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EXPLANATION of the Plate of Crystals of Diamond, Topaz, and Tourmaline. 



Diamond. 



Fig. I. — ^Primitive crystal, a regular octaedron, from which all the other forms 



may be conceived to be derived. 

 Fig. 2. — Octaedron with edges replaced by four faces, two and two making such 



obtuse angles at a a, as sometimes to be scarcely perceptible ; and then it passes 



into Fig. 3., when the faces become so broad as to meet in the point b, forming 



a three-sided pyramid on each face of the primitive crystal. 

 Fig. 4. — When the angles at a a. Fig. 2., are decidedly marked, and the faces 



meet, as at b. Fig. 3., a pyramid of six faces is formed on each plane of the 



primitive crystal. 



Fig. 5. — If the edges of the primitive crystal be replaced by planes a a, an octo- 

 dodecaedron, or solid of twenty faces, will be formed. 



Fig. 6. — If the planes a a o^ Fig. 5. be conceived to meet, as at b, a rhomboidal 

 dodecaedron will be formed. 



Fig. 7. and 8. — If the planes a a a of Fig. 6. be conceived to be extended as in 

 Fig. 7., or contracted as in Fig. 8., they may be considered as six-sided prisms, 

 terminated by three-sided pyramids, in which the terminal planes are set alter- 

 nately on the edges c, d, of the prism. 



Fig. 9. — Sometimes the faces a a a of Fig. 6. are divided in the direction of the 

 shorter diagonal, as shown in this figure. 



Fig. 1 o. — If the edges and the solid angles of the primitive crystal be replaced by 

 planes, the crystal (Fig. i o.) will be produced ; and if the edges be very slightly 

 and the angles very deeply truncated, a cube will be formed. This latter is 

 one of the rarest forms of the diamond. 



Fig. 1 1 . — If Fig. I . be conceived to be divided parallel to two of the opposite 

 faces of the octaedron, and through the middle of the crystal, and one of the 

 segments be turned a sixth part round, the form. Fig. 1 1., will be produced. 

 The dotted lines in the first six Figures, shew the position of the nucleus or pri- 

 mitive crystal. 



