221 JV. L. Bowen — The Ternary System : 



tion of the solid phases which can exist in equilibrium with 

 liquids represented by the various points along DE. 



The three-phase-boundaries NC, N,C, etc., are simply lines 

 radiating from the composition of the pure solid C and require 

 no special discussion. The three-phase-boundaries NK, N,K X , 

 etc., crossing the solid solution field, have a specific direction 

 determined by the properties of the system. Their usefulness 

 in discussing the crystallization of a mixture will appear later. 



Figure 11 illustrates the case of a ternary system in which 

 the components A and B form a stable compound AB and this 

 compound gives a complete series of solid solutions with the 

 third component C. In this case two systems of the type of 

 fig. 8 are combined ; each half C-A-AB and C-B-AB may be 

 treated separately and C-AB is a simple binary system of the 

 type shown in the side figure. The three-phase-boundaries 

 would then be of the type indicated in the figure (fig. 11). 



After discussing these simpler systems the case of a com- 

 pound unstable at its melting point, as in the system under 

 investigation, may now be proceeded with. In fig. 12 is shown 

 a theoretical case similar to the system under investigation. 

 We cannot in this case divide the system into two ternary sys- 

 tems, and the system C-AB cannot be treated as a binary 

 system. A solid solution of any composition between C and 

 K behaves on melting as a true binary mixture and its behav- 

 ior is expressed by the binary partial-diagram of the side 

 figure (a). A solid solution of any composition between K and 

 L behaves on melting as a true binary mixture until the tem- 

 perature of the isotherm passing through the point K is 

 reached and its behavior is again expressed by the side dia- 

 gram up to this temperature, but with further rise of tempera- 

 ture the component A begins to separate and the mixture 

 no longer behaves as a binary mixture. With solid solutions 

 between L and AB liquid begins to form and the component 

 A begins to separate simultaneously at a temperature higher 

 than that of the point K, the behavior throughout the melt- 

 ing being that of a ternary mixture. The three-phase-boun- 

 daries of this system are of the type shown in fig. 12 : NP, 

 MP, JS^P,, M^, etc. 



There may, also, be a minimum on the solid-solution line 

 between K and C as shown in fig. 12 (b). Accompanying this 

 there may be (or may not be) a minimum on one or both of 

 the boundary curves. The system investigated falls under the 

 type of fig. 12 (b). There is a very flat ill-defined minimum 

 close to diopside. On the boundary curves, as we have seen, 

 there are no minima. 



