388 Theory of Cryjlallifathn. 



If the decrements flop abruptly at a certain term, fo that 

 the pyramids are not terminated, the twenty-four trape- 

 zoids will be reduced to elongated hexagons, which will in- 

 tercept twelve rhombufes parallel to the faces of the nucleus. 

 This is the variety to which I have given the name of inter- 

 mediary garnet.. 



In the fulphure of zinc the regular o£taedron refults from 

 a decrement by a range around the eight folid angles, com- 

 pofed of three planes, viz. C, B, O, G, F, D, A, I (fg. 68.). 

 The fame fubftance affunaes alio the figure of a regular te- 

 traedron, by the help of a decrement by one range on four 

 only of the eight folid angles before mentioned, fuch as 

 C, O, F, A. This tetraedron is remarkable by its ftru&ure, 

 which prefents an affemblage of other tetraedra with ifofceles 

 faces. 



Cryjlah the Molecule of which are triangular Prijms, 



Oriental. 



I give this appellation to a kind of gem known under the 

 names of the ruby, japphirc, oriental topaz, according as it 

 is red, blue or yellow. It is fo rare to find cryftals of this 

 gem which do not exhibit marks of a precipitate formation, 

 or which have not been rolled, that hitherto we have had no 

 accurate defcription of its different varieties, nor any precife 

 indication of the nature of the particular angles of each va- 

 riety. The cryftals which enabled me to eftabliih the fol* 

 lowing refults were of a form fufficiently well chara&erifed. 



I. Primitive Oriental* 



It cryftallifcs in the form of a regular hexaedral prifm di- 

 viiible parallel to its bafes. Theory points out other joinings, 

 parallel to the planes, from which it follows, that the mole- 

 cula is n equilateral triangular prifm. The height of this J 

 prifm, fuch as given by theoretic calculation, is a little lefs 

 than three times the height of the triangle of the bafe. 



a, Efox- 



