714 



16. Without opening the appai'atus the experiments were now 

 continLicd witli the same mixture at 789". The tube was, therefore, 

 evacuated for tlie third time and then again a few times and each 

 time the equilibrium pressure was again measured. It now appeared 

 that the old pressure no longer set in, but that a lower pressure 

 was attained and the more so when more SO^ had been withdrawn. 

 In succession were found 93, 75, 6J, 54, 41, 34, and 28 m.m. 



This different behaviour can be explained in two ways. 



1. The equilibrium is no longer monovariant, but divariant. Instead 

 of three solid phases there are only two, one of w^hich possesses a 

 variable com|)osition. This phase might be a very basic sulphate 

 with a variable content iji PbO. The fusion diagram PbO — PbSO^ 

 of ScHENCK and Rassbach gives, however, but little support to this 

 conception. 



2. The pressures measured are not true eciuilibria pressures, but 

 indicate a stationary condition. 



For if, on evacuating, the pressure falls below the equilibrinm 

 pressure of equibrium (9) the basic sulfate (PbO)j PbSO^ can decom- 

 pose still further and give rise to the formation of (PbOjj PbSO^ . 

 Pb -f- PbO . PbSO^ then strive, according to reaction (8) towards the 

 pressure p^ , (Pb(>)3 PbSO, -|- S^» according to reaction (9), however, 

 in the direction ^- towards p^ . And when finally both reactions 

 take place with equal velocity, we obtain an apparent equilibrium 

 at a pressure between pg and p, and dependent on the quantities 

 of the different phases. 



It is even possible that PbO is also formed and that reaction (10) 

 thus takes place simultaneously. 



17. The second assumption was the most probable one. In order 

 to test it more closely a mixture of Pb and (PbO)3 PbSO^ was heated 

 in the pressure tube. From this mixture PbO oidy can be formed 

 as the third phase so that only one reaction, that of the monovariant 

 equilibrium (10), should be possible. 



(PbO), . PbSO^ was obtained by fusion of 1 PbS04 with more than 

 3 PbO. As porcelain is strongly attacked by PbO, the mixture was 

 heated in a magnesia boat previously heated and saturated with 

 lead oxide. 



The result of the measurement at 780° was p = 23, after evacua- 

 tion at the same temperature again 22 m.m., then at 800°, 38 and 

 after evacuation successively 30, 22, 16 m.m. Thus no constant 

 equilibrium is attained. 



On opening the pressure tube nearly all appeared to ha\'e been 



