212 



JV. L. Bow en — The Ternary System 



The Ternary System. 

 Preliminary Statement. 



The problem in the ternary system is the location of the 

 fusion surface for each of the solid phases. Considering com- 

 position alone, the solution of the problem consists in the 

 delimiting of the fields for the various solid phases. The field 

 for solid phase A is such that it includes all the points 

 representing the composition of all the liquids which can exist 



Fig. 3. 



J80Q 



160Q 



.400^ 



FORSTERITE & LIQUID 



C a M e S 



in equilibrium with solid A. Considering temperature alone, 

 the solution of the problem consists in finding the temperature 

 at which crystallization begins (or melting is complete) in each 

 mixture. 



The method of quenching is particularly useful in the 

 solution of the problem. If a mixture of known composition 

 is held at various temperatures and the temperature is deter- 

 mined at which the last crystals, say A, disappear (dissolve in 

 the liquid), then it is known that the composition dealt with 

 lies in the field of A and the temperature of the fusion surface 

 for that composition is also determined. (The crystals which 

 disappear last on heating are also those which appear first on 

 cooling and are called the primary phase.) If a mixture of a 

 different composition is taken and the primary phase is found 

 to be B, then it is known that the boundary curve between the 

 fields of A and B lies between these two compositions. By 

 taking intermediate mixtures, a point on the boundary curve 

 is fixed between as narrow limits as possible. Another series 



