ABSORPTION AND EMISSION CENTERS. 



17 



The substitution of saturated groups in benzene has been found to change 

 the absorption spectra but little. Unsaturated radicals like NIL, C.'OOH, etc., 

 change the spectra very greatly, so that there is hardly any relation to the 

 benzene spectra. In the various di-substitution products, the para com- 

 pounds retain the characteristics of the benzene absorption better than the 

 ortho or meta compounds. A considerable amount of work has been done by 

 Hartley, 1 Baly and Desch, 2 Ley and von Engelhardt, 3 Baly and Collie, 4 Ley,'' 

 Hartley and Hedley, 6 Baly and Tuck; 7 and others have worked on the halogen, 

 amino, and nitro compounds of benzene, the phenols, pyridine, and its substi- 

 tution products. Below are a few of the results obtained. 



Purvis 8 has studied the absorption spectra of the vapors of pyridine and 

 some of its derivatives at different temperatures and pressures. Following are 

 some of the bands: 



n. = narrow; wk. =weak; sh.=sharp; st.=strong; dif. = diffuse; wd. = wide. 



In the pyridine vapor spectrum it was found that the longer wave-length 

 bands became wider and increased in intensity as the temperature and pres- 

 sure were increased. In fact, all bands showed an increase in width and inten- 

 sity under these conditions, and the general absorption in the ultra-violet was 

 also greatly increased. The piperidine bands are wider and more diffuse than 

 the pyridine bands. Some of the piperidine bands were coincident with the 

 benzene bands. Benzene bands, however, have most of the sharp edges of the 

 bands on the red side, whereas the piperidine bands are wider and are diffuse 

 on both sides. 



Hartley has shown that the vapors of o-, m-, and p-xylenes, l-methyl-4- 

 propylbenzene, and 1:3: 5-trimethylbenzene have very few bands compared 

 with benzene vapor, which has 82 bands. The same is true of pyridine vapor, 

 in comparison with the much smaller number of bands of a-picoline and the 



1 Handbuch der Spectroscopic, vol. in. 



2 Journ. Chem. Soc, 93, 1345, 1747, 190: 



3 Ber. d. chem. Ges., 41, 2990 (1908). 



4 Journ. Chem. Soc, 87, 1344 (1905). 



5 Ber.d. chem. Ges., 41, 1637 (1908). 

 (1908). 6 Journ. Chem. Soc, 91, 319 (1907). 

 'Ibid., 93, 1902 (1908). 

 8 Journ. Chem. Soc, 97, 692 (1910). 



