54 G. S. A. Ranking — A Note on Indigo Manufacture. [No. 1, 



It is thus evident that in the case of the Sodium- silver- indigo salt 

 the resulting compound is made up of metallic silver, traces of AggO 

 and an Indigo-brown insoluble in water and alcohol but soluble in NH3* 



It would appear therefore that while with the other metals the In- 

 doxyl combines with them and is oxidised to Indigo-blue, in the case of 

 the Sodium-silver-indigo compound the Indigo is further oxidised, re- 

 ducing the silver to the metallic condition, itself being a brown substance 

 soluble in NH^HO but not soluble in HgO. Now this is interesting as 

 throwing light upon the way in which Indigo-brown (a source of great 

 loss in manufacture) is formed. The Indigo-brown however which re- 

 sults from the improper fermentation of Indigo liquor is soluble in water. 



Now it occurred to me to try whether, by a process of reduction 

 from this last product, we could get a less highly oxidised Indigo-brown, 

 and the results are most interesting. The solid extract obtained from 

 dry Indigo leaves a brown resinous looking substance soluble in wa^er, 

 obtained by evaporation to dryness of a watery extract of dry Indigo 

 leaves, was reduced in acid solution by means of Zinc : a brown powder 

 was thrown down and on separation by filtration and washing was found 

 to be insoluble in water but soluble in NH^HO thus behaving exactly 

 like the brown Indigo compound found to be in the Sodium-indigo-silver 

 compound. 



The inference is therefore possibly just that this Indigo-brown 

 occupies a position intermediate between Indigo-blue and the soluble 

 Indigo-brown resulting from the action of heat, alkalies, or improper 

 fermentation on Indigo Vat-liquor : inasmuch as it is produced by a 

 degree of oxidation which reduces the silver to the metallic state ; and 

 is also obtainable by reducing agencies from the soluble Indigo-brown 

 formed in dry Indigo leaves. 



It is therefore a further possible deduction from the facts observed, 

 that an improper oxidation of the Indigo Vat-liquor will result in Indigo- 

 brown instead of Indigo-blue : possibly this occurs during manufacture 

 owing to the prolonged " beating " which is necessary to secure the 

 deposition of the Indigo-blue in cases where fermentation has not gone 

 on properly, as it is only formed in a small quantity in laboratory 

 experiments with the plants, where all the stages can be more carefully 

 controlled than is possible in actual practice owing to the various dis- 

 tances from which plant has to be brought and the irregular steeping 

 which of necessity results. 



However this may be I am certain that alkalinity f of the water 



* Cf. Indihumin (Schunck.) 



t Acidity of the steeping water leads to the production of a red compound 

 soluble in water from which it may be separated by agitation with Ether PIndirubin. 



