MISCELLANEOUS COMPOUNDS. 67 



It is, of course, opaque to the visible rays, but, for the thickness used, it 

 transmits 45 to 50 per cent of the infra-red from 1.5 to 12^. This 

 great transparency to heat rays seem to be a property of the sulphides, 

 as will be noticed in sphalerite (ZnS). In this connection is to be 

 noticed that selenium is also very transparent to heat rays. 



From the reflection curve it will be noticed that the above thickness 

 of stibnite is practically transparent, instead of transmitting only 45 per 

 cent, as indicated above. From curves a and b, it will be noticed that 

 there is practically no difference in the transmission, although one is 

 twice as thick as the other. The greater transparency of the thicker 

 film at 7 p is not an error in observation, as was found on repeating the 



3456 8 10 II 



FIG. 55. Stibnite ; Brookite (rf) ; Rutile (e). 



work. In curve c, where the thickness is 4.9 mm., the absorption 

 (selective reflection ?) band beyond 15/4 becomes quite evident. In 

 this region the reflection curve suddenly decreases, indicating a region 

 of anomalous dispersion. 



BROOKITE (TiO 2 ). 

 (t = 0.37 mm. Curve b, fig. 55.) 



The specimen examined was subtranslucent, and by transmission was 

 of a grayish tinge. The curve shows no bands except a depression 

 at 3.1 ^ 



RUTILE (TiO 2 ). 

 (t = 0.2S mm. Curve c, fig. 55.) 



The section of this mineral was ground from a long, flat needle, which 

 transmitted a deep red. It is more opaque than brookite, from which it 

 differs in crystalline form. 



