18 Rankin and Wright — Ternary System CaO-Alft^SiO^ 



The starting point of a boundary curve 1-4 is a quadruple 

 point (1) in a binary system A-C\ as we proceed along the 

 boundary curve into the ternary system the corresponding 

 equilibrium temperature will fall, as postulated by the theorem 

 of Alkemade. The two phases in this case are the two com- 

 ponents A and O; according to the theorem, temperatures 

 along 4-1 must rise as it approaches A-C (temperatures along 

 1-4 must fall as 1-4 leaves A-C and enters the ternary sys- 

 tem). The point 4, at which the three boundary curves meet, 



Fig. 4. 



■nrt 



3 X > 3 4 



Fig. 4. Six Typical cases of Three- Component System. 



is a quintuple point at which A, B, and C are in equilibrium 

 with solution and vapor. Since 4 is the lowest melting tem- 

 perature for any mixture of A, B, (7, it is known as a eutectic 

 point. 



The next simplest cases of three-component systems are 

 those in which there is one binary compound ; these are illus- 

 trated by II, III, IV and V. 



In II A and B unite to form the compound AB which is 

 stable at its melting point. The binary system A-B will 

 possess then a maximum (the melting point of AB) and two 

 eutectics (3 and 4). Within the triangle there will be four 

 fields ; one for each of the components A, B, G and one for 

 the compound AB ' ; and five boundary curves of which 5-6 is 

 the only one which we need to consider especially, since the 



