709 



and as it had been shown that alwaj'S the old eqni librium again set 

 in, the oven was allowed to cool and the tube was withdrawn. 



The reaction product was strongl}^ caked and of a lighter colour 

 than the original mixture of PbS and PbSO^. Here and there were 

 visible granules with a strong metallic lustre so that at first the 

 suspicion was raised that lead had formed as a reaction product. 

 On closer examination by means of a magnifying glass these granules 

 proved to be very beautifully formed crystals of "galena" which 

 had deposited, besides in the reaction mass, also against the porcelain 

 boat and the extremity of the porcelain i-od. The analysis showed 

 this to be perfectly pure PbS, whilst in the reaction product itself 

 not a trace of free lead could be detected. 



Hence, no lead has formed so that the reaction product must be 

 basic lead sulphate. PbS -\- PbO.PbSO^ form the stable phase pair, 

 Pb -f- PbSO^ tJte metastable one. 



9. This was confirmed by the following expei-iment: 



An intimate mixture of PbSO^ and finely divided lead scrapings 

 in the proportion of 10 PbSO^ to 1 Pb was heated in an evacuated 

 and sealed tube for 3 hours at 600°. 



The product obtained gave with hydrochloric acid a very distinct 

 evolution of H2S. 



A weighed quantity was now heated in a small flask with strong 

 hydrochloric acid and the gas expelled collected in an ammoniacal 

 solution of hydrogen peroxide. This solution was boiled for a while, 

 then acidified, and the residting H2SO4 precipitated as BaSO^. 



Five grams of mixture containing originally 0,380 gram of Pb, 

 yielded 0,040 gram of BaSO^, equivalent to 0,041 gram of PbS. 



According to the equation : 



4 Pb -h 5 PbSO, =r 4 PbO . PbSO, -f PbS . . . (5) 



0,142 gram of Pb has been required for this 0.041 gram of PbS. 

 Although the lead has not yet entirely disappeared, a considerable 

 quantity of the same has been converted into PbS^). 



10. The pressures observed almost entirely agree with the values 

 found by Schenck and Rassbach for the mixture of PbS-f-PbSO^-f-Pb 

 as well as for PbS -|- PbSO^ + PbO. They conclude that the pressures 



1) Afterwards it came to our knowledo;e that also Jenkins and Smith (I.e. p. 691) 

 had already made an experiment from which this is shown. Tliey heated a mixture 

 of equal molecular quantities of Pb and PbSO^ in a porcelain crucible at dull red 

 heat for half an hour ; from the residual mass could be expelled with HGl 

 a quantity of HoS corresponding with 1,41% of sulphur. 



