6 2 Rankin and Wrigh I — Ternary System Ca 0-A l 2 3 - Si 2 . 



not follow the boundary curve B'-^t to the quintuple point 4. 

 That this conclusion is the correct one is evident also from a 

 consideration of the following facts : since 6^ disappears dur- 

 ing the cooling from t x to t 3 , and since there is no more 2 S to 

 so change and since 3 S 2 is stable at the temperatures in ques- 

 tion, the crystallization curve cannot follow the boundary 

 beyond t 3 . Consequently, on further cooling the crystalliza- 

 tion curve takes the only other possible course. 3 S 2 continues 

 to separate, the crystallization curve leaves the boundary, 

 crosses the field B'-B-k-Q, wherein 3 S 2 is the solid phase, 

 and intercepts the boundary B-6 at t±. In accordance with 

 the principles of equilibrium, the solution t A is saturated with 

 respect to OS as well as to 3 S 2 . Hence, on cooling the solution, 

 the crystallization curve passes along the boundary B-6, with 

 separation of OS and 3 S 2 . The mean composition of the 

 solid at t t is C 3 /S„ and at 6, which is the end of the curve, 

 it is a 4 . 



Thus in the course of the cooling of solution t there have 

 appeared consecutively pure 2 S ; C 2 S and 3 S 2 ; pure 3 S 2 ; 

 3 S 2 and OS, and finally 3 S 2 , OS and 2 AS. It is seen, there- 

 fore, that whenever there exists a ternary system which con- 

 tains a compound unstable at its melting point (in this case 

 3 S 2 ), solutions can be found in which crystallization curves do 

 not follow the first boundary curve intercepted to its quintuple 

 point, but which may pass into the field for the eompound and 

 follow some other boundary to another quintuple point. From 

 a consideration of fig. 18 it is evident that the crystallization 

 curve can leave the boundary only when there is reached some 

 point from which the line drawn through the point giving the 

 composition of the original solution, passes also through the 

 composition of the compound in question. Solutions for which 

 this condition holds lie within a limited field. By an inspec- 

 tion of fig. 18 one observes that such solutions involving 3 S 2 

 are possible only within the limits of the triangle 3 S.-B , -4 : . 

 These considerations obviously apply equally well to binary 

 and ternary compounds. 



In the study of the crystallization curves which proceed to a 

 boundary along which one solid phase wholly or partially dis- 

 appears, we have considered the case in which the boundary 

 curve separating the field of a compound stable at its melting 

 point from that of a compound unstable at its melting point is 

 a straight line. The same theory is also applicable when the 

 boundary is a curved line and holds whether the boundary 

 separates the fields for compounds stable or unstable at their 

 respective melting points. 



For the study of curved boundary lines of this class consider 

 the boundary curve 9-7 (fig. 17). The boundary curve 9-1 



