707 



PbS, PbSO,, PbO and region PbS, PbO, Pb) (o finally retain the 

 equilibriuni Pb + PbS or Pb -f PbO after eliminating the SO, as 

 much as possible. 



Hence, the reactions (1) and (3), which are generally qnuted as 

 taking place in the roasting reaction i)rocess cannot possibly indicate 

 both stable equilibria. 



Of the p-T-lines which Schenck and Rossbacu determined by addi- 

 tion of PbSO,, PbS and Pb and of PbS, PbO and Pb one at least 

 must, therefore, indicate an instable equilibrium or an equilibrium 

 between phases other than those which were brought together in 

 the reaction tube. 



We will see later that both equilibria are metastable and that the 

 pressure lines recorded by them relate to the equilibrium between 

 other phases. 



6. The supposition made in (2) sub a is not correct. Between 

 PbSO^ and PbO there still arrive three basic salts as intermediate 

 phases, namely PbO . PbSO, , (PbO), PbSO, and (PbO)3 PbSO,. The 

 first of these can be in equilibrium with PbSO^. 



The four monovariant equilibria mentioned in (3) now become: 



PbS + PbSO, = 2 Pb + 2 SO., (J) 



PbS + 7 PbSO, = 4 PbO . PbSO, + 4 SO, . . . . (2) 

 2 PbO . PbSO, + 3 PbS z= 7 Pb + 5 SO, . . . . (3) 



Pb + 3 PbSO, =: 2 PbO . PbSO, + SO, (4) 



and the alternative found must read : 



either Pb -[- PbSO^ stable and then Ih^pi^lh^ Ih and only fl) 

 and (2) stable, 



or PbS -j- PbO . PbSO, stable and then p^ < p^ <] p^ < />, and only (2) 

 and (3) stable. 



Experimental. 

 (conjointly with Dr. F. Goudriaan). 



7. In order to investigate which of these two phase pairs was 

 stable and at the same time to know the SO, -pressure of the stal)le 

 equilibrium, an intimate mixture of PbS and PbSO., (6 — 8 grams) 

 was heated in a porcelain tube connected by means of a ground 

 joint with a manometer and an air-pump. 



The lead sulphide was precipitated from a solution of lead acetate 

 with H,S and after washing, dried by heating in a current of nitrogen 

 at 200°— 300°. 



The lead sulphate was precipitated from a sohition of lead acetate 

 with sulphuric acid and also dried at 800°. 



