no 



THE POPULAR SCIENCE MONTHLY 



cule is the unit of the crystal structure. Consider the structure of the 

 simple cubic crystal of rock salt (sodium chloride). The structure 

 of the crystal deduced by Bragg is shown in Fig. 1. The sodium atoms 

 are marked by black spheres, the chlorine atoms by white spheres. The 

 simplicity of the crystal architecture is obvious, for all the atoms are 

 equi-distant. The structure of the diamond is more complicated but 

 it is one of great interest, for all the atoms in these cases are of one 

 kind, carbon. The structure found by Bragg is seen in Fig. 2 A. 



Fig. 2a. Arrangement of Carbon Atoms in a Diamond. 



The atoms are all equi-distant, but the general arrangement differs 

 markedly from that of rock salt. It is seen that each carbon atom is 

 linked with four neighbors in a perfectly symmetrical way, while the 

 linking of six carbon atoms in a ring is also obvious from the figure. 

 The distance between the planes containing atoms is seen to alternate in 

 the ratio 1 : 3. This variation of the grating space is brought out clearly 



Fig. 2b. Cubical, Arrangement of Carbon Atoms in a Diamond. 



from the study of the spectra, and is an essential feature of the struc- 

 ture of the diamond. The cubical arrangement is shown by turning 

 the model so that the lines joining the atoms are vertical and hori- 

 zontal (see Fig. 2B). 



