Page:Popular Science Monthly Volume 87.djvu/114

110 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.

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

from the study of the spectra, and is an essential feature of the structure of the diamond. The cubical arrangement is shown by turning the model so that the lines joining the atoms are vertical and horizontal (see Fig. 2B).