NATURAL HISTORY, 



'late 



62. Fig. 5. 



12. 

 14. 



The HexaheJral prifm, or equiangular fix- 



fided prifm 

 The Rhomboidal dodecahedron 

 The Pyramidal dodecahedron, or double fix- 



fided pyramid 



57. Crystallography, Plate II. 



Fig. 17 — 21. Mechanical difleaion of an hexahcdral 

 cryjlal of calcareous /par, and extrac- 

 tion of the primitive cryftal, or 

 nucleus 

 Obf. Lettered Jig. ij — 21. P/ate II. 

 Cryjlallography. Defcribed in article 

 Crystal, Vol. X. Part II. but erro- 

 neoujly referred to as Jig I — 5. Plate I. 

 22, 23. Mechanical divifion of the dog's-tooth 

 fpar, erroneoufly fig. 6, 7. Plate I. 



Fig. 24. Rhomboids of calcareous fpar, a fecon- 

 dary cr)-ft;al, with the primitive nucleus 



Crystallography, Plate V. 



Fig. j 6. The bafe of the ftxjided prifm, dinded by 

 feftions parallel to each of its fides, and 

 producing the triangular prifm, the ulti- 

 mate form obtained by mechanical divifion 



9. Crystallography, Plate IV. 



Fig. 48, 49. The cube divided by feftions parallel to 

 the fides, and producing a feries of 

 fmaller cubes, confidered as the form 

 of the integrant molecule 



Cystallograpiiy, Plate II. 



Fig. 26. The primitive rhomboid of the tourmaline, 

 with its difleftion. This crj'ftal is divi- 

 fible both in the direftion of the fix faces, 

 and in that of the (hort diagonals ; by 

 which latter feftions the rhomboid is re- 

 duced to fix tetrahedrons, furrounding 

 the nucleus, as here reprefented 



Decrements of the Edges of the Cryjlals. 

 &258. Crystallography, Plates II. & III. 

 Fig. 27, 28. The rhomboidal dodecahedron, which 

 figure may be formed from a cubic nu- 

 cleus, by the fuperpofition of decreafing 

 laminse 



I. Crystallography, Plate III. 



Fig. 29. Congeries of cubes, confifting of integrant 

 molecules, forming the cubic nucleus, 

 with the pyramids raifed on three of the 

 faces 



Crystallography, Plate II. 



Fig. 27. The form of the cryftal, produced by the 

 combination of thcfe integrant molecules, 

 when complete 



Decrement in Breadth and Height. 

 Crystallography, Plate III. 

 Fig. 30. A cryftal of iron pyrites, with twelve pen- 



Plate 

 258. 



Fig. 31' 



32' 



33- 



34- 



258. 



tagonal faces, in which the two kinds of 

 decrement are combined 

 The cubic nucleus of this variety is (hewn 

 The formation of the cryftal by decrements 

 Decrement of the dog's-tooth fpar, (the me- 

 taftatic cryftal of Haiiy,) reprefented com- 

 plete in Plate 257. fig. 22. Theprefent 

 figure fuppofes the hypothefis of a decre- 

 ment by two ranges in breadth. Itreprefents 

 the upper pyramid of this cryftal placed 

 on the upper planes of the primitive nu- 

 cleus, which being partly vifible, difclofes 

 more clearly the progreflive effects of the 

 decrement by two ranges 

 A fecondary cryftal, which is a rhomboid 

 much more obtufe than the nucleus, is 

 reprefented as furrounding the nucleus in 

 the variety of calcareous fpar, called by 

 Haiiy, equiaxe 

 The nucleus (prefumed to be the primitive* 

 rhomb of carbonate of lime) progreflively 

 diiTefted to explain its ftrudlure 

 The face at letter A, fig. 35, (hews the fame 

 face of the rhomb as is reprefented in fig. 

 34,butfymmetrically divided, and the fuite 

 ftiews the gradual divifion of the rhomb, 

 by mechanical feparation down to letter 

 \d 



Crystallography, Plate III. 

 Fig. 36. Decrements on the angles of a cryflal, fhewa 



in the 

 cube 



regular oSohedron formed on a 



259. Crystallography, Plate IV. 



Fig. 37. The arrangement of the integrant mole- 

 cules on one of the triangular faces of 

 the oftohedron 



259. Crystallogkafiiy, Plate IV. 

 Fig. 38. Rhomboid cryftal 



39. Another example of the decrements on the 

 angles, exemplified in the dilTeftion of the 

 rhomboid, fig. 38., which differs fomewhat 

 , from that of the cube, producing a very 



obtufe rhomboid, encircling the nucleus ; 

 found among the fecondary cryftals of 

 oligifte iron ore 

 Further illuftration of the different variations, 

 of which the decrements of the rhomboid, 

 both of the fuperior and inferior angles, 

 are fufceptible. The rhomboid is (hewn 

 at fig. 45. ; the difledtions at fig. 46, 47. 



Intermediate Decrements. 



Fig. 48. A parallelopiped undergoing a decrement, 

 by two ranges on the angle of its bafe 



A cryftal, in which all the three decrements 

 round the fame fohd angle are intermediate 



Another example of intermediate decre- 

 ment in one of the faces of a cubic nu- 

 cleus, taking place on the angles, by the 

 fubtraftion of double molecules 



The cubic nucleus, marking the decrements 

 parallel to the lines k m, &c. by fubtrac- 

 5 B 2 



49. 



50. 



