Ill 



point of the complex F -\- F' ; in n /^ 7 -diagram the foiir-pliase 

 ciii've then meets the ynblinialion cni-vc of the coni|)lex F -\- F' 

 (Point D in fig. 4). 



Further we liave: 



If in the four-piiase oquilibi-inni oocnrs a roaclion of gronp r (or ^/) 

 the four-phase curve meets in a l\ 7-(Magram the three-phase or 

 bouiKJary curve of F (or F'). 



We may readily deduce this last properly in llie following maimer. 

 Suppose in a point ,v. a[)pertaining to the temperature Tr and the 

 pressure Pj of the four-phase curve there occurs a thi-ee-phase reac- 

 tion of group c. If now we remove from the ecpiilibrium F-\-F' -\-L-\-(r 

 the compound F' we retain in the point .?; the three-phase equilibrium 

 F -\- L -[- G. As according to our assumption a reaction of group c 

 occurs between these three phases, they have such a composition 

 that between them a phase reaction is possible. Now, as we have 

 noticed previously, such an equilibrium F -\- L -\- G is represejited 

 in a P, 7^-diagram by a curve that we haxe called the three-phase 

 or boundary curve of F {KF in fig. 3 (III); inKFM in fig. 4 (IV). 

 From this it follows that the boundary curve of the compound F 

 passes through the point .r of the four-phase curve. 



dP A W 



As the direction of both curves is determined bv T — = 



^ JT A V 



and as in the point ,?; A W and A V are the same for both curves, 



these curves must meet in the point x. 



We may summarise the previous considerations as follows: if in 

 a point .1' of a four-phase curve occurs a three-phase reaction the 

 four-phase curve in the 7^7^-diagram comes into contact with a 

 three-phase curve appertaining to that reaction. To a reaction of 

 group a appertains the melting j^oint curxe, to one of group b the 

 sublimation curve of the complex F -^ F' ; to one of c the boundary 

 curve of F and to one of d the boundary curve of F' . 



From the reaction series 1—12 it appears that in each four-[)hase 

 equilibrium occurs one reaction of group a and one of group b. 

 In harmony with our previous considerations it follows that eacli 

 four-phase curve meets the melting-point curve and the sublimation 

 curve of F -\- F' , If now in a four-phase system occurs one of the 

 series 7 — 10 the four-phase curve meets the boundary curve of F 

 (series 8 and 10) or of F' (series 7 and 9); if one of the series J 1 or 

 12 appears, it meets the two boundary curves. These points of 

 contact lie between the minimum melting and the maximum subli- 

 mation point of the curves coming into contact with each other. 



If one of the reaction series 1 — (3 occurs the (uirves KF, K' F' 



