84 Journal of the Mitchell Society ^January 



chrome to the chromophore as frequently destroys color as in- 

 tensifies it but Kauffmann concludes that when the auxochrome 

 is attached indirectly to the chromophore by an aromatic nucleus 

 which can function as a chromophore, the color is intensified. 



Kauffmann, in a variation of the foregoing, offers what is 

 sometimes called the auxochrome theory. He considers that 

 those substances which at atmospheric pressure have the prop- 

 erty of luminescence possess certain characteristics in common 

 which form the basis of color. Luminescent compounds are 

 mainly benzene derivatives and contain certain groups which 

 Kauffmann terms luminophores. Benzene is regarded as the 

 seat of luminescence. Benzene itself is only feebly luminescent 

 but the effect may be intensified by the introduction of an auxo- 

 chrome, by the multiplication of aromatic nuclei, or by linking 

 nuclei with unsaturated carbon chains. Benzene is optically 

 colored for it produces in the ultraviolet a banded spectrum. 

 The effect produced by the introduction of auxochromes is to 

 shift the absorption toward the red and thus produce color. 

 Some of the absorption bands of nitrobenzene lie just within the 

 visible spectrum so a weak auxochrome produces visible color. 



Hantzsch has advanced a theory of color in compounds which 

 has become known as the theory of chromoisomerism. His 

 theory has been developed principally from observations con- 

 cerning the nitrophenols in which the chromophore, NOo, is 

 alone too weak to produce color until reinforced by the auxo- 

 chrome, OH, the chromogenic character of which is intensified 

 by conversion into the salt. Hantzsch holds that all true nitro- 

 phenols and their derivatives are colorless. For instance dini- 

 troethane CH3GH(N02)2, is colorless but on the formation of 

 the sodium salt, which is deep yellow, there is a change in the 

 form to CH3C(N02)N'0.0]Sra called the aci — form corres- 

 ponding to the pseudo acid and acid forms of phenylnitro- 

 methane. He expresses his general idea thus : " Every appear- 

 ance of color or change of color in salt formation with a color- 

 less metallic ion is due to isomeric change." The process of 

 isomeric change he terms chromotropism. The attempt to 

 extend the theory by further experimental observation has only 



