J^O Theory of CryjlalVifal'ian. 



the three angles of the fuperior bafe of the nucleus, fuch as /», 

 d,g, {Jig. 67.)? and on the intermediate angles of the inferior 

 bafe. It may be readily conceived, that in the decrement 

 which takes place, for example, on the angle g, the three 

 ranges, which remain unoccupied between that angle and 

 the corresponding edge ofthefirft lamina of fuperpofition, are, 

 1 ft, the fmall rhombus goip, which alone forms the firft 

 range ; 2d, the two rhombufes os ti, pzdi; $dj the three 

 rhombufes fituated on the fame line behind the two pre^ 

 ceding. 



Cryftals of the oriental are found particularly in the king- 

 dom of Pegu. There are alfo in France fapphires called the 

 fapphires of Puy. They are found at the diftance of a league 

 from Velai, on the banks of a rivulet near the village of 

 Expaillv, where they are mixed with garnets and hyacinths. 

 Thefe fapphires have all the characters of the ftone called 

 oriental ' Japphire, 



V. Difference between Structure and Increment. 



Tn what I have hitherto faid refpecting the decrements to 

 which the lamina? of fupcrpofition are fubjected, my only 

 view was to unfold the laws of ftructure ; and I am far from 

 believing that, in the formation of a dodcracdral cryftal, or. 

 one of any other form having a cube for nucleus, the cryf- 

 tallifation has originally produced that nucleus fuch as it is 

 extracted from the dodecaedron, and made it afterwards pafs 

 to the figure of that dodecaedron, by the fucceffive appli- 

 cation of all the laminae of fupcrpofition by which it is co- 

 vered. On the contrary, it feems proved, that from the firft 

 moment the cryftal is already a very fmall dodecaedron, con- 

 taining a cubical nucleus proportioned to its fmall fizc, and 

 that the cryftal afterwards increafes by degrees without 

 changing its form, by new layers which envelop it on all 

 fides, fo that the nucleus increafes alfo, always prcferving 

 the fame relation with the whole dodecaedron. 



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