COLORATION OF ANILINE: II. • 423 



The vibration causing the absorption must cease when the incident 

 light is removed or otherwise the substance would be self-luminous in the 

 dark. Moreover, the absorption band can not in any way be regarded 

 as being directly due to an oscillating atom, for the oscillation frequency 

 of the light waves absorbed is nearly the same for all substances examined 

 irrespective of the nature or mass 20 of the atoms in the so-called labile 

 condition and the vibration frequency of the atom can not be the same 

 as the oscillation frequency of the light absorbed. The frequency at- 

 tributed to atomic vibration is never so high as that of the absorbed 

 bands. 



We are therefore forced to conclude that the absorption of light is due to the 

 transformation which is expressed chemically by a change of linking. 21 



In the aromatic compounds the same type of absorption band is shown 

 by enol-keto compounds such as phenol. In this connection Baly and 

 Ewbank state. 22 



Now it has been shown that the tautomeric process, such as occurs in acetyl- 

 acetone and analogous aliphatic compounds, causes the formation of a band in 

 the absorption spectra of these compounds, the head of which lies very much in 

 the same region as does the band in the spectrum of phenol and its esters. 



The same type of tautomerism therefore occurs in phenol as in the 

 aliphatic compounds. 



That there is present in the aniline molecule (also methylaniline ) 

 the so-called labile hydrogen atom can hardly be doubted and there is 

 much evidence that- this condition very -strongly resembles that which 

 exists in the enol-keto tautomers. The absorption spectra of aniline and 

 phenol are of the same general type. The phenol absorption band in the 

 ultraviolet extends from 291.6 to 243.1 /a/x heading about 277 up. The 

 head of the absorption band due to the enolic-ketonic condition in phenol 

 lies about 10 jxjx further into the ultraviolet than the head of the cor- 

 responding band in aniline. The position of the aniline absorption band 

 may depend upon the free residual affinity of the nitrogen atom. 



The aniline system of molecular arrangement which is synchronous 

 with the light absorbed will therefore be expected to be much more 

 sensitive to the sunlight than the phenol molecule, for the reason that 

 the corresponding band in the latter lies nearer the ultraviolet limits 

 of the sun spectrum and in regions of lesser intensit} r , 23 a conclusion 



20 It was found that the oscillation frequency of the light waves absorbed is 

 nearly the same for all the substances examined, whether these contain a hydrogen, 

 glucinum, sodium or even a thorium atom in the so-called labile condition. Baly 

 and Desch. Jour. Chem. Soo. London (1905), 87, 766. 



21 Baly and Desch, Astropbys. Joiirn. (1906), 23, 117. 



22 Jour. Chem. Soc. London (1905), 87, 1347. 



23 Investigations of tropical sunlight which touch upon this phase of the 

 question are being carried on by the writer and will be ready for publication 

 shortly. 



