FERMENTATION OF INDIGO. 155 



according to Beyerinck's report, Bacillus cyaneo-fuscus has been 

 found to make itself unpleasantly apparent in a glue factory by 

 giving rise to black glue, which, by reason not only of its un- 

 desirable colour (due to the pigment developed by the microbe), 

 but also of its diminished setting power, was thus seriously 

 depreciated in value and merchantable quality. The source of 

 infection was discovered in a dirty pipe previously used for the 

 conveyance of ditch water, and afterwards for delivering the 

 finished glue into the setting pans. When this pipe was cleansed 

 the evil disappeared entirely. 



96. The Fermentation of Indigo. 



As is well known, the indigo so highly prized for dyeing, and 

 constituting such an important article of the world's commerce, is 

 obtained from certain species of the Leguminous genus Indigofera. 

 The province of Bengal alone produces from Indigofera tinctoria 

 over twelve millions of pounds of indigo per annum, the value 

 being about ^2,000,000 sterling. Some five hundred thousand 

 workers make a livelihood in that province by the cultivation and 

 treatment of this plant. The colouring matter does not exist 

 ready formed in the plant, but is developed therefrom by the 

 fermentation of a glucoside constituent known as indican. The 

 plants are cut down shortly before flowering-time, and are left 

 immersed in a five- to eight-fold quantity of water, to undergo a 

 continuous fermentation for eight to fifteen hours at an atmos- 

 pheric temperature of 2 5 "-3 5 C., the liquid gradually becoming 

 yellow in colour, with an alkaline reaction, and throwing up a 

 blue-violet scum. The most important of the fractional processes 

 constituting this fermentation is the splitting up of the indican 

 into sugar (indiglucin) and indigo white, which remains in solu- 

 tion in the alkaline liquor. The liquor is then drawn off into 

 another vessel and beaten with rods, whereby numerous and fresh 

 points of attack are presented to the air and the indigo white 

 (C 16 H 12 N 2 2 ) is oxidised by the atmospheric oxygen into indigo 

 blue or indigotin (C 16 H 10 N 2 2 ), which is precipitated as an in- 

 soluble body and deposited at the bottom of the vessel. The 

 sediment is then boiled in a pan, strained through cloths, dried, 

 and brought into commerce in the form of irregular fragments, 

 cubes, balls, &c. 



Several noteworthy observations on the organisms taking part 

 in this fermentation have been recorded by ALVAREZ (I.). The 

 fermentation may be performed on a small scale by comminuting 

 fresh leaves of the indigo plant and leaving them to stand in 

 water, whereupon fermentation, attended with evolution of heat, 

 will ensue in twelve to twenty-four hours, the surface of the liquid 

 becoming covered with a thin blue skin, which, when broken, 

 subsides to the bottom and is succeeded by a new one. Micro- 



