THE DETERMINATION OF REFRACTIVE INDEX 227 



In like manner if we have a series of crystals, or fragments of 

 transparent solids whose indices of refraction we know, it is 

 possible to roughly ascertain the index of a given liquid. 



The index of refraction is a constant for any given substance 

 of definite composition. Its determination often affords a ready 

 means of identification or differentiation and in many instances 

 is in fact the only simple means at our command for the recogni- 

 tion of a compound. By the index of refraction is meant the 

 ratio of the sine of the angle of incidence to the sine of the angle 

 of refraction. It is customary to refer indices of refraction to 

 that of air, which is taken as unity n = i. 



Although the identification of compounds through determina- 

 tions of their refractive indices by the immersion method and 

 the microscope has long been practiced by mineralogists, petrol- 

 ogists and microscopists, it is only within the last few years 

 that chemists have awakened to the value ot the data so easily 

 obtained. 



A determination of the refractive index is of special value in 

 the qualitative analysis of soils, sands, mineral fragments, etc., 

 in the examination of plant and animal fibers, 1 in the study of 

 crystalline residues, in the differentiation of isomeric compounds 

 and in the study of materials which, although pure, cannot 

 properly be separated from foreign matter. 



In order to determine the refractive index of crystalline solids 

 we may proceed as described below : 



Determination of the Refractive Index of Isotropic Substances. 

 -One or more tiny fragments or crystals of the material are 

 placed upon a clean slide, a small drop of a liquid of known 

 refractive index is placed upon a small, scrupulously clean cover- 

 glass and the cover with its drop is inverted and laid upon the 

 solid under investigation, care being observed in lowering the 

 glass with its drop of liquid to avoid the formation of air bubbles. 

 Place the preparation under the microscope with the Abbe con- 

 denser 2 raised as high as it will go. Focus with a 32-millimeter 

 or 1 6-millimeter objective. Under these conditions the prepa- 



1 See Herzog: Chem. Zeit. 40 (1916), 528. J. Soc. Ch. Ind. 35 (1916), 832, Abs. 



2 These directions refer specifically to the Chemical Microscope described on p. 19. 



