CHEMICAL MICROSCOPY 273 



As most crystals are anisotropic, the melting point can be deter- 

 mined by means of polarized light. To carry this out, a polarizer is 

 set under the hot stage and an analyser on the eyepiece, the work 

 taking place between polarizer and crossed analysers. The birefringent 

 crystals show up bright against a dark ground. When melted, they 

 vanish and the whole field becomes dark. Vanishing of birefringence 



• 



> 



^B ^ 



Fig. 11.7. Melting of azobenzene crystals. 



determines the melting point. However, it may occur that a crystal 

 is no longer birefringent at a temperature lower than its melting point. 

 It may yield an isotropic polymorph (carbon tetrabromide becomes 

 isotropic at 46-9° and melts at 92-5"'), or become an opaque mass of 

 disorientated crystals of a polymorph or, yet, lose the crystallization 

 solvent and become opaque. 



5. MEASURING THE REFRACTION INDEX OF A MELTED SUBSTANCE 



The refraction index is a useful datum for defining pure compounds 

 or analysing chemical binary systems. 



Kofler showed a method, based on glass powders, whose indices 

 are known, for measuring the refraction index of a melted substance. 



A specific powder is mixed with the substance in the solid state. 

 A few pellets of the mixture are then put on the hot stage and observed 

 through the microscope. As temperature rises, the crystals melt and 

 the glass particles are visible, immersed in the liquid. Kofler makes 

 use of Becke's line (see Chapter VII, §4) to determine the phase showing 

 the higher-index. The work is eff"ected in coherent light by reducing 



