71G 



according to the equation 5 PbO-f PbS = PbOPbSO,+ 4 Pb, 1 gmm 

 of the mixture should have yielded 154 mg. of BaSO^. Hence, a 

 large proportion of the PbO + PbS has been converted. ^) 



The pressures which Schenxk and Rassbach observed with a mix- 

 ture of Pb + P'^O -\- PbS do, therefore, probably not relate to an 

 equilibrium of these three phases with SO^, but to another equilibrium. 



By a comparison of their observations with our measurements it 

 appears that on heating at temperatures below 800° this is the 

 equilibrium : Pb -j- PbO . PbSO, + (PbO),PbSO„ and on heating above 

 800° and then cooling, the equilibrium : Pb -f PbS + PbO . PbSO,. 



Also below 800° however, this latter equilibrium sets in, which 

 with a sufficient excess of PbS is the most stable, as shown from 

 the following experiment: 



A mixture of 4 mols. of PbO to 1 mol. of PbS was heated in 

 a pressure tube. The evolution of gas started at 660°. After evacua- 

 tion the following change in pressure was observed at 750^. 



The pressure thus rises rapidly to =t 38 m.m. and then increases 

 gradually to 83. 



The first pressure falls on the ;>T-line of Pb + PbO.PbSO, + 

 + (PbO), PbSO,, the second on that of Pb + PbS + PbO.PbSO,. 



Similarly was found with a fresh mixture on heating at 790° a 



1) The high result of the sulphate content in the check experiment is very 

 striking, because both the PbO and the PbS employed were free from sulphate. 

 Evidently the conversion of PbS + PbO into sulphate takes already place at the 

 boiling heat in the aqueous solution, from which it follows that also at the 

 ordinary temperature PbS and PbO are not stable in each other's presence. 



I 



V 



