hi 



stable phase pair. We must, however, consider that the lead phase 

 need not be pure Pb, but may contain dissolved PbS and hence 

 there exists the possibility that the PbS found was present, not as a 

 free phase, but as a solution in the molten Pb. 



The amount of PbS was, therefore, determined ciuantitatively. 



From two grams of the mixture were obtained 17,4 mg. of BaSO^ 

 corresponding with 17,8 mg. of PbS. F'or the formation of 17,8 mg. 

 of PbS according to reaction (6) are required 70 mg. of Pb. Before 

 the heating 2 grams of the mixture contained 1,082 grams of lead. 

 Hence, there remains 1,022 gram of Pb, which in 100 grams contains 



— — X 100 = 1,77 grams of PbS. 

 1022 ^ ^ 



From the observations of Friedrich and Leroüx ^) it follows that 



the lead solution saturated with PbS at 680° contains 2,57o PbS. 



Hence the PbS will be present in the heated mixture not as a free 



phase, but as a solution in Pb, and Pb -|- PbO.PbSO^ will form 



the stable phase pair. 



13. This conclusion was fnrther confirmed by the dissociation 

 experiments, starting from a mixture of PbS and PbO.PbSO^. 



These experiments were conducted in a manner similar to that 

 in the case of PbS and PbSO,. The SO.,-evolution started at 680°. 

 The equilibrium set in quite as easily as with PbS -|- PbSO^ and 

 could be determined readily from both sides. Also, after removal of 

 larger quantities of SO.^, the same equilibrium pressure was again 

 always obtained. In order to prevent fusion the mass was not heated 

 above 800°. 



The results obtained are united in table 11 (Fig. 2, Curve II). 



TABLE II. 



1) Metallurgie 2, 536 (1905). 



Proceedings Royal Acad. Amsterdam. Vol. XVll. 



47 



