156 DYEING 



dyeing of textiles (though not in microtechnique), exhaustion is 

 often nearly complete. 



If, then, a substance acts as a dye, two problems present them- 

 selves: why is it coloured, and why is there an affinity between it 

 and the object that it dyes? The present chapter is concerned with 

 the first of these problems; the second will be considered in 

 chapter 10. 



All dyes, in the strict sense of the word, are organic compounds. 

 It is a remarkable fact that very few aliphatic substances are 

 coloured. Aromatic substances, on the contrary, have a tendency 

 to absorb electro-magnetic weaves. Whenever we can write two or 

 more equally good structural formulae for a particular substance, 

 it is supposed that there is a possibility of rapid change in the 

 configuration of the molecule between the various possible states, 

 and this change or resonance involves the absorption of electro- 

 magnetic waves. 



Alternative structural formulae for benzene 



If our eyes were sensitive in the ultra-violet, benzene would 

 appear coloured, for it has an absorption-band at wave-length 

 256 m^Lt; and other aromatic compounds have absorption-bands at 

 various wave-lengths in the ultra-violet. There is a tendency for 

 resonance to be transmitted through a path of alternate double and 

 single valency-bonds. Indeed, aromatic compounds tend to absorb 

 electro-magnetic waves at particular w^ave-lengths just because 

 there is this alternation of bonds. Annatto (bixin), a plant-product 

 used in the colouring of butter and cheese, is of particular interest 

 in this connexion, for it is one of the rare aliphatic compounds 

 that are strongly coloured, and its long molecule presents that 

 alternation of double and single bonds that is so characteristic of 

 aromatic substances. 



As a general rule, the alternation of bonds does not in itself 

 shake up the molecule sufficiently to extend the absorption-bands 

 into the visible region and thus produce colour. Any configuration 

 of the molecule that pushes the absorption bands downwards into 

 longer wave-lengths is said to be hathochromic^ and certain par- 

 ticular configurations are so effective in this respect that their 

 presence is always associated with colour. One of these is the 

 quinonoid arrangement. Quinone itself (parabenzoquinone) is 



