TORTOISE-SHELL 1023 



TOPAZ. The fundamental form is a scalene 4-sided pyramid ; but the secondary 

 forms have a prismatic character, and are frequently observed in 4-sidcd prisms, 

 terminated by 4 planes. The lateral planes of the prism are longitudinally striated. 

 Fracture conchoidal, uneven ; lustre vitreous ; colours, white, yellow, green, blue, 

 generally of pale shades. Hardness, 8 ; spec. gray. 3'5. Prismatic topaz con- 

 feists, according to Berzolius, of alumina, 57'45 ; silica, 34 '24 ; fluoric acid, 7'75. In 

 a strong heat the faces of crystallisation, but not those of cleavage, are covered with 

 small blisters, which however immediately crack. With borax, it melts slowly into a 

 transparent glass. Its powder colours the tincture of violets green. Those crystals 

 which possess different faces of crystallisation on opposite ends acquire the opposite 

 electricities ou being heated. By friction it acquires positive electricity. 



Most perfect crystals of topaz have been found in Siberia, of green, blue, and white 

 colours, along with beryl, in the Uralian and Altai mountains, as also in Karntschatka ; 

 in Brazil, where they generally occur in loose crystals, and pebble-forms of bright 

 yellow colours ; and in Mucla in Asia Minor, in pale straw-yellow regular crystals. 

 They are also met with in the granitic detritus of Cairngorm in Aberdeenshire. The 

 blue varieties are absurdly called oriental aquamarine by lapidaries. If exposed to 

 heat, the Saxon topaz loses its colour and becomes white ; the deep yellow Brazilian 

 varieties assume a pale pink hue, and are then sometimes mistaken for spinelle, 

 to which, however, they are somewhat inferior in hardness. Topaz is also dis- 

 tinguishable by its double refractive property. Tavernier mentions a topaz, in the 

 possession of the Great Mogul, which weighed 157 carats, and cost 20.000/. sterling. 

 There is a specimen in the Museum of Natural History at Paris which weighs 4 ounces 

 2 gros. Topazes are not scarce enough to be very highly valued. See GEMS. 



TORBAItflTE, or Torbane-Hill mineral. See BOGHEAD COAL. 



TORBITE. A preparation of Peat, for which works were established at Hor- 

 wich, in Lancashire. It does not appear that this manufacture was attended with 

 success. 



TOKREFACTIOXO 1 . Boasting ores to deprive them of sulphur, arsenic, or other 

 volatile substances. 



TORTOISE-SHELI.. or rather scale ; a horny substance that covers the hard 

 strong covering of a bony contexture, which encloses the Testudo imbrioata, Linn. 

 The lamellae or plates of this tortoise are thirteen in number, and may be readily 

 separated from the bony parts by placing fire beneath the shell, whereby they start 

 asunder. They vary in thickness from one-eighth to one-quarter of an inch, ac- 

 cording to the age and size of the animal, and weigh from 5 to 25 Ibs. The larger 

 the animal, the better is the shell. This substance may be softened by the heat of 

 boiling water ; and if compressed in this state by screws in iron or brass moulds, it 

 may be bent into any shape. The moulds being then plunged in cold water, the shell 

 becomes fixed in the form imparted by the mould. If the turnings or filings of 

 tortoise-shell be subjected skilfully to gradually increased compression between 

 moulds immersed in boiling water, compact objects of any desired ornamental figure 

 or device may be produced. The soldering of two pieces of scale is easily effected, 

 by placing their edges together, after they are nicely filed to one bevel, and then 

 squeezing them strongly between the long flat jaws of hot iron pincers, made some- 

 what like a hairdresser's curling tongs. The pincers should be strong, thick, and just 

 hot enough to brown paper slightly without burning it. They may be soldered also 

 by the heat of boiling water, applied along with skilful pressure. But in whatever 

 way this process is attempted, the surfaces to be united should be made very smooth, 

 level, and clean : the least foulness, even the toudi of a finger, or breathing upon them, 

 would prevent their coalescence. See HORN. 



Tortoise-shell is manufactured into various objects, partly by cutting out the shapes 

 and partly by agglutinating portions of the shell by heat. When the shell has become 

 soft by dipping it in hot water, and the edges are in the cleanest possible state without 

 grease, they are pressed together with hot flat tongs, and then plunged into cold 

 water, to fix them in their position. The teeth of the larger combs are parted in 

 their heated state, or cut out with a thin frame saw, while the shell, equal in size to 

 two combs with their teeth interlaced, as in fig. 2010, is bent like an arch in the 

 direction of the length of the teeth, as in fig. 2011. The shell is then flattened, the 

 points are separated with a narrow chisel or pricker, and the two combs are finished, 

 while flat, with coarse single-cut files and triangular scrapers. They are finally 

 warmed, and bent on the knee over a wooden mould, by means of a strap passed round 

 the foot, just as a shoemaker fixes his last. Smaller combs of horn and tortoise-shell 

 are parted while flat, by an ingenious machine, with two chisel-formed cutters placed 

 obliquely, so that each cut produces one tooth. See Eogers's Comb-cutting Machine, 

 ' Trans. Soc. Arts,' vol. xlix. part 2, since improved by Mr. Kelly. In making the 

 frames for eye-glasses, spectacles, &c., the apertures for the glasses were formerly cut 



