658 SCIENCE PROGRESS 



derivatives and corresponds in the chief cases to the following 

 three formulae : 



Diagonal. 

 I. 



The action of the colour-producing groups consists in altering 

 the contour of the benzene ring to produce those forms (II. and 

 III.) which contain the double bonds. These, as previously- 

 shown, have a strong chromophoric action under certain 

 conditions. 



In many cases the auxochrome-chromophor theory is very 

 unsatisfactory and leads to a number of contradictions. In the 

 azo compounds, for example, although in general the colour 

 intensity of the azo chromophore is continually increased by the 

 auxochromes combined with it, from the nearly colourless 

 azoisobutyricester to the red azobenzene and azodicarboxylic 

 acid derivatives, yet it is frequently found that the strong 

 auxochromes —OH and — NH 2 have no action, or indeed 

 sometimes a negative action, whilst other lesser known groups 

 produce strong colour intensification. 



The Quinone Theory 



The first attempt to find an explanation of colour apart 

 from Witt's chromophore-auxochrome theory was occasioned by 

 Armstrong's original discussion on the origin of colour about 

 fifteen years ago. 1 Armstrong pointed out that in the case of 

 those coloured compounds which had at that time been well 

 studied, by far the greater number could be given a quinonoid 

 structure, i.e. be formulated as derivatives of quinone. This it 

 was maintained was the cause of their colour, and indeed of 

 coloured organic compounds in general, the converse of this, 

 that quinone derivatives are always coloured, being known to 

 be true. Although these views were put forward without any 

 definite practical confirmation, they at once found a very wide 

 acceptance, and the formulae of many coloured bodies which 

 did not accord with them have been reconsidered and 

 reconstructed. 



1 Proc. Chem. Soc. 1888, 27, 1892, 101, 103, 143, 189, 194, 1893, 206, 1896, 42. 



