391 



deem it, tlierefore, desirable to consider the possibility that in fig. 1 

 there should be added another three-phase curve, viz. that for 

 Fe,0, + Fe, 0, + G lying under that for FeO -f Fe,0, + G. 



This situation gives rise to the question, w^hether this new three- 

 phase line can intersect another. If it intersected the three-phase 

 line for Fe -f- Fe, 0^ -|- G, the mutual relation would be as given 

 in fig. 6. 





Fig. 6. 

 The conclusion that the thtee-phase lines for Fe -)- FeO -f- G and 

 Fe^ 0^ + P^eO -|- G intersect, and that this point of intersection indicates, 

 therefore, the lowest temperature at which FeO can occnr stable 

 by tha side of the gas phase G, is entirely in accordance with the 

 sign of the conversion which must lake place in this point on 

 withdrawal of heat, viz. : 



4 FeO -^ Fe + Fe, O, + a cal. ') 

 When also the three-phase lines for Fe -|- Fe, 0^ -)- G and Fe, O3 -f- 

 ~h ^3 ^^1 4" Gr intersected in the way indicated here, then theconversion : 



3 Fe, 0, -^ Fe + 4 Fe, O3 

 would have to take place in this point of intersection on withdrawal 

 of heat, but this is in contradiction with the heat-effect of this reaction. 

 It follows namely from the measurements that: 



3Fe,0, -^ Fe + iFe^O»— b cal. ') 



The supposition expressed in fig. 6 should, therefore, be rejected. 



Now there remain two possibilities, namely these that the two 

 three-phase lines for Fe. 0, -f FeO + G and Fe, 0, -\~ Fe^ 0, -f G 

 intersect at higher temperature, but that melting sets in, before this 

 intersection takes place. In this case we get a situation as has been 

 schematically given by fig. 7. 



i) Comptes Rendus 120, 623 (1895). 

 ') loc. cil. 



