OXIDES. 



*5 



Selenium (Se). 

 (Curve d, fig. 7.) 



Selenium has practically the same reflecting power (18 to 20 per cent) 

 as molybdenite. Pfund (Johns Hopkins Univ. Circular No. 4, 1907) 

 has made a thorough investigation of the infra-red polarization of this 

 substance. 



OXIDES. 



In the previous examination only quartz (Si0 2 ) represented this im- 

 portant group of minerals. 



Magnetite (Fe 3 4 ). 

 (From Port Henry, Essex County New York. Curve a, fig. 7.) 



The surface examined was a triangular crystal, face about 3 cm. on 

 an edge. The specimen was very homogeneous, but did not take a high 

 polish, sufficient to observe the reflected image of an object at a small 

 angle of incidence. The reflecting power rises uniformly throughout the 

 spectrum, showing that the observations are affected by the lack of polish. 



Hematite (Fe 2 3 ). 

 (From Cumberland, England. Curve b, fig. 7.) 



The specimen examined was a very dense homogeneous concretion, 

 polished parallel to the radius of growth. The surface had a high polish 

 (better than that of magnetite), reflecting a strong image of a source at a 



40% 



:30 



u20 

 0) 





10 



<L 



^-* d ~ : 



. > . 



234-56 789/0// 



Fig. 7. Magnetite (a); Hematite (6); Chromite (c); Selenium. 



/2 



I3JU 



small angle of incidence. The reflecting power, in the infra-red, is far 

 below that of magnetite, although it is higher in the visible. The reflecting 

 power is uniformly 12 per cent, from which it would appear that this is 

 the normal value for hematite. 



Chromite (FeO, Cr 2 3 ). 

 (From Lancaster County, Pennsylvania. Curve c, fig. 7.) 



The surface of this dark specimen was mottled, parts of which took a 

 high polish. The reflecting power is uniformly 4 per cent throughout the 

 spectrum. 



It is of interest to note that the iron oxides show no bands of selective 

 reflection in the region of the spectrum up to 15 fi; zincite, ZnO, fig. 9, is 

 another example. 



