(185) 



ternperature wliieli wc might call the traiisforination temperature and 

 which I shall indicate by Tu- (fig". 5), 2"^^ a temperatnre a little 

 below the transformation temperatnre (fig. 4) and 3''^^ one a Utile 

 above Tt,- 



In the case that these sections are known for all possible tem- 

 peratures, the saturation surface is of course quite determined and 

 known, and so all other sections e.g. those normal to the tf-axis, 

 are also determined. But it appears from the given figures, that 

 though the realizable part of the saturation surface has a compara- 

 tivelj simple shape, the non-realizable part has a fairly intricate 

 course — and that it is necessary to know also that intricate portion 

 if we wish to get an insight into the course of the part that is to 

 be realized. 



To the intricacy of the hidden part it is due that though all the 

 sections normal to the .I'-axis are given by those normal to the 7- 

 axis, the shape of the {p, 7^).,-sections will not always be easy to 

 derive. Now that I for myself have obtained an insight into the 

 course of these sections I have tiiought it not devoid of interest to 

 try and make clear the properties of this curve by means of a 

 series of successive figures. 



If we wish to represent these (/>, T)x figni-es in a diagram, all 

 the surface must of course be known — in other words according 

 to the course of our derivation from the (p,d^T sections — all the 

 ii^, x)t sections must be known. 



Between two temperatures which are known by experiment, see 

 fig. 4, 5 and 6 I.e., such a {i),d^T section has two tops, viz. 7* and 

 (}. If T is raised, the part that has P as top, is narrowed, and 

 the part that has Q as top widens, and the reverse. This property 

 is perhaps not quite fultilled in the schematical tigures of the paper 

 mentioned, but it follows immediately from the fact that with con- 

 tinued rise of temperature the top P vanishes, whereas with sufficient 

 lowering of 7^ the top Q vanishes. Let us call the temperature at 

 which P vanishes 7 e and that at which Q disappears Ta. I choose 

 these symbols Te and 7o, because I think of the mixture of ethane and 

 alcohol as an example for the shape of the (/>, 7V?;)-surface discussed 

 here. Of these mixture the plaitpoint circumstances have been deter- 

 mined by KuENEN and Robson. At T^ the whole top the plaitpoint of 

 which is P, will have contracted, and the only trace left on the 

 outline of the (y>, ,*)-{igure of the complication found at lower values 

 of T, is a point, at which the tangent is horizontal, while at that 

 place there must be an inflection ]K)int in the (p, ,i')-curve, which 

 has for the rest a continuous course. For 7' equal to Ta this is the 



