RESIDUAL RAYS; CRYOLITE. 



31 



cases the transmission curves show complete opacity in the region of a 

 selective reflection band even for the thinnest film yet examined, viz, glass 

 (see Carnegie Publication No. 65, p. 65). That the band at 29.4 jj. is not 

 influenced by the structure of the crystal is proven by the fact that Asch- 

 kinass found this band in white marble. 



Magnesite (MgC0 3 ). 

 (Grating No. 2, fig. 22.) 



In this examination three reflecting surfaces of the massive material 

 were used, one of which did not have a high polish. The maxima at 

 B 1} C v Z>! correspond to wave-lengths 6.7, 11.3, and 30.7/* (5 52'), 

 respectively. In fig. 22 the maxima C\, C' 2 (=11.3/1) an d D\, D' 2 

 (=30.7 jj) are drawn to ten times the scale of B u B 2 . The band at D x is 



90 



777777 



80 



70 



60 



SO 



w 



c 

 o 



o 



0) 



40 



30 



20 



10 



7" 65432/ 1 2 3 4 5 6 7 8" 



Fig. 22. Magnesite. 



plotted to the same scale as B t but only two reflecting surfaces were used. 

 The magnesite maxima are identical with those of calcite (except at a 

 possible greater wave-length, at 30 fx) . This would seem to indicate that 

 coincidence of the reflection spectra of chemically related groups of sub- 

 stances is a property which obtains throughout the spectrum. This is to 

 be expected, but it seemed of interest to add further experimental evidence 

 to support this view, by extending the observations into the remote infra- 

 red. The bands at 29.4 y. are too weak to illustrate the effect of molec- 

 ular weight, noticed at 6.7 to 7.2 \i in the carbonates. 



RESIDUAL RAYS FROM CRYOLITE ( 3 NaF-AlF 3 ). 

 (Gratings No. 1; slits 4 mm.; fig. 23.) 



Reasoning from the fact that cryolite is a fluoride of Na and Al, and 

 from the behavior of fluorite (CaF 2 ), it was hoped to find this mineral to 







l^V 



