g6 INFRA-RED ABSORPTION SPECTRA. 



6.2 fi. The region at 3 /a is complex. In addition to the 3.45 /a band, 

 we have another at 3.7 /x, as in the other terpenes, and a strong band at 

 3 fi, which is also found in eucalyptol. Since eucalyptol is an oxide, 

 and since limonene, pinene, and Venice turpentine show small bands 

 in this region, for small dispersion, it would appear that the 3 fi band 

 is also characteristic of the terpenes, and not due to the OH-group. 

 The band for pure water occurs at 2.95 /x. 



pyridine; group. 

 Pyridine. CsHbN, (Figs. 118 and 120.) 

 The pyridine group of compounds is marked for its 

 C great opacity. A cell 0.19 mm, becomes quite opaque at 



^1l f ^ ^ 1^' ^^^ transmission is in two steps, the first ending at 



HCv^ ^CH 6 fi, while the second extends to 13 ju,, where the thick films 

 ^ are entirely opaque. It is well known that nitrogen exerts^ 



a great influence in the diathermancy of certain compounds. This work 

 shows where its effect is greatest. The thin films appear quite trans- 

 parent. This is due to the fact that the unusually numerous sharp bands 

 begin to overlap in the thicker films, and the general absorption increases 

 more rapidly than required by the law of variation of absorption with 

 thickness. Angstrom^ concluded that this is due to the fact that the 

 bands are complex groups of lines, not dispersed by the prism. Here 

 we have a practical demonstration of the validity of his conclusions. 



Pyridine has the 3.25 /x, 6.25 fx, 6.75 fi, as well as several other bands 

 in common with benzene. In addition it has the 2.95 fi band found in 

 ammonia and nitrogen compounds. The large spectrometer resolved 

 several lines at 6.75 fi, as shown in the curve. 



Alpha-Picoline. CsHiNCCHs). (Figs. 119 and 120.) 



jj This compound differs from pyridine in having a CH3- 



/C^ group. It is isomeric with aniline, and is an excellent 



II I demonstration of the effect of structure as well as the 



^C\ ^C-CH3 effect of the CHg-group on absorption. Its spectrum is 



quite different from pyridine. The band at 2.92 fi is much 



deeper. The next band, at 3.35 /x, is the mean of the 3.25 [x band of 



benzene and the 3.43 fx band of compounds predominating in CH3- 



groups. Taken in consideration with the facts gained from the spectra 



of mesitylene, toluene, and the xylenes, and also from the fact that the 



band is asymmetrical, it appears that the benzene, 3.25 /x, band still 



exists, but that it is not apparent because of the lack of dispersion. 



^Friedel, loc. cit. 

 ^Angstrom, loc. cit. 



