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 
