92 SCIENCE PROGRESS 



have its zinc and sulphur atoms respectively in two separate 

 intersecting face-centred cubic space-lattices, but the mode of 

 intersection is different from the former case (fig. 3). The one 

 set of atoms occupy the centres of four out of eight of the eight 

 small cubes into which the large cube is divided. Or, what may 

 not be so immediately obvious, each atom of either kind occupies 

 the centre of a regular tetrahedron, with four atoms of the other 



NaCl 



NaCl NaCl 

 ^(100) = a 



NaCl NaCl NaCl NaCl Na CI Na CI Na 

 tf(iio) = a/y/2 d m) = 2a/ v /3 

 Fig. 2. 



kind at the four corners of the tetrahedron. This is best seen 

 for the tetrahedron DLNQ of the figure. 



The next crystal, the diamond or crystal carbon, is one of 

 the most instructive. Its structure is identical with that shown 

 for zinc sulphide, with this difference, that both sets of atoms 

 are now alike. Hence crystal carbon is built up out of atoms 

 of carbon so orientated that each atom occupies the centre of a 

 regular tetrahedron the points of which are occupied by four 

 other carbon atoms. One can hardly appreciate too highly the 



