546 



side of llie line FF\ is iiieliistaltle. Revcrsally, if we have a s(al)le 

 vapour saliirated wilh F -\- F' , ilieii the li(iiiid saturaled with 

 F -\- F' , situated ou the same side of the line FF' is metastable. 

 Only in the case that a fourphase equilibrium F -\- F' -{- L -\- G 

 occurs, this liquid and vapour are stable at the same time. 



Let us now consider the occurrence of this system F-\- F' -{-L-{- G. For 

 the occurrence of this fourphaseequilibiium it is not sufncient that 

 the two threephaseequilibria F -\- F' ^ L and F -^ F' ^ G exist. 

 In addition it is also necessary for this, that the liquid Lof the one, 

 and the vapour G of the other threephase system shall be in equili- 

 brium with each other. The liquid curve ed of the heterogeneous 

 region LG then must go through the point of intersection of the 

 two saturationcurves and also the vapourcurve e^cl^ of the hetero- 

 geneous region must go through the point of inteisection of the two 

 vapoursaturationcnrves. Because this is not the case in tig. 1, no 

 fonrphaseequilibrium can occur, at the temperature and under the 

 pressure to which tig, 1 applies. In tig. 2 however, this is indeed the 

 case. Therein curve ed goes through the point of intersection ƒ of the 

 saturationcurves and also cur/e e^d^ goes through the point of inter- 

 section ƒ, of the vapoursaturationcnrves. At the temperature and 

 under the pressure, to which this figure applies, therefore (he system 

 7^+ i'^' + liquid ƒ -f- vapour /^ can occur. This is also the case 

 in tig. 3 wherein the fourphase equilibrium 7^-)- 7^' -|- 'iQ^iid >S -j- 

 vapour >Si occurs. 



Now we shall consider more in detail yet two points, nl. the 

 situation with respect to each othei- of the four points i^, i^', ƒ and /i 

 and also that of the three curves going through the points ƒ and /j. 



In the previous communications the first point has already been 

 treated here and there. We have seen there that the four points 

 can be situated with respect to each other in seven different ways, 

 so that between the four phases of the system F -\- F' -\- L -\- G 



one of the seven fourphasereactions : i^+^'+^2^^' ^-h^'-f-^^^» 

 F-^L^F'-^G, F'-\-L-^F-^G, F^F'-^L^G, ]i:^F'^L+G and 

 F'-^F-^ L-^ G occurs. 



In the particular case that three of the four points are situated 

 by chance on a straight line, (fig. 3) a threephasereaction occurs. 



Let us contemplate now the three curves, going through the points 

 ƒ and ƒ,. With the aid of the indicatrix theorem, we can deduce 

 the rule ^): 



When two equilibriumcurves (P and T constant) intersect each 



-) l'\ A. H. ScHREiNEMAKERS, Die heterogeiieii Gleichgewichte von H. W. Bakhuis 



ROOZEBOOM. IIP 116. 



