278 Experiments on Indigo, 



Carbonate of lime is seldom absent ; for whatever lime may 

 be in the plant, or in the water, is sure to be precipitated by 

 the carbonic acid developed during the fermentation. Some 

 manufacturers are in the habit of employing rain water pur- 

 posely to avoid this source of adulteration, but it is doubtful 

 whether the plant will not itself bring a portion of earths into 

 solution. 



Upon the whole, it may be safely decided, that the purer 

 the water is, and the cleaner and the more simply all the 

 operations of the manufacture are conducted, the more beau- 

 tiful and rich in colour will be the indigo : and it should be 

 the first maxim of the planter, that it is purity, and not weight, 

 which gives the value to his produce in the market. 



In conclusion of the present desultory notice, which will, I 

 hope, at least serve the purpose of leading others to bestow 

 further attention upon the subject, I annex an analysis of a 

 specimen of indigo, denominated fine blue in the Calcutta 

 market, made in the year 1820, to which I have referred in a 

 former part of this paper. 



Analysis of Calcutta Indigo. 



] 00 grains heated white in a closed platina crucible left a porous 



grey carbonaceous mass, with metallic lustre, weighing . . 49.0 

 Burnt with access of air, the 49 grains were reduced to . 7.42 



I. — Examination of Earthy Residue, 7. 42. 



1 . Boiled dry in nitric acid, and then digested in muriatic acid, 



a brown residue of oxide of iron and alumine remained . 2.7 



2. From the solution, ammonia threw down alumine . . 0.75 

 . Oxalate of ammonia— lime equal to . 0.9 



3. The clear liquor evaporated left red oxide of iron . . 3.05 



, 7.4 



2. — In the humid way. 



1. 100 grains of indigo digested in boiling water, some green and 

 dark brown matter was dissolved, which, when dry, weighed . 1 .6 



2. Alcohol boiled over the remainder became of a bright claret 

 colour, and yielded, on evaporation, a dark brown matter and a 

 little yellow resin, weighing 2.0 



3. Dilute muriatic acid then took up a mixture of greenish vegetable 

 matter and earths, which were afterwards separated by burning. 



The green matter was thus found 7.2 



4. The indigo now deemed pure weighed only 79.5 



