CHAPTER II. 



HISTORICAL. 



In 1882 Abney and Festing/ by means of photograph}', investigated 

 the absorption spectra of 52 compounds to i.2fi, which was the Hmit of 

 the sensibiHty of their photographic plates. They found many inter- 

 esting relations among the absorption bands, which are of the greatest 

 importance in the present investigation, since their work is in the region 

 which is difificult to explore on account of the small dispersion and the 

 numerous small bands which can not be detected without other measur- 

 ing devices than photography. Certain radicals were found to have 

 distinctive absorption bands at about 0.7 fi and 0.9 fj.. The ethyl, CH3, 

 series gave a line at 0.74 /x and a second band at about 0.92 fi. Hence 

 they decided that " when we find a body having a band at 0.74 fx and 

 another beginning at 0.907 /x and ending at 0.942 fx we may be pretty 

 sure that we have an ethyl radical present. In the aromatic group 

 (e. g., benzene) the critical line is at wave-length A =0.867/1. If that 

 line be connected with a band we feel certain that some derivative of 

 benzene is present." They found remarkable coincidences in ammo- 

 nium hydroxide, benzene, aniline, diethyl aniline, etc. The presence of 

 oxygen in a compound was observed to sharpen certain bands. They 

 call attention to the " remarkable fact that the 0.866 /x band of the sun 

 should be the basic lines of the benzene series," and " it would not be at 

 all surprising to find that A = 0.76 ix was another nucleus of a hydro- 

 carbon group." The recurrence of these lines is not to be overlooked 

 in considering others far in the infra-red, which, like those of Abney 

 and Festing, are broadened in some compounds and narrow in others. 

 They found a marked linear spectrum for chloroform, CHCI3, but could 

 not detect these lines in carbon tetrachloride, CCI4. Hence, they con- 

 cluded that the C and CI seem to have nothing to do with the linear 

 spectrum observed in chloroform and, since two lines of water are coin- 

 cident with those in the spectrum HCl, HNO3, H2SO4, NH^OH, etc., 

 that hydrogen must be the cause of the lines. 



Again, in some of the compounds containing oxygen, certain lines 

 coincided with the iodides, which were free from oxygen, and they 

 " were forced to the conclusion that there must be some connection 



lAbney & Festing : Phil. Trans., 172, p. 8S7, 1882. 



