wherry: crystal form and optical properties 277 



CRYSTALLOGRAPHY. — Certain relations between crystalline 

 form, chemical constitution, and optical properties in organic 

 compounds, — /. Edgar T. Wherry, Bureau of Chemistry. 



That definite relations can be traced between the refractive 

 indices and chemical constitution of substances has long been 

 recognized. Most work has been done with organic liquids, 

 and it has been found that each element possesses a characteristic 

 refractivity, that the sum of the refractivities of the constituents, 

 modified by their manner of combination, is equal to the molec- 

 ular refractivity of the compound, and that from this in turn the 

 refractive index can be derived.^ Data for crystalline inorganic 

 salts have been collected- and the relations shown to be similar; 

 in systems other than the cubic (isometric) the mean refractive 

 indices are usually employed. There is of course every reason 

 to expect that the mean indices of crystalline organic compounds 

 could be applied in like manner;^ but it seemed desirable to 

 inquire into the possibility of correlating the several indices of a 

 given substance with its crystal structure instead of concealing 

 in mean indices whatever relations may exist^. 



The refractive indices of substances may be connected with 



other properties by various formulas, of which the Lorentz- 



72.- — 1 

 Lorenz expression, which may be used in the form , - = 



— ^ — ^ is the most satisfactory. For short the left hand term 



may be referred to as the ''refraction," and denoted by the 

 letter R. If the refractive indices in different directions in a 

 single anisotropic substance be substituted for n, directional 

 values of R will be obtained; these may be called Ra, R&, and Re, 

 the first two of course being identical in uniaxial crystals. 



1 An excellent summary of this work has been prepared by Eisenlohr (Spektro- 

 chemie Organischer Verbindungen. Leipzig, 1912). 



2 Especially by Pope (Journ. Chem. Soc. 69: 1530. 1896). 



3 The refractivities of a few crystalline organic compounds have been calcu- 

 lated by Taubert (Zeits. Kryst. Min. 44: 313. 1910). 



* The theoretical relations between the refractivities shown in the different 

 directions in anisotropic substances and the electrical interaction of the atoms 

 have recently been discussed by Silberstein (Phil. Mag. 33: 92. 1917). 



