5-13 



direction that I he li(|ui(k'nrve ed of the heterogeneous region, and 

 therefore also the point / in fig. 2 shifts towards the line FF' , At 

 a definite temperature and pressure the points u and ƒ coincide 

 then on the line FF' and fig. 3 may arise ; herein however ƒ is 

 replaced by S and t\ by 6\. Therefore the fourphaseequilibriura 

 F -\- F' -\- liquid S -f- vapour S^ occurs, wherein the liquid S is 

 represented by a point on the line FF' . The pressure and tempe- 

 rature to which fig. 3 applies, agree therefore with the minimum 

 meltingpoint of the complex F -\- F' . 



From the situation of S with respect to F and F' it follows that 

 in fig. 3 a congruent melting of the complex F -\- F' is assumed. 

 With an incongruent melting either F or F' should be situated 

 between the two other points. 



Because the point ;S is situated on the line FF' , the two satura- 

 tioncurves must touch each other in S. We now imagine in S the 

 common tangent of the two saturationcurves and also the tangent to 

 curve ed to be drawn. For the salve of simplicity' we shall call, 

 the first the tangent >S' and the second the tangent de. 



Now, as is known, the tangent S and the line FF' are conjugated 

 diameters of the indicatrix in S ; the same applies to the tangent 

 ed and the line SS^. Because the indicatrix in S is an ellipse, on the 

 turning of a diameter its conjugated one moves in the same direction; 

 the lines SS^, FF' and the two tangents must therefore be situated 

 with respect to each other as in fig. 3. The point S^ must therefore 

 in fig. 3 be situated on the same side of the tangent S as the 

 point F' , when however F' is situated within the liquid region and 

 F outside that region, then S^ is situated on the same side of tan- 

 gent S as the point F. Also in the case, that the points F and F' 

 are situated both at the same time either within or outside the 

 liquidregion, the situation of S^, with respect to the tangent S is 

 easy to indicate. 



Besides the cases treated above, there are still numberless others 

 which the reader can easily deduce for himself. For that reason 

 we shall only still contemplate some points more in detail. 



We take at a definite P and T the two solid phases F and F' ; 

 perpendicular to the concentration diagram we draw the ? of these 

 substances ; we shall call these points {F) and [F'). 



When the point {F) is situated below the liquid- and the vapour leaf 

 of the S surface, then the compound F occurs in solid state. We can 

 then construct two cones, which have both their apexes in {F) and 

 of which the one touches the liquidleaf and the other the vapourleaf 



