COLOUR AND CHEMICAL STRUCTURE 



;67 



called pseudo-quinones. In this form the hypothesis is not 

 very different from the theory of chromophores. 



Hartley was the first to study the ultra-violet region of the 

 spectrum, and in 1887 he showed that the benzene spectrum 

 exhibits a number of narrow bands in this portion. He sug- 

 gested that absorption is caused by the vibrations of intra- 

 molecular particles synchronising with those of the incident 

 light waves, and that by the introduction of certain groups 

 such as Witt's auxochromes, or by the fusion of benzene nuclei, 

 the vibration period is slowed and the absorption bands shifted 

 towards the red . The residual affinity, if any, of the substituent 

 also alters the character of the absorption bands, and Formanek 

 has shown that colouring matters of the same class possess 

 similar absorption spectra. 



The effect of the mass of the substituent in slowing the 

 molecular vibrations is exhibited in the case of 



HO 



OH 



HO 



OH 



and 



Fluorescein (yellow). 



O 



Eosine (orange). 



The former has an absorption band in the blue region of the 



spectrum, producing a yellow colour ; the introduction of four 



bromine atoms shifts this band towards the green so that 



eosine appears reddish. The addition of a chlorine atom, or 



of two methyl groups, to chrysoidine or related substances has 



the effect of deepening the colour : 



Cl , 



NHjHCl 



Chrysoidine (yellow). 



— N :N- 

 NH 



-N : N- 



nh 2 L^Jnh 2 



Chlorochrysoidine (orange). 



NH 2 



r^cHs 



CH3 



HC1 



Benzene-5. azo-4 : 6. diamino- i : 3. 

 xylene orange). 



