204 PLANT AND ORGANIC CHEMISTRY 



of beams fixed into side posts, bamboo being placed under the 

 beams. Water is immediately run in, just sufficient to cover 

 the plant. The pure water from the Ganges is especially 

 sought for in these manufactories, and many indigo factories 

 line the river banks. The time for steeping depends much on 

 the temperature of the atmosphere, and can only be learned 

 by experience and careful watching of the vats, but in close, 

 sultry weather, with the thermometer at 96 in the shade, 

 eleven or twelve hours are sufficient. In cooler weather more 

 time is requisite. 



When fermentation is established, the surface of the vat is 

 covered with a violet scum. The liquid is drawn off through 

 plug holes in the wall of the vat. The fecula at the bottom 

 is then removed to the boiler. It is brought to the boiling 

 point as quickly as possible, and kept there for five or six 

 hours. While boiling it is stirred to keep the indigo from burn- 

 ing, and skimmed with a perforated ladle. When sufficiently 

 boiled it is run off to the straining table, where it remains 

 twelve or fifteen hours draining. It is then taken to the presses 

 and gradually pressed. This process takes twelve hours. It 

 is then ready to be taken out, cut, stamped, and laid in the 

 drying house to dry. 



In the manufacture of indigo the ordinary processes of 

 fermentation, drawing-off the liquor, beating, and collecting 

 the fecula, are generally well known and are followed with 

 but trifling variation in different provinces and manufactories 

 in India. The main points appear to be the watching and the 

 soaking of the plant so as to be able to tap off the infused 

 liquid at exactly the right point of fermentation, and next to 

 beat the liquid in the second vat long enough. 



Indigotin as it is contained in the vegetable tissues is color- 

 less, but it becomes blue on contact with air. If it is desired 

 to change indigo blue to indigo white, it is only necessary to 

 place it in the presence of a deoxidizing and alkaline liquid, 

 but as soon as air is admitted its blue color is resumed. 



The dyeing of fabrics is based upon the transformation of 

 indigo blue into soluble indigo white. The colorless matter 

 is placed on the stuff, which becomes blue by exposure. The 



