Relationship between density and refractive index of inorganic solids

The inter-dependence between refractive index and density of inorganic solids has been investigated. It is found that the available experimental data on refractive index n and density d may be expressed by the relations (n-1)/d=CM -u and (n 2 -1)/(n 2 +2)d=KM -v , where C and K are constants, the exponents u and v are approximately equal to 0.4, and M is the averaged atomic mass of a substance. For a large number of substances C=0.65 and K=0.375, but alkali halides are exceptions. These results are discussed in terms of the classical theory of dielectrics and the Phillips-Van Vechten description of the dielectric constant.