( 192 ) 



A plaitpoint always being a point where the stable and unstable 

 region meet, it would be incorrect to speak of stable, metastable and 

 unstable plaitpoints. But when we pay attention to the coexisting 

 phases in the neighbourhood of the plaitpoint, the preceding names are 

 appropriate for such phases according to the described situation of 

 the plaitpoints. As long as the plaitpoint lies on the external part 

 of the (2), T, ^r)-surface, the coexisting phases in its neighbourhood 

 are stable; as long as it lies on those parts of the loop which 

 may be considered as a continuation of the external branches, the 

 coexisting phases in its neighbourhood are metastable, and when 

 the plaitpoint lies on the remaining part of the loop, the coexisting 

 piiases in its neighbourhood are unstable. 



In the series of the figures {a, h, c, d etc.) is, besides the loop 

 of the {p, 7^).t-curve and the place of tlie plaitpoint, also the shape 

 of the spinodal curve indicated. This spinodal curve is the section of 

 the spinodal surface with the plane which has the chosen value of .r. 

 All the points of the loop which lie below the spinodal curve represent 

 unstable phases and those which lie above it, metastable or stable ones. 

 Thus e. g. in fig. 4, in which the plaitpoint lies on the retrograde 

 branch of the loop, the spinodal curve is a curve which cuts the 

 loop in two more points. In concordance with the figures 4, 5, 6 1. c. 

 are the points of intersection indicated by the letters D and C. By 

 raising the temperature in these figures, the point C is moved to the 

 left, and when the temperature is lowered, D moves to the right, 

 which makes it possible for them to come into the chosen ci'-plane. 



If from a (^>, 7^)^ -curve for a chosen value of a' the curve is derived 



fdp\ 

 which belongs to a value of x -f- dx, the value of I — - I must 



\cIvJt 



be known for e\ ery value of T. 



If f ) is = 0, the (/>, 7')rC*in-ve for the values x and x -\- dx, 

 \dxj 



must have the same value for p. If we draw both the (p,T).i-cnrve 

 and the curve {i}T)x-^jx as has been done in the figures 4, 5 and 6, 

 there will be intersection of these two (^>,7')-curves in all the points in 



which \\ =0. In the figures mentioned the curve ïoYx-\-dx\^ 

 \dxjT 



represented by . — . , and now the two (^>,7^)-curves will cut everj'- 



wliere where the spinodal curve cuts the first (^>,7')-curve, according to 



fdp\ AZ^$>y 



the property that for coexisting phases — J = when —- 1 = 0. 



\dxjT \dx J^T 



Also ill the point where the spinodal curve touches the curve {p, T)s, so 



in the plaitpoint, such an intersection of the two following (j>,7').rcurves 



