Mr. J. F. Daniell on Crystallization. 39 



external variety of solids which would be produced by the meet- 

 ing together of the various facets thus presented to the eye must 

 be derivable from one principle of internal arrangement. The 

 octahedron therefore and the cube could not depend, in this 

 instance, upon such opposite constructions as those represented 

 in Dr. Wollaston's hypothesis. But we are not on this ac- 

 count driven to abandon the spherical atoms, for, however 

 singular it may appear that this eminent philosopher should 

 have overlooked the fact, the cube may easily be shown to be 

 derivable from a structure precisely analogous to the octo- 

 hedron, the tetrahedron, and the acute rhombohedron. 



It has been already shown that by placing a sphere upon 

 two opposite faces of an octohedron, we convert the solid into 

 the latter figure: by placing one upon each face, it is as 

 simply converted into a cube as in the following figure. Now, 



such a cube would obviously be divisible by mechanical 

 force in directions parallel to the faces of the octohedron ; 

 because in those directions each particle is held by the attrac- 

 tion of three others only; while in the transverse direction each 

 is engaged by four others; and this is the direction in which 

 fluor spar yields. But now another difficulty occurs this 

 may be supposed to be a satisfactory account of the con- 

 struction of the cube with octohedral cleavages, but how shall 

 we explain the octohedron of sulphuret of lead, which splits 

 into cubic fragments ? This is one of a numerous class of sub- 

 stances exhibiting the same phenomenon of the cubic fracture 

 with the same series of crystals as fluor spar and others of the 

 octohedral class. The theory will be worth little if it should 

 be found applicable only to the latter. 



If we suppose ten spheres endued with equal powers of 

 attraction simultaneously exerting their powers upon each 

 other, their forces would be most equally balanced in the 

 cubic form, producing the compact and stable arrangemen 



