THE DETERMINATION OF REFRACTIVE INDEX 185 



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. 



Although the identification of compounds through determi- 

 nations 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 of 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 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 condenser 1 

 raised as high as it will go. Focus with a 32 millimeter or 16 

 millimeter objective. Under these conditions the preparation 

 will probably be flooded with light. Close the diaphragm two- 

 thirds or even more. If the crystal fragments are not now clear 

 and distinct with sharply defined contours lower the condenser 

 a trifle, but only a trifle. It is of course possible that in selecting 

 the liquid one of the same refractive index as that of the solid 

 may have been chosen; this is, however, very unlikely. 



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



