152 



The reaction product was obtained by evacuating a few times 

 more : it was a cindeiy, copper-coloured mass in whicli metallic 

 particles were easily discernible; sulphate could not be detected. 

 CuJJ-\-Cu^S tlius form a stable phase-pair and as the diagonal 

 CujO — Cu,S cuts the diagonal Cu — CuSO, (see fig. 1), it follows 

 that Cv-\-CuSO, must he metastable in presence of each other. 



3. By now combining the other phases that are stable between 

 ± 300° and 900° two and two with each other and heating in 

 evacuated, sealed tubes we were able to record which of these 

 phase-pairs can be considered as the stable ones. First of all were 

 chosen the combinations Cu^S -('uO and Cu^O — CuSO,. 



A mixture of equal mols. of the last named substances when 

 heated during 6 — 7 hours at 450° — 480° remained completely un- 

 changed, not a trace of sulphide could be detected. On the other 

 hand, mixtures of Cu^S and CuO appeared to undergo a very strong 

 change at this temperature; the colour changed from black to 

 brownish-red and on extraction with cold, recently boiled water a 

 blue-coloured filtrate was obtained which gave a strong sulphate 

 reaction. The sulphate soluble in water was determined. quantitatively 

 as BaS04. From 1.1182 grams of a mixture containing 2 mols. CuO 

 on 1 mol. CujS was thus obtained 0.1729 gram of BaSO^ corre- 

 sponding with 0.118 gram of CuS04 ; the mixture, after heating, 

 therefore contained 10.6 7o of CuSO^. This quite agrees with the 

 amount of CuSO^ calculated on the supposition that the conversion 

 of the mixture takes place according to the equation : 



Cu,S -f 9 CuO = 5Cu,0 + CuSO, 

 namely 10.58% of CuSO,. 



Hence, we come to tlie conclusion that Cu-^O -\- CuSO, forms 

 the stable and Cu^S-\-CuO tiie metastahle phase-pair. 



4. In a similar manner we investigated the combinations CiiS — 

 Ch,0 and CuSO, — CujS. It was a priori improbable that the first- 

 named pair should be stable as according to the observations of 

 Prkuner and Brockmöllp^r M, the dissociation of CuS into Cu^S -f- S 

 becomes already measurable at 450^. In fact, it appeared that the 

 colour of the CuS — Cn,0 mixtures had been changed from dark 

 brown to grey after 5 hours' heating at 300° — 320°; a considerable 

 quantity of sulphate had been formed. This, after extraction with 

 cold, recently boiled water, was determined as BaSO,. From 0.8722 



1) Zeitschr. phys. Chem. 81, 129, (1912). 



