572 SCIENCE PROGRESS. 



gases has shown that a refraction constant of an additive 

 nature — termed the molecular refraction — can be calculated; 

 if \V be the molecular weight of the compound, d its density 

 and n its refractive index for a particular wave-length of 

 lieht, the molecular refraction R is calculated from the 

 equation : — 



R=^'(«-,). 



On determining the molecular refractions for a large 

 number of organic liquids, Gladstone found them to be the 

 sums of so-called atomic refractions of the atoms composing 

 the molecule of weight W" ; these atomic refractions are 

 themselves calculated from the observed molecular refrac- 

 tions of many liquid organic compounds. It is found further 

 that the atomic refraction of a carbon atom attached to four 

 different atoms or groups of atoms is different from the 

 atomic refraction of a carbon atom bound to only three 

 atoms or groups of atoms ; the same is true of other atoms 

 bound to different numbers of other atomic groups. A 

 knowledge of the molecular refraction is frequently of great 

 value to the organic chemist, as for instance in determining 

 whether any particular organic compound is saturated or 

 not, namely, whether any of its carbon atoms are attached 

 to less than four other atoms or groups of atoms, because 

 the molecular refraction of a saturated compound differs 

 from that of an unsaturated compound of the same molecular 

 composition. On attempting to apply the idea of molecular 

 refraction to crystalline substances, difficulties arise, because 

 in the cases of most such bodies the refractive index varies 

 with the direction ; it is thus not at first sight clear how the 

 varying refractive index of a crystalline substance can be 

 dealt with so as to yield a single molecular refraction 

 constant from which results analogous to those obtained in 

 the case of liquids might be derived. It has, however, 

 been shown that from the several refractive indices of a 

 crystalline solid a certain mean refractive index can be 

 easily calculated which would be the refractive index of the 

 substance if the regular arrangement of its particles were 

 destroyed whilst its density were maintained constant ; then 



