70G 



and PbSO, is left. PbS and PbO are, therefore, not capable of 

 existing side by side of each other. 



If, conversely p^ < i\ the different reactions take place in the 

 opposite sense and Pb and PbSO, recede from each other. 



Hence, of the phase pairs Ph -\- PhSO, and PhS -\- PhO only 

 one can he stabk, the other forms a metastahle equilibrium. 



Here we have a case quite similar to that occurring with reci- 

 procal salt pairs where also only one of the two pairs can be stable. 



4. Let us now also consider the two other monovariant equilibria 

 which may be assumed to exist with SO^-vapour and which are 

 indicated by the equations 



2PbO.+ PbS ^3Pb +S0, . . . , . (3) 

 Pb -f PbSO,^ 2 PbO + SO, (4) 



In the case p^^ ih, it follows at once from the incompatibility 

 of the phases PbS and PbO that the equilibrium (4) can be stable, 

 but not equilibrium (3). 



Moreover, we then must have p^ > p^ for otherwise after the 

 reaction (1) in the direction -» might follow the reaction (3) in the 

 direction -^ which reactions might jointly cause the conversion (5) 

 in the direction <^-, which is in conflict with the premiss. 



Finally we shall have p,<Cp^, for then the stable equilibrium may 

 also be again attained by the reaction (2) in the direction ^^ followed 

 by (4) in the direction '^, namely: 



PbS + 3 PbSO, = 4 PbO + 4 SO, 

 4 SO, + 8 PbO = 4 Pb + 4 PbSO^ 



PbS + 4 PbO =4Pb4-PbS0, 



Hence, we get this result: 



If Ph -\- PbSO, forms the stable phase pair, then only the mono 

 variant equilibria (1) and (4) are stable and p^ > p^ > p^ > p^. 



If piQ J^ pi,S form the stable phase pair, all the reactions 

 occur in the opposite sense and only the monovariant equilibria (2) 

 and (3) are stable, whereas then p^^ p^^' Pi^ P»- 



5. Starting from the mixture of PbS and PbSO, (for instance a 

 in fig. 1) we will, on withdrawal of SO, travel either through the 

 monovariant equilibria (1) and (4) (region PbS, PbSO^, Pb and region 

 PbSO,, Pb, PbO of Fig. 1) or the equilibria (2) and (3) (region 



