Oil flic Intimate Stt'uct/rc of Ci-i/xfnls. 493 



" On the Intimate Structure of Crystals. IV. Cubic Crystals with 

 Octahedral Cleavage." By W. J. SOLLAS, D.Sc., LL.J)., F.lt.S. 

 Professor of Geology in the Tniversity of Oxford. Received 

 and read March 17, 1898. Revised December 10, 1900. 



IH<IIIIH<!. Carbon, m. w., 12 ; sp. gr., 3'51 ; m. v., 3-42. There is 

 good reason to believe, as will appear as we proceed, that the atoms 

 of carbon, of which diamond is composed, are disposed in the closest 

 possible order. This mode of disposition has . been described and 

 figured by Mr. Barlow,* in his work on " A Mechanical Cause of Homo- 

 geneity of Crystals." Each atom is in contact with twelve others, and 

 in planes normal to the trigonal axes of the cube, which they are sup- 

 posed to form ; they lie in closest contact, each sphere touching six 

 others in the plane. The volume of the atoms may be most readily 

 found by dividing the atomic volume by 1'35; it is 2'54, the diameter 

 of the atom is T693, and its gross volume 4-851. 



The cleavage of such a closest packed assemblage us we attribute 

 to diamond should be octahedral, for it is in planes pai'allel to the 

 faces of an octahedron that the atoms lie closest together. The 

 characteristic cleavage of diamond is thus readily accounted for ; not 

 so, however, its remarkable hemihedry. This has still to be explained. 

 Diamond is tetrahedral, and to account for this, we may suppose its 

 constituent atoms to be associated in groups of four, the centre of each 

 lying at the solid angle of a tetrahedron, or we may recall the tetra- 

 hedral nature of the carbon atom, and attribute the hemihedry of 

 diamond to an appropriate disposition of the poles of its atoms. 

 Whichever view is adopted, it will make no difference to the subse- 

 quent treatment of the question ; in either case, we have to consider 

 the manner in which tetrahedra may be arranged so as to give rise to 

 hemihedral symmetry. Two kinds of arrangement are possible; in 

 both, the trigonal axes of the tetrahedra lie on the trigonal axis 

 of a cube, but in one the tetrahedra are oppositely, in the other 

 similarly, orientated. On bringing the former case under the notice 

 of Mr. Barlow, he informed me that it was new to him, and subse- 

 quently he pointed out that the symmetry resulting from it is holo- 

 hedraljt while the arrangement in the second case is hemihedral ; we 

 must accordingly suppose that in diamond the constituent tetrahedra, 

 whether groups of atoms or the atoms themselves, are all similarly 

 orientated. 



* ' Sci. Proc. Roy. Dub. Soc.,' vol. 8 (X.S.), p. 533, 1897. 



t This arrangement is described by Mr. Barlow in the ' Sci. Proc. Eoy. Dub. 

 Soc.,' loc. cit., under tLe heading C (i), p. 542; the alternative arrangement is 

 piven under C (a). 



