AR A 



ARACHNI'DA {apdxvn, a spider). A 

 class of the Articulata, or the Diplo- 

 gangliata of Grant, comprising . arti- 

 cylated animals, generally with four 

 pairs of legs, without wings or meta- 

 morphosis, as the spiders, mites, and 

 scorpions. They are divided, by the dif- 

 ference of their respiratory organs, into 



1. Tracheata, which breathe by rami- 

 fied tracheae, as the phalangium, in 

 which there are only two abdominal stig- 

 mata ; and 



2. Pulmonata, which breathe by pul- 

 monary sacs opening by 2-8 transverse 

 abdominal stigmata, as in the scorpions 

 and spiders. 



ARiEO'METER (apati?, thin, /uerpoi/, 

 measure). An instrument, also called a 

 hydrometer or gravimeter, for ascertain- 

 ing the specific gravities of bodies, by the 

 depth to which it sinks in these bodies, 

 the amount being shown either by 

 weights, or by a scale attached to it. 

 Hence they are distinguished as weight 

 and scale araeometers, the former being 

 used chiefly for finding the specific gravity 

 of solids and fluids, whilst the latter 

 serve only for finding that of fluids, after 

 which, indeed, they are frequently named. 



Per Cent. Arceometer. An araeometer 

 made for particular and mixed liquors, 

 in which the scale is so graduated as to 

 express the component parts per cent., 

 either by weight or volume. It is named 

 after the fluid it is used to test, as spirit 

 of wine araeometer, beer araeometer. To 

 this class belongs the powder arcBometer, 

 for ascertaining the proportion per cent, 

 of saltpetre in gunpowder. 



ARALIA'CEiE. The Aralia tribe of 

 Dicotyledonous plants. Trees, shrubs, or 

 herbs, with, in all respects, the habit of 

 Umbelliferae, from which they are dis- 

 tinguished chiefly by their many-celled 

 fruit and more shrubby habit. 



ARANEI'D^ {aranea, a spider). A 

 section of the Arachnida, including the 

 Spiders, which are distinguished by hav- 

 ing the abdomen furnished with spin- 

 nerets, by means of which these animals 

 manufacture silken filaments for the 

 construction of their tveb. 



ARA'NEIFO'RMES {aranea, a spider, 

 forma, likeness). An order of edentulous 

 crustaceous animals, in which the extre- 

 mities are rod-like, long, and adapted for 

 walking. 



ARA'NGOES. A species of beads 

 made of rough carnelian, formerly im- 

 ported from Bombay, for re-exportation 

 to Africa. 

 36 



ARC 



A'RBOR. A term fancifully applied 

 to certain arborescent forms assumed by 

 metals under particular circumstances ; 

 thus arbor Diance signifies silver, when 

 precipitated from its oxide in the metallic 

 form by mercury ; arbor Saturni denotes 

 lead, when separated from its salts in a 

 metallic state by zinc. 



ARBORE'SCENT {arbor, a tree). 

 Having the characters of a tree, as dis- 

 tinguished from those of a shrub or 

 herb. 



ARC {arcus, a bow). A portion of a 

 curved line or circle. Thus the latitude 

 and declination are arcs of the meridian, 

 and the longitude is the arc of the 

 equator or parallel circle. 



1. Arc of direction. The arc which 

 a planet appears to describe, when its 

 motion is direct or progressive. 



2. Arc of retrogradaiion. The arc 

 which a planet describes whilst moving 

 contrary to the order of the signs, or 

 from east to west. 



A'RCAD^. Arch-shells ; a family of 

 the atrachian bivalves, named from the 

 typical genus area, in which the shell is 

 of various shapes, but the valves close 

 all round. 



ARCA'NUM. A secret ; a secret re- 

 medy, as arcanum duplicatum, an old 

 name for sulphate of potash; arcanum 

 tartari, deutoxide of mercury, &c. The 

 philosophers* stone was named by the 

 alchemists area arcanorum, a chest of 

 secrets. 



ARCH^'US {apxh beginning). A hy- 

 potlietical intelligent agent, adopted by 

 Van Helmont, resembling the anima of 

 Stahl. See.Anima. 



A'RCHIL. A violet red paste, pre- 

 pared from the lichen Parmelia, or Roc- 

 cella, and used in dyeing. The plant, 

 reduced to a pulp, and treated with im- 

 pure ammoniacal liquor, yields a rich 

 purple tincture, called litmus, or turn-^ 

 sole, used in chemistry as a test. 



ARCHIME'DES' SCREW. An ap- 

 paratus employed by Archimedes for 

 raising water and draining land in Egypt. 

 It consists of a large tube, coiled round 

 a shaft of wood, to keep it in place and 

 give it support. Both ends of the tube 

 are open, the lower one being dipped into 

 the water to be raised, and the upper one 

 discharging it in an intermitting stream. 

 The shaft turns on a support at each end, 

 the upper support being elevated in the 

 air, the lower being hidden beneath the 

 water. 



ARCHIPE'LAGO. A general term 



