50 Rankin and Wright — Ternamj System CaO-Al 2 0.-SiO^. 



(b) The straight line lies partially within the fields of the 

 two compounds whose compositions it connects. — For example 

 consider the line joining CAS, (CaO.Al 2 3 .2Si0 2 ) and AS 

 (Al 2 3 .Si0 2 ) in fig. 12. This line does not cross the boundary 

 (1-9) between CAS, and AS, so that there is no composition 

 in which CAS, and AS are in equilibrium with their own 

 solution; that is, the melting curves CAS,- U for CAS, and 

 AS-V for AS do not meet. These melting curves are sepa- 

 rated by the line U- U' which crosses the A field and is the 

 melting curve of A1 2 3 in solutions of AS and CAS, ; in other 

 words, neither CAS, nor AS are stable in solutions of the 



Fig. 16. 



Ca 2 Al 2 7 + Melt 



CaAi 2 Ov+Melt 



CaSi0 3 tCa 2 AI 2 Si07 



CaQ5i0 2 . 



Ca 2 Al 2 Si0 7 -t- CaAhO, 



2CaO. AI 2 v Si'0, 



CaO. Al 2 = 



Fig. 16. Temperature-concentration diagram of the binary system 

 CaO.Si0 2 -CaO.Al 2 3 . 



compositions on the line U- U' . This is, then, the case in 

 which two compounds stable at their respective melting points 

 and whose fields have a common boundary do not form a binary 

 series because the line joining the compositions of these two 

 compounds crosses the field of another compound. We have 

 three examples of this case ; the one already given ; the com- 

 position line for CJS and CA which crosses the field for C,AS 

 at X-X ; the composition line for C,AS and A which crosses 

 the field for <7 3 JL 5 at V-W. (Fip. 12.) 



(c) The composition line in this case connects a stable and 

 an unstable {at the M.P.) compound. — An example is the line 

 connecting the stable compound C,AS and the compound C S S, 

 unstable at the melting point. As is to be expected the stable 

 compound C^AS has a melting curve C,AS- Y but the unstable 

 compound C\S, has no melting curve, for the composition line 

 does not cross its field of stability. The composition line does 

 however cross the field for C,S and the line 6 r 3 $,- Y is the 

 melting curve of CJS m solutions of C Z S, and C,AS. There is 

 one other example of this case; the composition line for the 

 stable compound C,S and the unstable compound C\A, which 

 line crosses the fields for C 9 S and C 



