710 



relate to the equilibrium between the first three phases and tliat the 

 second trio is not stable. From the preceding it follows that both 

 equilibria are metastable and that the pressures recorded relate to 

 the equilibrium PbS, PbSO,, PbO.PbSO,, SO,. 



The fact that the lead present exerts so little influence on the 

 equilibrium pressure proves that reaction (4), which shonld lead to 

 a higher pressure p^ and the reactions (J) and (3) in the direction 

 «— which should lower it either compensate each other or, in com- 

 parison with the reaction (2) jn-oceed so slowly that they do not 

 perceptibly alter the pressure. Probably the moi-e finely divided 

 lead is soon converted and the remaining lead, united to larger 

 drops, offers such a small surface of attack that it can react but 

 very slowly. 



11. When now from the PbS + PbSO^ so much SO, has been 

 abstracted that all has passed into PbS -[- PbO . PbSO^ the equili- 

 brium has become divariant. Tlie residual phases will be capable 

 of existing by the order of each other in a series of pressures <^ />,. 



If, however, the pressure falls below a definite limit, a third 

 condensed i)hase appears. Two phases are concerned here, namely 

 Pb and (PbO),.PbSO,. 



As noticed in the case PbS -f- PbSO,, only one of the two can 

 be in stable equilibrium with PbS and PbO.PbSO,. 



This depends on which of the fjhase pairs PbS -|- (PbO), PbSO^ and 

 Pb -\- PbO . PbSO,, which can be converted into each other by 

 double decomposition : 



PbS + 5 (PbO), PbSO, ^ 4 Pb + 6 PbO.PbSO, ... (6) 

 is stable. 



12. In order to investigate this an intimate mixture of finely 

 divided lead scrapings and basic lead sulphate iji the molecular 

 proportion of 3 : 1 was heated for three hours in an evacuated and 

 sealed tube at 670'— 680\ 



The PbO . PbSO^ had been prepared by the moist process, accord- 

 ing to D. Strömholm ^), by digesting finely powdered PbSO^ with a 

 1—2"/, NHg-solution. The analysis of tlie product obtained gave 

 84.887„ PbO, theory for PbO.PbSO, 84,79V„. 



The heating of the mixture Pb + PbO.PbSO, yielded apparently 

 a but little changed product. It gave, however, a slight sulphide 

 reaction. Thus it seemed that the mixture selected did not form the 



') Zcitsclii-. f. anorg. Qliera. 38, 429 (1904). 



