wherry: crystal form and optical properties 323 



horizontal ones shown, the spacing of the planes of the layers 

 may be calculated in the usual way. In mellite x = 1, W = 

 62.95, ni = 1.64 x IQ-^", y = 64, p = 1.64, and c' = 1.055, 

 whence da = 2.10 X 10-« and dc = 2.22 X lO-* cm. The some- 

 what greater values obtained in this case are what would be 

 anticipated from the presence of the relatively large aluminium 

 atoms, although the structure is too complicated and too un- 

 certain for exact valuation of the effects of the several atoms. 



The double propionate group. 



The crystallographic and optical data in the literature^ upon 

 the double propionates of calcium with strontium, barium, and 



TABLE 6 

 Refraction Relations of Double Propionates 



lead respectively, show fair inverse agreement to exist between 

 their crystallographic-axial and refraction ratios, as brought 

 out in table 6. The barium salt is cubic, but this may be looked 

 upon as a limiting case of the tetragonal system, where axis c = 

 1, and the refraction ratio is of course also 1. 



The refractive indices and axial ratios have been accurately 

 determined for a large number of isomorphous mixtures of the 

 lead and barium salts; three of these are here listed as typical. 

 Toward either end of the series the refraction and axial ratios 

 show inverse agreement, but they deviate through the middle 

 portion. Evidently the irregularities in the layers of atoms that 



' Groth, op. cit., p. 203. 



