24 INFRA-RED TRANSMISSION SPECTRA. 



3, and 4.75 /x, beyond which point the opacity becomes too great for 

 further exploration. The crystals obtainable were small, so that the 

 section was not quite long enough to cover the whole slit. 



STILBITE (CaAl 2 Si 6 O w +6H 2 O). 

 (Transparent section cut perpendicular to optic axis. f = o.ii. Curve a, fig. 7.) 



The stilbite curve is to be noticed for its great transparency with all 

 the water bands superposed. There do not appear to be any important 

 bands belonging to the mineral itself. 



POTASSIUM ALUM (K 2 SO 4 Al 2 (SO4)a-|-24HaO). 

 (Cut perpendicular to axis. t = 0.085 mm.; transparent. Curve b, fig. 7.) 



The exploration of the spectrum extends to 3 /*. The water bands at 

 1.5 and 2 n are present. With 24 molecules of water and SO 4 bands 



01 a 3 4 5 6 



FIG. 8. Natrolite (a) ; Scolecite. 



at 455ft, to be mentioned later, it would not be possible to penetrate 

 beyond 3 /*, unless a thinner section could be made. 



NATKOLITE (Na 2 Al 2 Si s O 10 +2H 2 O). 



(Orthorhombic section ground parallel to m; subtranslucent ; f = o.u mm. 



Curve a, fig. 8.) 



The specimen from which this section was cut was a tuft of acicular 

 crystals. The section was made from the more massive part, and was 

 almost opaque for the thickness used. The curve (a) in fig. 8 shows 

 the water bands in their usual positions and proper intensities. 



