SULPHIDES. 93 



slightly higher reflecting power. The specimen has a slightly higher 

 reflecting power in the visible, and beyond 3 p a constant reflecting 

 power of about 32 per cent 1 to 14^. 



According to Reichenheim, 2 the electrical conductivity is variable for 

 different specimens on account of impurities, so that no comparisons 

 can be made with the computed reflecting power, such as have been 

 made by Hagen and Rubens 3 for metals. 



(GALENA (PbS). 

 (Cleavage piece, surface 2 by 2.5 cm. ; quite plane. Curve c, fig. 80.) 



The reflecting power appears to be slightly higher in the visible than 

 in the infra-red, where it is constant at 31 per cent to 14^. No doubt 

 the reflecting power would be slightly higher for a perfect specimen. 

 It was found impossible to grind a surface that was not full of small 

 depressions, due to chipping of the surface. 



SPHALERITE (ZnS). 

 (Curve d, fig. 80.) 



The specimen examined was a dark mass of crystals. The surface 

 showed several cracks. The reflecting power is no doubt low, even for 

 a perfect surface, as will be noticed from its uniformity throughout the 

 spectrum to I5//,. No large, clear specimen of this material was 

 obtainable. The refractive index in the visible is 2.369 (Na), while 

 computing from the reflecting power at 5 to 10 /JL would indicate a value 

 of n = 1.85 for R = 0.09. 



SULPHUR (S). 

 (Reflection from large crystal face. Curve b, fig. 81.) 



The reflection curve is low and uniform throughout the spectrum. 

 It is interesting to note that the absorption bands at 8 and 12 p. (found 

 in a previous examination) are too small to affect the reflection curve. 

 In this connection it will be noticed that all the sulphides examined, 

 except ZnS, have a much higher reflecting power, while the zinc sul- 

 phide reflection is practically the same as that of pure sulphur. 



1 Konigsberger and Reichenheim : Centralblatt fur Mineralogie, Jahrg. 1905, p. 

 465, "by extrapolation from the curve for the visible spectrum found a reflecting 

 power of 30 per cent." 



* Reichenheim : Inaug. Diss., Freiburg, 1906. 



"Hagen and Rubens: Ann. de Phys. (4), u, p. 873, 1903. 



