CHEMICAL MICROSCOPY 



275 



a crystallized organic compound and determining whether two com- 

 pounds are identical. 



The process is the following (Fig. 1 1.8(a), (b), (c)). One to two milli- 

 grammes of the least fusible substance B are melted first. The sub- 

 stance is set against the slip-cover, is flame-melted and spreads 

 under it. Good care is taken that the melted substance does not 



(a) (b) (c) 



Fig. 11.8. Mixed fusion — Kofler's contact method. 



extend over more than one half of the surface of the specimen 

 (Fig. 11.8(a)). When B has cooled off and solidified, the second sub- 

 stance A (Fig. 11.8(b)) is heated until it melts, spreads under the slip- 

 cover and comes in contact with B (Fig. 1 1.8(c)). If, during this process, 

 a portion of B were cooled, the whole is allowed to crystaUize again. 



(a) The substances A and B are identical 



In this case, there is no mixing zone. Crystals grow throughout 

 the specimen until the whole is soHdified. The mixing zone exhibits 

 no discontinuity either in the growth or shape of the crystals. 



(b) The substances A and B are not identical 



If the substances A and B are not identical, a break comes out 

 in the mixing area. 



(i) The two substances do not react and are miscible in any proportion. 



If the substances A and B are not identical a break appears in the 

 mixture area. In this case, they may furnish a continuous series of 

 mixed crystals whose properties change continuously within the range 

 of limit values corresponding to the forms of A and B. Plotting the 

 temperature as ordinates, and the mixture composition as abscissae, 

 gives the curve shown in Fig. 11.9. The dotted-line curve shows the 

 initial melting temperature and the full-line curve the temperature 

 when the melting process is over. During the cooling process only 

 one kind of mixed crystals is deposited. Under the graph is shown 

 a diagrammatic aspect of the specimen when heated at a temperature 



