713 



15. Pressures appertaining to the fust monovariant equilibrium 

 were obtained by starting from a mixture of Pb and PbO . PbSO^ . 



Not until 700° an evolution of gas was perceptible. The equilibrium 

 sets in with much greater diflficulty than in tiie first two cases ; 

 generally two or three hours were required. Probably this is due 

 to the fact that the metal conglomerates and thus offers but a small 

 contact surface with the basic salt. It is also very certain, liowever, 

 that the greater vapour tension of the PbS will have strongly promoted 

 the setting in of the pi-evious equilibria of which PbS was one of 

 the active phases. The equilibrium could again be attained from 

 both sides. 



The following pressures were measured (Fig. 2 Curve III) : 



TABLE III. 



The tube was subsequently evacuated at 789° and the equilibrium 

 pressure again determined. This proved to be unchanged. Even on 



m • 



zoo 



100 



50 



6(10' 



700' 



800' 



Fig. 2. 



evacuating a second time the pressure reverted to its old value. The 

 pressures measured therefore relate to a purely raonovariant equilibrium. 



