SULPHIDES. 13 



desirable to examine aragonite, CaC0 3 , which differs from calcite in its 

 crystalline form, to determine the effect of structure; this is given in 

 Chapter III. It was shown in Carnegie Publication No. 35 that isomeric 

 compounds have different transmission spectra; and one would expect also 

 the reflection spectra of isomers to be different. The unsteadiness of the 

 galvanometer prevented the location of the band, found by Aschkinass, 

 at 11.4/1. In this region the radiometer would have been more satisfac- 

 tory. 



Magnesite (MgC0 3 ). 

 (Massive. Curve b, fig. 5.) 



The sample examined was an opaque white mass which took a high 

 polish. The band of selective reflection is very similar to that of calcite 

 and consists of two maxima at 6.5 and 6.8 ;<, respectively. 



As a whole the carbonates are conspicuous for a double band of metallic 

 reflection at 6.5 to 7 /i which previously had not been resolved in calcite. 

 The shift of these bands with increase in molecular weight of the metallic 

 ion will be discussed on a later page. That the shift is not due to a change 

 in the adjustment of the apparatus was verified at the completion of the 

 observations by examining several of the specimens in succession, when it 

 was found that the maxima coincided with the values first observed. 



SULPHIDES. 



The reflection spectra of sulphur and of the sulphides of Zn, Sb, Fe, 

 and Pb were described in the preceding volume. Sulphur and sphalerite 

 (ZnS) have a low reflecting power of only about 8 per cent throughout the 

 spectrum to 1 5 ft. The remaining minerals have a high reflecting power 

 of 32 to 36 per cent throughout the spectrum to 15 /., where the reflection 

 of stibnite, Sb 2 S 3 , seemed to decrease. The present examination includes 

 four new sulphides. 



Molybdenite (MoS). 

 (Massive foliated. From South Australia. Curve a, fig. 6.) 



This mineral is very soft, like graphite. The specimens were folded 

 and distorted so that it was not possible to grind a surface parallel to a 

 cleavage plane. The surface took a high polish, but no luster as is found 

 in the cleavage lamina. The reflection curve is fairly uniform beyond 4 ft, 

 which would indicate that the lack of polish had no serious effect beyond 

 this point. The reflecting power is low and uniform (18 to 20 per cent, 

 as compared with stibnite, 35 per cent) throughout the spectrum to 14 /*, 

 which is to be expected from its electrical conductivity. (For further 

 references to electrical conductivity, etc., see Carnegie Publication No. 65, 

 p. 93; also Konigsberger, Jahrb. Radioaktivitat & Elektronik, vol. 4, 

 p. 161, 1907.) 



