12 Robinson, The Conjugation of Partial Valencies 



This expression as a vehicle for the explanation of the 

 behaviour of the compound has grave defects, and, since the 

 interpretation of the properties of the carbonyl group is very 

 much a matter of opinion and relations of colour to structure 

 still more so, the criticism of the formula may be confined to 

 its failure to indicate the non-basic character of indigotin and 

 the anomaly of the non-existence of cis-trans modifications. 

 There seems to be no reason why the isomeride, 



y\ 



\/\nh 



has never been isolated. All these and other objections are 

 completely met by regarding indigotin as a true indole deri- 

 vative. The series of formulae now suggested are : — 



C-OH 



iCH 



/VVV 



W 



Indole. 



Indoxyl. 



OH 



HO 



HH 



A/H 



Indigo White. 



+ r< 



Indigotin. 



