Han kin and Wright— Ternamj System CaO-Alfi-8iO n _. 57 



straight lines actually meet at the point representing the com- 

 position of the primary phase ; but if the compound is not 

 stable at its melting point these lines do not actually meet at 

 the point which is the composition of the primary phase, but 

 the direction of these lines is obtained with this point as an 

 imaginary origin. Thus 3Ca0.2SiO a is unstable at its melting 

 point and consequently its composition lies outside of its field ; 

 and the direction of the crystallization curve for any solution 

 within the field B-B'-^-b is obtained by drawing straight 

 lines, from the composition 3Ca0.2Si0 2 through this field ; as 

 is evident from the diagram, these lines do not meet. In the 

 same way lines have been drawn within the field 1 6-17-1 S of 

 the unstable compound 3CaO.SiO a , and within the field 

 D-ll-lft-lh-E oi the unstable compound 3CaO.Al 2 3 . 



It is evident, then, that if any solution of CaO,Al 2 3 ,Si0 2 is 

 cooled its crystallization curve follows a definite straight line 

 to a boundary curve, and the direction of falling temperature 

 along such a line is from the origin (the composition of a com- 

 pound) toward a boundary as represented in fig. 17 ; on reach- 

 ing a boundary, the crystallization curve follows the direction of 

 falling temperature as indicated by the arrows. 



The crystallization curve for solution b, it will be remem- 

 bered, on reaching the boundary followed the boundary, along 

 which two phases crystallize, until the ternary eutectic is 

 reached, when three phases crystallize and the solution solidi- 

 fies completely. All the ternary solutions of CaO,Al 2 3 ,Si0 2 

 whose crystallization curves proceed to a boundary which leads 

 directly to a eutectic will have this type of crystallization 

 curve. The ternary eutectics at which this type of crystalliza- 

 tion curve w T ill end are those indicated in the diagram by the 

 numbers 1, 2, 5, 6, 7, 12, 14, 15. It follows, therefore, that 

 the crystallization of any solution whose crystallization curve 

 intersects a boundary curve which with falling temperature 

 (i. e. the direction of the arrows) proceeds to one of these 

 eutectics, will be continuous until the solution solidifies com- 

 pletely. 



Certain pairs of solid phases behave as binary systems ; if a 

 solution of composition lying precisely on the line joining any 

 of these pairs — e. g., f on the line joining 2CaO Al o 3 .Si0 2 

 and CaO.Al 2 3 .2SiO— is cooled, 2CaO.Al 2 3 .Si0 2 will" crystal- 

 lize and the crystallization curve will proceed to the quadruple 

 point W where the solution will go solid, C^A8 and CAS a 

 crystallizing together. There are nine of these binary eutec- 

 tics, Z, M, j¥, O, P, Q, R, 8, and T 7 , included within the ter- 

 nary system. All crystallization curves which intersect a 

 boundary at one of these points will end at that point. 



In the ternary crystallization curves so far considered, crys- 



