388 



we stmt from Fe,0,, and every time take away a quantity of tlie 



ftjOj+L 



1^203+G 



FEgO^G 



Feo+g 



FeO+Re 



Fb-ho 



III 



FEaOjPfcjQjïbO 



Fig. 2. 



Fk 



vapour phase at the constant tempeiature of e.g. 1100°. The total 

 concentration will then change in the direclion from Fe,0, to Fe^O^, 

 and in this the pressure will also be subjected to a change. 



First the pressure will gradually descend from p to ƒ. During 

 this decrease of pressure two phases coexist, viz. mixed crystals rich 

 in Fe^Og and vapours consisting almost exclusively of oxygen. When 

 the pressure has I'allen to that of the three-phase equilibriuui e fg, 

 a mixed crystal phase g rich in FjO^ will be deposited by the side of 

 the mixed crystal phase ƒ rich in FejOj, aud a three-phase system 

 arises of which the phase rule demands that the pressui-e remains 

 constant in case of equilibrium. On continued withdrawal of a part 

 of the gas phase, during which tiie total concentration continually 

 moves to the right, the pressuie therefore remains constant until the 

 last trace of the mixed crystal phase rich in Fe^O» has entirely 

 disappeaied. At this moment only the vapour and the mixed crystal 

 phase (} rich in FejO^ coexist, and on further withdrawal of the gas 

 phase the pressure will again descend regularly, in which the solid 

 phase moves downward along gh. 



When we now draw the vapour tension a8 function of the total 



concentration, theory predicts Ihat 

 on partial mixing of the two oxides 

 FejOj and FejO^ in the solid state, 

 a broken line as is schematically 

 represented in fig. 3, will be found, 

 tiie middle part of which runs 

 horizontally. 



This is the theoretical curve, 

 and now it is directly to be seen, 



P 



7Ï2O3 



X 



Fig. 3. 



FÈ,a 



3"4 



