DIAMONDS — LOGIE 



363 



Figure 1. 



morphic, and a comparison of the properties of the two helps us to 

 understand what is important in determining those properties. 



Diamond has a density of 3.51 g./cc. against 2.25 g./cc. for graph- 

 ite, so that the carbon atoms are not packed together as closely in 

 graphite as they are in diamond. The X-ray analysis has confirmed 

 a structural difference between the two. Diamond crystallizes in a 

 cubic system with each atom symmetrically surrounded by four close 

 neighbors, all at the same distance and arranged at the corners of a 

 regular tetrahedron. (See fig. 1.) Diamond may also be thought 

 of as two interpenetrating face-centered cubic lattices, one of which 

 is displaced relative to the other by a quarter of the distance along the 

 main diagonal (fig. 2) . The angle which the bonds between the atoms 

 make with each other is 109°28'. Both the strength of the bonds and 

 their direction influence the properties of the material. The melting 

 point of substances is largely determined by the directed nature of the 

 forces between atoms, and where they are as strictly directed as in the 

 case of the diamond, there the melting point of the substance is very 

 high. In ionic crystals, where the bonds are not strictly directed and 

 the forces merely require that there shall be a great number of oppo- 

 sitely charged ions close to each other, the melting point is low. In 

 metals, we find frequently a tendency for each atom in the crystal to 

 gather romid it the greatest possible number of nearest neighbors (12) 



