88 CYTOLOGICAL TECHNIQUE 



Many aromatic, but few aliphatic compounds are coloured; 

 and this suggests that there must be something in ring-structure 

 that favours the production of colour : that is to say, that favours 

 the absorption of light of particular wave-lengths. This is true; 

 and all dyes are aromatic compounds. Benzene itself, the simplest 

 of this group of substances, would appear coloured if we could 

 see a short way into the ultra-violet, for it has an absorption- 

 band at the wave-length of 256 mjn. The molecule may be thought 



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Alternative structural formulae for benzene 



of as 'resonating' between different molecular configurations; 

 that is, undergoing rapid change from one to another. 



The alternation of single and double bonds between carbon 

 atoms is favourable to resonance and the associated absorption 

 of electro-magnetic waves; but there are certain molecular 

 arrangements that excite the molecule to a particular degree, so 



O 



II I! 



11 



The paraquinonoid 

 ring 



O 



Parabenzoquinone 



that light of greater wave-length is affected, and colour results. A 

 very frequent arrangement of this sort in dyes is the quinonoid. 

 All substances possessing a paraquinonoid ring are coloured. 



Parabenzoquinone is the simplest of such substances. It is a 

 pale yellow solid, dissolving in water to give a pale yellow solu- 

 tion, but it is not a dye. It dissolves to form a solution of mole- 

 cules, whereas dyes dissolve as ions. Parabenzoquinone has no 

 special tendency to attach itself to proteins or other tissue- 

 constituents in such a way as to impart colour to them: dyes 

 have such a tendency, powerfully developed. 



