60 Rankin and Wright — Ternary System CaO-Al 2 9 -Si0 2 . 



position along the boundary curve i?'-4 from r 1 to 4, it is seen 

 from the figure that the mean composition of the total solid 

 separated changes from CJS to cr. The mean composition of 

 that portion of the solid which has separated during the cooling 

 from r x to 4 is found by extrapolation of the line joining r x and 

 4 and is B '. However, B' lies outside of the line joining the 



Fig. 18. 



Fig. 18. An enlarged portion of fig. 17 to illustrate the phenomenon of 

 resorption. 



phases which separate, viz.: C^S, C 3 S 2 . Thus by no possible 

 combination of u 2 S and O^ can a mixture be made which has 

 a concentration B'. Therefore, C^S and O s S 2 can not both 

 have separated during the cooling from r x to 4, but in reality 

 (7 3 /6^ has been separating alone, while 3 S has been disappear- 

 ing. If more (7 a o disappears from the solid than unites with 

 the OS in solution to form 9 S 9 , then O^S must in reality be 

 dissolving. From the solution r, then, C^S will crystallize until 

 the crystallization curve intercepts the boundary B'-^t at r x . 



