715 



fused and run througli the boat although the te'.nperature liad not 

 got above 800°, whereas the eutecticum of PbO and (PbO)3 . PbSO^, 

 according to Schenck and Rassbach is at 820°. Probably the MgO is 

 attacked by the PbSO^. The want of a suitable material which is 

 attacked neither by PbO nor by PbSO^ or Pb at this high tempe- 

 rature renders a correct determination of dissociation pressuies for 

 reaction (9) and (10) a matter of great difficulty. 



If we accept the value of 20 mm. at 780" as the correct one for 

 the equilibrium (10), the y;-7Mine for this equilibrium would then 

 run as indicated by line V in tig. 2. 



The p-T-\me of equilibrium (9) then lies between III and V and 

 is indicated schematically in Fig. 2 by line IV. 



18. Although from the preceding it is evident that PbS and PbO 

 cannot be coexistent, a few experiments were made nevertheless in 

 order to confirm this opinion. 



ScHENCK and Rassbach in all their publications consider the equili- 

 brium PbS and PbO to be stable although in their dissociation expe- 

 riments it had already been shown that with such a mixture repro- 

 duceable pressures were not always obtained, for instance if the 

 temperature had been raised to above 800°. They also noticed the 

 formation of sulphate, but assume that this can only be formed at 

 a high temperature and then remains intact on sudden cooling to 

 the dissociation temperatures. 



We have now heated an intimate mixture of PbO -|- PbS for 

 some hours at 600 — 700° in an evacuated and sealed tube. 



The reaction product perceptibly contained sulphate. This was 

 estimated quantitatively by boiling a weighed quantity of the product 

 with aqueous sodium hydroxide and then passing COj. The PbSO^ 

 present is then converted into PbCOj. After filtering and acidifying 

 the filtrate the sulphate was precipitated as BaS04. ^ check analysis 

 was also made on a portion of the mixture that has not been sub- 

 jected to heating, under exactly the same conditions of boiling etc. 



Mixture of 4 mols. PbO to 1 mols. PbS. Temperature 670— 680 ' 

 Time of heating V/, 3 6 hours 



Gram of BaSO, per gram of mixture 0,0498 0.0758 0,1000 0,1121 

 Additional sulphate formed on 



heating at 680° 

 in gram ofBaSO, per gram of mixture — 0,0260 0,0502 0.0623 



If the mixture had been converted completely into basic sulphate 



