NATURAL HISTORY. 



Plate 

 259. 



tion of double molecules, in fuch a man- 

 ner as that three ranges be taken away 

 in the breadth, and one in height ; fo 

 that the decrements will be both inter- 

 mediate and mixed 



rz. A polyhedral cryftal of thirty faces, pro- 

 duced by the ceffation of decrementation, 

 before the formation of the pyramid round 

 each face of the nucleus 



cj. Example of intermediate decrements on the 

 two lateral angles of a rhomboid, (as at 

 fig. 47.), the decrements taking place by 

 ranges of double molecules, producing in 

 the complete rcfult a folid of twelve faces, 

 difpofed fix and fix towards each fummit, 

 as in one variety of calcareous fpar, or 

 double-pointed dog's-tooth fpar 



Compound fecondary Forms of Cryjlals. 



rig. 54. A fecondary cryftal, an tcofahedron bounded 

 by eight equilateral triangles, and twelve 

 ifofccles triangles. Occurs in iron pyrites 

 55. A calcareous fpar {^Analogique of Haiiy), 

 compofcd of twenty-four trapezoidal 

 faces, fix of which are vertical, and 

 twelve others difpofed iix and fix. The 

 different trapezoidal faces are (liewn in the 

 figure 



260. CnYSTALLOGRAPHY, Plate V. 



Fig. j6. The end of the regular fix-fided prifm, 

 which, for its molecule, prefents us with 

 the triangular, or three-fided prifm 



57. A cubic nucleus divided into its cubic mole- 

 cules 



59. Superior face of the fecond lamina, A 

 Ditto, further mechanical divifion of the 



integrant molecules, B 

 Ditto, ditto, C 



58. Explanation of vacuities on the edges of a 



cryftal, given by Haiiy 



60. An oblique prifm with rhomboidal bafes, fo 



fituated that the faces AH, ad, and C D, 

 c d, are vertical ; and B D are the acute 

 angles of the bafe, and that the laUer 

 proceed in an afcending dire£lion from 

 A to C 

 Interfeft this prifm into halves, by means of 

 a plane paffing by the diagonals, drawn 

 from B to D, and from b to d, fo that 

 the half fituated on the left remaining 

 fixed, the other is revcrfed without being 

 feparated, and the figure prefented will be 

 as fhewn at fig. 61. 



62. Another example of grouping, in which 



cryftals are inferted into each other, is ex- 

 tremely common. This combination is 

 illuftrated by a cube, and M N r, an equi- 

 lateral triangular facet, produced by 

 • a decrement of one range round the 



angle A 



63. A fecond cube modified in the fame manner, 



and affixed to the other by its corre- 

 fpondent facet, will aiford the double 

 cryftal reprefented 



Plate 



To illujlrate the Notation of Cryflals. 



Fig. 64. Reprefents an oblique parallelopiped, tl 

 faces of which have angles of differe: 

 meafures 



65. The effedl of decrement fhewn 



66. An oblique parallelopiped 



67. Primitive molecules 



68. Bibinary felfpar of Haiiy 



69. Primitive form of a reftangular prifi 



which has oblique-angled parallelograi 

 for its bafes, one of which is longer 



70. The obhque prifm, with rhomboidal bafes 



71. The reftangular prifm, with redtaiigul 



bafes 



72. The reftangular prifm, with rhomboid 



bafes 



73. The reftangular prifm, with fquare bafes 



74. The cube 



75. The moft common variety of chryfoberyl, 



cymophane, the nucleus of which is 

 regular parallelepiped,^ as reprefented 

 fig. 71. 



76. The prifm 



261. Crystallography, Plate VI. 



Fig. 77 — 84, &c. Several figures inferted to illufln 

 the manner in which the fymb( 

 employed in the definition oft 

 various modifications of cryfta 

 by Haiiy, and other cryftal] 

 graphers are exprefied 



80. Oftohedron, with fcalene triangles 



81. Oclohedron, another variety 



82. The regular oftohedron 



83, 84. The primitive oftohedron, compofed 

 eight ifofceles triangles fimilar, four a 

 four each 



85. The tetrahedron when become a primiti 



form 



86. The regular fix-fided prifm 



87. The fame, in which three folid angles, tab 



alternately, are replaced by faces, whi 

 the intermediate angles remain untouch© 



88. Rhomboidal dodecahedron, in which ea 



folid angle is compofed of three plan 

 and may be affimilated to a fummit oft 

 obtufe rhomboid Ji 



89. Primitive form of the tourmaline % 



90. One variety of the tourmaline 



Supplemental Plate, lettered Crystallization, Plate >^ 



262. Crystallization. See Mineralogy, j4ppen&t 

 Fig. I. Cube 



2. Rhomb 



3 — 5. 29. Varieties of the Prifm 



6. Pyramid 



7,8. Table 



9. Icofahedron 



10. Pentagonal Dodecahedron 



11. Lens 



12. Rhomboidal Dodecahedron 



13. Tetrahedron 



14. Double fix-fided Pyramid 



