sosman: problems of the oxides of iron 59 



at 1100° and 1200? These two oxides seem to form a continu- 

 ous series of solid solutions from Fe 2 3 to Fe 3 4 , or at least to a 

 point so near Fe 3 4 that it has not been possible to detect a 

 break in the dissociation pressure curves. 7 



The melting temperatures (solidus and liquidus curves) of 

 the series Fe 2 03-Fe 3 04 have not yet been determined. The 

 dissociation pressure rises so rapidly with increase of temperature 

 that at the melting temperature of the solid solution the pres- 

 sure of oxygen will probably be found to be of the order of several 

 atmospheres for compositions which depart much from Fe 3 4 . 



The melting point of Fe20 3 can be determined only under 

 sufficient oxygen pressure to prevent its dissociation, and it is 

 impossible to predict whether the melting point will be higher 

 or lower than that of magnetite. The form of the liquidus and 

 solidus curves (continuous, minimum, or maximum) can also 

 not be predicted. The form of these curves is of particular 

 interest in connection with the effect of changing oxygen pres- 

 sure. At a given temperature it is possible that increasing the 

 oxygen pressure might cause either fusion or solidification of the 

 oxide, or even fusion followed by solidification, depending upon 

 the form of the curves. 



The type of solid solution represented by these two oxides of 

 iron deserves a moment's consideration. The special feature 

 of this case is that the two constituents of the solution differ only 

 in the proportion of volatile component which is combined with 

 the non-volatile component. In other words one of the constituent 

 oxides of the solid solution is produced by direct dissociation of 

 oxygen from the other oxide. The case of Fe 2 3 -Fe 3 04 is 

 thus exactly analogous to the case of CoCl 2 .6NH 3 -CoCl 2 .2NH 3 , 

 in that we may consider Fe as replaced by CoCl 2 and O 

 by NH 3 . The system just referred to has been studied by 

 Biltz and Fetkenheuer, 8 who find a continuous series of solid 

 solutions, throughout which the NH 3 pressure falls continuously, 

 following a reversed curve of the same form as that shown by 

 the iron oxides. 



7 Sosman and Hostetter. Jour. Am. Chem. Soc, 38: 807-833. 1916. 



8 Biltz and Fetkexhetjer. Zs. anorg. Chem., 89: 106. 1914. 



