THE 



I 435 ] 



TIB 



cause, or from chemical changes. 

 Some thermal springs have flowed 

 without any known diminution of 

 temperature for nearly two thou- 

 sand years. The evidence that 

 many springs rise from considerable 

 depths, and possess a temperature 

 independent of solar influence, rests 

 on their great heat, which varies 

 from the boiling point of water 

 downwards, to ordinary tempera- 

 tures. It is impossible to account 

 for this, otherwise than by suppo- 

 sing such heat communicated to 

 the water in parts of the earth far 

 beneath the surface, and removed 

 from atmospheric influence. Many 

 thermal springs contain silica, 

 though this substance is of exceed- 

 ingly difficult solution. Dr. Tur- 

 ner found that some of the thermal 

 springs of India which yielded 

 twenty-four grains of solid matter 

 in a gallon, contained 21 -5 percent, 

 of silica, 19 of chloride of sodium, 

 19 of sulphate of soda, 19 of carbo- 

 nate of soda, 5 of pure soda, and 

 15*5 of water. Dr. Back gives the 

 following as an analysis of the 

 thermal waters of the geysers of 

 Iceland : soda 5*56, alumina 2*80, 

 silica 31*50, muriate of soda 14*42, 

 and sulphate of soda 8 '57. 



THEK'MOSCOPE. (from 0e/ytos, heat, 

 and aicoTreui, to explore, Gr.) An 

 instrument for measuring the 

 degrees of heat. 



THIN OUT. This is a term used by 

 geological writers to express the 

 appearance of a stratum which 

 gradually becomes thinner, 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 Dumbar- 

 ton, in Scotland, in trap. 



THORA'CIC. (thorachtque, Fr. from 

 thorax, 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. Oupa%, Gr. 

 thorax, Fr.) The cavity of the 

 chest, containing the heart, lungs, 

 &c., &c. 



THORI'NA. An earth discovered by 

 Berzelius, in Gadolinite. It bears 

 a strong resemblance to zirconia, 

 but differs from it in many particu- 

 lars. After being heated to redness 

 it is soluble in acids. No precipi- 

 tate is caused by the addition of 

 sulphate of potash to a solution of 

 it. Thorina is colourless, and infu- 

 sible after ignition. Dr. Ure states 

 that a strong solution of the sul- 

 phate becomes a thick mass by 

 boiling, but it is soluble in cold 

 water; a property which particu- 

 larly characterizes it. 



THORI'NUM. \ The metallic base of the 



THORI'UM. j earth thorina. 



THO'RITE. A mineral discovered 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. 



THU'LITE. A mineral of a peach- 

 blossom colour, occurring in Nor- 

 way ; it is very rare, and considered 

 to be a variety of epidote. 



THTJ'MERSTQNE. j A mineral, thus 



THT/MMERSTONE. j named, by Kir- 

 wan, from its being found, in 

 masses, near Thum, in Saxony. 

 It is the Axinite of Hau'y and 

 Brongniart ; the Axinit of Werner ; 

 La pierre de Thum of Brochant. 

 See Axinite. 



THYRSE. } (thyrsus, Lat) In botany, 



THY'RSUS. j a kind of inflorescence, 

 as when the middle branches of a 

 panicle are longer than the others. 

 The horse-chesnut, lilac, &c., afford 



TI'BIA. The name given to the shin- 

 bone, or large bone of the leg. It 

 is said to have received its name 

 from a supposed resemblance to a 

 pipe or flute. 



