TOR 



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TOMBAC. A white alloy of copper 

 with arsenic, called white copper. 



TOME'NTUM. A term applied, in 

 Botany, to the hairs of plants, when they 

 are entangled, and closely pressed to the 

 stem. 



TOOTHED WHEELS. Wheels which 

 are made to act upon, or, as it is called, 

 to drive one another by having the sur- 

 face of each indented with teeth, and 

 fixing the centres at such a distance from 

 each other that the teeth come succes- 

 sively into contact. The proper form for 

 the tooth of such wheels is a question of 

 much complexity. 



TOOTHINGS OF LEAVES. The pro- 

 jecting parts of the margin of leaves, 

 when the adhesion of the lobes is com- 

 plete, and the parenchyma which sepa- 

 rates the extremity only of the veins is 

 not extended to the extremity of the 

 principal veins, or beyond them. Such 

 leaves are said to be toothed or dentale ; 

 and, when the teeth or toothings are 

 rounded, they become crenels, and the 

 leaf is said to be crenelled or crenate. 



TOPAZ irundCtov). A mineral species 

 occurring massive in imbedded and 

 rounded crystals. It forms an essential 

 constituent of a particular mountain - 

 rock, which is an aggregate of topaz, 

 quartz, and schorl, and is called topaz- 

 rock. It comprises three sub-species, 

 common topaz, schorl ite, and physalite. 



TOPA'ZOLITE. A variety of pre- 

 cious garnet, found at Mussa, in Pied- 

 mont. 



TOPO'GRAPHY (tottoc, a place, 

 ypa^o), to describe). The description of 

 a place, as of a city, a town, a village; a 

 term differing from geography, as a part 

 differs from the whole. 



TORNA'DO (tornar, Span, to turn). 

 A sudden and violent storm of wind, 

 accompanied by thunder, lightning, and 

 rain, frequently occurring in the West 

 Indies, and other parts. The term also 

 denotes, generally, a typhoon or hurri- 

 cane. 



TO'RRELITE. A red mineral from 

 New Jersey, named from Dr. Torrey, and 

 consisting principally of silica, iron, and 

 lime. 



TORRICELLIAN EXPERIMENT. 

 The name given to the experiment by 

 which Torricelli, a pupil of Galileo, dis- 

 covered that the mercury remains in the 

 barometer tube at a height of nearly 

 30 inches above the level of the mercury 

 in the cistern. The tube was hence 

 called a Torricellian tube, and the vacuum 

 340 



in the tube above the surface of the 

 mercury is the Torricellian vacuum, by 

 which name it is distinguished from that 

 obtained by the air-pump, sometimes 

 called the Guerickian or Boylean va- 

 cuum. 



TORRID ZONE {torridus, burning). 

 That division, or belt, of the surface of 

 the earth, which lies on each side of the 

 equator, extending to the two tropics of 

 Cancer and Capricorn, and so called from 

 its excessive heat. See Ascii. 



TORSION BALANCE. An instru- 

 ment, invented by Coulomb, for the mea- 

 surement of small attractive and repul- 

 sive forces, by the torsion or twisting of 

 a fine metallic wire. The electrical tor- 

 sion balance is made of a thin thread of 

 shellac, carrying at its extremity a small 

 gilded pith ball ; this instrument forms 

 a very delicate electrometer, and is also 

 employed for estimating exactly the in- 

 tensity of magnetic forces. 



TORUS. A rope or cord made of 

 twisted grass or straw, on which the 

 ancients laid their skins or other furni- 

 ture for the convenience of sleeping; 

 hence the term is taken for a bed, and is 

 used in botany as synonymous with 

 thalamus or receptacle. 



TO'TIPALMES (tola palma, an entire 

 palm). The name given by Cuvier to a 

 group of birds whose hind toe is united 

 to the others by a continuous membrane ; 

 they comprise the pelican, the cormorant, 

 the boobies, the frigate-birds, &c. 



TOUCAN. A modern southern con- 

 stellation, consisting of nine stars. 



TOUCHSTONE. A variety of flinty- 

 slate, frequentl^y called Lydian stone. 



TOURMALINE. Schorl. A mineral 

 which is hard enough to scratch glass, 

 and becomes electric by heat. It is of 

 various colours and forms ; the blue 

 variety being called indicolite, and the 

 red rubellite ; it is transparent when 

 viewed across the thickness of a crystal, 

 but perfectly opaque when turned in the 

 opposite direction. The ancients called 

 it lyncurium. It occurs in gneiss, mica- 

 slate, talc-slate, &c. 



TRACHEA'TA (rpaxela, the trachea). 

 An order of the Arachnida, comprising 

 those species which breathe by means of 

 tracheae. See Pulmonata. 



TRACHE'LIDES (rpaxn^op, the neck). 

 A family of the heteromerous Coleoptera, 

 in which the head is separated from the 

 thorax by a kind of neck. They are 

 often of brilliant colours, as the blister- 

 ing-fly. 



