CHEMICAL MICROSCOPY 283 



(iii) The substance decomposes. 



(iv) The substance melts. Determination of its melting point, 

 (v) Measurement of the refraction index of the meked substance. 



(b) Observations made as the specimen cools off 



(i) Supercooling is taking place: the compound remains liquid 



below its melting point, 

 (ii) Observation of the rate of crystallization, 

 (iii) Observation of the crystallization front. As crystallization 



extends into the specimen, the boundary between the solid 



and liquid areas may exhibit three aspects: 

 (a) The front is smooth and uniform-orientated areas are forming 



(naphthalene) ; 

 ((i) Formation of discrete and variously shaped crystals (picric 



acid) ; 

 (y) The crystaUization front consists of small spherules which are 



developing from discrete nuclei (cholesteryl acetate). The shape 



of the crystallization front depends on substance purity and 



temperature. If the substance exhibits a polymorph, the latter 



does not exhibit the same crystallization front, 

 (iv) A second crystallization takes place after normal crystallization 



of the melted substance : this is a polymorphic transformation 



(carbon tetrabromide). 



(c) Observations made after the specimen is crystallized 



(i) Nature of the crystals formed. 



(ii) Birefringence — Observations in both parallel and converging 

 light. 



(d) Observations made applying the contact method using a reference 

 substance 



(i) Determining whether the investigated substance A is identical 

 with the reference substance B, thus making A identifiable. 

 When A and B are identical, crystals develop throughout the 

 specimen, without giving rise to a mixed area, 

 (ii) Plotting the composition-temperature diagram. SoUd solution. 

 Mixture with formation of a eutectic. Melting point and 

 measurement of the refraction index of the mehed eutectic. 

 Formation of an addition compound. 



