24 Rankin and Wright — Ternary System CaO-Al^O^-SiO^. 



side of the composition of a compound, then that compound is 

 unstable at its melting point and its composition lies outside of 

 the ternary field in which it occurs as primary phase. Such 

 unstable binary compounds are the following : 3CaO . A1 2 3 

 and 3Ca0.2Si0 2 . 



As for the unstable binary compound 3CaO.Si0 2 which does 

 not occur as primary phase in the binary system, it is not 

 possible to say from the binary investigation whether or not it 

 will have a field of stability in the ternary system or what the 

 nature of such a field might be ; except that if the field is 

 three-sided, the temperature along two of the boundaries would 

 rise to a common vertex which would point in the direction of 

 the binary system in which the compound 3CaO.Si0 2 occurs. 



In taking up, then, the experimental investigation of the 

 system CaO-Al 2 3 -Si0 2 , we know at the start that there are at 

 least 12 compounds, some of which occur in more than one 

 crystalline form, whose stability relations in the ternary system 

 must be established. Beside these 12 there may be any num- 

 ber of ternary compounds, all of which must have their con- 

 ditions of stability in the ternary system determined. 



The first step in this investigation consisted in the prepara- 

 tion of charges whose compositions represented intervals of 5 per 

 cent over the entire range of ternary mixtures. The primary 

 crystalline phase was then determined for each of these charges 

 which could be quenched at some temperature below 1600° in 

 such a way that a single crystalline phase imbedded in glass 

 could be obtained. From this preliminary reconnaissance a 

 general idea is obtained of the number and character of the 

 different crystalline phases which are stable in contact with the 

 liquid, and of their relations to each other in ternary mixtures. 

 In other words, we located approximately the fields with their 

 limiting boundary curves and quintuple points. With these 

 general data as a basis, other intermediate charges were pre- 

 pared and their primary phases determined ; this secured a 

 more exact location of the boundary curves and quintuple 

 points, the compositions along boundary curves (lines) being 

 accurate to within db 1*0 per cent and quintuple point composi- 

 tions being accurate within a radius of - 5 per cent. 



The limits of the fields of stability of the various crystalline 

 phases, determined in this way, are represented on the trian- 

 gular diagram (fig. 6). The dots given in the diagram 

 represent the compositions examined ; for the sake of clearness 

 they have been made different in adjoining fields ; these dots, 

 however, do not indicate all the concentrations examined or the 

 many heat treatments and examinations required to determine 

 the relations of any one concentration. The entire ternary 

 system has necessitated the preparation of about 1000 different 



