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T H U 



tortoise. This genus has been divided 

 into five subgenera. 



2. The land tortoise, a sub-genus of the 

 genus above mentioned. 



TETRACAU'LODON. An extinct animal of 

 the order mammalia ; allied to the masto- 

 don, and referrible to themiocene period. 

 One species only has been discovered 

 hitherto ; at Epplesheim, in Germany. 



TETRADA'CTYLOUS. (from rtrpatfa/cruXoc;, 

 Gr. quatuor digitos habem.} Having 

 four toes. 



TETRADYNA'MIAN. ) A term applied to 



TETRADYNA'MOUS. $ plants that have six 

 stamens, four of which are longer than 

 the other two : the wall-flower is a fami- 

 liar example. 



TE'TRAGON. (from rerpaywi'oc, Gr. tetra- 

 gonus, Lat. ) A quadrangle ; a figure that 

 has four angles, as a square. 



TETRA'GONAL. (tetragone, Fr.) Having 

 four sides and four angles ; four-cor- 

 nered. 



TE'TRAGYN. In botany, a plant that has 

 four pistils. 



TETRAHE'DRAL. Having four triangles, 

 equal and equilateral. 



TETRAHE'DRON. ) (from rsrpn, four, and 



TETRAC'DRON. ) tdpa, side, or base, Gr. 

 tetraedre, Fr.) The solid angles of a tetra- 

 hedron are formed by three equilateral 

 plane triangles, and the solid is bounded 

 by four equal and equilateral plane trian- 

 gles, therefore it is a pyramid ; a four- 

 sided solid contained by four equal-sided 

 triangles ; a solid contained by four tri- 

 angular surfaces. 



THA'LLITE. The name given by Lemethe- 

 rie to the Epidote of Haiiy, or Pistazit of 

 Werner. 



THA'LLUS. (from 0aXXoe, a green leaf, 

 Gr.) In botany, a name given to the 

 frond, or leaf-like part, of certain plants. 

 Lichens are stemless, leafless, plants, 

 consisting of a tough wrinkled substance, 

 called a thallus. 



THE'CA. (from 0jjo/, Gr. theca, Lat.) A 

 case or sheath. The sporules of plants 

 are contained within thecse. 



THECODONTOSAU'RUS. A recently disco- 

 vered genus of fossil saurians, found in 

 the magnesian limestone : the vertebrae 

 are deeply concave at each end. 



THE'RMAL. (thermal, Fr. 0ep/ioc, warm, 

 from 0/oo>, to make warm, Gr.) A term 

 applied principally to warm springs and 

 waters. The temperatures of different 

 thermal springs vary greatly from each 

 other, but the same spring is found to be 

 of a uniform temperature at all seasons of 

 the year. Thermal waters are found to 

 be, on the average, neither more nor less 

 pure than springs of common tempera- 

 ture. There is no one product of thermal 

 springs, constantly found in them, which 



never occurs in cold waters ; but it ap- 

 pears, from Dr. Daubeny's researches, 

 that nitrogen gas is very common in hot 

 springs, and, perhaps, very rare in cold 

 waters. Thermal waters prove the exten- 

 sive effects of subterranean heat, deriving 

 their temperature from a deep-seated in- 

 ternal source of heat, and not from any 

 local cause, or from chemical changes. 

 Some thermal springs have flowed with- 

 out any known diminution of temperature 

 for nearly two thousand years. The 

 range of temperature of the thermal 

 springs of Europe is from 66 to 165 

 Fahrenheit. 



THE'RMO-ELECTRI'CITY. Electricity deve- 

 loped by heat. Lyell. 



THIN OUT. This is a term used by geolo- 

 gical writers to express the appearance of 

 a stratum which gradually becomes thin- 

 ner, till it wholly disappears. 



THO'MSONITE. A mineral, thus named 

 after Dr. Thomson, a variety of zeolite, 

 crystallized in rectangular prisms. It is 

 found near Dumbarton, in Scotland. 



THORA'CIC. (thorachique, Fr. from tho- 

 rax, Lat.) Pertaining to the chest. The 

 name given to the duct into which the 

 absorbents empty themselves, namely, 

 the thoracic duct, and which terminates 

 in the left subclavian vein. 



THO'RAX. (thorax, Lat. Owpo?, Gr. tho- 

 rax, Fr.) The cavity of the chest, con- 

 taining the heart, lungs, c., c. 



THORI'NA. An earth discovered by Berze- 

 lius, in 1816 or 1817, in gadolinite. It 

 bears a strong resemblance to zirconia, 

 but differs from it in many particulars. 

 After being heated to redness it is soluble 

 in acids. No precipitate is caused by the 

 addition of sulphate of potash to a solu- 

 tion of it. Thorina is colourless and in- 

 fusible after ignition. Dr. Ure states 

 that a strong solution of the sulphate 

 becomes a thick mass by boiling, but 

 it is soluble in cold water ; a property 

 which particularly characterizes the new 

 earth. 



THO'RITE. A mineral discovered a few 

 years since, in Norway, by Esmark, and 

 named thorite by Berzelius. It is com- 

 pact ; of a black colour ; brittle. Specific 

 gravity 4'8. It is rare, and not used. 



THORI'NUM. ) The metallic base of the 



THORI'UM. $ earth thorina. 



THU'LITE. A mineral of a peach-blossom 

 colour, occurring in Norway ; it is very 

 rare. 



THU'MERSTONE. > A mineral, thus named, 



THU'MMERSTONE. \ by Kirwan, from its 

 being found, in masses, near Thum, in 

 Saxony. It is the Axinite of Haiiy and 

 Brongniart ; the Axinit of Werner ; La 

 pierre de Thum of Brochant. For a de- 

 scription of this mineral, see Axinite. 



