310 CHYMISTRY APPLIED TO AGRICULTURE. 



earthen pan, and pour over it four or five times its own 

 volume of very pure water ; stir the fecula very carefully, 

 raising it with the hand in. the water, and let this be re- 

 peated occasionally for several hours, after which it may be 

 allowed to settle; when the fecula is entirely deposited, 

 turn off the water and add more, and let this be repeated 

 till the water is no longer colored. As washing in cold 

 water will not remove all the foreign substances which in- 

 jure indigo, it is necessary to have recourse to hot water ; 

 but to perform the last washing economically, it is necessary 

 to collect the product of several cold washings, and to 

 operate upon large quantities. 



Before commencing the washing in hot water, the fecula 

 receives a certain degree of consistency by compression, 

 after which it is placed in a tub and allowed to ferment 

 during ten or twelve days, till it exhales a strongly acid 

 odor ; by this means a mealy portion, which escapes the 

 action of cold water, is decomposed. The process of wash- 

 ing in hot water is next performed in the same manner as 

 I have directed for the cold washing ; the operation may, 

 however, be shortened, and very nearly the same results 

 obtained by boiling the indigo in water, taking care to stir 

 it the whole time. 



To bring indigo to the greatest degree of purity, and to 

 give it the forms which it ought to have in commerce, it 

 must undergo certain other processes. 



The washings in water remove all those substances which 

 are capable of being dissolved; fermentation decomposes 

 certain principles which are foreign to the nature of indigo ; 

 but there still remain in it, in greater or less quantities, 

 certain earths, which, according to their several proportions, 

 adulterate it, and which should therefore be extracted; for 

 this purpose the indigo-paste is thrown into a vat furnished 

 with two or three stop-cocks situated at various heights, and 

 is there diluted with a large quantity of water. The indigo 

 is carefully mixed with the water, so that all the particles 

 of it may swim separately in the liquid ; the upper stop-cock 

 is then opened and the water drawn off into a bucket ; the 

 second is then opened, and afterwards the third, and the 

 indigo which the water carries off is allowed to precipitate 

 itself 



As the earthy deposit which is formed at the bottom of 

 the vat contains some indigo, it is washed in a great quan- 

 tity of water, which is drawn off in the same way as the 



