The Action of Acetic Acid on Indigo Blue. 165 



used in such excess as to exercise a destructive action upon 

 the product itself. The oxy-aceto-indigotin, as the prepa- 

 ration may be named for the present, does not admit of 

 purification by any process of dissolution and recrystallising ; 

 for practical purposes it is insoluble in all menstrua that I 

 have tried. In some directions it possesses considerable 

 stability ; mineral acids, unless concentrated, have no action 

 upon it ; a mixture of permanganate and sulphuric acid, 

 which destroys indigo rapidly, has no action upon it, and 

 may be used to free a product from uncombined indigo ; 

 chromic acid, chlorine, etc., in dilute solutions, have no 

 action upon it ; it is very readily decomposed by 

 alkalies. I cannot claim to know the real com- 

 position or constitution of this new and interesting 

 body ; it contains no indigo blue, no isatine and 

 no acetic acid. Combustions of the purest and most 

 successful preparations were made again and again ; in those 

 thought best the C was to the H as 20 to 15, never as 

 20:16, as it was thought the ratio should be ; but as I had 

 never a pure product to analyse, and as the tendency of the 

 impurity was to reduce the proportion of hydrogen to carbon, 

 I have for the present adopted the ratio of C20 to Hie, and 

 believe the following equation represents the chemical 

 change : — 



Indigo. Acetic Acid. 



C 16 H 10 N 2 O 2 + O + 2(C 2 H 4 2 ) = H 2 + C 20 H 1G N 2 O G . 

 The assumption of the separation of the elements of 

 water during the reaction is deduced rather from theoretical 

 considerations of the behaviour of the compound than from 

 any very reliable observations in practice, but I think there 

 is no doubt the excess of acetic acid beomes diluted, and is 

 no longer able to act as the monohydrated acid acts. The 

 calculated yield is 145%. as much as 140% has been got, and 

 often 125% and 130%. The decompositions of the oxy-aceto- 

 indigotin are numerous, and some very complex. I only 

 M 



