22 



INFRA-RED REFLECTION SPECTRA. 



80% 



3 

 Fig. 14 



3 9 



Quartz: (a) Crystalline; (b) Amorphous. 



iojul 



line [(CH 3 ) 2 NC 6 H 4 NO] coincide with the absorption maxima found by 

 the writer for a solid film of this compound. Furthermore, the writer 

 found that reflection spectra of solids in solution may or may not be iden- 

 tical with that of the solid, note- 

 worthy examples being the 

 sulphates of copper and of so- 

 dium. The cause of the reflec- 

 tion bands is therefore to be 

 sought within the molecule. 



From the fact that the phys- 

 ical and chemical properties of 

 quartz-glass are different from 

 the crystal, one would infer 

 that there is a difference in the 

 molecular structure. This is 

 well illustrated in fig. 14. The 

 reflection spectrum of the amor- 

 phous material is entirely differ- 

 ent from that of the crystal. It 

 seems to have no connection with 

 any of the silicates examined. The maxima of the reflection spectrum of 

 crystalline quartz occur at 8.4 and 9.02 p., while those of the amorphous 

 quartz are found at 7.8, 8.4, and 8.8 n, respectively. The intensity of the 

 maxima of bands of the amorphous quartz is not so great, neither does the 

 reflecting power, in the region of the spectrum just preceding a reflection 

 band, fall to so low a value as in crystalline quartz. 



In this connection it may be noticed that the various kinds of glass 

 (sodium silicates) previously examined have similar reflection spectra. 



Calcite and Aragonite (CaC0 3 ). 

 (Fig. 15. Curve a, calcite; curve b, aragonite.) 



From the fact that, although the carbonates examined belong to only 

 two crystal systems, 1 the reflection spectra are different, one would infer 

 that the cause is not to be attributed to crystalline form. Previous exami- 

 nations have shown that isomeric substances have different spectra. One 

 would therefore expect to find the spectra of calcite and aragonite to be 

 different, as was previously found for orthoclase and microclene (KAlSi 3 O s ) 

 at 8 to 10 /j.. The fact that the substances are inorganic, and that the re- 

 flection bands are far in the infra-red, is of interest but non-essential. 



In fig. 15, curve a gives the reflection from the plane, highly polished 



1 To the rhombohedral system belong CaC0 3 (calcite), MgC0 3 , FeCC>3, and ZnC0 3 ; to 

 the orthorhombic system belong CaC0 3 (aragonite), BaC0 3 , SrCOj, and PbC0 3 . In the 

 former group the band toward the shorter wave-length appears to be the most intense, while 

 in the latter group the bands seem sharper and of more uniform intensity. 



