INDIGO. 317 



is pursued, the chief apparatus consists of three 

 wooden vats of different sizes, placed on different 

 levels, so that the contents of the first may flow into 

 the second, and those of the second into the third. 

 The plants, on being cut, are laid in the first or 

 steeping vat, in sufficient quantity to fill it without 

 receiving pressure, and water is poured over them 

 until it rises about three inches above the level of 

 the top plants. A frame of heavy wooden bars is 

 then laid on the vat to prevent the plants from 

 rising when in fermentation. This state is generally 

 induced in less than eighteen hours. The contents 

 swell and foam; large bubbles of gas are formed, 

 which on being disengaged appear of a lively screen, 

 and tinge the whole vat of the same colour. When 

 at the highest, *he fermenting mass is covered with a 

 brilliant copper-coloured scum, which passes into 

 violet towards the end, but the pulp and liquor 

 remain green. The gas given off during the process 

 is inflammable *. The heaving of the scum is so 

 powerful as often to lift up the heavy wooden frame 

 above mentioned. This fermentation is carried ori 

 for the purpose of extracting all the grain or colouring 

 matter from the plant, and it is a nice point to 

 ascertain the exact period when it ought to cease. If 

 the fluid be drawn off too soon, much of the pulp is 

 left behind, and if too late, the tender tops of the 

 plant occasion putrefaction, by which all the dye is 

 destroyed. Many plans have been suggested to dis- 

 cover to a scientific certainty the most advantageous 

 degree of fermentation. Experiments were made at 

 St. Domingo, when the French possessed that island, 

 under the sanction and encouragement of the Chamber 

 of Agriculture; but the unsatisfactory result only 

 served to convince practical men that they could not 

 with safety trust to any test save that of experience. 

 * Aikin's Chemical Dictionary. 



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