TOP
TRA
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even a cuneiform fummit; the former termina-tion is oblervable in the Brafilian, and the latterin the Siberian topazes.
3. The preceding, with a hexahedral fummitdeeply truncated, fo as to form a regular hexa-gonal plane.
4. The preceding, with the edges of theprifm that are adjacent to the two fmaileft anglesreplaced each by two fecondary planes, thusforming a dodecahedral prifm.
5. The preceding, with the lateral, and fome-times the vertical edges of the fummit bevilled.
The crystals are generally fmall and middlefized; their lateral faces are generally longitu-dinally {freaked, and fometimes convex. Thela ft re both externally and internally, is ufuallyb ightly iliining, vitreous. The crols frattu.eis ftrait lamellar, the longitudinal is fmall andimperfect.y conchoidal. It varies from tranf-parent to tranllucent, and poflefies a double re-fraction. When in mafs it occurs in coarfe andfine granular diftindt concretions. It fcratchesquartz but is inferior in hardnefs to the ruby.It is eafily frangible. Sp. gr. 3.46—3.56.
It is infufib’e per fe before the blowpipe,hut melts without intumefcence by. the affift-ance of borax. The Saxon topaz by a gentleheat becomes colourlefs without undergoingany other change, and in this Rate is fometimesfold for diamond; at a higher heat however itbecomes opake. The Brazilian topaz, when’gnited in. a crucible, becomes of a rofe-redcolour, and is then often called by the jewellersBiazilian ruby.
The Brazilian and Siberian topazes are ren-dered eieftric by heat.
^recording to the lirft analyfis of this mineralby Vauquelin, its component parts appeared tohe, 31 Silex
68 Alumine
99
Klaproth however afterwards detected theprefence of fluoric acid in: confiderable abund-ance, which induced a fecond analyfis of it byVauquelin, 3 according to which it confifts of,
29 Silex50 Alumine
21 (or thereabouts) Fluoric acid100
Topaz occurs at Schneckenfiein in Saxony mixed with quartz, black fchorl, mica and li-
thomarga, forming an aggregate which has ob-tained the name of topaz rock. It is found inBohemia at Schlackenwald in veins mixed withtinflone, fluor fpar and mifpickel. The Siberian topazes are found in the Uralian mountains,near Ekaterinenburg, and in Daouria in themountain Odontchelon, imbedded in graphicgranite, and accompanied by blackifh quartzand aquamarine. Topaz alfo occurs in Brazil ,and at Muckla in Natolia , alfo in Pegu andCeylon.
It Tanks as one of the gems, but is not veryhighly valued.
TOPAZ Oriental. See Corundum .
TOPAZ falfe. See Fluor Spar andQuartz .
TOPFSTEIN. See Potstone.
TOUCHSTONE. See Kieselschiefer.
TOURMALINE . See Schorl in the Ad*denda at the end of the volume.
TRAGACANTH GUM. See Mucilage.
TRANSFER. An article of apparatus, fora defeription of which fee tire Appendix .
TRAP.
The term Trap is Swedifh, and fignifics a.flair. It was firlt applied about fixty yearsago, by fome Swedifh mineraiogifts to de-fignate a certain clafs of mountains, compofedof nearly horizontal ftrata with perpendicularbreaks, which thus prefented a rude refem-blance to a flight of flairs. In confequence ofthe vaguenefs of this effential character, manyfpecies of rock differing very much from eachother were called by the fame name, and inex-tricable confufion would foon have enfued, ifWerner had not by adopting and reflritflingthe term, bellowed on it a definite meaning.
According to Werner there are three dif-tin£l claffes or formations of rocks to whichthe term trap may be applied. Of thefe, thefirft clafs belongs to the primitive mountains,the fecond to the tranfition mountains, and thethird to the fecondary mountains.
j I. Primitive Traps.
The primitive traps are compofed eflentiallyof hornblende, not unfrequently mingled withfelfpar, and more rarely with pyrites and mica.There are four diftin£t fpecies of rocks belong-ing to- this formation, namely common horn-blende, fchiftofe hornblende, primitive grim-flein and fchiftofe grlinftcin.
The common hornblende rock is no other than,the fubftance already deferibed in the artide
* Jpurn, dcs Mines, No. 96, p. 4 6y.