366 SCIENCE PROGRESS 



N . C 6 H 5 . HC1 



II 



C 6 H 5 . NH . QH/ X C G H 4 . NH . C 6 H 5 



Aniline blue. 



In the colourless 



NH 2 



C. OH 



NH 2 . QH/ X C 6 H 4 . NH 2 



Pararosanilin base. 



the quinoid structure is absent. 



To account for the occurrence of colour in aliphatic substances, 

 Armstrong subsequently modified his views and suggested that 

 unsaturated groups or atoms might act as light-absorbing 

 centres and produce colour by their co-operation. In most 

 cases three such centres must be present to cause a pronounced 

 colour. Bodies with the quinoid configuration, for example 

 quinone itself, possess four of these centres : the two ethenoid 

 linkages in the nucleus and the two C=0 groups. When 

 less than three are present, as in />-diketohexamethylene, no 

 colour is seen : 



Aliphatic compounds which possess a sufficient number of 

 light-absorbing centres are coloured, as for example iodoform, 

 CHI 3 , with three iodine atoms. If one of the iodine atoms is 

 replaced by hydrogen we have the colourless body methylene 

 iodide, CH 2 I 2 . Such coloured aliphatic substances have been 



