282 



PROGRESS IN MICROSCOPY 



Balancing the indices thus allows one to determine the point M along 

 the straight line ab. The abscissa of M shows the composition of 

 the eutectic mixture. Adding to this measurement that of the eutectic's 

 mehing point, the position of the point E on the diagram is determined. 

 Procedure is the same with an addition compound (Fig. 11.19). Having 



B 

 Composition Pure 



Fig. 11.20, Determination of minimum melting composition {M'). 



previously determined the straight line ab with A and B being pure, 

 the point M is sought by balancing the particles index with the melted 

 addition compound. Composition of a maximum and a minimum 

 of the fusion curve may be determined in this way (Fig. 11.20). 



8. APPLICATION OF FUSION METHODS 



Observations are made during the temperature rise and cooling 

 of the specimen and, next, after crystallization. 



Experiments are complemented by application of the Kofler con- 

 tact method. We will now set forth, in accordance with the foregoing, 

 the more important properties that may be investigated together with 

 the feasible measurements when applying fusion methods. 



(a) Observations made as the specimen temperature rises 



(i) The compound is being sublimated. Observation of the cha- 

 racteristics of the crystals formed. 



(ii) A break in certain physical properties is evinced : a polymorphic 

 transformation is taking place. For instance, carbon tetra- 

 bromide changes over from the monoclinic form to the cubic 

 form at 46-9X. 



