68 



INFRA-RED TRANSMISSION SPECTRA. 



POTASSIUM NITRATE (KNO). 

 (f = o.32. Curve a, fig. 56.) 



The section of this substance was made from the commercial material. 



It is conspicuous for its large absorption band extending from 6.5 to 

 8.5 ft, and a slightly smaller band at 12 p.. 



There are absorption bands at 3.6, 4.0, 4.7, 5.65, 6.5 to 8.5, 9.6, 10.2, 

 12.0, 13.1, and 14.3 fji. Some of these bands coincide with sharp bands 

 in silver nitrate. The reflection curve (x-x-x-x) shows a maximum 

 at 7.15/1. 



AMBER. 

 (t = 0.8 mm. Curve b, fig. 56.) 



Amber is a fossil resin. The section examined is too opaque to give 

 us any information. 



-? 6 8 9 to II 12 



FIG. 56. Potassium nitrate ; Amber (b). 



15 16-fi. 



FLUORITE (CaF 2 ). 



In fig. 57 are shown the transmission curves of several pieces of 

 fluorite, examined several years ago, but never published. They are 

 given to show the difference between a perfectly white fluorite, curve a, 

 and a greenish tinged variety, curve b. The latter shows a slight 

 absorption band at i.4/i. In curve a, t = 2$ mm., and in curve b, 

 t = 4.75 mm., from which thickness it will be observed that if we cor- 

 rect for reflection, white fluorite is perfectly transparent, while the green 

 variety is likewise beyond 2.5 p. This transparency has been observed 

 by Rubens and others, and continues beyond 6 p., where it decreases to 

 zero at 10 /*,. 



From the curves it will be seen that for certain work on radiation the 

 green fluorites are just as serviceable as white ones, which latter are 

 becoming difficult to obtain. 



