308 CHYMISTRY APPLIED TO AGRICULTURE. 



In this operation, no foreign substance by which the 

 indigo can be adulterated is employed ; and it is therefore 

 obtained as pure as the best of the imported kind. 



When the leaves of the isatis are operated upon with 

 cold water by maceration, fermentation, or any other 

 method, the indigo cannot possibly be separated by beating. 

 The reason of this is, that the elevation of the tempe- 

 rature is not high enough to cause the combination of 

 oxygen with the indigo, and thus to ^ive it the color and 

 other characteristics which render it so valuable in the art 

 of dyeing. 



The substance which in these cases is most usually 

 employed to produce precipitation, is lime-water; but as 

 this process requires much attention, I shall describe par- 

 ticularly the use and action of this ingredient, that the man- 

 ufacturer may be the better able to direct it. 



After all the water which has been prepared in the course 

 of the day, has been collected in a tub, the operation of 

 precipitating the indigo from it is commenced in the fol- 

 lowing manner. 



The liquor is beaten almost uninterruptedly, and without 

 any particular method, for half an hour, the operation being 

 interrupted only to allow the foam to subside and exhibit 

 its color. When the liquor begins to appear of a deep 

 brown, five or six pints of lime-water are thrown into it. 

 The beating is continued, and the lime-water added at inter- 

 vals, till the liquor exhibits a greenish yellow, begins to grow 

 turbid, and to show in a state of suspension the substance 

 which is about to be precipitated. The quantity of lime- 

 water which is necessary to be used in this process, when 

 added at intervals, in the manner here directed, is never 

 more than one tenth of the volume of the liquor with which 

 it is mixed ; but if the lime-water be all thrown in at once, 

 the lime more than saturates the carbonic acid of the liquor, 

 and the carbonate thus formed, being precipitated, mixes 

 with and weakens the indigo. 



In the last described method of producing precipitation, 

 a large quantity of air is introduced into the liquor by 

 beating. This combines with th^ indigo, rendering it 

 insoluble in water, and forming at the same time a great 

 deal of carbonic acid. The admixture of a small quantity 

 of lime-water after each beating, produces an acidulated 

 carbonate, which remains in solution in the liquor, and a 

 kind of soapy combination with the extractive and vegeto- 



