390 



It seeins to ns tliat they overlook in tliis that in case of unmixing 

 it is by no means impossible that the phase rich in Fe,,!)^ of the 

 equilibrium of unmixing would show a much stronger refraction in 

 consequence of its content of Fe^O^ than the pure niagnetite. This 

 possibility is by no means improbable, because it already follows 

 from the above determinations that independently of the fact whether 

 or no unmixing is assunjed, the refi-action must diminish much 

 more rapidly somewhere in the optically not investigated region 

 than on the Fe.^Oj side. 



I( should besides be considered that, as also Sosman ^) remarked, 

 if we assume a continuous series of mixing between the hexagonal 

 hematite and the regular magnetite, (his would be an instance of 

 a continuous mixture between non-isomorphous substances, which 

 has not yet been experimentally observed in a single case. 



Now if we assume that from the ^../'-figure at 1100^ and 1200° 

 we musr actually conclude to a continuous mixed crystal-series 

 betvveen Fe^Oj and FcjO^, the said difficulty can still be obviated 

 by the assumption that at this temperature the two oxides are iso- 

 morphous ; as Sosman and Hostetter found that the homogeneous 

 mixed crystal phases are bi-refringent, magnetite would have to 

 possess a point of transition below 1100°, above which point the 

 regular foi'm is metastable. This not very probable change of crystal 

 class has, however, not been observed, and can besides not rendej- 

 the continuous mixing plausible for temperatures below that of the 

 point of transition. 



However this may be, the existence of a continuous series of 

 mixing of Fe, 0, -\- Fe, O4 does not seem proved to us, and we 



1) Journ. of the Washington Ac. of Science 7. 10 (1917). 



