contains from 2 to 3 per cent. of antimony, and I per cent. of iron and water; sp. gr. 5'75. It enters into most of the compositions for the specula of reflecting telescopes, and for other optical purposes. Its oxides are used in dyeing and several of the arts. Its sulphurets are valuable pigments. Its principal mineral compounds are:-Arsenic acid, composed of 2 equivalents or atoms of arsenic, and 5 of oxygen; atomic weight 115'4. -Oxide of arsenic, composed of arsenic and oxygen, colour snowwhite, sometimes reddish or greenish; cleavage, octahedral; fracture, conchoidal; taste, astringent. — Sulphuret of arsenic, or orpiment, a trisulphuret of arsenic, consisting of arsenic and sulphur; the colour is a lemon-sulphur, passing into goldyellow. Sulphuret of arsenic, realgar, or hemi-prismatic, a bisulphuret of arsenic, composed of arsenic and sulphur; it is of a brilliant red colour, and splendid vitreous lustre. Arsen'ical Soap, in taxidermy, a preparation used in anointing the skins of animals previous to stuffing. It is composed of arsenic, camphor, white soap, salt of tartar, and powdered lime. Arsen'ico-Sulphurets, in chemistry, compounds obtained by dissolving arsenic in a solution of caustic alkali, soda, and ammonia. Arsenite, in chemistry, a salt formed by the union of arsenious acid with a base; the oxide of arsenic, a very dangerous poison.

Arseni'uret, in chemistry, a combination of arsenic with a metallic or other base. The arseniurets are nu

merous: as, arseniuret of nikel, or antimonial silver; arseniuret of bismuth; axotomous arsenical pyrites; prismatical arsenical pyrites; white arsenic, or arsenious acid, &c. Artemis'ia, an extensive genus of plants, remarkable for the bitterness of some of its species; a term applied to southern-wood, or magwort, and wormwood.

Arteriot'omy (Gr.), the cutting of an artery: the operation of letting blood from the artery.

Art'ery, in anatomy, a vessel or tube which conveys the blood in a direction from the heart to all parts of the body. The blood of the arterial system, after having reached the extremities of its innumerable ramifications, passes through the capillaries into the veins, by which it is again transmitted to the heart.-Arteritis is an inflammation of the arteries. Arte'sian Wells (so named from Artois,

in France, where they were first brought into use), a perforation or deep boring made in the earth, through which the water springs, from various depths, to the surface. Arthrem'bolum (Gr. an inserted joint), in surgery, an instrument for rectifying disjointed bones.

Arthritis (Gr. arthron a joint), in pathology, any inflammation of the joints, especially the gout. Arthro'dia (Gr.), in anatomy, a joint in which the head of one bone is received into the socket of another; a ball and socket joint.-Arthrodic, pertaining to arthrodia. Arthrody'nia (Gr.), pain in the joints. Arthropuo'sis (Gr.), suppuration in the cavity of a joint.

Arthro'sia (Gr.), inflammation of the joints.

Artichoke (Fr.), a genus of composite plants, of the thistle tribe, Cardiaceæ. The Jerusalem artichoke (Helianthus tuberosus) is a plant with a root resembling the potato. Articulata (Lat.), one of the principal divisions of the animal kingdom, applied to those animals which have a jointed structure but no internal skeleton, and comprehending insects, spiders, crustacea, worms, &c. Articulation (Lat.), in anatomy, the adapting of one bone or joint in the skeleton of an animal to another. -In botany, the knots or joints inserted in certain plants, as the grasses, cane, &c. Artillery (Fr.), a term denoting the science or art which relates to the materials, ingredients, and composisition of whatever belongs to engines of war, as the composition of shot,

the arrangement of cannon in passes, or in the field, &c. Under this head are comprehended various implements of modern warfare, as cannon, mortars, howitzers, and other large pieces of ordnance, for discharging shot and shells.

Artiste (Fr.), in the arts, a term to denote one who is particularly dexterous in the art which he practises, of whatever nature it may be, as an operadancer, hair-dresser, cook, &c. Artocar pes (Gr. bread-fruit), a nat. order of plants, of which the Artocarpus, or bread-fruit tree, is the type. It is a native of the South-Sea Islands, and forms an important article of food to the natives.-The Artocarpus integrifolia is a native of the Indian Archipelago, and yields a fruit of sixty or seventy pounds weight.

Arts (Lat. artes), those branches of knowledge which require ingenuity and manual skill, and are used to denote the whole circle of the arts and sciences, or of an academical education; hence the degrees of A.B. Bachelor of Arts, and A.M. Master of Arts. They are divided into the liberal and mechanical arts: the former comprehending poetry, painting, sculpture, architecture, &c.; the latter, engineering, carpentry, masonry, smith-work, &c. Poetry, painting, music, sculpture, engraving, &c., are called the Fine Arts. Arum (Egyp.), a genus of plants belonging to the nat. order Aroidea (which see).-The A. maculatum, wake-robin, or cuckoo-pint, is the only British species.

Arundelian Marbles, certain tables containing the chronology of ancient history, particularly of Athens, from the year 1582 to 353 B.C. They were purchased by Thomas Lord Arundel, whence the name.

Arun'do (Lat. a reed), a genus of plants belonging to the Gramineæ, the water-reed.-The A. phragmites, or marsh-reed, is the only British species. In anatomy, the Arundo brachii is the radius of the arm. Arvicolæ (Lat.), a genus of rodent animals, of which the field-rat is the type.

Ary'tenoid (Gr. like a pitcher), in anatomy, applied to two small cartilages at the top of the larynx. Asafoetida (Lat. fœtid gum), a gum resin of a very fœtid description, obtained from the Persian plant, Ferrula asa fætida.

As'aphus (Gr. doubtful), in palæontology, a genus of Trilobites, found in the Silurian formation.