CHEMICAL MICROSCOPY 



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Experience has revealed that there may be two other types oi 

 diagrams: they are shown in Figs. 11.10 and 11.11. As temperature 

 of the specimen rises (as shown in Fig. 11.10), melting extends first 



Preparation 



Fig. 11.11. Temperature-composition diagram — The substances A and B are miscible 



in any proportions (type III). 



from the left, then from the right to reach the C area. When the curves 

 evince a minimum (Fig. 11.11), melting is initiated in the area cor- 

 responding to that minimum and gradually extends on either side. 



(ii) The two substances do not react and formation of a eutectic 

 mixture occurs. In this case, the crystals of the pure substances A 

 and B grow on either side up to the contact zone where the eutectic 

 is developed. Figure 11.12 shows the composition-temperature diagram 

 and the aspect of the specimen when the temperature slightly exceeds 

 the eutectic point. As the diagram shows, the eutectic that melts first 

 is in the area E. The eutectic, between crossed polarizers, looks like 

 a dark strip. The melting point of the eutectic was used by the Koflers as 

 an additional datum enabhng them to characterize organic compounds. 



(iii) Both substances react. The definite compound M is formed 

 in the contact area. Figure 11.13 is derived by placing side-by-side 

 the diagrams related to the systems AM and MB, diagrams which 

 are the same as those in Fig. 11.12 but the point M corresponding 

 to the definite compound, is shown by a rounded maximum. 



