191Jf\ Organic Compounds 85 



rendered the issues more complex and the explanations more 

 involved. 



Hartley offers a theory in explanation of color which is 

 markedly different from any of those discussed above. In a 

 discussion of his theory he says, " color may be visible or in^ 

 visible, but a visible color is one which causes absorption of any 

 rays with wave lengths not less than 3933, the more refrangible 

 rays in the violet H^ and H2, and not greater than 7951, the 

 deep red rays of rubidium." 



As stated earlier benzene shows a series of narrow absorption 

 'bands in the ultra-violet and these bands become displaced 

 towards the visible region of the spectrum by the introduction 

 of certain groups. Hartley contends that when two or more 

 benzene nuclei are fused or an auxochrome such as OH or ISTHs 

 is introduced into the nuclei there is a displacement of the bands 

 towards the visible region of the spectrum. Hartley's theory is 

 that the fusion of the nuclei or the introduction of an auxo- 

 chrome brings about a retardation of the period of molecular 

 vibration or damping of their oscillation; that is the absorption 

 bands are shifted towards the red. A chromogen is regarded as 

 an invisibly colored substance and a chromophore as an atom or 

 group which reduces the speed of vibration so that there is an 

 absorption of rays within the region of visibility. While the 

 linking of benzene nuclei may not produce color, it does reduce 

 vibration and bring therefore the absorption bands nearer color. 

 For example, triphenylmethane, though colorless or pale yellow 

 when fused, produces a broad band of the same general char- 

 acter as benzene but of greater intensity and much nearer the 

 margin of the visible spectrum. It is therefore a chromogen. 

 Para-nitrophenol absorbs faintly in the violet and possesses a 

 green tint though apparently colorless. When it is converted 

 into its sodium salt, the absorption shifts towards the visible 

 spectrum. There is not, as Hantzsch's theory would require, 

 any change of structure. Formanek has studied a large num- 

 ber of dye stuffs and the relation of color to structure in them. 

 He concludes that coloring matters which have analogous struc- 

 ture possess similar absorption spectra and the same is true of 

 those which have the same chromogen and the same number of 



