Rankin and Wright — Ternary System CaO-Al„0 3 -Si0 2 . 19 



others are altogether analogous to those already discussed. 

 The boundary curve 5-6, which lies wholly within the triangle, 

 delimits the fields for component and the compound AB. 

 Now if the line C-AB crosses 5-6, it follows from the theorem 

 of Alkemade that the point of intersection will be the maxi- 

 mum temperature on the boundary 5-6. The points 5 and 6 

 will, therefore, be eutectic points, as indicated by the arrows 

 which give the direction of falling temperature on the bound- 

 ary curves. If, however, (as is the case in III) the line C-AB 

 does not cross boundary 5-6, then boundary 5-6 does not 

 possess a maximum and temperatures along 5-6 will fall con- 

 tinuously from 5 to 6, the latter being a eutectic point. This 

 is a case where the boundary approaches a maximum, but the 

 compound ceases to be stable before the maximum is attained. 

 Point 5 is a quintuple point but not a eutectic ; that is, 5 is not 

 the lowest melting temperature on the three boundary curves 

 of which it is a common point. 



In IV AB is a compound, unstable at its melting point, 

 which dissociates at the temperature of point 4 into solid A 

 and liquid, so that the composition of AB (point X) lies 

 outside of the field 4-5-6-3 in which AB occurs as primary 

 phase. Point 4 is a quadruple point at which A and AB are 

 in equilibrium with liquid and vapor, but 4 is not a eutectic. 

 In the ternary system point 6 is a eutectic and 5 a quintuple 

 point not a eutectic. 



In V AB is an unstable binary compound which dissociates 

 into solid A and solid B, so that in binary mixtures AB does 

 not occur at the liquidus. In ternary mixtures, however, AB 

 is stable in contact with solution, because the melting tempera- 

 tures of certain ternary mixtures are below the dissociation 

 temperature of AB. The field for AB is bounded by curves 

 which are wholly within the triangle, and the temperature 

 along two of these curves will rise to a common vertex which 

 will point toward the binary system in which AB occurs. 



Case YI (Hg. 4) represents a system in which there is one 

 binary and one ternary compound (AB and ABO), each of 

 which has a definite melting point. The field for the ternary 

 compound is surrounded by boundary curves which lie entirely 

 within the triangle ; the composition and corresponding melt- 

 ing point of the ternary compound are represented by the 

 maximum within the field. 



If a ternary compound is unstable at its melting point it 

 may dissociate either into two solid phases and liquid, in which 

 case its composition will lie outside of its field ; or it may disso- 



