172 



ANNUAL REPOJIT SMITHSONIAN INSTITUTION^ 1915. 



chlorine atoms by white spheres. The simplicity of the crystal archi- 

 tecture is obvious, for all the atoms are equidistant. The structure 

 of the diamond is more complicated, l)ut it is one of great interest, for 

 all the atoms in these cases are of one kind, carbon. The structure 

 found by Bragg in seen in plate 1, figure 2. 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 neigh- 

 bors 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 plates 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 sliow^n by turning the model so 



that the lines joining the atoms 

 are vertical and horizontal (pi. 1, 

 fig. 1). 



Now^ that we have a method of 

 determining the arrangement and 

 distances apart of the atoms in a 

 crystal, the next step will be to 

 examine the intensity and type of 

 forces which are brought into play 

 to keep the atoms in equilibrium 

 and relatively fixed in their places. 

 It is to be expected that the atoms 

 are able to move to and fro about 

 their position of equilibrium, and 

 this is indicated by the effect of lowering the temperature of the 

 crystal, for the intensity of the diffraction spectra increases as the 

 amplitude of motion of the atom diminishes. The sharpness of the 

 diffraction spectra suggests that the atoms are not only arranged at 

 definite distances from one anothei-, but that each atom is orientated 

 in a definite position with regard to its neighbor. 



While varieties of crystals are known of all degrees of hardness, 

 the work of Lehmann has brought to light the unexpected existence 

 of crystalline arrangement in some liquids. These liquid crystals 

 are best shown in certain complex organic substances at a tem- 

 perature slightly above their melting point, and they are only observ- 

 able in the liquid by the patterns and colors developed when 

 polarized light passes through them. These crystals are mobile, like 

 a drop of oil in a solution, and can be squeezed into a variety of 

 patterns. Such results would indicate that the molecules of the liquid 

 have a tendency to arrange themselves in ordered patterns, although 

 it is difficult to understand how the freedom of relative motion that 



Fig. 1. — Arrangement of atoms in a rock salt 

 (NaCl) crystal. White circles represent 

 sodium atoms; black, chlorine. 



