109 



successivelj a pressure and a temperature niaximuin, the minimum 

 melting- and the maximum sublimation point. 



We have assumed in the above considerations that, in the three- 

 phase reaction appearing in the point *S', the volume increases. If 

 the volume decreases, the point ;S ai-rives on the branch il/iV of 

 the four-phase curve and the melting })oint line Sd" proceeds from 

 S towards lower temperatures. 



We ha\e noticed above that Ihe four-phase equilibrium 

 F -\- F' -\- L -]- G is represented in fig. 4 by jtoints of the curve 

 LSDR. On supply or withdrawal of heat or on change in volume 

 one of the four three-phase equilibria: 



F-{- F' + L, F^ F' -f U, F^ L + G or F -f L + 6r' 

 is formed from this equilibrium. 



The question now arises: by which points of the /*, 7^-diagram 

 are these equilibria represented. 



Let us take at the temperature T, and the ju-essnre Pr a corre- 

 lated point X of the curve LSDR. As we have already stated, to 

 such a point applies the rule that we find : 



to the right of x the three-phase sj'stems formed from the four-phase 

 system F -{- F' -\- L -\- G on supply of heat {LWyO), to the left 

 of X those formed on withdrawal of heat (A]F<[0), above ,i' those 

 formed with decrease in volume (A f' <^0) and below .i' those formed 

 with increase in volume (AT^^O). 



Here, the supply or withdrawal of heat must take place at the constant 

 pressure Px and the change in volume at the constant temperature Tx- 



In order to apply the above rule we must, of course, know the 

 reaction to take place in the point x. Let ns take as an example a 

 four-phase complex belonging to type 1 so that the reaction series 

 A and A^ appear. 



Let us first take the point x on the branch LM (fig. 4). From 

 series A it follows that in this point the reaction F -{- F' -{- G"^ L 

 takes place; further we have noticed that in this reaction A]" and 

 A W are negative in the point x (in the direction from the left 

 to the right). We now easily find which three-phase equilibria are 

 situated to the right or to the left of x and which above or below 

 that point. As this applies to all points of the branch LM we find : 

 at the right of and below branch LM exists the system i'^-J- 7^^' -|-6r; 

 at the left of and above this branch exist the three other systems : 

 f 4. F' + L. F -f /> + 6' and F' -f L + G. 



If we take tlie point x on branch MN (fig. 4) the reaction will 

 sMll be F -\- F' -\- Gt^L, but AH" is positive and A F negative. 

 We now find : at the left of and below branch MN exists the sys- 



