GEOPHYSICAL LABORATORY. 147 



(16) The feiTous ii'on content and magnetic susceptibility of some artificial and natural 



oxides of iron. R. B. Sosman and J. C. Hostetter. Bull. Am. Inst. Mining 

 Eng., 907-931 (1917). 



The percentage of ferrous iron and the relative magnetic susceptibility in 

 }iowder form have been determined on a number of artiJBcial and natural oxides 

 of iron. Artificial oxides made at 1100° and 1200° consist of a solid solution of 

 Fe304 in Fe203. Their relative magnetic susceptibility is approximately pro- 

 portional to their percentage of FeO, from FejOa over to Fe304. The devia- 

 tions may be partly accounted for by the effect of various factors, of which the 

 fineness of grain of the powdered oxide is the most important, especially in the 

 case of the more ferromagnetic members of the series. The colors of the pow- 

 dered oxides depend both on their chemical composition and on their physical 

 constitution, especiallj' the fineness of grain. 



In addition to the oxides whose susceptibility depends upon their content 

 of FeO, there exists also a highly ferromagnetic form of Fe203, which appears 

 to be rare in natural occurrence. 



The natural iron-oxide minerals are similar to the artificial in being in many 

 cases solid solutions of Fe304 in Fe203. Others are mixtures of Fe304 and 

 Fe203. If the ferrous iron is not in solid solution or in magnetite admixture, 

 the magnetic susceptibility falls below the normal. 



Some natural oxides can be magnetically fractionated; in these cases the 

 less magnetic portions are found to deviate more widely from normal than 

 the more magnetic. The cause of this deviation is not yet entirely clear. 



Martite is a pseudomorph after magnetite, but its constituent granules or 

 fibers consist usually of a soUd solution of Fe304 in Fe203. The ferrous iron 

 content and the magnetic susceptibility of the specimens examined suggest 

 that they have been produced at temperatures considerably higher than atmos- 

 pheric. 



(17) Zonal growth in hematite, and its bearing on the origin of certain iron ores. R. B. 



Sosman and J. C. Hostetter. BuU. Am. Inst. Mining Eng., 933-943 (1917). 



The pov/dered oxide from certain crystals of hematite from Elba contains 

 considerable FeO and can also be fractionated magnetically. It is therefore 

 not homogeneous, as would be the case if the crystal were a uniform solid 

 solution throughout. Analyses and magnetic measurements on a cross-section 

 of an Elba crystal showed that the magnetic susceptibility and percentage of 

 FeO varj^, not irregularly, but continuously, being highest at the base and low- 

 est at the free-growing tip of the crystal. The crystal is therefore zoned with 

 respect to its FeO content. 



Since Fe304 goes into solid solution in Fe203, forming a single solid phase of 

 varying composition and properties, a zonal distribution of FeO is to be 

 expected in an oxide of iron depositing from a vapor or solution. The occur- 

 rence of such zonal growth indicates continuously changing conditions of tem- 

 perature, pressure, and concentration during the formation of the crystals. 

 Several ore deposits of contact-metamorphic origin show a zonal distribution 

 of ferrous iron, probably arising from the same causes as the zoning of the 

 single crystals. 



(18) The petrographic microscope, a useful tool in applied optics. F. E. Wright. J. 



Optical Soc. Amer., 1, 15-21 (1917). 



In this paper the function of the petrographic microscope is outhned and its 

 apphcability to many problems in optics is indicated. It may be used to 

 advantage in the determination of non-metallic materials, whether crystal- 



