108 



direction, the quantity of vapour taking part in the reaction becomes 

 greater. Now, at the same P and T, a substance has in the gaseous 

 condition a much larger volume than in the liquid or solid state ; 

 at a definite position of .Y the small increase in volume that occurs 

 at the formation of liquid will be just compensated by the conden- 

 sation of the vapour taking i)art in the reaction. Let this be the 

 case in the point zY of fig. 4. In this point N, therefore, A F= 

 so that the tangent stands vertically. The point A', thus traverses a 

 curve as SN in fig. 4 on which the pressure increases w^ith a rising 

 temperature. 



If the point .Y moves further in the negative direction A F becomes 

 negative; A IF, however, is still {positive. Hence, the point X traverses 

 a curve like NM in fig. 4 on which the pressure increases with a 

 falling' temperature. 



When, however, the point A' moves away further from N, the 

 quantity of vapour taking part in the reaction accordingly increases. The 

 quantity of heat generated at the condensation of this vapour will, 

 therefore, also increase. This, in a definite point M will now just 

 compensate the heat required for the formation of liquid so that 

 A W becomes — 0. In the point M the tangent is therefore 

 horizontal. 



On further moving the point A^ in the negative direction A W 

 becomes negative ; A W and A V now being both negative the point 

 A^ will traverse a curve like ML in fig. 4 on which with a falling 

 temperature the pressure decreases likewise. 



We now allow the point A' to move away from S as far as just 

 past the point D (fig. 4). The three-phase reaction : 



F^F':^L or F':^F-\-L or F:^ F' -\-L 

 occurring in the point S then appears in one of the reactions: 



F-{-F' -^L:^G or F' -^ L:^F-\-G or F-^L:^F'+G. 

 This transition takes place according to one of the reaction series 

 1 — 12 and depends on the situation of the points F, F' , L and G 

 in regard to each other. As in each of these reactions A 11^ and A F 

 are positive the point A' will traverse a curve S D R on which the 

 pressure diminishes with a falling temperature. 



If the point X moves away further in a positive direction, other 

 reactions may again take place as we have already demonstrated 

 by a single example ; hence exists the possibility that the curve BR 

 in its further progress changes its direction. We will refer to this 

 later. Froui the above considerations follows : on moving in a 

 positive direction the fourphase equilibrium F -\- F' -\- L -{- G attains 



