282 Formation of Compound or Twin Crystals. 



angles of inclination, the axes continuing equal, are 90°, the ellip- 

 soid becomes a sphere with the axes of the cube, and generally the 

 vertical conjugate axis of the ellipsoid is greater or less than the lat- 

 eral, as this angle is greater or less than a right angle. 



Thus, then all the Primary forms of Crystals* proceed from one 

 simple solid, an ellipsoid (a sphere being a solid of this kind with 

 equal rectangular axes,) and all may result from a variation merely 

 in the length and direction of the conjugate diameter? of a solid of 

 this kind. It will be noticed that all possible positions of these di- 

 ameters occur in the forms of crystals, from an equality and rectan- 

 gularly in the Cube, through different variations in length and skua- 

 tion, to a general inequality in length, and a like inequality in their 

 mutual inclinations as in the Oblique Rhomboidal Prism. 



A few remarks on the situation of secondary planes, relatively to 

 the axes, in support of the hypothesis of the existence of these axes, 

 will finish the general exposition of the theory after which the main 



ted the same construction as in his Octahedron, the only " natural way," as seemed 

 to him, for union to take place. 



* The Octahedron and Dodecahedron have not been included above, it being 

 yet a subject of doubt, whether they depend on the three axes of the cube or have 

 peculiar axes of their own. The latter seems to be the most probable conclusion, 

 as solids then result cleavable parallel to their primary faces. The peculiar axes 

 of the Octahedron will be six in number, connecting the centres of the opposite 

 edges, the molecules touching one another in twelve points in this direction. 

 Such is Wollaston's Octahedron. The axes of the Dodecahedron will be four in 

 number, connecting the opposite obtuse solid angles. These appear to be the 

 only methods of constructing these figures so that they may have their natural 

 cleavage. It is possible however that some peculiar modification of the attraction 

 in the axes of the cube may give rise to the same arrangement of the particles as 

 would result from these peculiar axes. The Hexahedral Prism being a distinct 

 primary form, it will result from molecules with four axes, one of which is at 

 right angles with the other three. Fig. 8, is a horizontal section of the crystal and 

 molecule. 



If the existence of Crystallogenic axes is admitted, the Tetrahedron cannot be 

 considered one of the primary forms; for an unmodified axis must cause addition 

 of particles at each of its extremities, whereas in this solid, each face is opposite a 

 solid angle. Its origin is undoubtedly connected with the cause of the dissimilar 

 modifications at the opposite extremities of crystals of Tourmaline, at the opposite 

 angles of Cubes of Boracite, and also with that of the inequilateral Tetrahedrons 

 of Yellow Copper Pyrites. The electric polarity of such crystals, favors the sup- 

 position that it is connected with some peculiar disposition of the electric fluid. 

 The Tetrahedron of Copper Pyrites, deserves equally with the Regular Tetrahe- 



m 



dron, a place among the Primary forms. 



