16S 



set In \cl'y exactly on tlie lower value appertaining to this lowel' 

 temperature. 



A. few check experiments with ferric sulphate, executed in the 

 same apparatus yielded equilibria pressures agreeing entirely with 

 the values indicated by Bodenstkin '). 



The results are given below. The deviations answer to a ditlerence 

 in tem[)eralure of 1 — 2°. 



14. If we again consider the monovariant equilibrium, 

 CuO . CuSO, + Cu,Ü t; 4 CuO + SO, . . 



(V) 



we can imagine this to have originated in the following manner : 

 CuO . CuSO, :;t 2 CuO + SO» (VII) 



SO. :;f: SO, + 10, (IX) 



Cu,0 -\-hO,:^2 CuO (VIII) 



The gaseous phase consists both in reaction (V) and reaction I'VIl) 

 of a mixture of SO,, SO, and 0,. If we call the partial pressures 

 of these gases at a given temperature : 



for V respectively, ^pso^ > tPSO, , ^po, , the total pressure P^ 



„ VII „ ,pso,, ,pso, , ,po,, „ „ „ P, 



the homogeneous equilibrium in the gaseous phase will be as follows ; 



at V 



K^'-^ 



and at VII K. 



iPSOt 



;PSO, 



Hence it follows that, at the same temperature 



tPSO^ ■ ,po, 



,PSO„ ■ iPOt 



(«) 



iPSO, . jT'SO, 



The coexistence of the phases Cu,0 and CuO at V now demands 

 that the partial oxygen pressure in this equilibrium is equal to a 

 dissociation pressure of pure CuO into Cu,0 and O,. If we call 

 the latter P^, then jpo., must be = P^. 



Likewise does the coexistence of the solid phases CuO . CuSO, and 



1) Zeitschr. f. Elektrochemie 16, 912. (1900). 



