T U F 



[ 447 



T U K 



TUFF. | A name applied to 



TT/FA, VOLCA'NIC. j several different 

 substances the production of volcanic 

 eruptions. Generally, an aggregate 

 of sand, volcanic ashes, and frag- 

 ments of scoria and lava, united by 

 an argillaceous or muddy cement. 

 Sometimes it is composed of vol- 

 canic ashes and sand, transported 

 and deposited by rain water. 



Tuff, or volcanic tufa, as dis- 

 tinguished from calcareous tufa, 

 presents various shades of grey, 

 brown, red, yellow, &c., or it is 

 sometimes spotted. Hardness mo- 

 derate ; fracture dull and earthy. 



TUFA'CEOUS. Having the appearance 

 or texture of tufa. 



TU'NGSTEN. (from tung, heavy, and 

 sten, stone, Dan.) 



1. A greyish white metal, brittle, 

 and very hard. Specific gravity 

 17'4. Fusible in the most intense 

 heat only, its infusibility equalling 

 that of platinum. It has been 

 obtained only in the form of grains 

 of extreme hardness. It was dis- 

 covered by Scheele, and by "Werner 

 has been named Scheel ; by Haiiy, 

 Scheelin. 



2. A mineral, of a grey or yellow- 

 ish-grey colour, occurring in Bohe- 

 mia, Sweden, and Cornwall, mas- 

 sive and disseminated. Its external 

 lustre is shining and splendent; 

 internal lustre shining and resinous. 

 Specific gravity from 5-57 to 6 '06. 

 It can be scratched with a knife, 

 and is easily broken. It is infu- 

 sible, but before the blow-pipe it 

 becomes opaque, and decrepitates. 

 By digestion in nitric acid it is 

 converted into a yellow powder, 

 which is the oxide of tungsten. It 

 consists of oxide of tungsten 77*75, 

 lime 17.60, silex 3-00, according 

 to Klaproth : Berzelius gives its 

 analysis as consisting of oxide of 

 tungsten 80 '24, lime 19-40. It is 

 the Scheelin calcaire of Haiiy and 

 Brongniart; the schwerstein of 

 Werner ; the tungsten of Kirwan. 



TTTNICA'TA. (tunicatus, coated, Lat.) 

 The tunicated animals have no ex- 

 ternal shell nor internal solid parts, 

 but are covered with a tough, elas- 

 tic, homogeneous tunic, in the form 

 of an enveloping sac with a respi- 

 ratory and an anal orifice. This 

 exterior sac is the analogue of the 

 valves of conchifera, and has the 

 muscular fibres of the lining mantle 

 inserted into its inner surface. 



TTJ'NICATED. Covered with one or 

 more tunics or membranes. 



TUBBKLLA'RIA. According to Busk, 

 the sixth order of the class Scole- 

 cida. 



TU'BBINATED. (turbinatus, Lat.) Of 

 a spiral oblong form; in conchology, 

 applied to shells, broad at the base, 

 and becoming gradually narrower 

 till they are pointed at the apex. 



TURBINO'LIA. A genus of lamellated 

 stony polypifers ; free, (and in this 

 differing from the genus caryophyl- 

 lia, which is fixed,) simple, turbi- 

 nated or cuneiformed; longitudi- 

 nally striated on the exterior ; the 

 base pointed, the terminating cell 

 stelliformly lamellated and some- 

 times oblong. 



TU'RBO. (turbo, a top, Lat.) Plural, 

 turbines. A genus of marine uni- 

 valves, found on rocks and sea- 

 weeds, at depths varying to ten 

 fathoms. 



The turbo is a conoidal or slightly 

 turreted shell, the aperture com- 

 plete, rounded, and not toothed; 

 the margins disjoined in the upper 

 part ; the columella smoothed at 

 the base. In Turton's Linne one 

 hundred and fifty-one species of 

 tubines are described, sixty-nine of 

 which are indigenous to Britain. 

 Many species of turbines have been 

 described as found fossil. 



TTJRKO'IS. j (twquom, Fr.) A gem 



TuRQjro'isE. j of a blue or greenish 

 colour, and opaque, found in round- 

 ish masses, from the size of a pea 

 to that of an egg. The finest spe- 

 cimens are brought from Persia^ 



