704 



For a proper insight into this process and to answer the question 

 wliether these reactions actually do take place it is necessary to 

 study the equilibria between the different phases that may be formed 

 therein. 



After various older researches among which deserve to be men- 

 tioned those of H. C. Jenkins and E. H. Smith ^), a systematic 

 research as to these equilibria was carried out some years ago 

 by R. ScHENCK and W. Rassbach ^). They determined the equilibrium 

 pressure of the sulphur dioxide evolved when three of the four 

 phases PbS, PbSO^, PbO and Pb are heated together in an evacuated 

 tube at 550° to 900°. 



The conclusions which they drew^ from these measurements as to 

 the nature of the equilibria occurring therein could, however, not 

 be correct in many i-espects. Their idea has in fact been consider- 

 ably modified in various subsequent publications thereon ^). But 

 even the last concluding articles still contain many contradictions 

 so that it is not plain what equilibria they have actually determined 

 and which phases are stable in the presejice of each other. 



In the following will, therefore, be discussed (1) the different 

 equilibria imaginable in this system and (2) the results will be 

 communicated of researches which in consequence thereof have been 

 carried out conjointly with Dr. F. Goudriaan. 



2. The equilibria between Pb and the compounds PbO, PbS, 

 PbSO^ and SO^ may be considered as those in a system of three 

 components, namely Pb, 0, and S. The isotherm for the equilibria 

 between the different phases can, then, be indicated by a triangle 

 with these components as apexes. (See fig. 1). 



Let us now first suppose that 



a. only the phases Pb, PbS, PbSO,, PbO and SO^ are possible 



b. the gaseous phase is pure SO^ and the lead phase pure lead. 

 In the last supposition we therefore neglect the small amount of 



PbS in the vapour and the solubility of PbS in molten lead ; in the 

 first supposition no notice is taken of the basic sulphates which 

 according to the later researches of.ScHENCK and Rassbach ^) occur 

 as intermediate phases between PbS04 and PbO. 



1) Journ. Ghem. Soc. 71, 666 (1897). 



~) Ber. d. d. chem. Ges. 40, 2185 (1907). Metallurgie 4, 455, (1907). 

 3) Ber. d. d. chem. Ges. 40, 2947 (1907); 41, 2917 (1908). R. Schenck, Physi- 

 kalische Ghemie der Metalle. 



*) Ber. d. d. chem. Ges. 41, 2917, (1908). 



