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tallized in a (lightly acute rhomboid or doubletrihedral pyramid (its primitive figure) of whichthe obtufe' angles meafure 94 0 , and the acuteones 86°.
1. Sometimes the acute folid angles, or thefummits of the pyramids, are replaced by planesperpendicular to the axis, fituated either aboveor below, or exactly correfponding with thefhort diagonal.
2. Sometimes the edges of the bafe of theprimitive rhomboid are each of them replacedby a plane parallel to the axis, thus feparatingthe two pyramids by a hexahedral prifm withrhombic planes: when the prifm is long theterminal pyramids often entirely difappear.
3. Sometimes the flat angles of the primitivepyramids that reft on the common bafe arereplaced by two new planes, thus rendering thepyramids enneahedral as long as any of theprimitive faces remain, but when tliefe are ob-literated forming a dodecahedron, compofed oftwo hexahedral pyramids united bafe to bafe.This modification is often combined with No. 1.in confequence of which the fummits of thedodecahedron are more or lefs truncated.
4. The acute angles compofing the folidangle at the fummit of the primitive rhomboid,are fometimes replaced by fecondary planes foinclined to the axis as to form a very obtufefecondary rhomboid.
The variety in the form of a hexahedral prifm(No. 2.) appears fometimes to contain within ita fimilar prifm but of a different colour. Thecryftals are fmall and very fmall, rarely middlefized. It is commonly more or lefs tranfparentpalling into tranflucent. The luftre of the tranf-parent kinds is fhining and vitreous; of thetranflucent kinds generally gliftening. Whenthofe cryftals, in which the fummits of the pri-mitive rhomb are replaced by fecondary planes,are tranflucent, the planes juft mentioned prefentthat varying luftre which is called chatoyant :and if thefe cryftals are cut en cabochon, or inthe form of an ellipfe, taking care that the fum-mit of the ellipfe ffiall be fituated exactly underthe point correfponding with the fummit of therhomboid, there will then be produced the ap-pearance of a ftar with fix rays. Such ftonesare highly valued by the jewellers, and arecalled Jlar-Jlones.
The fra£ture of the blue varieties of Telefiais conchoidal palling into foliated; that of theOther varieties is foliated. The fragments areindeterminately angular. Its hardnefs is onlyinferior to that of the diamond, and the bluevarieties are uniformly fomewhat harder than
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the others. It gives fire with fteel, but not veryeafily. By collifion it becomes' luminous likequartz, and the light from the red varieties is avery deep fire colour or cherry-red. Its meanfpecific gravity is about =4. but it varies from3.907 to 4.161.
It is infufible per fe before the blowpipe, butwhen finely pulverized is foluble in borax andpartially in potafli. Its extreme hardnefs, andthe powerful affinity that fubfifts between alu-mine and filex, render it difficult to analyfe.According to Bergman, fapphire is compofed of35 filex, 58 alumine, 5 lime, and 2 oxyd of iron.An improved analyfis of the fame gem was madeby Klaproth (by means of cauftic potafli) withthe following refults:
98.5 Alumine,
0.5 Lime,
1.0 Oxyd of iron.
100.0
Finally, Mr. Chenevix fucceeded, by the helpof borax, in decompofing this ftubborn mineral,and his analyfes of the fapphire and ruby mayperhaps be looked upon as the moft correct ofany.
Telefia occurs in veins in granite and fyenite.It is found in the kingdom of Pegu and theifland of Ceylon in the Eaft Indies. It feemsto have been found by Count Bournon in theprovince of Forez in France , and has been metwith in the bed of the brook Expailly, in thefame country. It is faid alfo to have been dis-covered in Portugal and Bohemia ; and in theNorth of Italy by Profeflor Pini.
All the varieties of Telefia are valuable injewellery •, but rubies, if not very fmall, bear aconfiderably higher price in proportion to theirweight than fapphires or any of the other vari-eties. It is faid that the light violet colouredpieces may be rendered colourlefs by beingheated in a charcoal crucible, without impairingany other of their properties, and that fuchpieces are often fold for diamonds.
II. Adamantine Spar. ImperfeEl Corundumof Bournon,- Corund and Demantfpath Werner;Cor union Ilauy ; Spath adamantine Broch.
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