705 



We sliall see later how the 

 deduced eqiiihbria are being mo- 

 dified when we drop these sim- 

 plifications. 



3. Let us imagine PbS heated 

 at constant temperature between 

 600° and 800° in an enclosed 

 space wherein a limited quan- 

 tity of oxygen is forced. PbS 

 is then partly converted into 

 PbSO,. 



The two phases will be capable of existing in the presence of each 

 other and in an unchanged condition at a series of temperatures 

 and pressures. 



If on lowering the pressure this falls below a certain limit, one 

 of the two following reactions will take place 



PbS -[- PbSO, = 2 Pb +2 SO, (1) 



or PbS -f 3 PbSO, = 4 PbO + 4 SO, .... (2) 



In both cases there are formed in addition to the two existing 

 solid phases two new phases, namely fused Pb and SOj-gas, or solid 

 PbO and SO-^-gas. Hence, between these four phases a mono variant 

 equilibrium will set in, which, at a constant temperature, is possible 

 only at one special pressure. This will be p^ or p^. 



Only in a very special case, namely with an eventual transition 

 point where the five phases PbS, PbSO^, PbO, Pb, and SO, might 

 coexist, Pj and />, are equal. As a rule, however, they are not and 

 in consequence only one of the two monovariant equilibria can 

 be stable. 



For if py^ ^ p, the reaction (1), in the presence of the five phases, 

 will take place from the left to the right and the SO, formed act 

 on PbO according to equation (2) in the direction ■^—. Hence, the two 

 reactions together result in the following conversion : 



2 PbS + 2 PbSO, = 4 Pb +4 SO, 

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



PbS -f 4 PbO = 4 Pb 4- PbSO, 



(5) 



This transformation takes place until one of the phases of the 

 first member of the equation is used up, whilst the other with Pb 



